Geomorphology

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CONTENTS

CHAPTER 1

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1-23

NATURE OF GEOMORPHOLOGY

D e fin itio n and sc o p e o f g e o m o rp h o lo g y ; e v o lu tio n o f geom orphological thoughts; Indian contributions to g eo m o rp h o lo g y ; system c o n c e p t ; g eo m o rp h ic m odels ; m ethod s and ap p ro a c h e s to the study o f landform s.

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CHAPTER 2

FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

A

24-56

C o n c e p t s r e l a t e d to u n i f o r m i t a r i a n i s m , g e o l o g i c a l s t r u c t u r e , g e o m o rp h o lo g ic a l p rocesses, stages o f time, g e o m o rp h ic sc a le (tim e s c a l e - c y c l i c tim e , g r a d e d tim e a n d s te a d y tim e , s p a ti a l s c a l e ) , g e o m o rp h o lo g ic a l equation, com plex ity o f la n d fo rm s etc. CHAPTER 3

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TH EO RIES O F LANDFORM DEVELOPMENT

57-88

L a c k o f c o m m o n ly a c c ep tab le theo ry ; s ig n ific a n c e a n d g o a ls o f g e o m o r p h ic th e o ries ; historical p ersp ectiv e ; b ases a n d ty p e s o f g e o m o rp h ic theo ries (teleological theory, im m a n e n t th eory, h isto rical th e o ry , ta x o n o m ic theory, functional theory, realist theory, c o n v e n t io n ­ alist t h e o r y ) ; m a jo r g e o m o rp h ic theories o f G. K. G ilb ert, W .M . D a v is , W . P en ck , L. C. K ing, J. T. H ack, M . M o ris a w a an d S. A. S c h u m m ; g e o m o rp h ic th eories in Indian context. CHAPTER 4

CLIMATIC GEOMORPHOLOGY AND MORPHOGENETIC 89-104

REGIONS

D ia g n o s tic la n d fo rm s ; g e o m o rp h o lo g ica l p ro c e sse s and c lim a tic c o n ­ trol ; d ire c t co ntrol o f cl i m a t e ; indirect clim atic c o n t r o l ; c lim a tic c h a n g e s a n d la n d fo rm s ; m o rp h o g e n e tic regions. CHAPTER 5

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CONSTITUTION OF THE EARTH'S INTERIOR

105-113

S o u rc e s o f k n o w le d g e ; artificial sources, e v id e n c e s fro m th e th e o rie s o f th e o rig in o f the earth, an d natural so u rces ; e v id e n c e s o f s e is m o lo g y ; c h e m ic a l c o m p o s itio n and la y erin g sy stem o f the earth ; th ic k n e s s a n d d ep th o f different layers o f the earth ; recent view s - crust, m a n tle and core. CHAPTER 6

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CONTINENTS AND OCEANS

114-131

In tro d u c tio n ; te tra h e d ra l h y p o th e sis ; co n tin e n ta l d rift th e o ry o f T a y l o r ; c o n tin e n ta l d rift th e o ry o f W e g e n e r ; p late te cto n ic th e o ry . CHAPTER 7

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TH EO RY O F ISO STA SY

132-139

Introduction ; discovery o f the concept ; concept o f Airy ; concept o f Pratt; concept o f Hayford and B ow ie ; concept o f Joly ; concept o f Holmes ; global isostatic adjustment. CHAPTER 8

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ROCKS

140-157

Introduction; classification o f rock s; igneous rock s; sedimentary rocks ; metmorphic rocks. :

EARTH'S MOVEMENT

158-169

Introduction ; endogenetic forces (sudden forces and movements, diastrophic forces and movements - epeirogenetic movements, orogenetic m ovem en ts); folds ; faults ; rift valleys ; exogenetic forces.

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CHAPTER 9

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STRUCTURAL GEOMORPHOLOGY

CHAPTER 10

170-184

Geomorphic expressions of uniclinal structure ; topographic expressions of fault structure (fault geomorphology) ; topographic expressions o f folded structure (fold geomorphology), inversion o f relief, fluvial cycle of erosion on folded structure ; topographic expressions o f domed structure, fluvial cycle o f erosion on domed structure. :

CHAPTER 11

185*199

PLATE TECTONICS

Meaning and concept ; plate margins ; palaeomagnetism-source of g e o m a g n e ti c fie ld , r e m a n e n t m a g n e tis m , r e c o n s t r u c t i o n o f palaeomagnetism, reversal of polarity ; sea-floor spreading ; plate m o ­ tion ; causes of plate motion ; plate tectonics and continental d r i f t ; plate tectonics and mountain building ; plate tectonics and vulcanicity ; plate tectonics and earthquakes. CHAPTER 12

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200-215

VULCANICITY AND LANDFORMS

Concept of vulcanicity ; components o f volcanoes ; classification o f volcanoes ; volcanic types ; world distribution of volcanoes ; m echanism and causes o f vulcanism ; hazardous effects of volcanic eruptions ; topography produced by vulcanicity ; geysers ; fumaroles. CHAPTER 13

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216-246

MOUNTAIN BUILDING

Introduction ; classification of mountains ; block m ountains ; folded mountains ; geosynclines ; theories of mountain building - geosynclinal theory o f Kober ; thermal contraction theory of Jeffreys ; sliding co nti­ nent theory of Daly ; thermal convection current thery of H olm es ; radiactivity theory o f Joly ; plate tectonic theory. CHAPTE 14

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WEATHERING AND MASSMOVEMENT

247-266

M eaning and concept ; controlling factors o f weathering ; types o f weathering processes ; physical weathering ; chem ical w eath erin g ; biotic weathering ; biochemical weathering ; geom orphic im portance o f weathering ; m assm ovem ent and masswasting - m eaning and c o n c e p t ; classification o f m assm ovem ents ; factors o f m assm ov em ents ; slides; falls ; flows ; creep. CHAPTER 15

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HILLSLOPE

267-296

Classification o f s lo p e s ; slope e le m e n ts ; approaches to the study o f slope development-slope evolution approach and process-form approach (m ono­ process concept and poly-process concept) ; slope decline theory o f Davis ; slope replacem ent theory o f P enck ; A. W o o d 's m odel o f slope e v o lu tio n ; hillslope cycle theory o f L.C. K i n g ; co n ce p t o f R. A .S av ig ear ; F isher - L ehm ann model o f slope evolution ; pro cess-resp o n se m o d e l o f A. Y oung ; slope failure ; hillslope p rocesses and erosion. CHAPTER 16

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CYCLE OF EROSION, REJUVENATION AND POLYCYCLIC RELIEFS

297-307

Origin and evolution of the concept ; geographical cycle of Davis ; Penck's model of cycle of erosion; normal cycle of erosion; interruptions in cycle of erosion ; rejuvenation ; topographic expressions of rejuvena­ tion and poly (multi) cyclic reliefs.

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DENUDATION CHRONOLOGY, EROSION SURFACES AND PENEPLAINS

Meaning and concept ; erosion surfaces— meaning, identification o f erosion surfaces, dating of erosion surfaces ; erosion surfaces o f Cnotanagpur highlands ; denudation chronology o f peninsular India ; (ii) https://telegram.me/UPSC_CivilServiceBooks

CHAPTER 17

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308-333

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denudation chronology and erosion surfaces o f Belan basin ; denudation chronology and erosion surfaces o f Ranchi p lateau; p en ep lains; panplains. CHAPTER 18

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DRAINAGE SYSTEMS AND PATTERNS

334—352

M eaning and concept ; sequent drainage system s (consequent, sub se­ quent, obsequent and resequent streams) ; insequent d rainag e system (antecedent and superim posed drainage systems) ; drainage patterns (trellised, dendritic, rectangular, radial, centripetal, annular, barbed, pinnate, herringbone and parallel p a t te r n s ) ; river capture. CHAPTER 19

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MORPHOMETRY OF DRAINAGE BASINS

353-384

M e a n in g and c o n c e p t ; historical perspective ; shortcom ings ; d rain ag e basin : a geom o rp hic unit ; drainage basin : historical perspectiv e ; d rain a g e basin hydrological cycle ; basin m orphom etry ; linear aspects : stream ordering, bifurcation ratio, law o f stream num bers, length ratio, law o f stream length, sinuosity indices, stream ju n c tio n angles ; areal aspects : geo m etry o f basin shape, law o f basin perim eter, basin length a nd basin area, area ratio, law o f basin area, law o f allom etric g row th, stream frequency, drainage density, drainage texture ; relief aspects : h y p so m etric analysis, clinographic analysis, altim etric analysis, av era g e slope, relative reliefs, dissection index, law o f channel slope, profile analysis. RIVER V A LLEYS, GRADED RIVER AND PROFILE OF EQUILIBRIUM

385-395

F o rm s o f valley d ev elo p m en t ; valley deepening ; valley w id e n in g ; valley le n g th e n in g ; classification o f valleys ; graded curv e o f a riv er an d p ro file o f eq uilibrium : longitudinal profile and graded curve, c o n c e p t o f g rade, co n trollin g factors o f graded river, grading o f riv er ch an n e l a n d p ro file o f eq u ilib riu m ; disturbed and regraded cu rv e : effects o f r e ju v e ­ n ation , effects o f deposition. CHAPTER 21

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CHANNEL MORPHOLOGY

396-412

C h a n n e l g e o m e try o r form ; hydraulic g eo m etry (at - a station re la tio n ­ ships, d o w n stre a m variations in channel form s, bed and b a n k m a te ria ls a n d h y d ra u lic g eo m etry , sed im en t load and h y d raulic g e o m e t r y ) ; c h a n ­ nel b ed to p o g rap h y ; ch annel types (b ed ro ck c h a n n e ls and allu v ial c h a n n e ls ) ; ch ann el patterns (straight ch annel, m e a n d e r in g c h a n n e l, b raid e d ch an n e l, a n a s to m o s in g channel and a n a b ra n c h in g ch a n n e l). CHAPTER 22

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FLUVIAL GEOM ORPHOLOGY

413-434

Erosional work of rivers; types of fluvial erosion ; base-level o f erosion ; erosional landforms (river valieys-gorges and canyons, waterfalls, pot holes, structural benches, river terraces, river meanders, ox-bow lakes, and peneplains); transportational work of stream s; depositional works o f streams ; depositional landforms (alluvial fans and cones, natural levees, delta). :

KARST GEOMORPHOLOGY

435-446

Groundwater: meaning and concept; geomorphic work o f groundwater ; erosional work ; depositional work T lim estone (karst) topography ; distribution o f karst areas ; erosional landforms (lapies, solution holes, polje, sinking creek, blind valley, karst valley, caves or cavern s); karst cycle o f erosion. ( iii) https://telegram.me/UPSC_CivilServiceBooks

CHAPTER 23

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CHAPTER 24

447-46*

COASTAL GEOMORPHOLOGY

A gents o f coastal erosion ; sea coast and sea shore ; processes and m echanism o f marine erosion ; erosional landform s (cliffs, w ave-cut platform, natural chimneys, stack, blow h o l e ) ; transportational w o rk , depositional landforms (beaches, bars, barriers and associated f e a t u r e s ) , classification o f coasts, and shorelines ; developm ent o f shorelines and marine cycle o f erosion along a shoreline o f su bm ergence and e m e r­ gence. :

CHAPTER 25

ARID AND SEMIARID GEOMORPHOLOGY

463-477

Aeolian environm ents ; erosional works o f wind ; erosional la ndform s ; transportational works o f w i n d ; depositional w ork o f w i n d ; depositional landform s ( b e d f o rm s ) ; fluvial desert landform s (badland, playas, p e d i­ ments, b a j a d a s ) ; arid cycle o f erosion ; savanna cycle o f erosion. CHAPTER 26

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478-491

GLACIAL GEOMORPHOLOGY

Ice and related pheno m en a ; types o f glaciers ; m o v e m en t o f glaciers ; ero s io n a l w o rk o f g laciers ; erosional and residual la n d fo r m s ; tran sportational and depositional w orks o f glaciers ; dep o sitio n al landform s ; glacio-fluvial deposits and landform s ; glacial geo m o rp h ic cycle ; ice ages and pleistocene glaciation. CHAPTER 27

492-505

PERIGLACIAL GEOMORPHOLOGY

M ean in g and concept ; periglacial clim ate ; periglacial areas ; p e r m a ­ frost ; active l a y e r ; m echanism o f periglacial processes (congelifraction, frost heaving, congelifluction, nivation, fluvial process, and aeolian pro cess) ; genetic classification o f periglacial land fo rm s ; periglacial cycle o f erosion. :

CHAPTER 28

REGIONAL GEOMORPHOLOGY

506-553

K u m a u n H im a lay a region ; G an g a plain ; S. E. C h o ta n a g p u r re g io n ; R an ch i p l a t e a u ; P alam au u p la n d s ; B elan b a s i n ; B h a n d e r p l a t e a u ; G irn a r hill region ; w est coastal plains.

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CHAPTER 29

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APPLIED GEOMORPHOLOGY

554-563

M e a n in g and co n ce p t ; applied g e o m o rp h o lo g y in I n d ia n c o n te x t ; g e o m o rp h o lo g y and regional pla n n in g ; g e o m o rp h o lo g y and h a z a rd m a n a g e m e n t ; g e o m o rp h o lo g y and u rb an iz atio n ; g e o m o rp h o lo g y an d e n g in e e rin g w orks ; g eo m o rp h o lo g y an d h y d ro lo g y ; g e o m o rp h o lo g y an d m in eral exploration.

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CHAPTER 30

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564-589

Meaning and concept; historical perspective; man's impacts on environ­ mental processes; man and hydrological p rocesses; man and weathering and massmovement processes; man and coastal p rocesses; man and river p rocess; man and periglacial processes ; man and subsurface processes ; man and pedological processes ; man-induced soil erosion ; man and sedimentation. :

CLIMATE CHANGE AND QUATERNARY GEOMORPHOLOGY

590-629

Indicators o f climatic changes; causes and theories o f climatic changes; quaternary climatic changes and landforms. REFERENCES

631-639

INDEX

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CHAPTER 31

ANTHROPOGENIC GEOMORPHOLOGY

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NATURE OF GEOMORPHOLOGY

D e f i n i t i o n a n d s c o p e o f g e o m o r p h o l o g y ; e v o lu tio n o f g e o m o rp h o lo g ic a l thoughts; Indian contributions to geomorphology ; s y s t e m c o n c e p t ; g eom orphic models ; methods and approaches to th e s tu d y o f landform s.

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CHAPTER 1

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1

NATURE OF GEOMORPHOLOGY

forms (morphe) of the earth’s surface. T o be m o re precise, forms mean topographic features o r g e o ­ metric features (relief features) o f the earth s sur­ face. P.G. Worcester (1940) prefered to d efin e geomorphology as the intepretative description o f the relief features of the earth's surface while W .D . Thornbury (1954) pleaded for the inclusion o f su b ­ marine forms in addition to surface reliefs in the realm of geomorphology.

The rapidly evolving discipline of geomorphology has undergone seachange in methodol­ ogy and approaches to the study of landforms and related processes since 1945 when R.E. Horton introduced quantitative methods for the analysis of morphometric characteristics of fluvially originated drainage basins. A clear-cut cleavage surfaced in the discipline in the form of evolutionary approach involving progressive changes in landforms through long time periods and process-response approach involving equilibrium model and steady state of landform development after 1950. Thus, the need of the hour is to integrate the cyclic concept involving long-term historical evolution of landlorms and noncyclic concept involving dynamic equilibrium, func­ tional and process-reponse models on the one hand and m icro-geom orphology involving smaller spa­ tial and temporal scales and mega-geomorphology involving larger spatial and longer temporal scales

Geomorphology may be defined as the scien ­ tific study of surface features o f the earth's surface involving interpretative description o f landform s, their origin and development and nature and m e c h a ­ nism of geomorphological processes w hich evolve the landforms with a view that ‘all landform s can be related to a particular geologic process, or set o f processes, and that the landforms thus developed may evolve with time through a sequence o f form s dependent in part, on the relative tim e a particular process has been operating’ (Easterrook, 1969). A.L. Bloom (1979) also defined g eom orphology as the systematic description and analysis o f land­ scapes and the processes that change them.

on the other hand. 1.1 DEFINITION O F GEOM ORPHOLOGY

Geomorphology is significant branch of physi­ cal geography (geomorphology, oceanography, cli­ m atology and b io g e o g r a p h y ). The term geomorphology stems from three Greek words i.e. ‘ge’ (rtieaning earth), ‘m orphe’ (form) and logos (a discourse). Geomorphology, therefore, is defined as the science of description (discourse) of various

1.2 SCOPE OF GEOMORPHOLOGY

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The subject matter o f geom orphology m ay be organized on the bases o f (i) dim ension and scale o f relief features (landforms), (ij) processes that shape the landforms, and (iii) the app ro ac h es to the

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g e o m o r ph o l o g y

(2) RELIEF FEATURES OF THE SECOND ORDER

geomorphic studies. In fact, geomorphology, being a study of landforms, has a well defined framework of its subject matter. The systematic study of landforms requires some fundamental knowledge of geology as the genesis and development of all types of lan dform s p rim arily d ep en d on the m aterials (geomaterials or structure) of the earth's crust and partly on the forces coming from within the earth (endogenetic forces).Based on this connotation geomorphology is, some times, equated with geol­ ogy (W.D. Thornbury, 1954) and sometimes is con­ sidered a branch of geology (A.K. Lobeck, 1939). In fact, geomorphology has originated from geology and in most of the American Universities it is still housed in geology departments. Thus, some aspects o f geology, even today, are included in the descrip­ tion and analysis of landforms e.g. structural and dynamic geology. Theoretical geology helps in un­ derstanding the nature of landforms and, therefore, the origin of different types of reliefs like mountains, plateaus, continents and ocean basins on which the microlandforms are evolved must be properly un­ derstood. Endogenetic forces particularly diastrophic and sudden (vulcanicity and seismic events) should be taken note of as they introduce irregularities on the earth's surface which generate variety in landforms.

The structural forms developed over a con ' nent or part thereof as mountains, plateaus, lakes faults, rift valleys etc. constitute the category 0f relief features o f the second order. These forms owe their genesis mainly to endogenetic forces particu­ larly diastrophic forces. The nature, mode and rate of operation of these endogenetic forces must be stud­ ied properly so that general characteristics, nature and mode of origin of the second order relief fea­ tures, upon which the third order reliefs are pro­ duced, are well understood. These are called as constructional landforms. (3) RELIEF FEATURES OF THIRD ORDER

Micro-level landforms developed on second order relief features by exogenetic denudational processes originating from the atmosphere are in­ cluded in this category. These landforms may be erosional (e.g. glacial valley, river valley, karst valley, cirques, canyons, gorges, terraces, yardangs, sea cliffs etc.), depositional (e.g. drumlins, eskers, flood plains, natural levees, delta, sea beaches, sand dunes, stalactites, stalagmites etc.), residual (e.g. monadnocks, inselbergs or bornhardts etc.) and some times minortectonic features (by endogenetic forces). In fact, the relief features of the third order are given more importance in geomorphic studies as they constitute the core of the subject m atter of geomorphology. Besides, the nature, mode and rate of operation of denudational processes, which pro­ duce the relief features of the third order, are also studied at varying spatial and temporal scales. Be­ sides natural g e o m o rp h o lo g ic a l p ro c e s s e s , anthropogenic processes are also attached due im­ portance in geomorphic investigation because the role of man as ‘economic and technological m an’ through his economic activities has augmented the rate of natural processes beyond imagination (chap­ ter 30).

Thus, on the basis of dimension and scale, the relief features of the earth's surface, the core subject matter of geomorphic study, may be grouped in three broad categories of descending order. (1) RELIEF FEATURES OF THE FIRST ORDER

‘On the smallest scale and covering the larg­ est area is world geom orphology’ (C.A.M. King, 1966) which includes consideration of continents and ocean basins. The consideration and interpreta­ tion of worldwide erosion surfaces requires the de­ scription and analysis of the characteristics and evolution o f continents and ocean basins. Thus, continents and ocean basins become the relief fea­ tures of the first order. The consideration of conti­ nental drift, in one way or the other, caused either by the forces coming from within the earth (thermal convective currents) involving plate tectonics or from outer sources (tidal forces, gravitational forces etc.), becomes desirable for the analysis of major morphological features of the earth's surface. Plate tectonics help in understanding the origin of conti­ nents and ocean basins.

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The subject matter of geomorphology may also be organized on the basis of geomorphic proc­ esses (both endogenous and exogenous) that shape the landforms and approaches to the study of landforms. Davisian dictum that ‘landscape is a function of structure, process and tim e’ and K.J. Gregory's geomorphic equation (F=f (PM)dt, where F = landforms, f = function of, P = processes, M = geomaterials, dt = mathematical way of denoting

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n ature o f g e o m o r p h o l o g y

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change over time) clearly reveal that any geomorphic study r e q u i r e s care fu l in v e stig a tio n of geomorphological pro cesses (m ainly denudational processes), geom aterials (lithology, disposition of rock beds and co m p o sitio n o f rocks, collectively known as structure) and tim e factor, though the advocates o f dynam ic equilibrium theory have pleaded for exclusion o f tim e factor on the basic premise that the landform s are tim e-independent. G eomorphic studies incorporate tw o m ajor approaches viz. his­ torical studies in v o lv in g historical evolution of landforms and functional studies involving timeindependent series o f landform evolution reflecting association b etw een landform characteristics and existing en viron m ental conditions. Both the ap­ proaches have their relevan ce in geomorphological investigations.

(1) ANCIENT PERIOD

T h ough ‘geom orphology has d ev elo p ed from the w ork o f late eighteenth and nineteen th century geologists and hyd rolo gists’ (C .A .M . King, 1966) but som e ideas regarding landform s w ere indirectly postulated even in the ancient period w h en ph iloso ­ phers and historians o f G reece, R om e, E g y p t etc., the principal seats o f ancient culture an d civilization, took the initiative in this precarious field. H ero d o tu s (485 B.C.— 425 B.C), a noted G re e k historian, made significant contribution in the field o f r iv e r s alluvial behaviour during his ex tensiv e jo u rn e y o f Egypt. After having a close observation o f depositional work of the Nile he postulated that ‘E g y p t w a s th e g ift o f N ile ’. He fu rth er re la te d th e s h a p e o f depositional feature at the m outh o f the r iv e r to Greek letter A and nam ed this feature as d e lta . H e also postulated that ‘there is gradual g ro w th o f d elta towards the sea. On the basis o f the p re s e n c e o f marine fossils in the alluvium o f the N ile far inland he opined that ‘the level o f sea is not p e r m a n e n t b u t there is occasional rise and fall w hen sea a d v a n c e s landw ard ( tr a n s g re s s io n a l p h a s e ) a n d r e tr e a ts (regressional phase)’ Thus, we can infer the co n c e p t of transgressional and reg ressio n a l p h a se s o f the sea from the statements o f H erodotus.

1.3 EVOLUTION OF GEOMORPHOLOGICAL THOUGHTS T he present status o f geom orphology is the result o f gradual but successive development of geomorphic thoughts postulated in different periods by i n n u m e r a b l e p h i l o s o p h e r s , e x p e r t s and geoscientists in the subject and out side the subject. Thus, the developm ental phases of geomorphology indicate its dynam ic nature. After taking its birth in the philosophical ideas o f the ancient Romans and Greeks the su b ject has b lo sso m ed through the geom orphological m ethodological nutrients of the 18th and 19th century and reached its golden status in the 1st and 2nd decades o f the 20th century with the postulation and w ider acceptance of cyclic con­ cept o f landscape d ev elo p m en t and denudation chro­ nology world over. After 1950, the science o f geomor­ phology w itnessed a m ajo r change in the m ethodo­ logical aspect in the form o f rejection o f Davisian model o f cyclic dev elo p m en t o f landforms, intro­ duction o f quantitative m ethods in geomorphological studies, postulation o f dynam ic equilibrium theory of landscape d ev elo p m en t based on the concept o f time-independent series o f landform evolution, more emphasis on process geom orphology (process re­ sponse m o d e l) , e m e r g e n c e o f e n v iro n m e n ta l geomorphology, shift from mega-geomorphology to micro-geomorphology, from longer temporal scale to shorter tem poral scale, and more attention to­

A ristotle (384 B.C.— 322 B.C.), a rep u te d Greek philosopher, presented som e very interesting ideas regarding w ater spring, origin o f stream s an d behaviour of seas and oceans. A ccording to h im spring-fed streams are seasonal and ephem eral (n o n ­ permanent). Limestones cannot m aintain p erm a n e n t surface drainage as m ost o f the stream s d isap p ea r and form subterranean drainage.' A cco rd in g to him water springs get supply o f w ater through (i) ra in w a ­ ter, which reaches underground through percolation and seepage, (ii) condensation o f underg ro und satu­ rated air, and (iii) w ater vapour. He w as also aware o f changing nature o f sea-level and deposition o f eroded materials by the rivers in the form o f allu­ vium. Strabbo (54 B.C— 25 A.D.). a noted h isto ­ rian, made significant contributions in the field o f depositional work o f the rivers. A ccording to him thc^ size and shape o f delta depend on the nature o f terrain through which the river makes its course. Am extensive region having com paratively weaker rocks gives birth to larger delta as weak rocks through erosion yield more sedim ents to maintain large delta

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wards applied aspect o f the subject.

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GEOMORPHOLOGY

in the 18th century A.D. through his well knit con­ cept o f uniformitarianism but the postulation of coherent scientific thoughts in the field of geomorphologv already began in the 15th, 16th and 17th centu­ ries when the preexisting concept o f everlasting (permanent) landforms was rejected and theirchanging nature through weathering and erosion was very much realized. L eonardo da V inci (1452— 1519 A.D.) was o f the opinion that the rivers formed their valleys themselves through vertical erosion and de­ posited the eroded materials elsewhere. Buffon (1707— 1788 A.D.) rejected the catastrophists’ pos­ tulation of very little age o f the earth (thousands of years). He further opined that the rivers were the most powerful agent of erosion and they were capa­ ble of eroding the uplifted high land mass to sealevel. T argioni Tozetti (1712— 1784 A.D.), an Ital­ ian thinker postulated that the irregular courses (sym­ metry and asymmetry of the valleys) o f the rivers depended on the nature of rocks through which they flow. The regions of massive and resistant rocks maintain deep and narrow courses (valleys) whereas broad and meandering courses are developed in the regions of soft and less resistant rocks. Thus, this concept gives the glimpse of differential erosion. According to G u e th a r d (1715— 1786 A.D.) not all the eroded sediments are deposited by the rivers in the seas rather some parts are also deposited in the courses of the rivers as flood plains. He also at­ tached importance to the erosive power o f the m a­ rine processes. Dim arest (1725— 1815 A.D.) was of the opinion that ‘the valleys through which rivers flow have been formed by themselves through the process of valley deepening’. He was probably the first to postulate the concept of development o f landforms through successive stages.

while the region of resistant rocks maintains smaller delta because resistant rocks are less eroded and hence produce less sediments. Thus, we may infer an indirect glimpse of the concept o f differential ero­ sion from the statements o f Strabbo. Seneca main­ tained that ‘the rivers deepen their valleys through abrasion.’ It may be mentioned that some incoherent ideas were forwarded by ancient philosophers and historians but they could not collectively come to any definite conclusion. (2) DARK AGE

V iith the fall of Roman empire a lull prevailed in the development of geographical as well as geomorphological thoughts for a very long period of 1400 years (from 1st century A.D. to 14th century A.D.). Besides, some glimpses of geomorphological ideas put forth by few thinkers e.g. Aviecena (980— 1037 A.D.), an .Arabian thinker, broke the academic monotony. According to him mountains should be divided into two categories i.e. (i) mountains origi­ nated due to upliftment and (ii) mountains origi­ nated due to erosion by running water. (3) AGE OF CATASTROPHISM

The long continued academic silence of 1400 years was suddenly broken by the emergence of catastrophists who believed in the quick and sudden origin and evolution of all animate and inanimate objects in very short period of time and thus new pages of peculiar and fantastic concepts were added to the treasure of geomorphological and geographi­ cal literature. The age of the earth was calculated to be a few thousand years. Only those events could be given cognizance which occurred in the life-time of the people. It may be pointed out that sudden endogenetic forces like volcanic eruption and earth­ quakes may be held responsible for convincing the thinkers to postulate such fantastic and unreaslistic ideas not only related to the landforms but to all of the animate and inanimate objects. The concept of sudden change and evolution also swept the biolo­ gists who believed in sudden evolution and destruc­ tion of all the living organisms.

The 18th century appeared with a new wave of uniform itarianism on the academic stage of geomorphology, with Jam es H utton as its postulator. His concept of uniformitarianism is based on the basic tenet that the same geological processes which operate today operated in the past and therefore the history of geological events repeats in cyclic pattern. His concept of ‘present is key to the p ast9 aimed at the reconstruction of past earth-history on the basis of the present. According to him the nature is sys­ tematic, coherent and reasonable and thus destruc­ tion ultimately leading to construction indicates

(4) AGE OF UNJFORMfTARtANISM

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The concept of catastrophism was finally rejected and gradual cyclic nature o f earth's history was postulated by Jam es Hutton (1726— 1797 A.D.)

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NATURE o f

5

g eo m o r ph o lo g y

orderliness o f nature. He w as the first geologist to observe cyclic natu re o f the ea rth ’s history. His

glacial erosion, marine erosion, fluvial processes and erosion, arid and karst landscapes.

work was published in the form o f a research paper ‘theory of the earth : or an investigation of laws observable in the com position, dissolution and res­ toration of land upon the g lo b e’ in the Transactions of the Royal Society o f Edinburgh in 1788. Later on, his major work was published in the form o f a book entitled, ‘Theory o f the Earth with Proofs and Illus­ trations' in two volum es in 1795. His concept, ‘that topography is carved o u t and not built-up’ is a significant contribution in geomorphology. John Play-fair (1748— 1819), a professor of mathemat­ ics and a close friend o f Hutton, after making some suitable modifications in the Huttonian concept and adding some valuable contributions of his own elu­ cidated the Hutton's views on uniformitarianism through his book entitled ‘Illustrations of Huttonian Theory of the Earth' in 1802. Playfair also visual­ ized the erosive and transporting powers of fluvial and glacial processes. On the origin of valleys Playfair was also far in advance o f the views current at his time’ (C.A.M. King, 1966). C harles Lyell (1797— 1873 A.D.), one o f the most active followers of James Huttcn, laid the foundation of modern histori­ cal geology and he defined geology ‘as that science which investigates the successive changes that have taken place in the organic and inorganic kingdoms of nature.’ Most o f his works appeared in his two books ; ‘Principles o f G eo lo gy’(in two volumes) and T h e Geological Evidences of the Antiquity of Man’ in 1863. C.G. G reenw ood came to light through his paper entitled ‘rain and rivers : or Hutton and Playfair against Lyell and all com ers’ in 1857 and was accepted as the father of modern subaerialism. ‘He put forward the idea o f the base-level o f erosion before Powell in A m eric a’ (C.A.M. King, 1966).

Sir Charls Lyell ( 1797— 1873 A .D .) not only endorsed the concept o f uniform itarianism put forth by James Hutton but also popularised the concept through his books, ‘Principles o f G eology (two volumes). His significant contributions in biology became the base of ‘p r ig in o f S p e c ie s’ o f Charles Darwin. His book entitled, ‘T he G eological E v i­ dences o f the Antiquity o f M a n ’ (published in 1863) accommodated most o f the concepts o f H utton. Credit goes to E u ro p ea n school o f g e o ­ morphology for identification and recognition o f ice ages. The geoscientists collected sufficient and c o n ­ vincing evidences in support o f total glaciation o f northern Europe during Pleistocene period. L ou is Agassiz (1807— 1873 A.D.) is given credit for an early start in this precarious field. T h ou gh J ea n d e C h a r p e n tie r postulated his concept o f continental glacier and ice ages in 1841 but A gassiz is given credit for the recognition and identification o f the presence of ice age during Pleistocene period as he presented his ideas in 1840. They opined that m o st parts of northern Europe were covered w ith thick sheets of continental glaciers during Pleistocene period. It may be mentioned that the process o f study of glaciation was started m uch earlier by J oh n Playfair in 1815; V enetz o f Sw itzerland in 1821 and 1829, Norweigian scholar E sm ark in 1824, G erm an scientist Bernhardi in 1832, Jean de C harpentier o f Switzerland in 1834 than Louis Agassiz. T h e S co t­ tish geologist Jam es G eikie studied different as­ pects of ice age and published his ideas through his book entitled. ‘The Great Ice A g e' in 1894. A ccord­ ing to him an ice age involving longer geological period of time is comprised of distinct several glacial periods which are separated by w arm interglacial periods. A Penck and B ru ck n er after their observa­ tions of Pleistocene glaciation o ver the A lps identi­ fied four glacial periods during Pleistocene ice age e.g. Gunz, Mindel, Riss and W u rm w hich were separated by three warm interglacial periods.

(5) MODERN AGE (NINETEENTH CENTURY)

Geomorphology became an independent disci­ pline and a major branch o f geology at the beginning of the 19th century w hen the developm ent of geomorphic thoughts took place at regional level and two distinct schools o f geomorphic thoughts can well be identified e.g. (i) European School and (ii) American School. (A) E u rop ea n S ch o o l— S ignificant c o n ­ tributions were made in the fields o f recognition and identification o f Pleistocene Ice Age and glaciation,

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In the field of m arine erosion , corrasion by sea waves was given more attention and importance. Sir A ndrew R am say (1814— 1891) presented de­ tailed description o f marine platforms made by ma­ rine erosion in W ales and S.W. England. It may be mentioned that previously Ramsay attached more

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GEOMORPHOLOGY

ft

them into antecedent, superim posed, consequent vU||cys etc. His most significant contribution is the postulation o f lim it o f m a x im u m vertical erosion (valley deepening or dow ncutting) by streams to which he proposed the term o f base level, which is determined by sea-level. Later on, C.A. mallot (1928) inferred three types o f base level from his writings viz. ultimate, local and temporary base levels. He also opined that if the fluvial processes (streams) were allow ed to e rode the landmass unin­ terruptedly for fairly a long period o f geological time the high landmass m ight be ero d ed d ow n to a level plain which may be slightly abo ve the sea level. This erosional level plain was later term ed by Davis as peneplain. He also observ ed the nature o f narrow­ ing and shifting o f w ater divides th ro u g h the process

significance to murine abrasion hut in Inter part ol his life he gave more importance to Hu vial erosion. Baron Ferdinand N on Richthofen (1833— 1905) made significant contributions in the field ol marine erosion during his visit to China. He ‘produced his work on the genetic treatment o f landforms, in which he supported a marine origin for plains found be­ neath marine transgressions, these being produced when sea-love I is rising slowly' ( C A M . king. 1966). C. G , G re e n w o o d , a British geologist, made significant contribution in the field of subacrial erosion. He is considered to be the first geoscientist to postulate the concept o f base level o f erosion even before M ajor Powell in the U.S.A. J u k e s (1862) divided rivers into two categories e.g. (i) transverse streams which flow across the geological structures and (ii) longitudinal streams which follow the direc­ tion o f strikes o f rock beds or (low parallel to the geological structures. According to him longitudi­ nal streams are subsequent to transverse streams i.e. transverse streams originate prior to longitudinal streams. Jukes also described various aspects of river capture.

o f lateral erosion. G . K. G ilb e r t (1 843 — 1918 A .D .) is consid­ ered as the first real g eo m o rp h o lo g is t o f A m erica because ol his significant c o n trib u tio n s in system ­ atic and quantitative g e o m o rp h o lo g y . In fact, he was much ahead o f his lime and p o s tu la ted such concepts which still hold today. ‘He stressed the im portance o f creative im agination, o f testin g a n u m b e r o f h y p o th e se s , an d o f a n a l o g i e s in th e fie ld o f geom orphology’ (C .A .M . King, 1966). G ilbert never preferred to be called as the o re tic ian rather he took him self as an investigator. A fte r a th o ro u g h study o f different localities o f A m e r ic a (e.g. G reat Basin, Bonnevile Lake, artesian w ells o f G reat Plains, Henry M ountains, Siera M o u n ta in s etc.) he propo und ed a num ber ot laws i.e. law o f u n ifo rm slope, law of structure, law of divides, law o f in c re a sin g acclivity, law ot tendency to e q u ality o f actio n s, dynamic equilibrium, law o f the in te rd e p e n d e n c e etc. He was the first geoscientist to p r o p o u n d the concept of graded profile ot a riv er and to e s ta b lis h relationship am ong load, v olum e, velo city a n d ch a n n e l gradient on the basis o f q u a n tita tiv e a n a ly s e s o f these vari­ ables. His co n trib u tio n s h a v e b e e n elaborated in m uch detail in the 3rd Chapter o f th is book.

(R) A m e ric a n S chool— American school is credited for making m axim um contributions in the field o f geom orphology. In fact, the last two decades o f l^th century and first two decades o f 20th century (i.e. from 1S75 to 1920) are considered as ‘golden a g e’ not only o f American geomorphology but also o f world geom orphology because it was this period when for the first time general theory o f landscape developm ent was propounded by VV.M. Davis and the landform analysis attained its final shape. The concept o f sequential changes o f landforms through successive developmental phases in terms o f time hased on the basic tenet o f time-dependent concept o f Divisian model o f geographical cycle o f erosion became the core o f landform analysis and guide-line for geom orphologists and geologists not only in North America but world over. Pow ell, G ilbert, D utton and Davis made significant contributions in the field o f subaerial denudation.

C. F. Dutton (1843— 1912 A.D.) was the first gcoscientist to use the term isostasy to denote equilibrium condition of upstanding and downstanding landmasses of the earth’s surface. During his study and investigations of Colorado Plateau and Grand Canyon of the Colorado river he opined that the present canyon was the result o f long continued

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M ajor J.W . Pow ell (1834— 1902 A.D.), a major in American army after a thorough study o f Colorado plateau and Uinta mountains (1876) sug­ gested geological structure as a basis for the classi­ fication o f landforms. He attempted a genetic classi­ fication o f river valleys and consequently classified

J

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7

NATURE OF GEOMORMOl.OOY

period of fluvial erosion to winch he assigned ihe term of the period of great denudation. He ulso presented evidences in support of Powell's concept of base level of erosion.

W. Penck in Germany. His classical model o f geo­ graphical cycle propounded in 1899 and defined by him ‘as a period of time during which an uplifted landmass undergoes its transformation by the proc­ esses of land sculpture ending into a low featureless plain (peneplain)’ dominated the geomorphological investigations all over the world throughout 1st half of the 20th century inspite o f its stiff opposition by W. Penck and others in Germany. His model of geographical cycle was variously termed, popular­ ised and applied by his followers world over e.g. no rm al cycle, erosion cycle, g e o m o rp h ic cycle, hum id cycle etc. It may be mentioned that his ‘geographical cycle’ does not represent his general theory of landscape development as his general theory states ‘that th e re is se q u e n tia l c h a n g e in la n d fo rm s th ro u g h successive stag es an d the changes a re directed to w a rd s a d efin ite end i.e. attain m en t of featureless p la in (p e n e p la in ) ’. The main goal of his theory was to present systematic description and a gcnetic classification of landforms. Davis also identified 3 basic factors which control the evolution of landforms viz. ‘landscape is a function of s tru c tu re , p rocess and tim e’, which are termed as ‘trio o f D avis’. His concept o f geo­ graphical cycle was later on applied with all other (other than fluvial) processes by Davis and his fol­ lowers e.g. arid cycle of erosion (Davis, 1903, 1905 and 1930), glacial cycle of erosion (Davis, 1900 and 1906), marine cycle of erosion (Davis, 1912, D.W. Johnson, 1919), karst cycle of erosion (Beede, 1911, Cvijic, 1918), periglacial cycle of erosion (L.C. Peltier, 1950). His model was modified and pre­ sented in revised forms by a few geomorphologists after 1950. Davis concept of historical evolution of landscape became the pivot for the classical concept of d en u d atio n chronology and erosion (planation) surfaces in U.K. D avis’ major contributions (re­ search articles, papers and addresses) were pub­ lished in a book form entitled ‘G eograp hical E s ­ says’ in 1909. He is considered as a great definer, analyser, interpreter, systematiser and synthesiser. Only two quotes from S.W. W ooldridge and S. Judson that ‘Davis towers above his predecessors and successors, like a monadnock above one o f his own peneplains’ (S.W. Wooldridge), and ‘his grasp of time, space and change, his com mand o f detail, and his ability to order his information and frame his

W.M. Davis (1850— 1934) was a professor of physical geography at Harward University. He is considered to he the patron o f the science of geom orphology because o f his significant contribu­ tions in different fields of geomorphology and for giving new direction to landform study. He covered almost every nook and corner of geomorphology. He is given credit to systematize and integrate hith­ erto seaitercd ideas of American geomorphologists to present them in coherent and well defined frame­ work. His contributions were so significant and lie w as so d o m in a n t am o n g the A m erican geom orphologists that the American school of geomorphology was recognized as Davisian school o f geom orphology. Davis is credited for the postu­ lation of first general theory of landscape develop­ ment which, is in fact, a synthesis of his three major concepts viz.. com plete cycle of river life (1889), geographical cycle (1899) and slope evolution. He emphasized progressive developm ent of erosional stream valleys through the concept of complete cycle of river life while sequential changes of land­ scapes through time involving historical evolution of landforms (time-dependent series of landforms) or cyclic developm ent of landform s were high­ lighted through the concept o f geographical cycle. ‘The reference system of Davisian model/theory of landscape development is that the landforms change in an orderly manner as processes operate through time such that under uniform external environmen­ tal conditions an orderly sequence of landforms develops’ (Robert C. Palmquist). Since Gilbert and Davis also stepped in the 20th century and hence their further contributions to the geomorphological thought are considered in the succeeding heading. Further, the contributions of Davis will be elaborated in detail in the 3rd chapter of this book. (6) MODERN AGE (20TH CENTURY : FIRST HALF)

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The beginning o f the 20th century was her­ ald e d by m e th o d o l o g i c a l r e v o lu t io n in geomorphological studies brought in by W.M. Davis and his followers at home (UiS.A.) and abroad and

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GEOMORPHOLOGY

arguments remind us again that wc are in the pres­ ence o f a giant’ (Sheldon Judson. 1975) are suffi­ cient enough to demonstrate the greatness of Davis in the e n r ic h m e n t and a d v a n c e m e n t of geomorphological knowledge. C.G. Higgins' (1975) remark that ‘Davis’ rhetorical style is justly admired and several generations of readers became slightly bemused by long though mild intoxication on the limpid prose of Davis’ remarkable essays’ speaks of the academic calibre o f W.M. Davis. The American school of geomorphology was further entriched by significant contributions of a host of geomorphologists e.g. D.W. Johnson (ma­ rine process and coastal geomorphology), C. A. Malott (fluvial processes and erosion), H.A. Mayerhoff and E.W. Olmsted (evolution of Applachian drainage), R.P. Sharp. C.P.S. Sharp. A.K. Lobeck, W.D. Thornbury etc. During the 1st half of the 20th century Euro­ pean school of geomorpholgy made significant con­ tributions in the advancement of geomorphological thoughts. British geomorphologists made their inde­ pendent identity and there emerged an entirely dif­ ferent school o f geom orphology which laid empha­ sis on the chronological study of landscape develop­ ment in historical perspective better known as d en u ­ d ation ch ro n o lo g y based on the co n ce p t of p alim p sest S.W. Wooldridge (his famous book being the Physical Basis of Geography : An Outline of Geomorphology, published in 1937), J.A. Steers (The Unstable Earth, published in 1832) etc. made significant contributions in different branches of geomorphology.

A new branch o f geom orphology in the form of climatic geom orphology was developed in France and Germany on the basic tenet that ‘each climatic type produces its own characteristic assemblage o f landform s’. Sauer (1925), Wentworth 1928), Saper (1935), Friese (1935) etc. paved the way for the p o s tu la tio n o f the c o n c e p t o f clim atic geomorphology and m orphogenetic or morpho climatic regions by Budel (1944, 1948) and L.C. Peltier (1950) in Germany. This concept of climatic geomorphology was further advanced and estab­ lished by Tricart and C ailleux in France in the 2nd half ot the 20th century. The statistical techniques were first intro­ duced by Krumbein in geology in 1930s and the work ol American engineer R.E. Horton (1932 and 1945) brought quantitative revolution in the field of geomorphology when he presented quantitative analy­ sis ot morphometric characteristics o f fluvially origi*

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The Davisian model of geographical , cycle met with strong criticism and his concept of rapid and erosionless upliftment became the crux of criticisms by the opponents of cyclic concept of the evolution o f landforms particularly by the German geoscientists. The German critics of Davisian model of cycle o f erosion fall in two categories viz. the first category of opponents pleaded for outright rejection of cyclic concept while the second category of critics suggested modifications and presented entirely new model. According to Penck landform development is not time-dependent as envisaged by Davis rather it is time-independent. W. Penck, through his ‘M or­ phological Analysis’ and ‘Morphological System ’

tried to reconstruct and interpret past events of crustal movements on the basis o f exogenetic proc­ esses and morphological characteristics. The refer­ ence system of Penck's model o f landscape develop, mcnt is that the characteristics of landforms of a given region are related to the tectonic activity of that region. The landlorms, thus, reflect the ratio between the intensity of endogenetic processes (i.e. rate of upliftment) and the magnitude of displace­ ment o f materials by exogenetic processes (the rate o f erosion and removal o f materials). According to Penck landforms development should be interpreted by means of ratios between diastrophic processes (endogenetic or rate of upliftment) and erosional processes (cxogen^tic, or rate of vertical incision). ‘Penck is supposed to have deliberately avoided the use of stage concept in his model of landscape development either to undermine the cyclic concept of W.M. Davis or to present a new m o del’ (Savindra Singh, 1995). In the place of D avis’ stage he used the term entw ickelung meaning thereby development. In the place of youth, mature and old stages he used the terms aufsteigende entw ickelung (waxing or accelerated rate of development), gleichformige entwickelung (uniform rate of development) and absteigende entw ickelung (waning or decelerating rate of development). Detailed account of Penck's contributions will be presented in the third chapter of this book.

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9

NATURE OF GEOMORPHOLOGY

nated drainage basins. The criticism of Davisian model o f landscape development and descriptive geomorphology gained currency after 1940 and si­ ren was raised for the rejection and replacement of time-dependent evolutionary concept of landscape development. It may be mentioned that at a time (1950) when majority of the geomorphologists world over became fed up with evolutionary model ot Davis and pleaded for alternative theory of land­ scape development which may envisage time-inde­ pendent series of landforms Pelltier presented the concept o f periglacial cycle o f erosion in 1950 in Germany which offered support to Davisian model of cycle o f erosion. (7) RECENT TRENDS (SECOND HALF OF 20TH CEN­ TURY)

Post-1950 geomorphology has undergone seachange in the methods and approaches to the study o f landforms, conceptual framework, paradigm and thrust areas o f study. The recent trends in the field of geomorphological studies since 1950 include in­ creasing criticism o f Davisian model of cyclic de­ velopment o f landforms, concerted efforts for the replacement o f cyclic model by non-cyclic (dy­ namic equilibrium) model, descriptive geomorpho­ logy (qualitative treatment o f landforms) by quanti­ tative geomorphology, inductive method of landform analysis by deductive method, introduction of m od­ els and system approach, emergence of process geom orphology, climatic geomorphology, applied geomorphology,environmental geomorphology, shift from larger spatial and longer temporal scale to sm aller spatial and shorter temporal scale etc.

The landscapes were taken as open systems which are in steady state of balance through continuous input of energy and matter and output o f matter. Though Hackian model o f landscape devel­ opment envisaged landscapes as the result o f bal­ ance between the resisting force of geomaterials and erosive force of the geomorphological processes acting on them but he laid more em phasis on geo­ logical control as he opined that ‘differences and characteristics of forms are explicable in term s o f spatial relations in which geologic patterns are the primary consideration’ (Hack, 1960). It may be pointed out that even Hack could not escape from evolutionary concept as he h im self adm itted ‘that evolution is also a fact of nature and that the inher­ itance of form is always a possibility’ (H ack, 1960). R.C. Palmquist has opined that ‘Hack (1965) para­ phrases Davis’ ideal geographical cycle in term s o f equilibrium concept and develops a sim ilar ev olu­ tionary scheme. An initial disequilibrium stage (youth) of rapid stream incision is followed by an eq uilib­ rium stage (mature) wherein the rounded interfluves are lowered as potential energy decreases though they do not change in fo rm ’ (R.C. Palm quist). It may be mentioned that continued criticism o f cyclic m odel of landform development and ultimately its rejec­ tion caused a conceptual vacuum which could not be filled up even by dynamic equilibrium theory. R e ­ cently, a few alternative geom orphic theories have been advanced e.g. ‘geom orphic th resh old m o d e l’, ‘tectonic-geom orphic m od el’ (M. M orisaw a), ‘e p i­ sodic erosion model* (S.A. S chum m ) etc. The most outstanding contribution to the ad­ vancement of geom orphological know ledg e in this period is the adoption of quantitative approach based on deductive scientific m ethod to the study o f landforms and processes at short spatial and tem p o ­ ral scales. The time factor w hich w as taken as a process in the landscape d evelopm ent in the cyclic model has now been accepted as a variable. The maga and m eso-scales used for landform studies have now been reduced to m icro-scale w herein the m echanism o f processes can be properly understood through field instrum entation and m easurem en t of the mode and rate o f operation o f geom orphic pro c­ esses. Thus, ‘form g eo m o rp h o lo g y 9 has been re­ placed by ‘p rocess geom orphology*. This quantita­ tive approach resulted in the form ulation o f 4func-

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The decade 1950— 60 was devoted more for the quantitative study o f landforms and processes and the consideration o f geom orphic theories occu­ pied a back seat. This is the reason that a set o f basic concepts o f ‘the landscape cycle, the epigene cycle’, ‘the pediplanation c y c le ’ and ‘hillslope cycle' pos­ tulated by L.C. King and his ‘C anons o f L andscape’ (published in 1953) could not win support. The rejec­ tion o f D avisian concept o f ‘cyclic m o d e l’ based on ‘time dependent landform e v o lu tio n ’ culminated in the postulation o f ‘dynam ic equilibrium theory’ of landscape dev elopm en t by J.T .H ack, R.J. Chorley and others based on the concept o f ‘tim e-independ­ ent evolution o f la n d sca p e’, and ‘system co n ce p t’.

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fTf/Mtmnunsxn

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The decade* 1950-60 and 1960-70 saw a real take off in the geomorphological research** wherein more attention wa* paid towards the study o f differ­ ent physiographic regions o f peninsular India Sig­ nificant contributions came from R.P. Singh, A.K. Sen Gupta, E. Ahmad, S.C. Bo**. W.D. West and V.D. Chaubey, R. Vaidyanadhan. B. Venketesh. G V. Rao etc. The 21st International Geographical Congress held in 1968 in New Delhi aroused deep interests in Indian geographers forgeom orpholopcal researches in various parts of the country. Signifi­ cant co ntrib ution s in the field o f system atic geomorphology came from B.C. Acharya *floods of Mahanadi;, G.K. Datta (origin and evolution of la n d fo rm s in L o w e r S o n e V a iJ e y ), M .K . Bandopadhyay (glacial landforms;. D. Suza (evolu­ tion of drainage pattern of Goay, R.N. Mathur (geohydrology of Meerut district/. H.S. S h an na {ra­ vine erosion/, A.K. Sengupta (denudation on ^.cntrai Ranchi plateau;, L>. Niyogi. S.K. Sarkar and S. Mallick (geomorphic mapping;, D. Niyogi (river terraces;, A.K. Pal (Balasan river basin;. S. Subba Rao (landforms of Deccan traps, physical features of Girnar hills;, A.B. Mukerjee (inland streams in Haryana;, S. Sen (outer bank slope steepness in meandering rivers;, E. A hm ad (gull, erosion in India;. H.R. Betal (identification of slope categories in Damodar valley;. S.C. Boseed on topographical maps was initiated bv R.L. Singh in 1967 when he presented an exhaustive paper on Morphometric Analysis o f T errain ’ in the form o f presidential address at the joint session o f geologygeography section o f Indian Science Congress held in 1967. H isefforts culminated in the presentation o f a few Ph. D. dissertations on ’landforms and settle­ ments in the department o f geography, Banaras

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n a ture o f g eo m o r ph o lo g y

profiles on some residual hills in the Jamalpur-Kiul Hills’ by Anil Kumar (981) in the prestigious jo u r­ nal, Zeitschrift fur Geom orphologie became a sin­ gular contribution in field-based geom orphology bsed on A. Young's method of slope profiling. The other sign ificant c o n trib u tio n s w ere m a d e by S.C.Mukhopodhyay (1982, Tista Basin), R.K. Rai (1980, Sonar-Bearm a Basin), Prudhvi Raju and R. Vaidyanadhan (1981, Sarda Basin), B.S. M arh (1986, Ravi Basin) etc. Three im portant contributions in the form of international publications (in International Geomorphology edited by V. Gardiner, 1987) cam e from H .S. S h a r m a ( c l i m a t e a n d d r a i n a g e morphometric properties), R.K. Rai (evidences o f rejuvenation of the Deccan foreland) and S avindra Singh and R.S. Pandey (m orphological analysis and development of slope profiles over B han der Scarps). V S. Kale published a field-based significant paper on ‘western ghats’ in Zeitschrift fur geom orphologie. A research paper entitled ‘rill and gully erosion in the subhumid tropical riverine environment o f Teothar tahsil, M .P.’ by Savindra Singh and S.P. A gnihotri published in Geografiska A nnaler (1987) is a sig ­ nificant contribution in the field and laboratorybased geomorphological study o f m a n -im p ac ted gullied area. The fluvial geom orphology w as e n ­ riched by substantial work undertaken by A.B. Mukerjee, S.K. Pal. V.S. Kale, S.R. Jog etc. T he International Conference on G eo m orph olo gy and Environment held in 1987 at A llahabad U niversity rejuvenated geormorphological researches in India and encouraged field m easurem ent of s p atio -tem p o ­ ral variations in landform characteristics. R iver-bed m o rp h o lo g y , alluvial m o r p h o lo g y a n d c o a s ta l geomorphology became the centre o f intensive study by Poona School o f G eom orphology led by K.R. Dikshit, V S. Kale, S.R. Jog, S.N K arlekar and their associates. The other positive result o f the said conference was the establishm ent o f the Indian Insti­ tute of Geom orpliologists with its headquarters at geography departm ent, A llahabad University. The annual conferences organized at different places o f the country under the agies o f the aforesaid organi­ zation since 1988 have encouraged several young researchers from different parts o f the country to peruse field-based geom orphic studies.

Hindu University, Varanasi, (e.g. S.C. Kharkwal, 1969. V.K. Asthana, 1968. K.N. Singh, 1967,Meera Agarwal, 1970. O.P. Singh. 1977 etc.). Besides, significant contributions were made in different as­ pects o f In d ia n g e o m o rp h o l o g y by S.C. M ukhopadhyav (1968. geo m o rp h o lo g y of Subamarekha basin), E. Ahmad (Ranchi to Ra jaroppa, 1969), S.C. Chakravarti (1970, geomorphological evolution of W. Bengal), Swami Pranawanand (1970, Sources o f four great rivers o f India), J.P. Singh (1970. geomorphological evolution of Meghalaya), K.R. Dikshit (1970, erosion surfaces and ploycyclic reliefs of Deccan trap). R.P. Singh (1969. denuda­ tion chronology of C hotanagpur plateau, 1970, periglacial cycle of erosion), Savindra Singh (1977, altimeteric analysis as a significant morphometric technique), K.R. Diksshit, S.N. Rajguru, N.S. Gupta and J.P. Jog (1972. geomorphology of southern K o n k a n a r e a ), S .C . M u h o p a d h y a y (1 973, geomorphology ofSubam arekhabasin). Anil Kumar (1974. morphological classification of landforms of S.W. Ranchi plateau, 1979. geomorphology of Simdega and its adjoining area), Savindra Singh and Renu Srivastava (1976, denudation chronology and erosion surfaces of the Belan Basin), Savindra Singh (1977. tors o f Ranchi plateau). R K . Rai etc. The recognition of drainage basins as ideal geomorphic units for geomorphological investiga­ tions resulted in the systematic morphometric analy­ sis o f drainage basins consequent upon the presenta­ tion o f doctoral thesis on 'drainage basin character­ istics o f the Belan river’ by Renu Srivastava in 1976 in the departm ent o f geography, Allahabad Univer­ sity. This was followed by presentation of a number o f doctoral theses in Allahabad University e.g. small drainage basins o f Ranchi plateau (Savindra Singh, 1978), m orphom etric study of small drainage basins o f P a l a m a u u p la n d (S .S . O jh a , 1981), geom orphological study o f small drainage basins o f S.E. Chotanagpur region (D.P. Upadhyav. 1981) etc. The decadc 1980-90 was characterized by the study o f causal relationship between landform s and processes and formulation o f models and techniques. Estimation o f drainage density on the basis o f drain­ age texture by Savindra Singh (1976 and 1981) is a significant contribution in theoretical geomorphology. The publication o f the study o f ‘nature o f slope

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A few centres o f geom orphology have co m e up in the c o u n try . T h e A lla h a b a d C e n tr e o f

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GEOMORPHOLOGY

The Calcutta Centre of geomorphology is given credit for early start in geomorphological researches. S.P. Chatterjee and K. Bagchi paved the way forthe initiation and development of geomorphic researches through their pioneer works in this field. Presently, the department o f geography, Calcutta University is widely known for researches in differ­ ent branchesof geomorphology. M.K. Bandopadhyay is actively eng ag ed in the stud y o f glacial geomorpthology of the Himalayas and has regularly monitored the recession of glaciers on the basis of field studies. S.C. Mukhopadhyay has made signifi­ cant contributions in fluvial geomorphology while landslides in the eastern Himalaya are regularly monitored by S.R. Basu. The geomorphologists o f the Central Arid Zone Research Institute (CAZRI), Jodhpur,e.g. Bimal Ghose, Surendra Singh, P.C. Vats and Amalkar have done outstanding researches in arid geomorphology and applied geomorphology on the basis o f intensive field surveys and remotely sensed data. R.K. Rai and his associates are actively engaged in the field of fluvial geomorphology, structural geomorphology, karst geomorphology, etc. at Shillong. Besides, geomorphological researches are being persued at Bhagalpur (Anil Kumar and his team), Jaipur (H.S. Sharma), Delhi (S.K. Pal), Jamm u (M.N. Kaul), Thanjavur (Victor Raja Manickam), Almora (J.S. Rawat and R.K. Pandey), Varanasi (K. Prudhvi Raju), Srinagar-Garhwal (Devidatt) etc. A very out­ standing contribution in the form of development of a c o m p u te r s o f tw a r e for the i n t e r p r e ta tio n (geomorphological) of satellite imagery has been developed by S.R. Jog (Pune).

geomorphology has initiated geomorphological re­ searches since 1971. In the beginning, attention was focused on the morphometric study of drainage basins based on topographical maps and limited field observations. The detailed field studies started in the decade 1980-90 wherein probably the 1st d o c to ra l d is s e r ta tio n on environmental geomorphology was produced by Alok Dubey un­ der the supervision of Savindra Singh in 1985. Besides fluvial geomorphology, a new branch of urban geomorphology has been developed by Savindra Singh and a few doctoral theses have been produced. The doctoral theses on solution topogra­ phy of Rohtas Plateau by M. S. Singh (1991) and applied geomorphology of Belan-Son interstream area by Neera Rastogi (1994) are significant contri­ butions. The geochemistry of cave water and mor­ phogenesis of Guptadham cave (Rohtas plateau, Bihar) based on laboratory analysis of water, solutes and rock samples for 36 months was subsequently published in Zeitschrift fur Geomorphologie by Savindra Singh, M.S. Singh and Alok Dubey in 1992. Recently, micro-level study of rill and gully erosion has been initiated by Savindra Singh and Alok Dubey. A major research project on 'gully erosion and man­ agement’ of a micro-man-impacted gully basin (about 56.000 m2. area) funded by the DST, New Delhi, has been completed (1991-95) wherein the meteorologi­ cal, hydrological and geomorphological variables have been recorded through field instrumentation for three wet monsoon months of 1991 to 94 and soil erosion and soil loss, sedimentation, discharge etc. have been regularly monitored.

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The Poona Centre of geomorphology is char­ a c t e r iz e d by s e r io u s r e s e a rc h e s in flu vial ( 1. 5 SYSTEM C O N C EP T geomorphology. structural geomorphology, river bed The system concept was adapted in the expla­ morphology, alluvial geomorphology and coastal nation of geomorphic problems after the postulation geomorphology. The gemorphological researches o f ‘general system theory’ by Von Bertalanffy in were initiated by K.R. Dikshit. He encouraged young 1950. ‘A system may be defined as a set of objects geomorphologists for field instrumentation of the that are considered together by studying their rela­ processes. Consequently, V.S. Kale and S.R. Jog tionships to each other and their individual attributes’ m a d e s ig n if i c a n t c o n tr ib u tio n s in flu vial (C.A. M King, 1966). A geomorphic system is an geomorphology. A number of research projects funded integrated complex of mosaic of geomorphic fea­ by the U.G.C., D.S.T. and other organizations have tures and this system functions under definite condi­ been undertaken by K.R. Dikshit, V.S. Kale and S.R. Jog. S.N. K arlekarand their associates have studied tions through the input of energy (precipitation, extensively the western coasts of Maharahtra and insolation, upliftment etc.) and output o f matter. A have made significant contributions in coastal critical balance between the input o f energy and geomorphology. output of matter is a prerequisite condition for the

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13

n atu re o f g e o m o r ph o lo g y

successful functioning o f a geomorphic system. In fact, ‘a geomorphic system is a structure of interact­ ing processes and landforms that function individu­ ally and jointly to form a landscape com plex’ (R.J. Chorley, S.A. Schumm and D.E. Sudden, 1985). The system state includes its composition, organi­ zation and flow of energy and matter wherein the geomorphic system may be in a steady state, dy­ namic equilibrium state or in changing state in terms of time. Further, a super geomorphic system consists of several subsystems of different suites of landforms and these subsystems are interconnected through the input-output linkages.

effected by any of the external factors (say input factor) which regulates the equilibrium condition o f the geomorphic system, is counter-balanced by changes in other system components, this is called ‘hom eostatis’ or negative feedback mechanism. It is apparent that closed geomorphic system is regu­ lated through positive feedback mechanism and leads to progressive changes in landforms through time in such a way that a featureless plain with minimum relief (peneplain) is produced in the end while an open geomorphic system operates according to nega­ tive feedback mechanism and thus the geomorphic system remains in equilibrium.

Geomorphic systems are divided into closed and open system s. A closed system has well defined boundary wherein neither energy nor matter can cross this boundary. Davisian ‘geographical cycle’ is an example of closed geomorphic system which begins to function with the input of initial potential energy through short-period rapid rate-upliftment. With the march of time both height and energy decrease progressively due to denudation resulting into minimum height and energy at the attainment of peneplain stage. Sometimes there may be temporary increase in energy due to rejuvenation caused either by upliftment or by negative change in sea-level but ultimately the system runs down when the land is eroded down to peneplain and the sum of available energy and the work to be done equals zero resulting into maximum entropy. On the other hand, an open geomorphic system is characterized by continuous renewal of energy and removal of matter from the system which functions in such a way that it attains steady state. A drainage basin is an example of an open geomorphic system which receives energy through insolation and rainfall and releases water and eroded material from its mouth.

Explanation-A simple example may explain positive feedback— increased amount o f rainfall (in­ crease in input) causes phenomenal increase in the overland (low and surface runoff which accelerates soil erosion leading to removal o f surficial soil cover and exposure of underlying resistant rock cover which discourages infiltration ol water and augments soil erosion resulting in the lowering of relief. Negative feedback— a profile ol equilibrium of a stream means equilibrium o f works o f the stream pertaining to erosion, transportation and deposition. A graded stream having attained the profile o f equi­ librium is such that there is equilibrium between transporting capacity of the stream and total load (sediments) to he transported and thus a graded stream neither erodes nor deposits in short term. Suppose, there is sudden increase in the sediment load of the stream due to accelerated rate of erosion consequent upon increased rainfall. This situation disturbs the equilibrium condition because the work to be done (i.e. sediment load to be transported down stream) exceeds the transporting capacity (available energy) of the stream. This change forces the stream to deposit extra load till the channel gradients (steep­ ening of gradient due to deposition) becomes such that it provides required velocity and hence required energy to transport increased sediment load so that equilibrium condition is re-attained and the stream is regraded.

The internal structure of a geomorphic sys­ tem is controlled by feedback mechanism. ‘Posi­ tive feedback occurs whenever externally induced changes of input produce changes in the same direc­ tion as the input changes (i.e. lead to progressivelychanging ‘timebound’ state). Negative feedback operates when changes in the system input result in changes in other system components which regulate the effects of the changed input such as to bring a new ‘timeless’ equilibrium or steady state’ (R.J. Chorley, 1967). In other words, when any change

1.6 GEOMORPHIC M ODELS

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Model is generally defined as simplified ap­ proximation of external real world. A model may be in the form of structured idea to represent real situation, an hypothesis, a theory or a law (H. Skilling, 1964). ‘It can be a role, a relation or an equation. It

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GEOMOR PHOLOGY

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(i) Models are selective approxim ation s as they include some of the relevant and fundamental aspects ol real world while they ignore detailed aspects ; (ii) Modes are s tr u c tu re d ideas about the real world i.e. the selected relevant and fundamental aspects are well interconnected in such a way that the real world may be projected in simple and general­ ized form; (iii) Models are suggestive in nature i.e. these incorporate scope for their future extension and generalization; (iv) Models are analogies; (v) Models have the quality of reapplicability to the real world etc. The functional role of models includes (i) psychological aspect which 'enables some groups of phenomena to be visualized and comprehended which could otherwise not be because of its magni­ tude or complexity’ (P. Haggett and R.J. Chorley, 1967; (ii) acquisitive aspect which provides scopc for the acquisition of data, information and ideas for the formulation of models; ( iii) logical aspect which enables the geographer (investigator) to explain the details of data and information; (iv) normative aspect, which includes provision of comparison of selected phenomena (not previously known with precise perfection) with already known situation; (v) constructional aspect includes provision for for­ mulation of theories and iaws etc. Models are classified on different bases— (1) On the basis of familiarity o f situation and existing reality models arc divided into (i) descriptive m o d ­ els and (ii) norm ative models wherein descriptive models involve description of real situation having empirical information whereas normative models are concerned with description of a less familiar situation on the basis of more familiar situation. (2) On the basis of stuff models are classified into (i) h a rd w a re models, physical models and experi­

mental and (ii) theoretical m odels, sym bolic mod­ els, conceptual m odels ctc. (3) On the basis of system concept models are divided into 'i) synthetic system models, (ii) partial system m odels, (iii) balck box models. According to R.J. Chorley (1967) the concep­ tual geomorphic model system may be approached in 3 ways e.g. (i) in terms o f time and space, (ii) in terms of physical system, and (iii) in terms of general system. The translation of systematic geomorphic views in time or space yields natural analogue system. Natural Analogue System — A natural ana­ logue system is such wherein geomorphological phenomena of a geomorphic system arc described on the basis of such analogous natural system which is simple and better known and similar to the original system. The natural analogue system is divided into (i) historical natural analogue system when time factor is taken into consideration and (ii) spatial natural analogue system when space becomes main consideration. The historical natural analogue model implies the concept of ‘time-controlled se­ quences’ i.e. many gcomorphic activities are re­ peated through lime and thus the past geomorphic history has relevance to the present history. Thus, the past geomorphic history of a given region may be reconstructed on the basis of present geomorphic processes and their responses (resultant features). James Hutton's concept of ‘present is key to the p ast’ and ‘no vestige o f a beginning : no prospect of an end’ is a line example of historical natural analgue model. In the spatial natural analogue model the geomorphic features of the original region are described on the basis of identical and contiguous region which is better known. In other words, the original area is described on the basis of comparison ol another area which is similar to original one but is better understood. Fenneman's physiographic re­ gions' (1914), ‘tectonic or structural provinces’ on the basis of morphotectonics, ‘m orphogenetic regions’ on the basis of the concept that ‘each climatic type produces its own characteristic assem­ blage of landforms' etc. are a few examples. P hysical S ystem involves dissection of geomorphic problems into several component parts and the study of operation of each part and intercon­ nections between the parts presents a complete syn­

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can be synthesis of data. Most important from the geographical view point, it can also include reason­ ing about the real world bv means of translations in space (to give spatial models) or in time (to give historical models' (P. Haggett and R.J. Chorley, 1969). The main characteristic features o f a model are—

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NATURE OF GEOMORPHOLOGY

dom unpredictable effects o f natural processes which obscure simpler deterministic relationships i.e. cause and effect relationships. S toch astic m athem atical m odels remove such w eakness o f deterministic models. Stochastic models arc, infact, statistical models wherein besides mathematical variables, pa­ rameters and constants, certain aspects c5f natural processes are also included so that the simpler deter­ ministic relationships are also revealed.

thesis of the entire physical system comprising all com ponent parts. Physical system approach of geomorphological investigations is based on quanti­ tative method. Physical system includes three inter­ related models e.g. (i) hardware model, (ii) math­ ematical model and (iii) experimental design model. H ardw are m odel involves simulation of natural geomorphic complexes in the laboratory involving similar natural conditions but on very smaller spatial and shorter temporal scales e.g. development of river meander, development of rills and gullies in the laboratory etc. The construction of hardware models in geomorphology has not been very successful because (i) the natural geomorphic system is very complex and (ii) this complexity imposes problems of scale, both spatial and temporal. M athem atical geom orphic m odels are abstract forms of equations wherein phenomena, forces, processes, events, fea­ tures etc. o f natural geomorphic systems are re­ placed by mathematical variables, parameters, sym­ bols. letters, constants etc. For example, Davisian model o f ‘landscape is a function of structure, proc­ ess and tim e’ has been paraphrased into mathemati­ cal model by K.J. Gregory as a geomorphological equation—

Experim ental design m odels are constructed on the basic premise that ‘within a given range o f observational data exist certain meaningful c o m p o ­ nent parts which can be identified by em ploying a suitable experimental design' (quoted by R.J.Chorley, 1967). T h e design, ,which is derived from past observation, logical deduction, intuition, or a c o m ­ bination of these provides a structure within w hich other data are collectcd and then analysed by c o n ­ ventional statistical means to produce some gener­ alization’ (quoted by R.J. Chorley, 1967). The c o n ­ struction of such models very often incorporates the use of simple and multiple regression analysis, h ar­ monic analysis, spectral analysis ctc. A.N. Strahler's model of linear relationship between channel slope and ground slope and linear relationship between discharge and stream width, depth and velocity (A.N. Strahler, 1950) etc. are exam ples o f experi­ mental design models.

F = f(M P )t where, F = forms (landforms) f = function of M = maternal (geomaterials) P = process t = time Mathematical models are classified into (1) deterministic mathematical models and (ii) stochastic mathematical models. The deterministic m athem ati­ cal m odels are constructed on the basis of exact predictable relationships between independent and dependent geomorphic variables i.e. relationships between cause and effect. Horton's laws of stream numbers and stream orders, and stream lengths and stream orders (exponential function model) are good examples of such model. Law of allometric crowth (power function model) stating proportionate growth in all components o f drainage basins with time is another exam ple of deterministic mathematical models. Though nearly all of the variables of com ­ plex natural situation are included in de*erministic mathematical models yet there a^e certain such ran­

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G eneral system involves the consideration of groups of geomorphic phenom ena which are structured into a broad general system wherein ‘e m ­ phasis lies in the organization and operation o f the system as a whole or as linked com ponents rather than in detailed study of individual system elem ents’ (Von BcrtalanlTy, quoted by R J . Chorley, 1967). Within geomorphology a 'geomorphic system’ is consiuereu as a general system wherein detailed study of geomorphological processes operating within the system and their responses (resultant landforms) provides explanation oflandform characteristics. ‘A geomorphic system is a structure of interacting proc­ esses and landforms that function individually and jointly to form a landscape com plex' (Chorley, Schumm and Sugden. 1985). A fluvially originated drainage basin may be cited as an example o f a geomorphic system which operates through input of energy (solar energy and precipitation) and output of energy and matter

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16

Geomorphic models should be such that they can be applied for practical purposes. It may be mentioned that natural physical system is character­ ised by ‘homeostatis mechanism’ involving nega­ tive feedback which counterbalances any change effected by natural factors in any component of the natural physical system and thus regulates the sys­ tem and maintains equilibrium. But the changes and interventions effected by human activities are some­ times so enormous that they exceed the resilience of the system and upset the balance. ‘Geomorphologists should therefore ensure that any intervention in landform systems is thoroughly regulated so as to exploit the system successfully, rather than cause its degradation. Such intervention must therefore be based on proven geomorphological models which can accurately predict the likely impact of any planned intervention in the system’ (A Goudie, 1981). 1.7 METHODS AND A P P R O A C H ES TO THE STUDY O F LANDFORMS

The main task of a geomorphologist is to study the evolution and characteristics of erosional

and depositional landforms and geomorphological processes operating therein. The entire practice and exercise of landform studies may be grouped into three closely linked steps e.g. (A) main tasks, (B) approaches and (C) methods (of data collection and of analysis). A geomorphologist has three main tasks o f (i) description, (ii) classification and (iii) explanation of landforms. The description and explanation of landforms may be approached in a variety of ways viz. (i) qualitative Vs. quantitative (empirical) approach or (ii) systematic Vs. regional approach while the methods of analysis may be (i) inductive or (ii) deductive or (iii) analytical. The landforms may also be analysed by adopting system approach. (A) MAIN TA SK S

The first and foremost task of a student o f the science of landforms is (i) to describe the landform characteristics either subjectively or objectively on the basis of detailed information available to him, (ii) to classify the ladforms either genetically or quantitatively, and finally (iii) to explain the evolu­ tionary processes of the concerned landforms. (I) DESCRIPTION OF LANDFORMS

Landform characteristics may be described in a variety of ways depending on the audience to which the description is addressed and the nature of problems needing description and explanation. Gen­ erally, landform description involves (a) subjective description, (b) genetic description and (c) objective or quantitative scientific description. (a) S ubjective d e scrip tio n involves general­ ized and literary presentation of physical landscapes in a stylish manner by the non-specialist person. Such description depends upon the thinking of the individual as how he looks at the problems. Thus, the description becomes highly subjective and totally unscientific and hence has no geomorphological significance. (b) G enetic description involves besides general information of landform characteristics, rev­ elation of causes and factors o f origin and develop­ ment of landforms. For example, if the hillslope of any given region is undergoing the process o f de­ cline or water divide is being narrowed down, then one must also describe the processes and causes of slope decline and shifting of interfluves. If the rivers

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General system is divided into (i) synthetic system , (ii) partial system and (ii) black boxes. Process-response model is ultimate result of syn­ thetic system. In fact, process-response model is constructed on the basis of structure and analysis of geomorphic events and final conclusions. The main goal of partial system is to establish workable rela­ tionships between sets of factors or subsystems of a geomorphic system wherein detailed understanding of internal functioning of the sub-systems is not considered to be necessary but the information of the interrelationships between the sets of factors or sub­ systems enables the investigator to determine and predict the behaviour of the entire system under different input conditions (R.J. Chorley, 1967, p. 84). A black box is that wherein no detailed knowledge of the internal structure of different com­ ponents of the geomorphic system is required. ‘The black box models are constructed on the basis of assumptions and not on the basis of detailed knowl­ edge of geomorphological processes. Examples of such models are ‘dynamic equilibrium model’ of G.K. Gilbert, ‘climatic geomorphology’ of German and French geomorphologists (e.g. Budel, Peltier, Cailleux, Tricart etc.)’ ‘geographical cycle’ of W.M. Davis, W. Penck's ‘morphological system’ etc...

GEOMORPHOLOGY

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MATURE o f

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g eo m orph o log y

on deferen t scales) and rela tio n sh ip s b etw ee n morphometric variables.

have developed meandering courses, then the mode of development o f meanders should also he de­ scribed. W.M. Davis adopted entirely genetic ap­ proach for describing landform characteristics of any physiographic region having certain environ­ mental conditions. He described the landforms in terms of youth, mature and old stages. T h e Davisian method of genetic description has ...... been very widely applied in geomorphology but it is unfortu­ nately a decidedly clumsy tool, lacking in any real precision’ (R.J. Small, 1970). A.N. Strahler (1950) has also criticised Davisian method of genetic de­ scription of landforms as he remarked ‘a generalized overall scheme o f landscape evolution stated in terms of youth, maturity and old age contributes next to nothing to the understanding of factors determin­ ing the mechanism and intensity of erosion on slopes’ (A.N. Strahler, 1950).

(II) CLASSIFICATION OF LANDFORMS

An investigator after having an observation of physical landforms and processes and their distri­ bution patterns in the field attempts to classify the landforms and processes into identifiable catego­ ries. The landforms may be classified on two bases i.e. (a) quantitative basis (quantitative or non-genetic classification) and (b) genetic basis (genetic classification). (a) Q uantitative (n on -genetic) cla ssifica ­ tion involves numerical data which are obtained through morphological mapping, field instrum enta­ tion and interpretation of air photographs and satel­ lite imageries and is descriptive in nature as it does not include the consideration of mode of origin and nature of development of landforms, which, nodoubt, (c) O bjective description also called as quan­ is very important aspect of geomorphology. A hillslope titative or scientific description involves math­ profile may be classified on the basis o f slope angle ematical and statistical techniques. The relevant and slope plan into summital convex, free-face, data and information required for scientific descrip­ rectilinear and basal concave slope. The m easure­ tion of landscape characteristics of a given region ment of slope angles of hillslope profiles in the field arc gathered through precise measurements of facilitates the geomorphologists to classify slopes landforms in the field, or data are derived from into (i) level slope (0°— 0.5°), (ii) almost level slope topographical maps, air photographs and satellite (0.5°— 1°), (ii) very gentle slope (1°— 2°), (iv) gentle imageries and the data so derived are analysed through slope (2°— 5°), (v) moderate slope (5°— 10°), (vi) appropriate statistical techniques. Quantification is moderately steep slope (10°— 18°), (vii) steep slope applied not only to landscape forms, giving rise to (18°— 30°), (viii) very steep slope (30°— 45°), (ix) the branch of modern geomorphology known as precipitous to vertical slope (45°— 90°) (A Young). morphom etry, but also to processes such as river Fluvially originated drainage basins arc divided into flow, movement of sediments, types and rates of 1st, 2nd, 3rd, 4th............. order basins on the basis of weathering, soil creep, solifluxion and so on' (R.J. stream ordering and hierarchical order of the streams. Small, 1970, p. 4.). Exam ple : An ideal hillslope On the basis of periodicity of water flow streams are profile may be quantitatively or objectively de­ divided into ephemeral, seasonal and perennial scribed as follows— the hillslope is characterized by streams. ‘Indeed, classifications o f this kind are limited submittal convexity which is succeeded (down normally a prelude to the development o f hypoth­ the slope profile) by free face element of more than eses of origin, and really represent an organization 70° angle, middle rectilinear element having slope of the evidence on which such hypotheses are to be angle of more than 25° and thin vineer of debris and founded’ (R.J. Small, 1970, p. 6). basal concave element (pediment section) having (b) Genetic classification involves division o t landform assemblage o f a given geomorphic region into certain categories on the basis o f their mode of origin. For example, slopes can be geneti­ cally divided into tectonic slope (due to faulting, folding, warping etc.), erosional slope, slope of accumulation (depositional slope) etc. Streams may

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slope angle ranging between 7° and 0.5°. Similarly, a flu v ia lly o r ig i n a te d d ra in a g e b asin is morphometrically described on the basis of hierar­ chical position o f different tributary streams (stream ordering), stream number, stream lengths, basin areas, bifurcation, length and area ratios (the data for all aspects are derived from the topographical maps

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GEOMORPHOLOGY

la n d fo rm s’. T h o ugh the advocates o f climatic geomorphology have attempted to relate particular landform or landform suites to a particular climate (e.g. pediments to semi-arid climate, tors to periglacial climate, convexo-concave slope to humid climate etc.) but they have not succeeded as the so-called diagnostic landforms o f a particular climate have been found in more than one climatic regions. For example, tors are found right from tropical climate to periglacial climate, pediments have developed in many climatic regions except glacial and periglacial climates. Similarly, the presence o f tors in the areas having granites, sandstones, quartzites and even limestones has put big question mark before the advocates of structure-form approach.

be classified into sequent and insequent streams. Sequent streams (which follow the regional slope) are further divided into consequent, subsequent, obsequent and rcsequcnt streams whereas insequent streams (which flow across the geological structure and regional slope) are grouped into antecedent and superimposed streams. Besides individual landforms, landform assemblage may also be collectively di­ vided e.g. youthful landscape, mature landscape and old stage landscape as envisaged by W.M. Davis. On the basis o f cyclic origin of landforms they may be divided into mono-cyclic landforms, poly-cyclic la n d fo r m s , r e ju v e n a te d la n d fo r m s , e x h u m ed landforms etc. Landform assemblages are also clas­ sified morphogenetically on the basis of basic tenet o f climatic geomorphology that ‘each climatic type produces its own characteristic assem blage of landform s’ into (a) humid, sub-humid, arid, semiarid and glacial landscapes (W. Penck), (b) glacial, periglacial, boreal, maritime, selva, moderate, sa­ vanna, semi-arid and arid landscapes (L.C. Peltier).

The historical or chronological approach of landform explanation is based on the concept ‘that there is sequential change in landforms through time’, and on the ‘principle o f uniformitarianism^ (that ‘all the physical laws and processes that operate today operated throughout geological periods not necessarily with same intensity as now ’ and ‘present is key to the past’), cyclic nature o f earth's history, 'the cunccpt of palimpsest topography' and Davisian model o f ‘cyclic evolution o f la n d fo rm s'. The landform development is described in term s o f ev o ­ lutionary stages of youth, mature and old as envis­ aged by W.M. Davis. The main goal of this approach is to reconstruct the chronological history o f d en u ­ dation of a given region known as denudation chronology and to 'identify, date and interpret plan­ tation surfaces developed in past cycles and subcycles of erosion' (R.J. Small, 1970, p. 9). This approach also suffers from several shortcom ings w hich would be detailed out in the succeeding subsections.

(Ill) EXPLANATION OF LANDFORMS

The origin and development of landforms are explained on the basis of available information de­ rived through their description and classification. The explanation of landscapes may be approached through (a) establishing relationships between landforms and climate (clim atic geom orphology a p p r o a c h ) or between landforms and structure or rock types ( s tr u c tu r e - fo rm a p p ro a c h ), (b) through seeking landform origin and development in histori­ cal perspective (chronological or historical a p ­ p r o a c h ) and (c) through establishing relationships between landforms and processes (process-form a p p r o a c h ).

The p ro c e ss-fo rm a p p r o a c h o f landform explanation involves establishment o f relationships between geomorphological processes and landform s on the basis ot the concept that ‘each geom orphic process produces its own assem blage o f landform s.’ This approach further involves detailed study and m o nito ring o f m ode and rate o f o p e ra tio n o f geomorphic processes in terns o f w eathering, ero­ sion, transportation and deposition on one hand and their relationships with individual and groups o f landforms on the other hand. A few geomorphologists have also expressed reservations against this ap­ proach. For example, S.W. W ooldridge remarked, ‘I

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The s tr u c t u r e - f o r m a p p r o a c h of landscape explanation is based on the basic tencnt of structural geomorphology that ‘geological structure is a dom i­ nant control factor in the evolution of landforms . Thus, the influences of geological structure and lithological characteristics on the evolution o f indi­ vidual landforms (e.g. hillslopes, scarps, valleysides, tors) or general landforms and landtorm as­ semblage (e.g karst topography) are studied. C li­ m a te (through processesj-lan dform a p p r o a c h of landform explanation is based on fundamental co n­ cept o f climatic geomorphology that ‘each climatic ty p e produces its own characteristic assemblage of

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19

nature o f g eo m o r ph o lo g y

esses in smaller areas during shorter period o f time. The description o f morphological characteristics o f larger areas may also be approached in two ways e.g. (i) historical o r c h ro n o lo g ical a p p r o a c h and (ii) em pirical a p p ro a c h . Alternatively the explanation of landform characteristics may be approached ei­ ther through (i) regional a p p r o a c h or through (ii) system atic a p p ro a c h .

regard it as quite fundamental that geomorphology is primarily concerned with the interpretation of forms, not the study of processes’ while A.N. Strahler cautioned that ‘geographically-trained geom orpho­ logists are not well qualified to work in the field of process.’ Though process-form approach is more sci­ entific and involves mathematical and statistical techniques but it also suffers from certain shortcom­ ings. (1) The mechanism of all geomorphological processes is not the same. Some processes operate so slowly (e.g. soil creep or chemical weathering) or so intermittently (e.g. rainwash) that their precise and careful measurement in the field becomes necessary so that reliable data my be obtained. (2) The changes in some landform take place at exceedingly slow rate over long period of time that it becomes virtually impossible to measure them within a life-time of the investigator. (3) It becomes difficult to relate all the landforms to the present processes as many of the landforms are in fact ‘relict’ or ‘fossil’ features, the result of past processes (e.g. granitic tors of Dart­ moor of England). (4) ‘Another fundamental prob­ lem is the sheer difficulty of proving a causal rela­ tionship between process and form. How can it be demonstrated conclusively that a particular process results in a particular form?’ (R.J. Small, 1970, pp. 11 -12) because many processes operate together and thus it becomes difficult to isolate one process from other processes. For example, most of the weather­ ing processes (physical, chemical and biological) operate together (physico-biochcmica! weathering). This approach will be further elaborated and exam­ ined in the succeeding sections.

(I) HISTORICAL APPROACH

Historical approach o f landform studies in­ volves description of landform evolution through successive stages of geological time or say cyclic time involving larger spatial and longer temporal scales. ‘In this type of analysis the em phasis is placed on the historical development o f the land­ scape, based on the cyclic concept o f Davis, on the assumption that evidence of the past character o f the landscape is still apparent in its present form ’ (C.A.M. King. 1966, pp. 15-16). In fact, historical approach is based on the concept of ‘p a lim p se st to p o g r a p h y ’ which means such a surface which bears the imprints of geomorphological processes during past geologi­ cal periods after partially erased initial imprints (features) in the beginning. Palimpsest refers to that manuscript which has been written, erased and re­ written several times. Similarly, palimpsest topog­ raphy represents complex topographic features of a region which have been written (characterized by topographic features) by geomorphological proc­ esses, erased (previous geomorphological features partially destroyed by succeeding processes) and re­ written (production of new reliefs on older surfaces) several times. An attempt is made to reconstruct (reproduc­ tion) the past geomorphic history of the region concerned on the basis of present and remnant landforms following the dictum o f ‘p re s e n t is key to the past. This method of landform study is popularly known as d enudation chronology (denudational history of a given region). ‘The principal objective (of this method) is to identify, date, and interpret plantation surfaces developed in past cycles and sub-cycles of erosion’ (R.J. Small, 1970, p. 9) on the basis of evidences of drainage development, relic surfaces and past tectonic events. The degree of precision of landform analysis rests on deductive power of the researcher and level of qualitative and quantitative description of relic features.

(B) A PP R O A C H ES TO GEOM ORPHOLOGICAL AN ALYSIS

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The explanation of morphological character­ istics of a given region may be approached in a number of ways depending on spatial and temporal scales and goals of the geomorphologists. Based on conceptual bases the geomorphic studies may be approached in two ways e.g. (i) historical a p p ro ac h and (ii) functional a p p ro a c h . The historical ap­ proach is adopted when geomorphological evolu­ tion of larger areas is traced through long geological period while functional approach is adopted when landform characteristics arc related to present proc­

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GEOMORPHOLOGY

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This approach suffers from ccrtain perceptiblc weaknesses. This approach is highly deductive because unknown events and their responses are described on the basis of very limited known infor­ mation and evidences. In fact, the past geomorphic history is reconstructed on (he basis of very small parts ol pre-existing landforms. ‘An important criti­ cism which has been levelled against the denudation chronology approach is that it succeeds in explain­ ing directly only very small parts of the existing landscape, namely the fragments of former (erosion) surfaces which have been dissected and almost to­ tally destroyed in some cases by more recent ero­ sion' (R.J. Small, 1970). Secondly, historical ap­ proach is highly speculative because the old erosion surfaces and remnant forms have been so greatly modified by subsequent processes that it becomes difficult or say impossible to find out their original forms and initial heights. The dating of erosion surfaces is also highly speculative as valid geologi­ cal evidences are not available.

drainage density and drainage texture and carto­ graphic presentation of their spatial patterns) ; and r e l i e f aspect (computation of altimetric, hypsometric and clinographic variables and determination of relationships between area and height, height and slope angles, determination o f altitudinal frequency ipaxima for the identification o f erosion surfaces, calculation of hypsometric and erosion integrals for the determination of stages of cycle of erosion, computation of relative reliefs, dissection and ruggedness indices, slope angles and measurement of slope profiles etc.) This quantitative approach was developed in the U.S.A. in 1940s and was subsequently adopted by geomorphologists worldover. It may be pointed out that the results derived through morphometric analysis are sometimes misleading and erroneous and if they are not verified on the basis of field checks thes,e may lead to wrong conclusions about the geomorphological problems.

(II) QUANTITATIVE AND EMPIRICAL APPROACH

Regional approach involves study of land­ scape assemblage of a geomorphic region at large spatial and long tem poral scales e.g. m egageomorphology an d m eso-geom orphology. In fact, regional approach also involves theoretical studies of ‘cyclic evolution of landforms and more practical studies of denudation chronology’ at different spa­ tial scales varying from regional to continental scales. Similarly, the approaches to the study of the mega­ scale landforms may be grouped into 3 sub-catego­ ries e.g. (i) explanation of present landscape charac­ teristics and their evolution with reference to palaeoprocesses involving spatial scales varying from re­ gional to subscontinental areas and temporal scales of 108 years to 10s years ; (ii) examination and explanation of ‘present processes and the dynamic balance between process and form on sub-continen­ tal and regional scales* (Rita Gardiner and Helen Scoging, 1983) involving temporal scale of 1 to 100 years; and (iii) examination and explanation of sig­ nificant determinants of geomorphological proc­ esses i.e. climatic and sea-level changes and re­ gional to global tectonics. Thus, regional approach lor the study of mega-geomorphology aims at, mega­ scale, ‘an accurate understanding o f the nature of the past environmental conditions and associated proc­ esses; for an appreciation of how and when these

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Quantitative or empirical approach as alter­ native to historical approach is adopted to explain landform characteristics of such larger areas where sufficient evidences for historical study are not avail­ able because of destruction of relic forms due to g r e a te r d eg ree o f dissectio n by subsequent geomorphological processes. The empirical approach to study geomorphic problems of the large-scale geomorphological features involves the measure­ ment of geometry of different aspects of landscape and their quantitative interpretation. A fluvially origi­ nated drainage basin is selected as an ideal geomorphic unit for morphometric study wherein measurable properties of different aspects are measured, com­ puted and tabulated for reasonable explanation e.g. linear aspect (determination of hierarchical orders of streams, computation of stream numbers and bifurcation rattio, measurement of stream lengths and basin areas and computation of length and area ratios and establishment of relationships between these morphometric variables and examination of morphometric laws of stream numbers, stream lengths and basin areas based on exponential function mod­ els and law of allometric growth based on power function model) ; areal aspect (measurement of basin shapes and computation of stream frequency,

(Ill) REGIONAL APPROACH

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nature o f g e o m o r ph o l o g y

past processes m oulded the surface of the earth; and for description and models of the present dynamic interaction between process and form ’ (Rita Gardiner and Helen Scoging, 1983, p. xi). It is apparent that in one way or the other the regional approach is analo­ gous to historical approach o f landscape studies. It may also be pointed out that the historical or regional approach has been overshadowed by process-form approach involving micro-geomorphology (spatial and temporal scales both being very small). (IV) SYSTEMATIC (FUNCTIONAL ) APPROACH

Systematic approach of landform studies in­ volves the measurement and analysis o f operation of geomorphological processes which shape different suites of landforms in varying environmental condi­ tions. The conceptual base o f systematic approach comprises functional studies o f reasonably contem­ porary processes and the behaviour of earth material which can be directly observed and which help the geomorphologist to understand the maintenance and change of landform s’ (Chorley, Schumm and Sugden, 1985). Functional approach lays more emphasis on the observation and monitoring of operation of present day processes at very small spatial and short tempo­ ral scalcs and establishment of causal relationships between process and form which becomes process geom orp h ology which aims at prediction of likely responses (effect) to be produced by causative fac­ tors i.e. independent variables. Systematic approach is further divided on the basis of major causative factors of landscape development into (a) processform approach and (b) structure-form approach.

21 evolution of form over tim e’ (Rita G arddiner and Helen Scoging, 1983). Thus, a gcomorphologist's task is to (i) have detailed instrumentation and study o f micro-processes so as to understand the physical and chemical works performed by them, their co m ­ plex interactions and responses (effects) in the evo ­ lution of morphological features, (ii) reconstruct chronology of environmental changes w hich might have occurred during geological past, identify palaeoprocesses and their probable relationships with landforms, and (iii) ‘analyse m ega-scale (regional and continental) dynamic systems existing at present because the independent variables controlling the development of the landform may change totally as the scale changes from mega to micro levels. O nce these aspects of geomorphology have been ev alu ­ ated and combined we will better understand, model, and predict the morphological developm ent o f the surface of the earth’ (Rita G ardiner and Helen Scoging, 1983). (C) R E S E A R C H METHODS

Explanation of processes and landform s and building of models require data acquisition from various sources. R.J. Chorley (1966) has outlined 3 steps and methods of data acquisition which ulti­ mately lead to theoretical work. The integrated approaches to research methods in geom orphology include, according to R.J. Chorley, field observa­ tions, laboratory observations, office observations and theoretical work. ‘O bservation in the field plays a very large part in geomorphological work, w hatever the aim o f the particular study or whatever the method o f ap ­ proach’ (C.A.M. King, 1966). Field observation involves qualitative as well as quantitative methods of data acquisition depending on the approaches of landform studies. For example, the geomorphologists of the school o f denudation chron ology used to derive information about chronological evolution o f landscapes and erosion surfaces at regional and mega scales through qualitative field observations and through ‘subjective map analysis’ but the emer­ gence of functional and process-from approach to landform studies dem anded accurate quantitative data regarding forms, processes and materials (rocks and soils). Thus, quantitative data are obtained through numerical measurement o f forms (e.g. slope angles,

The process-from approach envisages that ‘an understanding o f the erosional and depositional processes that fashion the landforms, their mechan­ ics and their rate o f operation must also be obtained in order that the past evolution can be explained and future evolution predicted. The aspects and short­ comings o f process-form approach and structureform approach have already been detailed in the preceding section.

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It may be concluded that ‘if geomorphology is to continue to exist as an independent discipline, and not to be subsumed within earth sciences, geol­ ogy, engineering, hydrology and so on, it must attempt to explain the relationships between form and process, both in past and present, as well as the

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GEOMORPHOLOGY

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absolute and relative reliefs, various properties re­ garding height, dimension etc. of landform compo­ nents) and processes (e.g. measurement of discharge, infiltration, evaporation, sediment load, rainfall, runoff etc. in the case of fluvial process) in the field through appropriate instruments.

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Laboratory observation involves experimen­ tation and numerical measurements of samples col­ lected in the field (e.g. chemical and mechanical properties of soil and rock samples, chemical prop­ erties of water, grain-size measurement and deriva­ tion of chemical properties of suspended sediment load and other eroded materials etc.) and measure­ ment derived through controlled experiments in the laboratories (e.g. nature and rate of rill and gully development, rate of meander development, rate of soil erosion and sedimentation, rate of nick point recession etc.). The examples may be cited from other processes and related landforms of different environmental conditions. Office observation comprises data deriva­ tion from map analysis say map work. These days a mass data set is being derived through measurement and computation involving numerous useful tech­ niques from topographical maps, air photographs and satellite imageries pertaining to different com­ ponents of landforms. The measurement and deriva­ tion of data of geomorphological significance from air photographs and satellite images at regular time intervals has enabled the geomorphologists to moni­ tor geomorphic changes. The recent work by Savindra Singh and Alok Dubey (1991-1995) on ‘gully erosion and manage­ ment’ in sub-humid tropical riverine alluvial envi­ ronment of India incorporates almost all the steps referred to above for the study of genesis, micro­ geometry, morpho-cyclcs and integrated manage­ ment of gully watersheds as the gully basin was surveyed thrice ( 19 9 1, 1992 and 1994) and contours at the interval of one meter were traced on the ground for the derivation of morphometric data of gully basin, the meteorological (rainfall, temperature, rela­ tive humidity, evaporation etc.), hydrological (dis­ charge, runoff, infiltration, hydrological budget etc.) and geomorphological (rate of erosion, deposition, suspended sediment load etc.) data were obtained (hi i ugh field instrumentation during 3 wet monsoon months of July, August and September for 1991,

1992, 1993 and 1994 and the mass data set, so derived, were processed in the computer lab. besides analysis o f w ater and s e d im e n t loads in the geomorphological laboratory. The quantitative ap proach gave birth to morphometric analysis o f linear, areal and relief aspects of fluvially originated drainage basins which have been recognized as ideal geomorphic units since 1945. Detailed data pertaining to linear aspect (e.g. hierarchical orders of streams, stream number, stream lengths, sinuosity, m eander properties etc.), areal aspect (stream frequency, drainage density, drainage texture etc.) and relief aspect (relative relief, average slope, dissection index, altimetric, hypsometric and clinographic properties etc.) are derived from topographical maps o f different scales. Theoretical \york involves data processing and formulation of models and theories e.g. laws o f stream numberand stream lengths (R.E. H orton)and calculation of mathematical models. (D) METHODS O F A N A LYSIS

There are three alternative routes for satis­ factory scientific explanation o f geom orpholoigcal problems e.g. (i) inductive m ethod, (ii) ded u ctive method and (iii) analytical m ethod, all o f w hich are based on data acquisition, their classification, and analysis so as to come to certain ‘conclusions con­ cerning the nature and genesis of the particular feature, investigated, whether it be a whole conti­ nent or one small slope or spit’ (C. A.M. King, 1966). In du ctiv e m e th o d of argum ent and analysis of geomorphic problems involves, in successive steps, arrangement ol unordered facts in logical order on the basis ot correct definition and classifi­ cation of observed facts of the given problem s so that one (fact) leads to another and then to the final conclusion (C.A.M. King, 1966), inductive gener­ alization and linal conclusion resulting into formu­ lation ol laws and theories which offer satisfactory explanation of geomorphic problem. It may be pointed out that in inductive methods data are collected first, t ey are defined and classified and final conclusion about the real world is drawn (i.e. model or theory Ul, ' ng) ' n *ast stagc. In other words, inferences and final conclusions are drawn on the basis of o served tacts. ‘As a method it is best suited to a air y simple problem, the solution o f which is based

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NATURE OF GEOMORPHOLOGY

23

only those facts which validate the tentative hypoth­ esis and may ignore those facts which do not favour his deductions. T h e quality of results will depend on the nature of deductions and the closeness of comparisons. When there are many complex or peculiar deductions then there is a better chance that the comparison will be valid, and the theory will be more strongly supported’ (C.A.M. King, 1966).

on a wide field of observation and relevant data, so that it is not necessary to invoke theoretical reasoning' (C.A.M. King, 1966). This method suffers from the shortcomings that no generalization about the real world is made in the beginning and hence a lot of labour in the collection of data is wasted and since only one conclusion is derived at the end and hence such conclusion may be questionable or sometimes may be even false because some of the facts, which may be geomorphologically significant but may not be favourable to the final result, are deliberately or subconsciously ignored. But, ‘it is sometimes help­ ful to give at least some indication of the final conclusion nearer the beginning of the argument* (C.A.M. King, 1966).

The fundamental difference between induc­ tive (the method of ruling hypothesis) and deductive (the method of working hypothesis) is that in the 1st method theory is formulated in the last stage on the basis of observed facts while in the second method a working hypothesis is deduced in the beginning and the fieldwork and data collection is accom ­ plished according to the demand o f the deduced hypothesis. The analytical m ethod involves deduction and formulation of more than one alternative hy ­ potheses (multiple hypotheses) and thus data are collected according to alternative hypotheses and hence the investigator does not have bias to a par­ ticular hypothesis. The observed facts and deduc­ tions of all the alternative hypotheses are compared and finally only that hypothesis is approved and retained which conforms with the greatest number of observations derived through filed work. Thus, it is obvious that the analytical method of landform analysis overcomes the shortcomings of deductive and inductive methods.

The deductive m ethod of explanation of geomorphic problems involves formulation of a tentative hypothesis regarding the real world (i.e. geomorphic problems under investigation) in the beginning. After the formulation of tentative hy­ pothesis its consequences are deduced in advance, facts are collected according to the demand of de­ duced hypothesis, actual field observations are com­ pared with deduced consequences and finally it is argued whether the hypothesis is approved or re­ jected. In case the tentatively deduced hypothesis is not approved or it becomes unsuccessful, original hypothesis is revised and the entire process as re­ ferred to above is repeated but if the hypothesis becomes successful after comparison of deduced and observed facts, it leads to the construction of laws and theory which may offer reasonable expla­ nation o f the real world. This method suffers from the weakness that there is every likelihood that the investigator may become biased in the matter of collection o f data and information as he may retain

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It may be concluded that ‘main essential in all the other methods discussed is that observations should be accurate, as far as possible quantitative, and carried out on a systematic basis, while imagina­ tion and integrity are required in the development and testing of hypotheses’ (C.A.M. King, 1966).

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

C o n c e p t s r e l a t e d to u n i f o r m i t a r i a n i s m , g e o l o g i c a l s t r u c t u r e , g e o m o r p h o l o g ic a l processes, stages o f time, geom orphic scale (time s c a l e - c y c l i c tim e , g r a d e d tim e and ste a d y tim e, sp a tia l s cale ), g e o m o r p h o l o g ic a l equation, com plexity o f landforms etc.

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CHAPTER 2

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24-56

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

T h e d e v e l o p m e n t o f g e o m o rp h o lo g ic a l thoughts through different periods of evolution of geom orphological know ledge and associated re­ searches pertaining to the understanding and expla­ nation o f landform characteristics and geomorphic processes associated with their genesis and meth­ odological development o f geomorphic research have enabled the geom orphologists to conceive a few fundamental concepts which generalize the landform developm ent. W.D. Thornbury (1959) has presented a su m m a ry o f a few fundam ental concepts in geom orphology. It is, thus, desirable that the readers s h o u ld be a q u a i n te d w ith su ch fu n d a m e n ta l geom orphic concepts. CO N CEPT 1

The sam e p h ysica l processes and laws that operate today, o p era ted throughout geological time, although no t necessarily always with the same intensity as now ’ (W.D. Thornbury)

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The present conccpt is fundamental principle o f modern geology which is very often popularly known as ‘p rin cip le o f u n ifo rm ita ria n ism ’ which was first postulated by renow ned Scottish’ geolo­ gist, Jam es H utton, in 1785. This concept was furthei modified and developed by his disciple Jhon P layfair i n 1802. S ir C harles L yell popularized this concept o f uniformitarianism by giving suitable place

to it in his famous book ‘p rin cip les o f g eo lo g y ’. It may be pointed out that H utton's original concept was a bit different from the co n ce p t stated above and suffered from some sh o rtcom ing s. F or example, Hutton stated that ‘geological processes w ere active with same intensity during each period o f geological tim e’ and thus he postulated an o th e r principle on this concept e.g. ‘the p resen t is k ey to th e p a st’ and ‘no vestige o f a b eg in n in g an d n o p ro sp ect o f an end.’ It is inferred from his co n cep ts that all the geological processes affecting the earth's crust, w hich operate at present, were also active in the geological past and hence the past geological and g e o m o rp h ic history of the earth may be reco nstructed on the basis o f present processes and their topographic expressions (landform characteristics). Hutton's concept ‘that physical processes were always active with sam e intensity throughout geo­ logical periods is erro n eo u s and confusing. For example, g laciers w ere m ore activ e during Carboniler.ous and P leistocene p erio d s than other pro­ cesses. At the sam e time, they w ere m ore active during aforesaid periods than the present glaciers. The temporal variations in the m ag nitude o f opera­ tion ol processes are because o f clim atic changes and there are definite ev id en ces for several phases of climatic changes du ring past geological times. Thus, the distributional p atterns o f d ifferent climatic types

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

25

have registered spatial shiftings during geological past. For example, some areas, which are presently characterized by humid climate and dominance of fluvial process, have been dominated by dry climatic conditions and aeolian process. Similarly, some of the present dry desert areas have been humid regions in the past. For example, the fossils of coal found in Great Britain are indicative of vegetation commu­ nity of equatorial climate, which forcefully proves that Great Britain, which enjoys humid temperate climate at present, was characterized by hot and humid equatorial climate during Carboniferous pe­ riod when the present-day tropical areas were domi­ nated by glacial climate. For example, ample evi­ dences are available to elucidate several phases of climatic changes in India. There is presence of glacial boulders and boulder clay just below the Talchir coal seams in Orissa. Most of the coal seams of India were formed during Gondwana period, which means belore the formation of Gondwana system o f rocks (sedimentaries including coal), the regions having coals in India were glaciated. The coal seams overlying glacial boulder indicate the prevalence of hot humid climate. Similarly, vulcanicity was not uniformly active throughout geological pe­ riods. It was more active during Cretaceous period than today. The Cretaceous lava flow was so wide­ spread that extensive lava plains and plateaus were formed in almost all o f the continents including basaltic lava flow over Peninsular India. The m ou n­ tain building was confined to certain periods only e.g. pre-Cambrian. Caledonian. Variscan(hercynian) and Tertiary periods of mountain building.

believed in orderliness o f nature i.e. the nature evolves in orderly course. According to him the_ nature is systematic, orderly, coherent and reasonable i.e. destruction leads to construction while construction results into destruction. For example, denudation of uplands (destruction) leads to sedimen­ tation in lowiying areas giving birth to alluvial plains (construction). Continuous sedimentation leads to subsidence of ground surface. The nature has inbuilt self regulatory mechanism known as hom eostatis mechanism which acts in such a manner that any chang e effected by natu ra l fa c to rs (w h e th e r endogenetic or exogenetic) is suitably com pensated by changes in other components of the natural system. Hutton was the first scientist who postulated the concept of cyclic nature o f earth's history. All major geological activities are repeated in cyclic manner. For example, there have been four major periods of mountain building viz. precam brian, Caledonian, hercynian and tertiary periods o f m oun­ tain b u i 1d in g a n d ^ a c h jn o u n ta i^ ^ succeeded by a period of quiescence. Similarly, glacial periods during Pleistocene ice age w'ere sepa­ rated by interglacial periods. There are ample evi­ dences to validate the observations that each geo­ logical process has completed several cycles during geological past but it becomes difficult to find out as to when a particular geological process began to work and it is equally a difficult task to predict as to when a particular process would cease to work. Based on this connotation Hutton postulated his concept, ‘no vestige o f a b eg in n in g : no prospect o f an end.

It is. thus, obvious that geomorphic and tec­ tonic processes were active in all the geological periods and their mode o f operation was the same as today (e.g. rivers formed their valleys through ver­ tical and lateral erosion in the past in the same manner as they are forming their valleys to day, sea waves shaped coastal areas in the same manner as they are doing today, the glacial movement and erosion was controlled by the same laws and princi­ ples during Carboniferous and Pleistocene periods as they are controlled today etc.) but the intensity of erosional and depositional works differed tempo­ rally.

The examples of denudation chronology o f the Applachians and Peninsular India may dem on­ strate the cyclic nature of earth's history as envis­ aged by Hutton. The Applachian revolution during Permian period resulted in the 1st upliftment of the Applachians which was followed by long period of active denudation culminating into the development of Schooley peneplain which w'as again uplifted and then was peneplained to form Shenondoah peneplain. The third phase of upliftment was again followed by active denudation resulting in the formation o f Harrisberg peneplain which was again uplifted in the recent past and fourth cycle o f erosion is in operation. Peninsular India has passed through van-

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The processes (mainly endogenetic) which affect the earth's crust act in a cyclic manner. Hutton

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GEOMORPHOLOGY

26 ous phases o f cyclic development e.g. D h a r w a r landscape cycle, Cuddapah-Vindhyan landscape cycle, Cambrian landscape cycle, Gondwana land­ scape cycle, Cenozoic landscape cycle etc. (R.P. Singh), (see Chapter 17).

CONCEPT 2 ‘G eologic structure is a dom inant control fa c to r in the evolution o f landforms and is reflected in them. ” (W.D. Thombury) The above concept demonstrates imposing influence of geological structure on primary and secondary landforms (produced by exogeneticdenudational processes). W.M. Davis included ‘struc­ tu re’ in his ‘t r i o ’ namely structure, process and time, as important controlling factors of landscape development through his postulate that ‘landscape is a function o f structure, process and time’ but he gave more importance to ‘tim e’. A few usages like ‘rocks and reliefs’, ‘geological structure and landforms’, ‘geologicalgeomorphology’ (Chorley, Schumm and Sugden, 1985), ‘structural geomorphology’, ‘vol­ c a n ic la n d f o r m s , ’ ‘a re n a c e o u s la n d fo r m s ’, ‘argillaceous landforms,’ ‘calcareous landforms,’ ‘igneous landforms’, ‘metamorphic landforms’ etc. c le a rly d e m o n s tra te the view s o f a host of geomorphologists about strong control of geologi­ cal structure and lithological characteristics on mor­ phological characteristics of a region. Even the modem geomorphologists like J.T. Hack, R.J. Chorley, S. Schumm, D.E. Sugden etc. have clearly outlined influences of geological struc­ ture on landforms. ‘Exposed rocks are immediately acted upon by exogenetic weatheripg and erosional processes to form secondary landforms, which re­ flect geologic controls at both global and local scales (p. 7 8 )............ The distinctive characteristics o f landscape are commonly a complex response to variations in rock type (lithology), to primary struc­ tures within the rock units, to secondary structures involving groups of rocks units mainly due to diastrophic processes, to the effects of different exogenetic processes and to the geomorphic history’ (Chorley, Schumm and Sugden, 1985, p. 150).

domi n a n t that they overshadow the control of geokaL cal structure. Som e um es geological structure - ^ ^ T j S v e factor in the evolution o f landforms. ‘There is tendency to regard structure as the domi­ nant control of surface form and no doubt this is true in many instances. But structure is not always the principal control and never the only one’ (E.H. Brown) and thus ‘thejandform s_cannpt be rn nne cause, but are the result o f a complex inter. several factors and processes, both ^ ^ i d r T o r T g i n a t i n g from within the earth's m i r t nnH i " H a tin g structure and rock-type) and ^ r r l r w i r (originating from the atmosphere and' T n du din gw eath ering , transportation and erosion’ (R.J. Small, 1970). If structure is used in narrow sense of the term then it includes only deformation and arrangement of rocks due to earth-movements (endogenetic forces) but if this term is used in w ider sense then structure includes (i) nature o f rocks (lithology, meaning rock types), (ii) arran gem en t o f rocks (widely known as structure) and (iii) rock characteristics. Here, ‘structure’ is used in w ider sense o f the term so as to demonstrate influences o f all the aforesaid aspects of geological structure and landforms. 1. Lithology or Nature of R o cks

Lithological aspect o f geological structure includes types of rocks (e.g. igneous, sedimentary and metamorphic groups o f rocks). Lithological c h a r a c te r i s t ic s h a v e g r e a t e r s ig n if ic a n c e in geomorphology because these determ ine and con­ trol the evolution o f landform s and nature of land­ scape. Considering this fact S.W. Wooldridge and R.S. Morgan aptly remarked, ‘rocks whether igne­ ous or sedimentary, constitute on the one hand the manuscripts of the past earth-history, on the other, the basis for contem porary scen ery ’. In fact, differ­ ent types of rocks differ considerably as regards their composition and chemical characteristics and hencc weathering and erosional processes act upon them at varying rates thus giving birth to variations in landform characteristics. ‘Lithological controls over landforms produce a large num ber o f variations and, more important, these variations may be associated with a wide range o f discrete regions varying in size from a distinctive outcrop o f a few square metres to areas of uniform rock type extending over hundreds of

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This does not mean that geological structure is always and only dominant control factor in the evolution o f landforms as sometimes exogenetic (denudational) processes become so effective and

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

near Khandala (between Bombay and Pune). The Yellowstone river has dug out a large canyon in the Columbian lava plateau o f the U.S.A.

square kilometres’ (Chorley, Schumm and Sugden, 1985). The relatively hard rocks (most o f igneous and metamorphic rocks) give birth to bold topogra­ phy. Sometimes, the influence of some rocks on geomorphic features is so dominant that the resul­ tant landscape is named after the rock group or individual rock e.g. granitic landforms, karst or limestone landforms, chalk landforms etc. The associa­ tion of few rocks and their topographic expressions (landforms) may be examined to elucidate the con­ cept in question.

If the sills are intruded in the tilted or inclined sedimentary layers and if they are more resistant than the surrounding sedimentary' rocks, the latter are enoded more than the former and thus resistant sills project above the general ground surface as cuestas and h o g b ack s (fig. 2.1 >. Granitic rocks when subjected to exfoliation or onion w e a th e rin g give birth to dom eshaped landforms known as exfoliation d o m e s. Several exfoliation domes o f granite-gneisses are seen over the Ranchi plateau, for example. Kanke Dome near Ranchi city, a group of gneissic dom es near Buti village (near Ranchi city).

Igneous Topography

Variations in structure and composition of igneous rocks o f a particular area exert strong influ­ ence on the genesis, development and nature of landscape. Further, intrusive (e.g. granites) and ex­ trusive (e.g. basalt) igneous rocks influence land­ form characteristics differently depending on their degree o f relative hardness. M assive lava flows over extensive areas re­ sult, after cooling and consolidation, in the forma­ tion o f lava plateaus the surfaces of which are least affected by fluvial erosion because ‘the drainage is conducted underground by the joint systems, perme­ able ash and flow cavities, but deep weathering of basalt (especially where closely jointed in the humid tropics) and areas o f poorly welded tuffs may lead to considerable piecemeal reduction of volcanic pla­ teau by ero sio n ’ (Chorley et. al. 1985) but the rivers, which develop over the basaltic plateaus and are subsequently fully established, resort to vigorous valley deepening through active downcutting with the result the extensive basaltic plateau is seg­ mented into num erous smaller plateaus character­ ized by flat tops and steep slopes on all sides. Such features are called as m e sas and bu ttes. Basaltic plateaus and plains give birth to picturesque land­ scapes after continued weathering and erosion. Very deep and long gorges and canyons have been formed by the source segm ents o f the Saraswati (draining towards Arabian Sea) and the Krishna rivers (drain­ ing towards the Bay o f Bengal) through their vigor­ ous vertical erosion in the massive and thick basaltic covers o f M ahabaleshw ar plateau (about 100 km south-west o f Pune). Similarly, the Ullahas river has entrenched a very deep gorge in the basaltic plateau

Fig. 2.1 : Landforms resulting from differential erosion o f sills and surrounding rocks.

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Massive granitic batholiths. when exposed to the earth's surface due to removal o f superincum bent load of overlying rocks through continued erosion, become interesting landforms. These dom e-shaped hills project above the general surface. Such ex­ posed granite-gneissic domes are very often found on Ranchi Plateau. The granitic batholiths were intruded in the Dharwarian sediinentaries during Archaean period. After a long period o f prolonged subaerial erosion the Dharwarian sedimentaries have been removed and the batholiths, regionally known as Ranchi Batholiths, have been exposed well above the ground surface (50 to 100m from the ground surface). M urha Pahar near Pithauria village, lo­ cated to the north-west o f Ranchi city, is a typical

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GEOMORPHOLOQy

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exam ple of exposed grantic-gneissic batholithic domes. These exposed batholithic domes have suf­ fered intense fracture because of the removal of superincumbent load o f Dharwarian sedimentaries and hence resultant massive joints have been re­ sponsible for the development of different types of ‘t o r s ’. Extensive granitic domes of Yosemite P a rk , Sierra Nevada, S to ne M o u n ta in of Georgia (U.S.A.) and S u g a r L o a f of Rio de Janeiro (Brazil) are other exam ples o f such granitic domes which have been formed due to unloading of superincumbent load (sedimentaries) consequent upon prolonged erosion. Fit’. 2.3 : An example o f volcanic butte.

The differential erosion of the basaltic ‘cap r o c k s ’ (fig. 2.2) produces interesting features like m e sas and buttes. Mesa is a Spanish word meaning thereby a table. Mesa, in fact, is such a hill which is characterized by almost flat and regular top-surface but by very steep slopes (wall-like) from all sides. When mesas are reduced in size due to continuous weathering and erosion, they are called buttes. Messas are locally called as ‘P a ts ’ or ‘P a tla n d ’ 011 the Chotanagpur plateau of south Bihar. Jamira pat, Netarhat Pat, Bagru pat, Khamar pat, Raldami pat, Lota pat etc. are typical examples of lava-capped messas of the western Chotanagpur High Lands. Mahabaleshwar plateau and Panchgani plateau (of the Western Ghats, Maharashtra) are characteristic representatives of well developed basaltic mesas. Grand Mesa and Raton Mesa of the state of Colo­ rado, USA, are typical examples of extensive mesas. Grand Mesa rises more than 1500m (5,000 feet) higher than the surrounding ground surface.

Sometimes magma is injected in a vertical columnar form in the sedim entary rocks. The upper portion of vertical column of magma appears as butte when the overlying rocks arc eroded down. Such butte is called as ‘volcanic b u t t e ’ (fig. 2.3). The grantic rocks having rectangular joint patterns are weathered and eroded along the inter­ faces of their joints and thus smaller tables or blocks are separated by the eroded narrow clefts developed along the joints. Such granitic topography develops rectangular drainage pattern (fig. 2.4).

Fig. 2.4 : Development ofrectangular topographicfea­ tures on granitic rocks having rectangular joint pattern.

The igneous rocks having columnar joints give birth to hexagonal landforms after weathering and erosion (fig. 2.5). Scoria and ash cones when subjected to fluvial erosion develop radiating rills and gullies whereas strato-valcanic cones, after prolonged erosion, are c aracterized by n u m e ro u s ra d ia tin g valleys

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Fig. 2.2 : Lava-capped mesa and butte.

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

1

Fig. 2.5 : Development o f hexagonal landforms on igenous rocks having columnar joints.

known as ‘b a r r a n c a s ’. The valcanic pipe filled with breccia is exposed after prolonged erosion above the ground surface and is called d ia tre m e . Shiprock (fig. 12.8) o f New; M exico (USA) is fine example of diatreme which projects 515m above the surround­ ing surface composed of sedimentary rocks. If magma is intruded as sills into inclined sedimentary beds of weak resistance then the sedimentary beds are eroded and the sills being resistant project above the ground surface.

Fig. 2.6 : Formation o f tors.

Similarly, mesas and butles are co n fin ed not only to basaltic plateau but these have also been found over sandstone rocks where these overlie weak shales and siltstones. M orchapahar(H azaribagh plateau, Bihar, India) is a fine exam ple o f sandstonecapped mesa. Similarly, B hander plateau (M.P., India) having Vindhyan sandstones over w eak shales and siltstones is an example of extensive m esa. It may, thus, be concluded that the d ev elo pm ent o f mesas and buttes is no doubt lithologically co ntrol­ led but these are not confined to a particular rock type. They may be formed through active fluvial erosion in humid and subhum id climate w henever relatively resistant rock overlies weak rock.

Well jo inted granitic rocks give birth to very peculiar landform s such as to rs which ‘are piles of broken and exposed masses o f hard rocks particu­ larly granites having a crown o f rock blocks of different sizes on the top and clitters (trains of blocks) on the sides. The rock-blocks, the main com ponents o f tors, may be cuboidal, rounded, an­ gular etc. in shape. They may be posted at the top of the hills, on the flanks o f the hills facing a river valley or on flat basal p la tfo rm ’ (Savindra Singh, 1977, p. 93, N ational G eographer, Vol. 12(1) (fig. 2.6). A few alternative hypotheses o f tor formation have been put forth e.g. pediplanation theory o f L.C. King, deep basal w eathering theory o f D.L. Linton,

Sedimentary Landforms

The landform s developed over different sedi­ mentary rocks (e.g. arenaceous— siltstones, m u d­ stones, sandstones; argillaceous— clay and shale; calcareous— limestones, dolom ites etc. rocks) are called sedimentary landforms. Som etim e, the co n­ trol of a particular sedim entary rock on landform characteristics is so dom inant that particular rock is p refix e d w ith g e o m o rp h o lo g y e.g. ‘lim e sto n e g eom orp h ology 9 (Stephen Trudgill, 1 9 $ 5 )o rk a rst geom orp h ology etc. Sandstones having silica ce­ mentation are resistant to chem ical w eathering and hence give birth to bold topography and developm ent o f low drainage density while sandstones cem ented

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periglacial theory o f J. P alm er and R.A. Neilson, two-stage theory o f J. D einek, glacial theory ot R. Dalh etc. but there is no unanim ity am ong the exp o­ nents becausc tors are not confined to a particular rock type and clim ate as tors have been found over granites (even basalt), sandstones, limestones etc. right from hum id tropical to periglacial climate.

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GEOMORPHOLOGY

by ferrous contents are subjected to rapid rate of oxidation and fluvial erosion and hence give birth to undulating and rolling terrain. The argillaceous rocks e.g. clay and shale are less resistant to erosion and thus low relief is associated with them. Argillaceous rocks respond differently in humid, arid and semiarid environment e.g. in humid regions these are characterized by low relief, low to gentle slope angles (less than 8°), moderate drainage density, dendritic drainage pattern, convexo-concave hills ; subhumid and semi-arid regions having clay-shale rocks are characterized by the development of badland topography with high drainage density (due to numerous rills and gullies) and subdued reliefs, the gully valleys having steep valley sides (30°-60° and sometime 70°-80°) are separated by narrow ridges. Calcareous rocks (e.g. limestones, dolomites and chalk) are subjected to solution under humid conditions and give birth to solution holes and de­ pressions of varying shapes and dimensions (e.g. sink holes, swallow holes, dolines, polje, uvala etc.), underground solution networks (caves and associated features), disorganized and poor surface drainage etc. The landforms developed on carbonate rocks are collectively called as k a rs t topography. In humid tropics two special types of karstic topography have been identified e.g. cone karst, in the ‘cockpit country* of Jamaica and Cuba, characterized by steep sided rounded hills, and tower karst, in monsoon land of China and Vietnam, characterized by isolated very steep sided (almost vertical ) narrow but high pillars (upto 300m). Wherever sandstones overlie shales and siltstones majestic mesa and butte are formed and escarpments are crowned by stupen­ dous steep scarps (e.g. Rewa escarpments, Bhander escarpments, Rohtas plateau escarpments etc. where Vindhyan sandstones lie over shales and siltstones). Metamorphic Landforms

2. Arrangement of Rocks

Arrangement of rocks means disposition of rock beds mainly of sedimentary rocks due to de­ formation processes. Sedimentary rocks are gener­ ally deformed due to isostatic, tectonic and orogenetic mechanisms into folded, faulted, domed, homoclinal (uniclinal) structures etc. Horizontal dis­ position of sedimentary beds denotes least deforma­ tion but these may be subjected to upwarping. Such geological structures exert strong influence on land­ form characteristics. (I) FOLDED STRUCTURE AND LANDFORMS

Sedimentary rock beds are sqeezed and buck­ led and folded into anticlines and synclines due to lateral compressive forces. The folded structure ranges from simple folds to complex folds (i.e. recumbent folds depending on intensity of compressive forces). Simple folded structure is characterized by sequence of anticlines and synclines and in the initial stage trellis drainage pattern evolves over such structure. Such drainage pattern is characterized by the devel­ opment ot consequent, subsequent, obsequent and resequent streams. The Fegion of folded structure when subjected to continued fluvial erosion for longer period experiences the process of inversion of relief wherein original anticlines (due to more erosion) are eroded down and become anticlinal valleys where as synclines (due to less erosion) become synclinal ridges (fig. 2.7). For details see chapter 10 and figs. 10.9, 10.10 (chapter 10). examples ot inverted reliefs are found in Jura moun­ tains and southern Applachians.

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Unlike sedimentary and igneous rocks meta­ morphic rocks are not pronounced in the develop­ ment of landforms because these (e.g. quartzite, slate, schist, gneiss etc.) have uniform resistance to erosional processes though the process of meta­ morphism-‘coverts rocks of lower resistance (e.g. shale and sandstone) to those of higher resistance (e.g. slate and quartzite). Although metamorphic rocks generally present more resistance to erosion than do their sedimentary counterparts, it is not easy

to identi fy a separate class of distinctly metamorphic landforms' (Chorley, Schumn and Sugden, 1985). Quartzitic sandstones when lie over shales and siltstones give birth to stupendous escarpment char­ acterized by upper free face and rectilinear segment and basal concave pediment section (last two devel­ oped on shale and siltstone). Quartzite^ are on an average resistant to mechanical and chemical weath­ ering and produce bold topography having very high reliefs. Slates are more succeptible to erosion and are associated with subdued reliefs while resistant schist rocks produce highland topography. Gneissic rocks form domes and tors.

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

faultline s carp is formed due to renewed downward erosion caused by further fall in base-level of ero­ sion. In fact, resesequent scarps result from the reversal of obsequent scarp and it is oriented in the direction of the original normal or consequent scarps but is much older than the latter (fig. 2.8(4)).

Fig. 2.7: Development o f landforms over folded structure. (II) FAULTED STRUCTURE AND LANDFORMS

A fault is a fracture in the crustal rocks wherein th* rocks are displaced along a plane called as fault plane. In other words, when the crustal rocks are displaced due to tensional movement caused by the endogenetic forces along a plane, the resultant struc­ ture is called a fault. Different types of faults are created due to varying directions of motion along the fault plane e.g. normal faults, reverse faults, lateral or strike-slip faults, step faults, transform faults etc. Differentfaulttypesproduce,aftererosion. landforms of varying characteristics. Take the case of normal fault where downthrown block is displaced down­ ward along the fault plane giving birth to fault scarp which is, without doubt, structural in genesis. Such fault scarps after prolonged erosion produce differ­ ent types of erosional landforms e.g. (a) consequent faultline scarp is formed due to erosion of weak rocks of downthrown blocks. Such fault scarps are oriented towards the direction of original fault scarp (fig. 2.8 (1) ; (b) reverse o r obsequent faultline s c a rp developes in opposite direction to the original fault scarp due to erosion of weaker strata of the upthrown block of the fault. Such fault line scarps are formed at much later date at relatively lower height (fig. 2.8 (3)). ‘An obsequent fault-line scarp will normally represent a later stage o f development than a consequent scarp, though this is not invariably the case__ the reversal of the fault line scarp is possible only because a Iall in base-level has ex­ posed to denudation the weak rocks on the upthrown side of the fault* (R.J. Small, 1970). (c) Resequent

Fig. 2.8 : Developmen t ofdifferent types o f fault line scarps over normalfaults e.g. 1. consequent or normal, 2. obliteration o f scarps by erosion^ 3. ob­ sequent and 4. resesquent fault-line scarps. (Ill) DOMED STRUCTURE

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Domed structure results either due to upw'arping of crustal surface effected by diastrophic force or due to intrusion of magma into surficial rocks. The superincumbent material is removed due to pro­ longed erosion and the underlying structure is ex­ posed to the surface and few typical features like cuesta, hogback and ridges are formed. Domqs formed due to upwarping are characterized by the development of radial or centrifugal drainage p a tte rn having a set of sequent streams which fol­ low the slope gradient e.g. consequent, subsequent, obsequent and resequent streams (fig. 2.9). For de­ tails, see ‘fluvial cycle of erosion on domal struc­ ture’ (chapter 10).

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GEOMORPHOLOGY

Fig. 2.9: Development o f erosional landforms over domed structure. (IV) UNICLINAL/HOMOCLINAL STRUCTURE

Homoclinal structures are those which repre­ sent inclined rock strata at uniform dip angle caused by general regional tilt. ‘These structures are formed in two main ways, either by the uplift of a sequence of off-lapping coastal plain sediments or as part of one limb of a large dome or fold' (Chorley, Schumm and Sugden, 1985). Such structure^ involve both

hard and soft rocks and sometimes there are alter­ nate bands of soft and resistant rocks and hence these are subjected to differential erosion with the result rivers form their valleys along soft rocks giving birth to the formation of strike vales while resistant rock beds arc less eroded and hence become lines of asymmetrical hills known as cuesta having one side of steeper scarp slopes while other side represents gentle slope. Homoclinal structure formed due to general tilting of sedimentary beds of coastal plains and retreat of sea water presents ideal condition for the development o f consequent and subsequent streams. The consequent streams drain seaward across resistant and weak rock beds alike but the lateral subsequent streams develop on the less resistan rocks. Thus, lines of asymmetrical cuesta features having steeper landward facing scarp slopes and gentler seaward facing dipslopes are formed parallel to the coast lines (fig. 2.10).

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Fig. 2.10 Development o f trellis drainage and cuesta on uniclinal strata of coastal plain, after Von Engeln. 1948.

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(V) HORIZONTAL STRUCTURE AND LANDFORMS

33

3. Rock Characteristics

If the regional sedimentary formation has developed well defined horizontal beds o f resistant rocks, say sandstones, then after fluvial erosion tabular topography is formed. The uplifted hori­ zontal thick beds of relatively resistant rocks (e.g. sandstones) lying over shales and siltstones, when subjected to erosion from all sides, produce isolated flat-topped hills known as m esa (ot large size) and butte (of smaller size). Such numerous features have developed over Rew a and Bhander plateau (M.P.). In fact, Bhander plateau having massive sandstone capping over shales and siltstones of Vindhyan formation is itself an example of very extensive mesa while a few smaller mesas have developed around Bhander plateau (fig. 3.8). Look hill in Jawa block of R ew a district (M.P.) is fine example o f mesa capped with Vindhyan sandstone overlying shales. The horizontal structures having alternate bands of sandstones and shales or sand­ stone - limestone - shale, are sub jected to differential erosion and give birth to step -lik e scarps and bench topography (stru ctu ra l ben ch es). The Grand Canyon (Colorado, U.S.A.) having horizontal beds of alternate bands o f sandstone, limestone and shale presents a picturesque view of well pronounced structural benches flanking the deeply entrenched canyon of the Colorado river. Even horizontally dis­ posed basaltic beds of different phases of lava flow sometimes are of varying resistance and after vigorous erosion produce picturesque stepped topography (e.g. source tributaries of the Savitn and the Krishna rivers have produced Grand-Cany on - like topography around M ah ab alesh w ar plateau in Maharashtra). T ooth -lik e top ograp h y develops over resistant quartzitic sandstones whereas impervious and insolu­ ble resistant rock produces rounded topography.

The rock characteristics include chemical and mechanical composition of rocks, permeability and impermeability, joint patterns, rock resistance etc. Chemical composition determines nature o f chem i­ cal weathering of rocks which in turn determines resultant landforms. For example, limestone co m ­ posed of calcium carbonate is very much prone to intense chcmical weathering under humid condition and hence running and groundwater, when acts on carbonate rocks, produces picturesque limestone landscape (karst topography). Dolomite having m ag­ nesium carbonate as principal constituent is also readily attacked by acidulated water. Some sandstones having calcareous or ferrous cements undergo the process of chemical erosion under warm and humid climatic conditions. The prolonged chemical action on some common minerals and rocks produces dif­ ferent kinds of clay (e.g. terra-rosa on limestone and dolomite, kaolinite on granite and gneiss, clay on. chalk etc.) the thick accumulation o f which on sur­ face causes soil crecp and slumping resulting in gentle rounding of the existing landscape. The re­ sultant soil creep produces convex slope. Rock joints are considered to be significant attribute of rock characteristics which influence landform characteristics both at macro-and microscalcs because rock joints determine permeability of rocks, their weathering and erosion and detailed shape of some landforms. A well jointed rock being more permeable is subjected to intense chemical weathering because it allows dow nward movement of corroding agent (solvent water). Similarly, rocks having well developed joint pattern are vulnerable to mechanical disintegration into big rock blocks. A permeable rock having well developed joint system reduces surface drainage by allowing efficient dow n­ ward movement of water and hence fluvial erosion and transportation at the surface is remarkably mini­ mized. Joint pattern also influences development of drainage pattern at least on well jointed rocks. Widely jointed granites after weathering produces ‘tors’ while poorly jointed rocks like besalt are chemically decomposed enmass. ‘Perm eability refers to the capacity of a rock for allowing water to pass through it. A prime factor determining the degree of permeability is the pres­ ence of bedding planes and joints, but in some

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Fig. 2.11 : Development ofstripped and structural plains on horizontal structure, after W.M. Davis and C.A. Cotton (in Chorley et. al, 1985).

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34

GEOMORPHOLOGY

instances porosity can promote and enhance perme­ ability. Porosity refers to the presence of small gaps between the constituent mineral particles of a rock’ (R.J. Small, 1976). Highly permeable rocks disfa­ vour erosion as these allow more efficient perco­ lation of water and hence form high relief topogra­ phy e.g. high plateaus, escarpments and ridges (for example, sandstones and limestones) while imper­ meable rocks (e.g. clay and shale), which are me­ chanically weak, discourage percolation of water and hence are more readily eroded and produce undulating vales and lowlands. Rock h a rd n e ss is always considered in rel­

ative sense because a particular rock may be resis­ tant to weathering and erosion in certain environ­ mental condition while the same rock may be less resistant or weak in other environmental conditions. For example, limestone becomes weak rock in hu­ mid climatic conditions because of active dissolution of rock but the same rock becomes relatively resist­ ant in hot and dry climate due to absence of water. Normally, less resistant rocks (e.g. clay, shale) are more rapidly eroded and give birth to lowland while resistant rocks produce bold topography due to less erosion. It may be mentioned that ‘however, the relationship between rock strength and erosive proc­ esses is by no means straightforward’ (R.J. Small, 1970). It may be concluded that geological structure and lithological characteristics no doubt are impor­ tant factors in influencing landform characteristics in different environmental conditions but it is not the only factor controlling landscape development and landform characteristics. CO N CEPT 3

‘Geomorphic processes leave their distinctive imprints upon landforms and each geomorphic proc­ ess develops its own characteristic assemblage o f landforms. ” ’ W.D. Thornbury Meaning

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Geomorphic process and geomorphic agent are considered separately for different meaning by a few geomorphologists. According to W.D. Thornbury geomorphic processes include all those physical and chemical changes which affect earth s surface and are involved in the evolution and development of landforms of varying sizes and magnitudes, while

geomorphic age it is medium through which eroded materials are transported from the place of erosion to the place of deposition. On an average, geomorphic process and geomorphic agent should be considered as synonym. In fact, geomorphic processes include those physical processes which operate on the earth s surface both internally and externally (Savindra Singh, 1991, p. 277). ‘In geomorphology the word process is a noun used to define dynamic actions or events in geomorphological systems which involve the appli­ cations of forces over gradients. Such actions are caused by agents such as wind and falling rain, waves and tides, river and soil water solution (J.B. Thorns, 1979). Types of Processes

On the basis of source-place geomorphic pro­ cesses are divided into two broad categories e.g. endogenetic and exogenetic processes. The inter­ nal or endogenetic processes originating from within the earth fostered by diastrophic and sudden forces, caused by thermal conditions of the interior of the earth and varying physical and chemical properties of the materials of which the earth’s interior has been composed of, introduce vertical irregularities on the earth's surface and create various suites of habitats for biotic communities. The significant endogenetic or hypogenous processes include diastrophic, seis­ mic and volcanic activities. The external or exogenous (epigene) processes originating from the atmos­ phere driven by solar energy change the face of the earths surface through erosional and depositional activities. Exogenetic processes include running water (rivers— fluvial process), groundwater, sea waves (marine process), wind (aeolian process), glacier (glacial process), periglacial process etc. Besides, weathering and mass translocation of rockwaste are also included in this category. There are certain extraterrestrial processes (e.g. fall of meteorites) which are neither related to the interior of the earth nor to the atmospheric conditions. The endogenetic and exogenetic processes are considered competing forces which are engaged in continual conflict. Thus, the interactions between endogenetic and exogenetic processes produce com­ plex sets of physical landscapes. Endogenetic pro­ cesses are considered as constructional processes as these produce surface irregularities in the form of mountains, plateaus, faults, folds, volcanic cones,

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

35

Mechanism of Processes

depressions etc. on the earth’s surface. On the other hand, exogenetic processes are called as grada­ tional or planation processes because these are continuously engaged in removing vertical irregu­ larities created by endogenetic processes through denudational mechanism (including both weather­ ing and erosion) and depositional activities. The planation work o f the earth s surface irregularities is accomplished through (i) degradation (e.g. weath­ ering and erosion wherein upstanding landmass is lowered dow n by weathering (disintegration and decomposition and consequent downslope transfer of weathered materials) and erosional activities (this mechanism o f planation is called as level down) and (ii) aggradation (deposition, this mechanism of planation is termed as level up).

Exogenetic processes are generally called as erosional processes which perform three-phase work i.e. erosion, transportation and deposition. These external processes are also known as destructional processes because these are con­ tinuously engaged in the destruction o f relief fea­ tures created by the endogenetic forces through weathering, erosional and depositional activities. The erosional work by differrent processes is performed through the mechanism o f chem ical erosion (corrosion or solution), corrasion or ab ra­ sion, attrition, hydraulic action, deflation, plucking, polishing, crvoturbation etc. 1. Erosional Work

(l) The mechanism o f corrosion involves dissolution of the soluble materials (carbonate rocks) through the process of disintegration and decom po­ sition of carbonate rocks. Solution refers to dissolu­ tion of soluble particles and minerals from the rocks with the help of water (having dissolved carbon dioxide in it) in motion. Solution o f rocks depends on the nature of rocks, solubility o f solids, ratio between the volume of solvent (water) and the solids and contact time of solvent and solids. Running water (streams), groundwater and sea waves effec­ tively corrode carbonate rocks. Streams remove soluble materials from the parent rocks and the chemically eroded sediments are suspended in the running water of the streams. Most o f the salts are removed from the bedrocks through the process o f carbonation and are suspended in river water. A c­ cording to the estimate of Murray every cubic mile water of the river contains about 7,62,587 tons o f suspended minerals of which about 50 per cent is calcium carbonate. On an average, the world rivers discharge about 6,500 cubic miles o f water into the oceans ever)' year. On the basis of Murray’s estimate it may be inferred that about 5 billion tons o f miner­ als are removed from the bedrocks by the world rivers every year. Groundwater is the most effective efficient process of corrosion o f carbonate rocks. Rainwater mixed with atmospheric and organic car­ bon dioxide (C O J becomes active solvent agent and disintegrates and dissolves carbonate rocks at the sunface and below the surface to form numerous types of solutional landforms. It may be pointed out that amount of dissolution o f carbonate rocks by

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A. E pigene or Exogenous Processes (gradational/planation/denudational processes) 1. D egradational work (i) weathering (ii) massmovement of rockwaste (iii) erosion fa) running water (rivers) (b) groundwater (c) marine process (sea waves) (d) aeolian process (wind) (e) glaciers ( f ) periglacial process 2. Aggradational work Deposition of weathered and eroded sediments (a) running water (rivers) (b) groundwater (c) Sea waves (d) wind (e) glaciers B. H ypogene o r E n do g en o u s Processes (constructional forces; 1. D iastrophic movements (i) Epeirogenetic force (a) emergence (b) submergence (ii) Orogenetic force fa) faulting (b) folding (c) warping C. Extra-terrestrial Process .D. Anthropogenous G e o m o r p h o lo g ic a l Processes

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36 g eo m orphology

lateral abrasion leading to erosion of valley walls Lateral abrasion causes valley widening while the vertical abrasion leads to valley incision wherein the erosion tools drill the valley floor through the mecha­ nism of pot hole drilling resulting into the forma­ tion of pot holes (cylindrical depressions in the valley floors). Vertical abrasion becomes most ef­ fective when the erosion tools are of large size (boulders and cobbles), and of high angularity (high calibre) and the channel gradient is steep causing (2) Abrasion or corrasion involves the re­ high velocity of running water. Vertical abrasion moval ot loosened materials of the rocks by different and valley incision (downcutting) becomes more erosional processes in different manner. The degree effective during juvenile (youthful) stage of river o f abrasion depends on a host of variables, e.g. and valley development when channel gradient and nature of erosion tools, nature of erosional processes velocity are very high. A b rasion by groundwater (e.g. rivers, groundwater, seawaves, glacier, wind is not effective because of exceedingly slow move­ etc.), nature of geomaterials (rocks), force of ero­ ment of water and very fine sediments, that too in sional processes, nature of ground surface, gradient solution form. A brasion by sea waves is very effec­ etc. Erosion tools refer to all those solid materials tive because high-energy storm waves charged with (boulders, cobbles, pebbles, sands etc.) with the help large cobbles drill out circular pot-holes and abrade of which erosional agents attack and abrade the the standing bedrocks. W ind armed with entrained rocks. The efficiency of abrasion depends on size, sand grains as tools of erosion attacks the rocks and amount and calibre of erosion tools. Calibre of erodes them through the mechanism of abrasion, erosion tools means shape and angularity of eroding pitting, grooving and polishing (collectively called materials (e.g. whether rounded or angular in shape). as sandblasting). Aeolian abrasion is minimum at Generally speaking, large-size and quantity and high ground-level because wind velocity is retarded by calibre (more angular) of erosion tools make the friction. Similarly, wind ceases to become an ero­ erosional processes most effective abrading agents. sive agent beyond the height of 182 cm frcm the Nearly all of the erosional processes resort to abra­ ground surface level because normal wind cannot sion work but the mode of abrasion differs from lift and carry particles of average size. Thus, maxi­ process to process. mum abrasion occurs at the height between 20-25 Abrasion by running water (rivers) refers to cm from the ground surface. A b rasion by glaciers the breakdown of rocks and removal of loosened depends on the rate o f movement of glaciers, gradi­ materials of rocks of valley walls and valley floors ent and nature of erosion tools. Normally, glacier with the help o f erosion tools as referred to above. erodes its bed and valley walls with the help of The erosional tools or river loads move down the erosion tools (coarse debris) through the m echanism channel gradient along with water and thus strike of abrasion. against the rocks which come in contact with them. The repetition of this mechanism weakens the rocks (3) Hydraulic action involves the break which are ultimately loosened, broken down and down ot rocks due to pressure exerted by water dislodged. The nature and magnitude of abrasion by currents ot the rivers and sea waves. In fact, hydrau­ rivers depends on the nature, size and calibre (angu­ lic action is the mechanical loosening and rem o v al larity) of erosion tools, channel gradient and How ol materials of rocks by water alone (without the velocity. Boulders, cobbles and pebbles of various help of erosion tools). It may be pointed out that sizes and angularity are by far the most important chemical erosion (corrosion), abrasion and hydrau tools of erosion which arc generally called as drill­ lie action are so intimately interrelated that it ,s ing tool*. The erosional mechanism of abrasion unwise to think of exclusively pure action operates in two ways e.g. (i) vertical erosion leading without chemical erosion and abrasion. The rivers to erosion and deepening of valley floors and (ii) erode their valley walls through hydraulic action-

groundwater depends on temperature, partial pres­ sure ot atmospheric carbon dioxide, organic carbon dioxide, chemical composition of carbonate rocks (e.g. calcium carbonate - limestone, magnesium \.arbonate - dolomite etc.). rock joints, nature and velocity of flow o f groundwater, contact time of groundw ater with the rock etc. Sea waves also resort to corrosion o f coastal rocks and form numerous coves and caves of varying dimensions.

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h y d r a u l i c

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37

Sea waves are more powerful agents of hydraulic action which refers to impact of gushing water on the coastal rocks. Powerful storm sea waves attack the coastal rocks with enormous hammer-blows amounting.to 50 kg f per square centimeter (gravity force (f) is 9.81 and hence sea waves normally hurl a force of 50 kgf per square centimeter of the coastal rocks). Repeated blows of striking sea waves enlarge the incipient joints, fracture patterns and thus help in breaking the rocks into smaller joint-bounded blocks. The waves are capable of dislodging larger fragment of rocks weighing several tonnes in weight. This process of displacement of rock fragments is also called as quarrying and sulcking.

tion (frost weathering), congelffluctlon (soil creep), frost heave (bulging and subs'dence), nlvationfsnow patch erosion) etc. are significant weathering and transportation rnecahnisms performed by periglacial processes. T he mechanism o f erosion, though very slow and insignificant, by periglacial processes is cryoturbation. 2. Transportational Work

T he tra n sp o rta tio n w o rk by different gcmorphic processes is accomplished through flota­ tion, suspension, traction, saltation , solution etc. Running water (rivers) transports sedim ents through traction, saltation, suspension and solution. G.K. Gilbert has propounded a law o f stream tran sp or­ tation based on the relationship between stream velocity and its transporting power. T he law is known as Gilbert's Sixth Pow er L aw according to which the transportation power o f the streams is proportional to the sixth power o f their velocity (transportation power a stream velocity*). The mecha­ nism ofsaltation by streams involves the transport of load with water currents wherein coarse load moves downward by leaping and jum ping through valley floors. This mechanism is extremely slow. The downstream movement of loose materials on the valley floor is called traction. The bed-load being transported by traction method consists o f gravels, pebbles, cobbles and boulders. The m ateri­ als of medium size are suspended in water (called as suspended load) due to buoyancy. The transporta­ tion by streams is unidirectional (downstream). The soluble materials are dissolved in water and become invisible and are transported downstream in solution. The groundw ater transports dissolved materials in suspended form.

(4) A ttrition refers to mechanical tear and wear of erosion tools suffered by themselves. The boulders, cobbles, pebbles etc. while moving down­ stream with water collide against each other and thus are fragmented into smaller and finer pieces in the transit. The rock pieces are so broken down that ultimately they are comminuted into coarse to fine sands which are transported down the channel in suspension. Attrition by marine process involves mechanical tear and wear and consequential break­ down of rock fragments due to their mutual collision effected by backwash and rip currents which remove the fragments from the cliff base and transport them towards the sea. A ttrition by wind involves me­ chanical breakdown o f rock particles while they arc transported by wind through the processes ofsaltation and surface creep. Saltating grains frequently rise to a height of 50 centimeters over a sand bed and upto 2 meters over pebbly surface by combined action of aerodynamic lift and the impact of other saltating grains which return back to the ground surface. Thus, the particles, while they are moving, collide against each other and are further comminuted in finer particles.

The transportational w ork o f sea w aves varies significantly from other agents o f erosion and transportation. For example, the backw ash or un ­ dertow currents (moving from the sea coasts and beaches towards the sea) pick up the eroded materi­ als and transport them seaward but the uprushing breaker waves or su rf curents pick up these mate­ rials and bring them again to the coasts and beaches. Thus, the transportation o f materials takes place from the coastland towards the sea and from sea towards the coast (i.e. to and fro transportation). Longshore curents transport the materials parallel to the coast and shorelines. The materials involved in

(5) D eflation, the process of removing, lift­ ing and blowing away dry and loose particles of sands and dusts by winds, is called deflation (de­ rived from Latin word deflatus, which means blow­ ing away). Long continued deflation removes most of loose materials and thus depressions or hollows known as ‘b low ou ts’ are formed and bedrocks are exposed to wind abrasion.

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(6) The mechanism o f periglacial processes is quite different to other processes i.e. congelifrac-

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g e o m o r ph o l o g y

the transportation by sea waves includc sands, silts, gravels, pebbles, cobbles, and some times boulders. The transportation by sea waves is bi-directional. The tran sportation al w ork o f wind differs significantly trom other agents of erosion because the direction ot wind is highly variable and hence wind-lransportation is m ulti-directional. Wind trans­ port involves cntrainm cnt of loosened grains of sands and dust in the air and their movement to new locations. Very tine materials with a diameter of less than 0.2 m m are kept in suspension by upward m oving air. Such materials kept in suspension are called dusts and extremely fine particulate matters arc called haze or snioke. The materials larger than 0.2 m m in diam eter are transported through the m echanism o f bouncing, leaping or jumping, which is know n as saltation whereas the loosened materi­ als transported through surface creep or traction alw ays touch the ground. A very significant aspect o f wind transport is that materials are transported at the ground surface and above the ground surface. Only very fine materials are transported to greater distances in one step while coarser materials are transported in stages and steps by rolling, leaping and jum ping. G lacial sedim ents (glacial drifts) are trans­ ported along the sides and floors of the glacial valleys and snouts o f the glaciers. The debris falling directly into the galcier is transported without touch­ ing the bottom of the glacier while the debris falling on to the surface of a glacier is transported downslope with the moving ice mass. The materials derived from the bed by subglacial erosion are transported by touching the bottom. The mechanism o f transportation of materials in periglacial areas has been described variously e.g. con geliflu ction , congeliturbatityi (it is also used for erosion) and gelifluction etc. Solifluction or co ng elifluction involves only soil-flow in the periglacial areas having permafrost below activc layer. According to K. Bryan (1946) cryoturbation includes all types o f massmovement of regolith in periglacial environment. Recently, gelifluction is used in place o f congelifluction. 3. Depositional Work

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The deposition o f load carricd by the streams is effected by a variety of factors e.g. (i) decrease in

channel gradient, (ii) spreading o f river water oVer large areas, (iii) obstruction in channel flow, (iV) decrease in the volum e and discharge o f water, (V) decrease in stream velocity, (vi) increase in sedi­ ment load etc. The decrease in stream velocity re­ duces the transporting pow er o f the streams which are forced to leave additional sedim ent load to settle down. Sedimentation takes place in the river beds, flood plains and at the river m ouths (to form deltas). Depositional work by groundw ater takes place when solvent (water) becomes oversturated. As the chemical erosion o f carbonate rocks contin­ ues, the groundwater or say solvent receives more and more solutes and becomes saturated with dis­ solved sediments. Since the m ovem ent of ground­ water is exceedingly slow it cannot transport enough sediments. Thus, chemical erosion (dissolution) and sedimentation (deposition) take place together. Largesized sediments immediately settle down whereas suspended fine sediments kept in supended form are deposited due to following factors— (i) due to ob­ struction in the flow path of groundwater and conse­ quent decrease in the flow velocity of solvent, (ii) due to evaporation of water because of increase in temperature and consequent decrease in the volume of groundwater and increase in solute-water ratio, (iii) due todecrease in solution capacity of groundwater etc. Deposition of sediment takes place at various places in different forms e.g. (i) at the floor of caves, (ii) along the ceiling o f caves, (iii) in the rock joints etc. All the deposits in the caverns are collectively called speleothem s of which calcite is the common constituent. Banded calcareous deposits are called travertines whereas the calcareous deposits, softer than travertines, at the cave mouths are called tufa or calc-tufa. The calcareous deposits from dripping water in dry caves are called dripstones. Deposition by m arine processes (sea waves) is most variable and temporary in character because surl currents or breakers abrade the coasts and back-' wash or undertow curents and rip currents bring them seaward and deposit at the lower segments of wave-cut platforms but these sediments are again picked up by surf curents and breakers and are brought to the coasts. Thus, marine sediments are reworked by sea waves again and again. When there isequilibrium between incoming supplies of sediments

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PUNDAMBNTAI, r'ONCHKrS IN OliOMOKl'HOUKiY

39

Procaaa-Raaponaa (Landform*)

by backwash on the wave-cut platforms, a profile of equilibrium it achieved If the wavc-cui rock plat­ form i» characterised by steep slope towards the oceanic slope, Ihc destructive waves become very aciive and thus resultant powerful backwash re­ moves sediments from the landward side so lhal Ihc slope ol the platform is lessened, On the other hand, if the slope of wave-cut platform is less sleep, constructive waves become more effective as they favoui sedimentation and beach deposition on Ihc landward side so that the slope of the platform becomcs steeper. Beaches, cusps, bars and associ­ ated features arc formed due to marine sedimenta­ tion but since the depositional work depends on a variety of factors and fiencc these features are sel­ dom permanent as they are built and depleted and rebuilt.

fl is evident from the aforesaid analysis o f the mechanism o f the operation (erosional and depositional work) of exogenetic processes that the mode of operation of each geomorphic process is different from the other process and hence the landforms produced by each process may be differentiated if wc accept the m ono-process concept e.g. dissected by streams, abraded by wind, glaciated by glaciers etc. Before the emergence o f process geomorphol­ ogy, landscape characteristics o f a gi ven region were studied as a response of com bined actions o f all processes operating in that region (p oly-p rocess approach) but now operational mechanism (ero­ sional, transportational and depositional works) o f each gcomorphic proccss and resultant landforms (erosional, depositional and relict) are studied sepa­ rately. Bccausc of distinctive characteristics the landforms produced by one particular process may be dif ferentiated from those produced by other proc­ esses. For example, alluvial cones and fans, flood plains, gorges and canyons, natural levees, river meanders, and deltas arc indicative o f the work o f fluvial process (streams) while solutional holes and depressions (sink and swallow holes, dolines, polje, uvalas etc.), limestone caves, stallectites and stalag­ mites arc the products of the erosional and d e­ positional works of groundwater on carbonate rocks. Sand dunes indicate the depositional work by winds, moraines, drumlins, eskers etc. and U-shaped valley with hanging valley, cirque, aretes etc. denote the product of glacial proccss whereas patterned ground t.stone circles, stone nets, stone polygons etc.), pingo, thermokarst, solifluctatc lobes and terraces, stone glacier, blockfields, altiplanation terraces.nivation hollows etc. arc the exclusive responses o f periglacial processes.

D e p o s itio n a l w ork by w in d is gcornorphologically very important because significant features like sand dunes and loess arc formed. Deposition of wind blown sediments occurs due to marked reduction in wind speed and obstructions caused by bushes, forests, marshes and swamps, lakes, big rivers, walls etc. Sands arc deposited on both windward and leeward sides ol fixed obstruc­ tions. '/b e accum ulated sand mounds on cither side of the obstructions arc called sand shadow s whereas accumulations o f sands between obstacles arccallcd sand drifts. 'I hc rock debris carried by glaciers arc collcctively callcd as glacial drifts which include (i) till, (n) ice-con tact stratified drift, (iii) outw ash etc. 'Ibe unsorted arid non-stratificd glacial drifts arc called tills which arc further divided into ( IJ basal or lodgem ent till and (ii) ablation till. I he basal or lodgement tills are com pact, tough, dense and rich in clay. These arc deposited at the base of the glaciers. 'Ihc ablation tills are poorly consolidated and lack in fine grain *ize. The ice-contact stratified drifts are modified glacial debris by inellwater. Till is also known as b oulder clay. Glacial debris arc divided into 3 type* on the basis of location e.g. (i)en glacial d ebris, which is transported within the glaciers, (ii) supraglacial d ebris, which exists on the surface of the glacier and (in* su bglacial d eb ris, which is found at the base o f the glacier. The glacial deposi­ tion it generally called m oraine.

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On the basis o f landform assem blage having d istin ctiv e ch a ra cteristics produced by each geomorphic proccss the landforms may be classified genetically as initiated by W .M. Davis. The genetic classification o f landforms enables us to understand the mode of origin o f particular landform, sequence o f developm ent and gcomorphic history. Generally, a few terms arc used to indicate certain sets o f general landforms which do not give any clue for their genesis e.g. ridge, gorge, scarp, column, mound, table, hole, depression, valley, trough, cave, dune,

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40

GEOMORPHOLOGY

terrace, bench, cone, fan, creek, plain, hummocks, cliff, polygon etc. If these and many more forms are associated with the processes which have formed them, then we may have knowledge of their genesis and developmental mechanism. For example, plain is formed by several processes e.g. flood deposition (flood plain), peneplanation (peneplain, all by flu­ vial process), karst plain (by groundwater), pediplain (by scarp retreat and pedimentation in semi-arid climate), panplain (by coalescence of flood plains caused by lateral erosion by fluvial process), etchplain (by etching and washing of debris by streams in savan na region), alluvial plain (deposition by streams), outw ash plain (due to fluvio-glacial ac­ tion), cryoplain (due to cryoplanation) etc. The following additional examples support genetic as­ pect of landforms and processes responsible for their formation.

thermokarst (frost thaw, periglacial process); hum. mocks -earth hummock (frost weathering, periglacial process), turf hummock (frost weather ring, periglacial process); polygon - frost polygon (frost weathering, periglacial process), stone polygons (frost heave’ periglacial process), cliffs - river cliff (fluvial), sea cliff ( erosional, sea waves) ; platform - wave-cut platform (erosional, sea waves), wave-built plat­ form (depositional-sea waves) etc.

Ridg e— anticlinal ridge (tectonic), synclinal ridge (erosional, streams), hogback ridge (tectonic and erosional), beach ridges (depositional, sea waves), morainic ridge (deposition, glacier), nivation ridge (depositional, periglacial process) etc.; gorge-river g o rg e ; scarp— faultscarp (tectonic), fault-line scarp (erosional, fluvial process), normal, obsequent and resequent fault-line scarps (erosional, fluvial proc­ ess), resurrected scarp (erosional, fluvial) e t c .; val­ leys- (V-shaped valley-fluvial), rift valley (tectonic), hanging valley (both fluvial and glacial), karst val­ ley, blind valley, solution valley (solution by groundwater), glacial valley (U-shaped, glacial ero­ sion), dry valley (periglacial process) etc.

(e.g.) in v o lu tio n s , h u m m o c k s , pingo, thermokarst, frost cliffs, frost polygons etc.).

(1) CONGELIFRACTATE LANDFORMS

(due to frost weatehring and frost-heave)

(2) PATTERNED GROUND

(due to frost heave and solifluction) (e.g. stone circles, stone nets, stone polygons, stone garlands, stone stripes) (3) CONTORTED SURFACE

(due to frost heave and congelifraction) (4) SOUFLUCTATE/CONGELWLUCTATE LANDFORMS

(due to differences in the movement of so­ lifluction) (e.g. solifluction terraces, solifluction lobes, talus, stratified scree). (5) ALTIPLANATION LANDFORMS

(e.g. altiplanation terraces, altiplanation cliffs, tors, frost-riven cliffs, blockfields, stone streams) (6) NIVATION LANDFORMS

(e.g. nivation hollows, nivation platforms, nivation ridge, nivation fans) (7) PERIGLACIO-FLUVIAL LANDFORMS

(e.g. thaw gullies, thaw ravines— thaw badland) It may be pointed out that it is easier, theo­ retically, to associate a particular landform with * particular process but very few landforms are of mono-process origin because most o f the land­ forms have been developed by more than one pro­

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> Holes — sink hole, swallow hole (solutional work by groundwater), thaw sink (periglacial proc­ ess), pot-hole (fluvial process, erosional) etc.; moundmima mound (congelifluctate, periglacial process); dunes-sand dunes (aeolian, depositional), bencheswave-cut benches (erosional, seawaves), structural benches (tectonic and structural), giant benches (ero­ sional, glacier); terraces-river terraces, paired ter­ races, fluvial terraces (both erosional and depositional, streams), marine terraces (erosional, sea waves), solifluction terraces( soil creep, periglacial process), altiplanation terraces (frost action, periglacial pro­ cess), nivation tcrraces (depositional, periglacial process); cone - alluvial cone (depositional, streams), volcanic cone (depositional, vulcanicity); karst (solution al,. g r o u n d w a te r, c a rb o n a te rocks),

Savindra Singh's genetic classification of periglacial landforms (1974) presents an ideal ex­ ample of process-related and mechanism-related (weathering, erosion, transportation and deposition) landforms developed in periglacial areas as fol­ lows—

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41

FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY c

It has been accepted that geomorphic pro­ cesses play significant role in the evolution and changes in the form of hillslopes but there is con­ trasting opinion about the evolution of slopes in terms of mono-process or poly-process origin. Con­ vexity and concavity have been related to soil creep and rainwash respectively. Fenneman (1908) ex­ plained the evolution of convexo-concave slope through rainwash alone. H. Baulig (1950) postulated the concept of poly-process origin and development of hillslope wherein soil creep and rainwash were accepted as the most important processes. The summital convexity o f a convexo-concave hillslope in humid temperate region results due to soil creep as it becomes more active than rainwash due to less volume of rainwater while basal concavity is formed by rill and gully erosion because soil creep becomes less effective due to abundance of surface water (coming from upslope). A few geomorphologists are of the view that soil creep and rainwash instead of working separately work together to form different slope forms. The advocates o f climatic geomorphology have pleaded for the study of landforms association of a climatic region together involving all the pro­ cesses active therein and have suggested to divide the world into morphogenetic regions e.g. L.C. Peltier (1950) has divided the world into glacial, periglacial, boreal, maritime, selva, moderate, savanna, arid and semi-arid morphogenetic regions (see chapter 4). CONCEPT 4 “As the differen t erosional agencies act on the earth’s surface there is p ro d u ced a sequence o f landform s having distinctive characteristics at the successive stages o f their developm ent. ” — W.D. Thornbury

The present concept is related to one o f ‘trio o f D avis’ (landscape is a function o f structure, process and tim e ) which was given more impotance rather was overemphasised by Davis. The stage concept is based on the concept o f ‘cyclic tim e’ which involves long geological period o f millions o f years and larger spatial areas. It may be pointed out that Davis used ‘tim e’ as a p ro cess’ rather than ‘an attribute’ of landscape developm ent wherein he envisaged sequential changes in landform s through tim e.’ ‘For Davis, the concept o f evolution implied an inevitable, continuous and broadly irreversible process of change producing an orderly sequence o f landform transformation, w'herein earlier forms could be considered as stages in a progression leading to later forms. By this model, time becam e not a tem ­ poral frame work within which events could occur, but a process itself leading to an inevitable p rogres­ sion of change’ (Chorley, Schum m and Sugden, 1985, p. 17). Thus, following Davis there is progressive change in landform characteristics with the passage of time. Davis’ model o f cycle o f erosion is based on the conccpt of ‘low -entropy closed sy ste m ’ w herein initial potential' energy in the closed system is p ro ­ vided by initial rapid rate short-period upliftm ent o f landscape. With the passage o f time and continuous erosion there is equal distribution o f energy in the geomorphic system so that all com ponents o f the system are characterized by equal energy levels and hence in the absence o f difference in the energy levels of different com ponents of the system, the state of m axim um disord er and hence m axim u m entropy is achieved wherein no further w ork is performed because there is no energy flow and the ultimate result is the developm ent o f peneplain. Though this concept o f Davis (closed geom orphic system characterized by evolutionary changes in the landform geom etry) is subject to severe criticism but ‘for Davis, each stage or his cycle w as associated with declining potential energy as the relief was worn down, and each stage was characterized by an assemblage o f landforms (i.e. valley-side slopes, drainage patterns etc.) having geom etries appropri­ ate to the local potential energy expressed by the difference in level between the land surface (ridge crest or top o f w ater divides) and some, low er elevation (base level, valley floor) tow ards which

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cesses i.e. they are of poly-process origin as dif­ ferent geomorphic processes seldom operate in iso­ lation. For example, even in periglacial environment (as referred to above) different geometrical patterns (very commor.iy called as patterned ground having definite geometrical patterns such as circle, net, polygon, stripe etc.) are formed due to combined actions of frost heave and solifluction whereas invo­ lutions, h um m ocks and pingo are formed by congelifraction (frost weathering) and altiplanation landforms (as referred to above) are the result of combined actions of solifluction, ni vation, frost heave and congelifraction.

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GEOMORPHOLOGY

42

mum clue to high velocity o f flow rate and high kinetic energy because o f very steep channel gradient High transporting capacity enables the rivers to carry big boulders (tools o f erosion) o f fairly good

degradation was directed’ (Chorley, Schumm and Sugden, 1985). W.M. Davis divided the whole time span of geographical cycle of erosion (fig. 3.1) into three distinct stages of varying landform geometries on the basis of time span of human life e.g. (i) youthful stage characterized by higher energy landform s, (ii) m ature stage of m edium -energy landform s and (iii) old or penultim ate stage of low but equal energy-landform s. Based on further variations in landform characteristics he further divided each stage into early, m iddle and late e.g. (i) early youth, middle youth and late youth, (ii) early mature, mid­ dle mature and late mature and (iii) early old, middle old and late old stages. Based on Davisian model of normal cycle of erosion in humid temperate regions the following sequences of landform evolution through successive stages of youth, mature and old stages may be presented in the support of the above con­ cept.

size (large size) and calibre (angular boulders) which help in the p othole d rillin g o f the river beds. It may be mentioned that pothole drilling is the mostactivc and powerful process o f vertical erosion (valley deepening) in the juvenile stage o f the normal cycle

1. Youthful stage

The region experiences rapid short-period upliftment resulting into m axim um potential en­ ergy and m inim um entropy. ‘The potential energy of landform o f initial uplift is the dominant source of energy input (potential energy) and that, thereafter, there is an irreversible equalization of energy levels throughout the landform assemblage, leading ulti­ mately to a spatially uniform terrain-the peneplain or peneplane’ (at the end of the cycle i.e. old stage) (Chorley, Schumm and Sugden, 1985).

River capture is the m ost characteristic fea­ ture of the juvenile stage o f the normal cycle of erosion. Main rivers having steeper channel gradi­ ents and more volume o f w ater capture smaller streams of relatively low channel gradient through headward erosion. 2. Mature stage

Marked valley deepening through vertical erosion uring youthful stage results in pronounced ecrease in channel gradient and consequent de­ crease in flow velocity with the result the arrival of y maturity is heralded by marked decrease in ey eepening due to (i) decrease in channel gra lent, (ii) decrease in the velocity o f river flow,

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Consequent streams (which follow the re­ gional slope) are originated with the upliftment of land area due to endogenetic forces. In the begin­ ning, the streams are less in number and short in length. Very few tributaries of the master conse­ quent streams are originated. The slopes are domi­ nated by numerous rills and gullies rather than big streams. These rills and gullies lengthen their lon­ gitudinal profiles (increase their lengths) through headw ard erosion. Gradually and gradually the main streams deepen their valleys. The origin and evolution of tributaries of master streams give birth to the development of dendritic drainage pattern. The rivers are continuously engaged in rapid rate o f downcutting o f their valleys (valley incision) be­ cause the transporting capacity o f the rivers is maxi­

of erosion. The valley becom es very narrow and deep with almost vertical side walls due to continuous active downcutting o f the valley floors at exceed­ ingly fast rate. The valley side slopes are convex in plan. Thus, the resultant ju v e n ile valleys are Vshaped and are called gorges and canyons. The valley floors are studded with num erous pot holes which are the result of pothole drilling. The inter­ stream areas or w ater d iv id es (land area between the valleys of two major stream s) are extensive and wide and these are least affected by denudational processes because valley w id en in g by lateral ero­ sion is less effective in the early and middle youth stages. The valley thalw egs (longitudinal profiles of the rivers) are characterized by num erous rapids and waterfalls which always recede upstream. Most of the waterfalls and knick points disappear by late youth. The rivers are underloaded (not having the required amount ol sedim ent load according to their transporting capacity) and thus available energy is more than the work to be done. The rivers are well integrated by the end o f youth when maximum relative reliefs are formed.

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

(iii) decrease in the transporting capacity etc. Conse­ quently, valley w idening through active lateral erosion dominates over valley incision through

43

downcutting. The convex slope o f valley sides is progressively transformed into u niform or recti­ linear slope and the gorges and canyons charac­ terized by deep and narrow valleys are replaced by broad and flat valleys. The rivers deposit big boulders at the foothill zones due to sudden decrease in channel gradient and hence marked decrease in the transporting ca­ pacity of the rivers. These materials form alluvial fans and alluvial cones. The gradual expansion o f these fans and cones due to their continuous grow th result in the formation of extensive p ied m o n t plains through the coalescence o f several fans and cones. Interstream areas or water divides are continuously narrowed due to backw asting caused by active lateral erosion and valley widening. Thus, inter­ stream areas are transformed into narrow ridges. T he major river erodes down to its base level (sea level) and becomes ‘graded’. Thus, the longitudinal pro ­ file of the master river becomes the p rofile o f eq u i­ librium wherein there is balance between available energy and the work to be done i.e. balance betw een the transporting capacity and total sedim ent load to be transported. Because of marked decrease in channel gradient rivers adopt sinuous courses and develop numerous m eanders and loop s in their courses. Extensive flood plains are formed due to sedimentation o f alluvia. Rivers frequently change their courses because o f gentle to level slopes o f the flood plains. Numerous ox-bow lakes are formed due to straightening o f highly m eandering loops. Deposition of sediments on either side of the river valleys leads to the formation o f natural levees. 3. Old Stage

The old stage is characterized by further de­ crease in channel gradient, almost total absence o f valley deepening, decrease in the num ber of tribu­ tary streams and flattening o f valleys. Tributary streams also attain the base level of erosion and are graded. Lateral erosion and consequent backwasting eliminates most of interstream areas. Valleys be­ come broad and flat characterized by concave slopes o f valley sides. Downcutting o f the valleys is totally absent. Weathering processes are most active. Thus, lateral erosion, downwasting and weathering con­ tinuously degrade the land resulting into gradual lowering of absolute altitude and water divides. Interstream areas and water divides are remarkably

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Fig. 2.12 : Stages o f landform development - 1. initial stage. 2. early youth. 3. late youth. 4. early m aturity. 5. m aturity and 6. old stage (peneplain).

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kinetic energy (through precipitation and channel flow), o f thermal energy (through insolation from the sun) and o f chem ical energy (through disintegra­ tion and decom position o f rocks) and there is con­ tinuous export o f energy and m atter out o f the system and hence the geom orphic system tends to be in equilibrium condition. Thus, the Davisian concept o f sequential changes o f landform s through succes­

reduced in height and are changed to lowland but they still rise above the surrounding areas. Trans­ porting capacity o f the rivers becomes minimum because o f very low channel gradient and thus the rivers becom e overloaded. Consequently, sedi­ m entation becom es m ost active during this stage. The rivers adopt highly meandering courses. The extensive flood plains with level to gentle slopes (2°5°) and very low channel gradient make the river flow so sluggish that the main channel of the river is divided into num erous distributaries and thus the river becom es braided. Valley sides are bordered by extensive natural levees which are also known as bluffs which denote the farthest limit of recurrent floods o f the concerned rivers. Rivers deposit and form extensive deltas at their mouths if other envi­ ronm ental conditions remain favourable for delta formation.

sive stages is not tenable. Moreover, it is argued that the life cycle of landform development cannot be equated with hu­ man life cycle because the time span o f three stages o f the latter (youth, mature and old) is almost fixed and one stage changes to the next stage after certain time period but this is not possible in the case of landscapes because a region having weak and less resistant rocks is quickly eroded dow n and youth stage advances to mature stage within shorter period of time but if the region is characterized by hard and resistant rocks then the period o f youth stage is lengthened and change from youth to mature stage is much delayed. This is why W. Penck pleaded for the rejection of Davis’ concept, ‘landscape is a function of structure, process and time (stage)’, and postu­ lated the concept that, ‘landforms reflect the ratio between the intensity of endogenetic processes (i.e. rate of upliftment) and the magnitude o f displace­ ment of materials by exogenetic processes (the rate of erosion and removal o f weathered and eroded materials)’. Inspite o f some inherent weaknesses in Davisian model the stage concept cannot be alto­ gether discarded. Even Penck is supposed to have deliberately avoided the use o f stage concept in his model of landscape development either to under­ mine the cyclic concept o f W.M. Davis or to present a new model. According to Von Engeln (1960) Penck found escape from the concept o f cyclic change marked by the stages youth, maturity and old age . In the place o f stage he used the term entwickelung meani ng thereby d ev elo p m en t Thus, in place of youth, mature and old stages he used the terms aufsteigende entw ickelung (waxing or ac­ celerated rate of developm ent), g le ic h f o r m ig * entwickelung (uniform rate o f developm ent) and absteigende enlw ickeluge (waning or decelerating rate of development). In fact, stage does not mean specified absolute period o f time rather it denotes the phase of landform development and hence ‘stage’

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The entire landscape is converted into exten­ sive flat plain o f undulating surface except a few residual convexo-concave hills which project above the general flat surface and thus break the monotony o f reliefless flat plain, called as peneplain. These residual hills, the result of differential erosion, are called m onadnocks on the basis of monadnock hills o f the North-East Applachians in New England region (USA). The whole landscape is dominated by concave slope, minimum available energy, both potential (because o f very low height) and kinetic energy (due to very low channel gradient) and m axi­ m um entropy (means maximum d i s o r d e r ^ relief, as the whole area is characterized by featureless peneplain). The Davisian model of sequential changes in landforms through youth (maximum relief, maxi­ m um potential and kinetic energy, narrow and deep valleys with convex valley sides and minimum en­ tropy), maturity (graded stream profile, broad valley with rectilinear valleysides) to old stage (equally distributed energy, broad and flat valleys with con­ cave slope, featureless plains-peneplain, minimum potential and kinetic energy and maximum entropy) is possible only in low-entropy closed geomorphic system but the geomorphic systems having different landform assemblages are open systems wherein t h e r e is continuous input of potential energy (through upliftment of landscape, plate tectonic theory has demonstrated continuous tectonic activities), of

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLtXJY

45

should be used in relative sense and not in absolute sense.

It may be pointed out that time and space are no longer passive factors rather they are active independent variables which influence both proc­ esses and landforms at micro-meso and macro ncale resolution levels. ‘At different scale resolution lev­ els, which are mapped out according to our aims and abilities, different problems arc identified; different types of explanation arc rele v an t; different levels o f organization arc appropriate: different variables are dominant; and different roles o f casue and effect are assigned’ (Chorley, Schumm and Sugden, 1985).

It may be further argued that each stage of geomorphic cycle docs not have same time-period. Further, if the landscape development in different regions is passing through similar stage (say youth stage) it does not mean that the time-period of similar stage is the same in all regions. If two regions are characterized by same stage of landscape devel­ opment the landform characteristics in both the regions may be similar but not the same.

The geomorphic scales, very often used in geomorphological investigations, arc o f two types e.g. (i) time scale and (ii) spatial scale. The scale level resolutions depend on the objectives o f study. For example, if the evolutionary phases of landscape development ever long period of time involving larger areas are to be reconstructed, the model o f Davisian cycle of erosion involving cyclic time (millions of years) may be more apropriate but if a component of landform assemblage is to be studied, a shorter time scale would be more appropriate. It may be mentioned that conclusions derived about landform development and processes at one spatial and temporal scale may not be applicable to other scales because the influence of dominant variables changes from one scale to another scale.

CO N CEPT 5

1. ‘G eom orphic scale is a significant param ­ eter in the interpretation o f landform development and landform characteristics o f geomorphic sys­ tems. ' 2. ‘Landscape is function o f time and space \ The geomorphic investigation requires study of different geomorphic processes (both mode and rate of operation ) and related landforms of a spatial unit over definite time-span for having ‘postdiction (extrapolation from the present to the past of con­ temporary ‘process-form interrelationships) and prediction’ (future development of landforms). Both gemorphological processes and landforms are con­ sidered at various levels of spatial and temporal resolutions. The detailed study of processes through field instrumentation in small areas over small time span has revealed significant results regarding their mode and rate of operation and their influences on landform characteristics under varying time-intervals. ‘Certainly one major result of process study has been the relegation of time to the position of a parameter to be measured rather than a process (as envisaged by W.M. Davis) in its own right. Another major result of the change in gemorphological em­ phasis has been a reduction in the spatial and tempo­ ral scales within which landforms are now consid­ ered’ (M.G. Anderson and T.P Burt, 1981). In 1965 an important contribution to the development ol landform as a function of lime and space (area) was made by Schumm and Lichty. rhcy argued that the kind of model we construct for the study of landform development depends upon the length ol the time-span wc have in mind (P. McC ullagh, 1978).

Time Scales

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Generally, temporal scales are considered at three resolution levels e.g. macro-temproal scale involving millions of years for the study o f mega­ geomorphology, meso-temporal scale involving thousands of years and micro-temporal scale in­ volving shorter time-span involving tens and hun­ dreds of years. For geomorphic evolution and inter­ pretation temporal scales are, alternatively, consid­ ered at three resolution levels e.g. cy d ic time, graded time and steady time. Time scale assumes greater significance in the study o f the rates of operation of processes and changes occurring in landscapes. Generally, no perceptible change may occur in the morphological features during short period of time because either the force exerted by the processes may not be enough to introduce significant change or the processes might have not operated for desired sufficient length of nine. Any changc in the rate of the operation of geomorphic process is supposed to

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GEOMORPHOLOGY

46

limc-span involves progressive but slow change in both process rate and landforms. In a cyclic time •landforms slowly lose energy and mass as agents of denudation reduce altitude’ (P. M cCullagh, 1978). Davisian model o f cycle o f erosion .s based on cyclic time wherein there is progressive sequential change in landforms through time i.e. as the erosion begins with the completion o f upliftm ent there is continu­ ous lowering of reliefs and loss o f energy in such a way that there is equal distribution o f energy in geomorphic system so that all co m po nents of the system are characterized by equal energy levels and hence in the absence o f difference in the energy levels o f different co m ponents o f the system the slate of m axim um d isord er and h ence m axim um en tro p y is achieved wherein no further w ork is performed because there is no energy flow and the ultimate result is the dev elopm ent o f peneplain. Cyclic time is punctuated by g ra d ed tim e (fig. 2.13 A) having a time-span o f 100 to 1000 years.

bring corresponding changc in the landforms. ‘Some times the response is instantaneous, as when a large flood passes through a channel. At other times, the response may be quite slow or there may be ‘dead tim e’ when nothing happens to land forms to reveal the change in process. The time taken for the system to respond to externally imposed changes is called its 'reaction tim e’ (J.B. Thornes, 1979). Cyclic Time

Cyclic time involves longer geological pe­ riod ol time measuring millions o f years (say 10,000,000 years) and very larger spatial areal unit measuring thousands of square kilometers of arca.This

t

®

■e C y c l i-c T i m ( 10,000 000 Y e a r s )

h2

S. A. Schumm and R.W. Lichty (1965) have identified ten drainage basin variables (10) and their relative importance in term s o f cyclic, graded and steady time-scales.As regards the cyclic develop­ ment of landforms, tim e, in itial r e li e f (representing difference of height between ridge crest and valley doors or between highest and low est parts created by tectonic events-upliftment and subsidence, vulcanicity or sea-level changes), geology (both structure-folds, faults ctc. and lithology-rock types) and clim ate (precipitation and insolation) are in dep en den t vari­ ables which control landform d e v e lo p m e n t involv­ ing cyclic time-span (long geological period o f time ranging in millions o f years), w h ereas vegetation (type and density, d e p e n d in g on p recip ita tio n , insolation and geological characteristics), re lie f or volume o f landmass above base level, h yd rology (runoff and sedim ent yield p er unit area within the system-drainage basin), d ra in a g e n etw o rk m orP ° ogy (diainage density ex p essed as total stream

Gr aded T i me

LU

TIME

CYCLIC

Gr aded Time ( 100 -1000 Y e a r s ) r y

Steady Time

t & f ---- ---------

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GRADED

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S t e a d y Time C 10 Y e a r s . )

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,nn!f K I*0 *1 ^aSm area)’ h illslo p e m o rp h ology, f .I r° (discharge o f water and sediment 0,11' . f ^ ? tCir^ 3re ^e Pendent variables which are n ro e y aforesaid four independent variables ime, initia relief, geology and clim ate) but time is

I n s t a n t a n e o u s Ti me ( O n e day ) S TE ADY

T I ME

concenKnnrCant,1|nuC^en^Cnt varia^*e - There are three 2 14A» t h e r e |U br, Um * d e c a * “ l^ i b r iu m (figis progressive but slow rate o f decline

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Fig. 2.13 : Timcscales-(A) cyclic tune, (/ij graded tii and (C) steady time.

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

47 * energy level wherein erosional processes act in epi­ sodic manner as envisaged by S.A. Schumm and R.W. Lichty (1965). Based on the concepts of geom orphic thresholds and com plex system re­ sponse Schumm postulated that some changes in the fluvial system are not effected by external factors (isostatic upliftment) rather these are caused by inherent geomorphic controls in the'eroding system e.g. due to erosional and depositional activities. According to him effective erosion is not a continu­ ous process rather it is episodic in nature and thus the valley floors are not continuously deepened but are reduced in discontinuous manner as periods o f ero­ sion are separated by periods of deposition of sediments to an unstable condition. In other words, the period of erosion (period o f instability) is fol­ lowed by period of deposition of sediments.W hen the sediment storage in the valley crosses the thresh­ old value and channel gradient is steepened then the system becomes unstable and active erosion is initi­ ated resulting in the downcutting (excavation o f deposited sediments and valley floor) of valley floor. The process continues till the sediments are flushed out and again period of deposition is initiated due to lessening of channel gradient. Thus, the valley floor becomes stepped. It is apparent that there is period o f dynamic equilibrium between periods of instability occasioned by episodic erosion (see chapter 3, and fig. 3.7). The result is stepped valley floor (fig. 2.14 D=a, b. c, d indicate steps in the valleyfloor). T h i s dynamic metastable equilibrium model of eipsodic erosion shows, in addition, that many of the details of the landscape (e.g. small terraces and recent alluvial fills) do not need to be explained by the influence of external variables because they devleop as an integral part of system evolution’ (Chorley, Schumm and Sugden, 1985, p. 40).

in form through time leading to establishment of equilibrium condition in the penultimate stage-old stage-ot Davisian cycle of erosion), dynamic equi­ librium (tig. 2.14 C) (indicating a condition of forms oscillating around a moving average value but also characterized by continuous decline in form through time e.g. a river’s long pofile characteized by alternate actions of erosion and deposition) and dynamic metastable equilibrium (fig. 2 . 14D) (rep-, resenting ‘a condition of oscillation about a mean value of form which is trending through time and, at the same time, is subjected to step-like discontinuities as a threshold effect appears to promote a sudden change of form' (Chorley, Schumm and Sugden, 1985) i.e. a condition of equilibrium at insufficient

*

,r4v/Va / I(/V a aVA Y A V*, v ay I St eady S ta t e

aa

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*-

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Graded Time

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rig. 2.14 : Equilibrium types : A—decay equilibrium. B—steady stale equilibrium. C—dynamic equilibrium, D—dynamic mestastable equilibrium (based on A\ J. Chorley and R. B. Beckinsale, /980 and SA. Schumm, 1975). a, b, c and d indicate stepped valley floor.

The time-scale having shorter period (say 100 to 1000 years), during which smaller streams or parts ot big streams and individual hillslopes in adrainage network achieve graded stage of steady state equi­ librium (where geomorphic forms of a system, say drainage basin, oscillate around a stable average value) due to self regulatory mechanism (i.e. nega­ tive feedback mechanism), is called graded time. As the timc-span of landscape development is re­ duced the number of controlling (of landforms)

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factors (i.e. independent variables) increases and number o f dependent variables decreases. For exam­ ple, in a drainage basin time, initial relief, geology and climate are independent (controlling) variables in cyclic time but in graded time besides these four variables, vegetation (type and denisty), relief (above base level) and hydrology (runoff and sediment yield per unit area within the system) also become independent variables (which are dependent vari­ ables in cyclic time). It may be mentioned that time and initial reliefs, which are very significant control­ ling variables (of landforms) in cyclic time become insignificant in the development of landforms in graded time while drainage network morphology, hillslope morphology and discharge of water and transport o f sediment out of the system remain dependent variables e.g. they are controlled by afore­ said independent variables.

may be studied in terms of graded or steady timescale while larger area should be studied in terms of cyclic time-scale. Spatial Scales

There has always been shift in the selection of ideal geomorphic unit having specific areal cover­ age for the study of landforms and geomorphic processes with varying view points and objectives. If we go in historical perspective, spatial scales have varied considerably i.e. from ‘physiographic re­ gions’ of N.M. Fenneman (1914) through Hortons (1945) ‘drainage basin’ as ideal geomorphic unit to J.F. Gellert's ‘m orphotops’ or ‘m orphofacies’ (1982). Fenneman's physiographic regions of N. America on the basis of chronology and uniformity of geological history and structural geology repre­ sent large spatial scale i.e. macro or mega scale and further subdivisions of major physiographic regions into smaller units involved small spatial scale i.e. meso and micro scales. Bourne ( 1932) based on his concept of ‘characteristics-site-assem blage’ rec­ ognized morphological regions at two levels e.g. (i) ‘regions of first level were distinguished on the basis of morphological features produced by ero­ sional and depositional features’ and (ii) regions of second level were identified on the basis of areal units having similar environmental conditions for the development of pedogenic processes, vegetation etc. R.E. Horton ( 1945) recognized ‘erosional drain­ age basin’ as ideal spatial geomorphic unit for the study of drainage basin processes and forms. R e­ cently, J.F. Gellert( 1982) recognized ‘m orphotops’ or ‘morphofacies’ as basic units for morphological regionalization and ‘suggested a uniform shape (mor­ phology, morphometry), homogeneous lithological structure, uniform origin and d e v e lo p m e n t (morphogenesis, morphochronology) and uniform present-day processes (morphodynamics) as the characteristic features for the identification of geomorphological regional units’ (Mamta Dubey, 1993). It is apparent that spatial scales have changed from macro or mega-scale (of earl ier gemorphologists dealing with the cyclic development o f landforms and denudation chronology) through m eso-spatial scale to present - day m icro-spatial scale (in the case of process geomorphology).

Steady Time Still shorter time-span (10 to 100 years), during which a very short reach of the stream or a single slope segment (e.g. convex or rectilinear or concave segment) involving very small area reaches steady state, is called steady time in which there is balance between erosion, transport and deposition. The aforesaid seven variables (e.g. time, initial re­ lief, geology, climate, vegetation, volume of relief above base-level, runoff and sediment yield per unit a re a w ithin the s y s t e m , d r ai na ge , which are indipendent variables in cyclic and graded time plus drainage network morphology and hillslope mor­ phology (which are dependent variables in graded time) becom e independent variables and only one variable (i.e. discharge o f water and sediment out of the geom orphic system (say drainage basin) be­ com es dependent variable in steady time. The in­ s ta n ta n e o u s tim e re fe rs to the condition of form at a single day. ‘It will be seen that time can be considered as the most significant independent variable in landform studies, or regarded as o f relatively little signifi­ cance, depending upon the time-span involved (and the size o f spatial unit-areal coverage). It is generally true to say that most modern geomorphological emphasis is upon studies concerned within graded or steady tim e’ (P. McCullagh, 1978, p. 11). The geomorphic system having smaller areal coverage

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It may be mentioned that spatial scale has much significance in controlling the rate and mecha­

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

nism of operation of processes and their responses (landforms) as the areal coverages of study areas change. For example, if a small area (less than one square kilometer) of gullied zone is selected for the study of behaviour of runoff, discharge, soil erosion, sediment transport etc. during strong rainstorms associated with thunderstorm, the fluvial process is highly accelerated and the rate of erosion becomes very high becuase of maximum runoff and discharge but if the study area is a large drainage basin then the effect o f strong rainstorm of short duration is ob­ scured as only the part of the basin is affected by high intensity rainstorms. The post- 1950 geomorphology lays more emphasis on the study of different aspects of processes on the basis of field instrumentation and laboratory experiments. This requires shorter tem­ poral scale (time scale) and smaller spatial unit. It may be concluded that ‘at different scales of space different variables become dominant, different lev­ els of generalization may be employed and even different problems identified’ (Chorley, Schumm and Sugden, 1985). C O N C EP T

6

A sim ple geom orphological equation may be envisaged as a vehicle for the explanation o f landforms as fo llo w s — F = f (PM ) dt

K.J. Gregory, 1977

This geomorphological equation envisages that ‘the landform (F) is the function of process (P), material (M, geomaterial) and change through time (dt)’. Gregory stated (1977) that ‘morphology (F) = function o f processes (P) on materials (M) over tim e ( t) \ According to him ‘morphology refers to the form of earth's surface or landform; processes include the geomorphological processes associated with weathering, wind, water, ice and massmovement, and materials connote the rock, soil and superficial deposits upon which processes operate (Gregory, 1977, p. 137). He has identified four aspects of interest wherein the equation may be studied at four

the equation (e.g. between form, proccss and materials) at specific time. Level 3 : Differentiating the equation, involving the investigation o f the way in which some relationships between form, proc­ ess and materials vary over time. Level 4 : Applying the equation i.e. to apply the results drawn through aforesaid three levels of investigation for solving the environmental problems. Study of Elements of the Equation

It is necessary to study detailed aspects of forms (landforms), geom aterials (of which the landforms have been formed) and processes (which shape the la n d fo rm s th r o u g h e r o s io n a l and depositional activities) independently so that the landscape of a particular geomorphic unit o f a spe­ cific spatial scale may be studied in right perspec­ tive. Different aspects of forms (landforms) have been widely studied and given more attention right from the beginning of geomorphological investiga­ tions to the development of the branch of landform geography (B. Zakrzewska, 1967). Morphometric techniques have enabled geomorphologists to study different morphometric aspects (shapes, amplitude and dimension) of landforms produced by various denudational processes. Information derived from aerial photographs and satellite imageries have also enriched landform geography. ‘Although the study of form is a necessary p re-req u isite to later geomorphological analysis it has been argued that it should not be an end in itself because it is very difficult to understand the past development o f form, the present significance or future character, from morphology alone’ (E. Derbishire, K.J. Gregory and J.R. H ails, 1979) and h e n c e m a te r ia ls and geomorphological processes should also be studied with equal emphasis. Geomaterials, of which the landforms are composed, have not been studied in right perspec­ tive inspite of the fact that geological structure plays an important role in the evolution of landforms (see concept 2). Generally, geomaterials include rock types, geological structure (disposition of rock beds e.g. folded, faulted, uniclinal, domal etc. structures),

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levels— Level 1 : Study of elements of the equation, i.e. investigation of three elements of the equation (e.g. form, process and materi­ als) independently. Level 2 : Balancing the equation i.e. tu obtain relationships between ihe elements ot

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GEOMORPHOLOGY

rock characteristics (mechanical and chcmical com­ position), weathered materials, surficial deposits and soils. Traditionally, geological structure includes three aspects viz. lithology or nature of rocks (igne­ ous, sedimentary and metamorphic rocks), arrange­ ment and disposition of rock beds (folded, faulted, uniclinal, domal etc.) and rock characteristics (chemi­ cal and mechanical composition, permeability and impermeability, joint patterns, rock resistance etc.).

esses (driving force- operation of processes) and materials (resisting force) leading to the attainment of equilibrium when driving force equals the resist­ ing force (see chapter 3, Gilbert’s model). •

Differentiating the Equation

Differentiating the equation requires to find out ‘the way in which geomorphological systems change or adjust over time’. In fact, the geomorphic investigation requires study of different geomorphic processes (both mode and rate of operation) and related landforms o f a spatial unit over definite timespan for having ‘postdiction (extrapolation from the present to the past of contemporary process-form interrelationships) and prediction’ (future develop­ ment of landforms). ‘Inclusion of time dimension is necessary because periods of time may be necessary for a certain process or assemblage of processes acting upon particulate materials to produce a spe­ cific form’ (Gregory, 1977). The changes in landforms may be studied through varying temporal scales e.g. macro-time scale (cyclic time, involving millions of years), meso-time scale (involving thousands of years) and micro-time scale (involving tens of years). Alternatively, landform changes may be investi­ gated through cyclic time (involving long-term pe­ riod of millions of years), graded time (hundreds of years) or steady time (tens of years). Time scale assumes greater significance in the rates of opera­ tion of processes and changes occurring in land­ scapes. Generally, no perceptible change may occur in the morphological features during short period because either the force exerted by the processes may not be enough to introduce significant change or the processes might have not operated for desired sufficient length of time. Any change in the rate of operation of geomorphic process is supposed to bring corresponding change in the landforms, ‘som e­ times the response is instantaneous, as when a large flood passes through a channel. At other times the response may be quite slow or there may be ‘dead time when nothing happens to landforms to reveal the change in the process’ (J.B. Thornes, 1979).

Processes (see concept 3) constitute third element o f the equation and include those physical processes which operate on the earth's surface both internally and externally (i.e. endogenetic and exogenetic processes). A detailed investigation re­ garding three-phase work of geomorphic processes (i.e. erosion, transportation and deposition) is needed to understand the mode of origin and development of landforms of varying scales. The detailed study of exogenetic geomorphic processes (denudational proc­ esses e.g. fluvial, coastal, glacial, aeolian, periglacial, groundwater etc.) through field observation and instrumentation and laboratory experimentation has gained currency since 1950. Balancing the Equation

After the detailed investigation of form (landforms), materials and processes individually and independently, attempt is made to produce a general model o f ‘form -processes-m aterials rela­ tionships.9 In other words, an attempt is made to establish relationships between landform and mate­ rials (structure, see concept 2), between form (landforms) and processes (see concept 3) and be­ tween form, materials and processes leading to for­ mulation of functional theories of landscape devel­ opment. ‘The system approach is ideally suited to the identification o f the relationships between the elements o f the equation and has been instrumental in clarifying the diverse ways in which indices of materials, o f process, and of form are related’ (Der­ byshire, Gregory and Hails, 1979). It may be men­ tioned that not only perceptible relationships be­ tween form, processes and materials in any specific area having definite climatic conditions are investi­ gated but spatial contrasts of the elements of equa­ tion and interrelationships are also studied. The introduction of equilibrium concept has enabled the geomorphologists to envisage the landscape devel­ opment on the basis o f relationship between proc­

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Recently, the role o f man (through his eco­ nomic activities) as geomorphic agent has increased significantly and thus it has becom e necessary to study the influences of man on geomorphological processes and their responses (forms) in a particular area at different stages. For example, the rate o f soil

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

the measurement o f contemporary environmental (geomorphological) processes since 1950 ushered in a new era o f realization o f significance o f human a c tiv itie s a ffe c tin g the en v iron m en tal (geomorphological) processes (Savindra Singh, 1991).

erosion in man-impacted gully basins has increased alarmingly (Savindra Singh, e l at 1995). Similarly, the impact of human activities on hydrological, fluvial, coastal, periglacial processes etc. has .in­ creased many fold (see chapter 30). ‘It is possible to envisage several geomorphological equations each pertaining to a particular time in an area and each relating to a particular degree of m an’s influence' (Gregory, 1977).

CONCEPT 7 4C om plexity o f geom orphic evolution is m ore common than sim p lic ity .9 W .D. Thornbury Generally, landform characteristics are ex ­ plained on the basis o f most dom inant controlling factor on the basic premise that majority o f landforms are simple and have less com plex geom orphic ev o ­ lution but in reality most o f the landform s are the result of poly-factor rather than m ono-factor. S ec­ ondly, mono-process evolution o f landform s o f topofunction or of lithofunction or o f tectonofunction or o f pedofunction or of clim o-function etc. has been recently refuted by majority o f geom orphologists and they have been considered to be the o u tc o m e o f poly-process evolution. In fact, 'the crux o f the problems o f landform evolution as to w heth er there is sequential change in landscape ecology w ith the march of time (time-dependent approach-cyclic ev o ­ lution of landforms). or an individual process is competent enough to evolve its own characteristic landforms (process- form approach), or steady state of operation o f processes leads to tim e-independent series of landform (dynamic equilibrium — non-cyclic evolution of landforms), or geologic structure is the most dominant controlling factor in the evolu ­ tion of landforms (structure-form approach, litho­ function), or each climatic type produces its own distinctive assemblage of landforms (clim ate-process-form approach— climo-function) etc. still re­ main unresolved' because o f the fact that ‘the basic factors controlling the genesis and developm en t o f landforms based on the param eters o f geologic struc­ ture (lith o-fu n ction ). tectonics (tecto n o -fu n ction ), climatic elements (clim o-function), processes (process-resp o n se), vegetal cov ers iflo ro -fu n ctio n K pedological characteristics (p ed o-fu n ctioii), human interference with physical environm ent ( a n th r o p o function), and topographic factors (top o-fan ction ) su bstantially vary both spatially and temporally* (Savindra Singh. 1985).

Applying the Equation

The knowledge derived through the analysis of geomorphological equation at three levels viz. (i) study o f forms, materials and processes individually and independently, (ii) establishing relationships between form and materials, between form and proc­ esses and between form-processes-materials, and (iii) investigation of changes in geomorphic system and landforms over time (cyclic time, graded time and steady time) is utilized for 'estimation of the behaviour of geomorphological systems either in locations where processes have not been measured (spatial prediction) or in the future (temporal pre­ diction)' (Derbyshire, Gregory and Hails, 1979). This becomes the field o f applied geomorphology having varying dim ensions e.g. environmental geomorphology, urban geomorphology, geomorphic engineering etc (see chapters 29 and 30). ‘We can think o f environment as a machine which we need to control. However, such control can be achieved only if we fully understand how the geomorphological machine w orks’ (Derbyshire et. al., 1979). T h e equation outline is tentatively offered as a basis for synthesising contemporary approaches to geom orphology and it could be extended to physical geography as a w hole’ (Gregory, 1977). The useful­ ness of geomorphic investigations depends on the successful application of geomorphic knowledge in ameliorating different environmental problems cre­ ated by hum an a ctiv ities and acceleration of geomorphological processes by man as a potent geomorphic agent. T h e under-emphasis on the study o f m an's role in c h a n g in g the en v iro n m en ta l (geomorphological) processes till 1950 was because of lesser attention paid towards the measurement of contemporary geomorphological processes and quali­ tative assessment o f the reconstruction o f the effects of palaeoprocesscs. Increased enthusiam towards

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It may be mentioned that landscape mosaic of any physiographic region or morphogenetic region

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GEOMORPHOLOGY

52

causc interruptions in cycles o f erosion w hich co m ­ plicate the landform s through rejuvenation and ini­ tiation o f new cyclcs o f erosion,

having a distinctive clim atic regim e is the result o f a variety o f factors but it may be that one o f the factors may be most dom inant in shaping the landforms. The variations and com plexity in landform s arc introduced due to follow ing reasons—

(v) C h anges in base-lcvcls o f erosion cause by negative or positive changes (fall and rise) in seaIcvcls either due to tectonic ev ents (rise o f sea-floor or subsidence of coastal land— rise in sea level positive changc or fall in sea-lev el— e ith er due to subsidence of sea-floor or d u e to uplift o f coastal land - negative changc in sca-level)or climatic changes (fall in sea-level or negative ch an g c d u e to glacial ice

(i) T h e p r e s e n t la n d s c a p e s o f d iffe re n t physiographic regions at least at macro-spatial scale (m egageom orp h ology) are exam ples o f p alim p sest top ograp h y ('lik e surfacc which has been written on m any times after previous incriptions have been only partially erased; G reek : palin -'‘a g ain ’, psegma‘rubbed o ff’-Chorley et a i , 1985) because these regions have experienced several phases o f cycles of erosion and the landform s have evolved very slowly over long period o f geological time and thus the landscapes having superim posed effects of climate and tectonic factors show evidences o f poly-cyclic evolution and com plexity in their general character­ istics. In fact, successive cycles o f erosion introduce com plexity in landforms. Fo rex am p le, most parts of peninsular India exhibit a fine exam ple o f palimpsest topography having polycyclic reliefs characterized by different erosion (planation) surfaces at different elevations.

age or rise in sea-level due to intcrglacial period) are responsible for the initiation o f successive cy cles o f erosion and hence polycyclic landform s. On the basis o f variations in landform ch a ra c ­ teristics H orberg (1952) divided the la n d sca p es o f the globe into five principal categories viz. (1) sim ­ ple landscapes, (2) co m pou nd lan dscapes, (3) m onocyclic landscapes, (4) m ulti-cyclic landscapes, and (5) exhum ed or resurrected landscapes. Sim ple La n d scap e s

Simple landscapes are those w hich are gener­ ally devoid o f com plexity and are the result o f m ono­ process acting during a single cy cle o f erosion. For exam ple, if we take the case o f a region having sedim entary rocks consisting o f alternate bands o f relatively resistant (sandstones) and soft rock beds (shales) and river as agent o f erosion, the differential fluvial erosion will give birth to step p ed landscapes. It may be adm itted that even sim ple la n d sca p e is not the resulf o f a single g eom orph ic process but for simplification and generalization the m o st d o m in an t process is given due im portance and landscape d e ­ velopm ent is studied in term s o f m ost d o m in an t process (e.g. fluvial landscapes, glaciated landscapes, periglacial landscapes, aeolian or arid landscapes etc.). For exam ple, if the landscape o f a given region is evolved due to the work o f ru nn ing water (river), the fluvial process undoubtedly is the m ost effective geom orphic agent but w eathering process (corrasion) and m assw asting and m asstranslocation (slum ping, soil creep, mud flow etc.) also play significant role. Similarly, the solution (corrosion) m echanism is m o s t d o m i n a n t d e n u d a t i o n a l m ech a n ism by groundw ater in the areas o f carbonate rocks but surface w ater (surface ru n o ff resulting form rainfall) also helps in the evolution o f landforms. In fact, the

(ii) The operation of several geom orphic proc­ esses even during a single cycle o f erosion intro­ duces com plexity in landforms. Forexam ple, though wind is the most dom inant geom orphic process in warm and hot arid regions but fluvial process be­ com es occasionally very active when there is occa­ sional heavy rainfall through strong rainstorm (though very rarely). Consequently, besides aeolian landforms {e.g. inselbergs, yardang, zeugen, sand dunes etc.), very interesting fluvial landforms (pediments, bajadas, playas and badland) are also formed. Similarly, besides the developm ent o f pure glacial landforms in glaciated regions, fluvio-glacial landforms (e.g. kame, eskers, outw ash plains etc.) are also evolved. (iii) The spatial variations in landform-controlling factors (e.g. lithology, geological structure, climatic parameters mainly temperature and pre­ cipitation, vegetation, soils, human activities etc.) within a physiographic or m orphogenetic region introduce complexity in the landforms,

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(iv ) T e c to n ic ev en ts (u p w arp in g , downwarping, upliftment, subsidence, folding, fault­ ing etc.) are very important factors for creating variations in landform characteristics. Tectonic events

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FUNDAMENTAL CONCEPTS IN GEOMORPHOLOGY

Compound Land scap es

The landscapes, produced by more than one geomorphic processes and landform controlling fac­ tors, are called as com pound landscapes. In fact, com pound landscapes are more common in reality than simple landscapes. The landscapes produced during Pleistocene glaciation present examples of com pound landscapes as glacial geomorphic fea­ tures (both erosional and depositional) are found at higher altitudes while fluvial landforms (produced by rivers) are found at lower levels. Besides, aeolian features mainly depositional forms have also devel­ oped. Several exam ples o f compound landscapes are seen in Utah, New Mexico, Arizona, Nevada etc. of the U.S.A. where volcanic cones and related vol­ canic landform s and lava-flow related features have developed in the fluvially originated river valleys. Tectonic events also introduce complexity in the landscapes. Com posite fault-line scarps are such examples. Such features bear the characteristics of fault plane as well as erosional surface. Such co m ­ posite fault-line scarps are formed when fault scarp is originated due to faulting resulting in the dow n­ ward m ovem ent o f down thrown block along the fault plane and subsequent erosion of lower segment o f fault scarp. Thus, the upper segment is technically formed (due to faulting) while the lower segment is erosional. Mono-cyclic La n d scap e s

The landform s produced in a physiographic region during a single cycle of erosion are called

m onocylic landform s. Like sim ple landscapes, monocyclic landscapes are less com m on in reality. Monocyclic landscapes may be possible along coastal plains provided that the coastal plains are not affected by several phases o f em ergence and sub­ mergence. Monocyclic landforms generally develop over volcanic cones, lava plains and lava plateaus, newly formed domes etc. It may be pointed out that monocyclic landscapes may be both sim ple and compound. Poly-cyclic Landscapes

Landscapes produced due to com pletion o f several cycles of erosion (successive cycles o f e ro ­ sion) in a region are called as poly (multi) cyclic landscapes (example of palimpsest topography). M ost of the present-day landscapes are the exam ples o f multicyclic landscapes which have developed d u r­ ing more than one cycles o f erosion. It m ay be mentioned that landsforms o f older cycles are not found in their original forms because they are m o d i­ fied by succeeding phases o f cycle o f erosion and hence only relic features of older cycles are p r e ­ served. Polycyclic landscapes are identified on the basis of a few diagnostic and representative landforms e.g. valley in valley topography (multi-storyed valleys, topographic discordance), rejuvenated river valleys, uplifted peneplains, incised m eanders, nick points or heads of rejuvenation etc.). T he m u lti­ cyclic landscapes are evolved due to rejuvenation consequent upon lowering o f base level o f erosion cither due to upliftment or negative ch ange in sealevel (fall in sea-level). Applachian highlands o f the USA present fine exam ple o f polycyclic landscapes which have developed because o f three successive cycles of erosion (viz-Schooley, H arrisberg and Sommerville cycles o f erosion). T h e D am o d ar river valley at Rajroppa in Hazaribagh (Bihar, India) and the N armada valley at B heraghat (near Jabalpur, M.P.) present ideal exam ples o f rejuvenated valleys having three-tier te rra c e s on e ith e r side. T he Chotanagpur region in general and Ranchi plateau in particular represents exam ples o f polycyclic land­ scapes. Hundrughagh falls on the Subam asekha river, Johna or G autam dhara falls at the confluence d f the G unga and the Raru rivers, D assam ghagh falls on the Kanchi river (a tributary o f the Subarnarekha) etc. indicate heads o f rejuvenation along the junction of the central and eastern Ranchi plateau (Bihar).

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concept o f ‘m ono-process landform 9 is related to the concepts o f clim a tic g eom orp h ology and m orphogenetic regions wherein it is envisaged that ‘each c lim atic typ e (and hence the resu ltan t geomorphic process) produces its own characteris­ tic assemblage of landforms’. L.C. Peltier’s classifi­ cation of climatogenetic landforms into nine catego­ ries and division of world landscapes into nine morphogenetic regions (e.g. glacial, periglacial, boreal, maritime, selva, moderate, savanna, semiarid and arid morphogenetic regions) is based on the concept o f climatic geomorphology but it may be p o in te d o u t that the a d v o c a te s o f c lim a tic geomorphology have not succeeded in presenting ample convincing evidences in support of their argu­ ments through diagnostic landforms (e.g. lateritic feature, inselbergs, pediments, tors etc.).

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GEOMORPHOLOOY

Resurrected Landscapes The resurrected or exhum ed landscapes are those which were covered with either lava flow (volcanic eruption) or sedimentation (mainly on the coastal plains) after their formation but were uncov­ ered at a later date due to denudational processes. Majority o f landscapes were covered with thick ice sheets during Pleistocene ice age in North America and Eurasia but these reappeared after deglaciation o f ice sheets. M any of the landscapes were buried under lava sheets in Peninsular India during Creta­ ceous vulcanicity and a few o f them have r^ow been exhum ed due to erosion of lava cover. CONCEPT 8 ‘Little o f the earth's topography is older than Tertiary and m ost o f it no older than Pleistocene. ’ W.D. Thornbury It is argued by majority of geomorphologists that most o f the present-day landforms are the result o f geomorphic processes which operated in the T er­ tiary and Quaternary times as the landforms older than Tertiary have been either obliterated by the dynamic wheels o f denudational processes or have been so greatly modified that they have lost their original shapes and cannot be properly and accu­ ra te ly id e n tif i e d . On the o th e r han d , som e geomorphologists also argue that the present-day landform assemblages are the examples of palimpsest topography and are the result o f past (palaeo) and present processes.

Though the Himalayan orogeny began either during late Cretaceous period (M esozoic era) or Eocene period (Tertiary) but it was not complete until Pleistocene period but most o f the topographic details were carved out during Quaternary epoch by the fluvial processes. The H imalayas are character­ ized by young and rejuvenated landforms e.g. deep, long and narrow valleys (gorges and canyons), three paired terraces, waterfalls and rapids etc. The side effects of the Himalayan orogeny are well observed in the present-day topographic features o f the Chotanagpur (Bihar, India). Tertiary epoch regis­ tered three phases o f upliftment and hence interrup­ tions in fluvial cycles of erosion occurred several times mainly in Palamau uplands and Ranchi Pla­ teau. The marginal areas o f the Ranchi plateau (including ‘paltands’) characterized by waterfalls (Hundrughagh falls, G autam dhara or Johna falls, Dassamghagh falls, Pheruaghagh falls etc.), nick points and breaks in slopes and juvenile characters of the rivers where these descend from the escarp­ ments, tell the story of Tertiary upliftments. The formation of the Gangetic trough consequent upon the Himalayan orogeny rejuvenated the foreland of Indian peninsula which is evidenced by the presence of a series of waterfalls on the northward flowing rivers which after descending through the foreland meet the Yamuna and the G anga rivers right from the extreme western point of the Rewa plateau (M.P.) to Rohtas plateau in the east (Bihar) e.g. Tons or Purwa falls (70 m), Chachai falls (127m), Kevti falls (98m), Odda falls (148 m, all in M.P.), Devdari falls (58 m), Telharkund falls (80m), Sura falls (120m ), Durgawati falls (80 m), Dhuan Kund falls, Rahim Kund falls (168 m) etc. (all in Rohtas plateau, Bihar). ‘It is now clear that an understanding of the new geology ahd o f tectonics is essential to under­ stand landforms, and not only first order landforms...... and there is an increasing concern with the older landscapes’ (C.D. Oilier, 1981). It may be mentioned that erosional and weath­ ering processes, responsible for the creation o f most of the third order landforms are largely determined by climatic conditions and hence climatic changes

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The advocates o f this concept (aforesaid) argue that pre-existing earth's surface was greatly affected and modified by global Tertiary orogeny (formation of Alpine-Himalayan chains, Rockies, Andes, Atlas, Island arcs and festoons of east Asia etc.) and related rejuvenation o f existing cycles of erosion and initiation o f new cycles resulting in the origin of new sets o f landforms world over. The Quaternary epoch experienced global climatic change and Pleistocene ice age comprising four glacial periods (Gunz, Mindel, Riss, Wurm in Europe and Nebraskan, Kansan, Illinoin and Wisconcin in. N. America) and alternated by four interglacial periods (warm period) obliterated and modified nearly all of the pre-existing landscapes in most o f the regions of North America and northern Eurasia as the advanc­ ing ice sheets filled up the lowlying areas and low­

ered and rounded sharp peaks and hills. The retreat­ ing ice sheets left morainic deposits behind and thus numerous morainic ridges and glacial lakes were formed in North America and Europe.

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f u n d a m e n t a l c o n c e p t s in g e o m o r p h o l o g y

through g e o lo g ic a l tim e s h av e g re a te r geomorphological significance. ‘Weathering fea­ tures preserved in rocks show that climate and weath­ ering havcchangcd not only in Quaternary times, but through all geological time’ (C.D. Oilier, 1969). For example, latcrite profiles of Tertiary period have been covered by lava sheets in Ireland. It is iilso well known fact that latcrites arc formed under warm and humid climate and hence the latcritcs of Ireland cannot be attributed to present climatic conditions. ‘In Triassic time England was largely a desert, as was Scotland in Torridonian (Prccambrian) lime. In contrast, South Africa, India and Australia had gla­ cial climates in Permo-Carboniferous time’ (C.D. Oilier, 1969). Some of the relics of landforms resulting from weathering and erosional processes as a conse­ quence of climatic changes through geological times have been preserved. For example, most of the southern hemisphere (S. America, Africa, India, Australia etc.) were glaciated during upper Carbon­ iferous time. ‘In Mesozoic times the whole world experienced a warm phase, and glaciation was com­ pletely absent. World climates in the Jurassic were particularly uniform, but in the upper Jurassic and Cretaceous climatic variations once again became important......... at the start o f the Tertiary the world was still considerably warmer than it is now. There were no ice caps, trees grew in polar regions, and the climate was more uniform over the earth’ (C.D. Oilier, 1969).

"Ifie earth's surface contains many relics of former gcomorphic processes— landforms that were created long ago, and remain at the earth's surface. So in the thinking of time-scale we are concerned not only with the formation, but also the preserva­ tion of landforms. There are places where actual landforms, such as river valley systems, have been preserved for hundreds o f millions o f years' (C. D. Oilier, 1981). It is pertinent to point out that time scale is also o f paramount significance in the evolu­ tion of landf orms. For example, some landforms are created instantaneously following tectonic activity (e.g. faults and fissures due to tensional forces or due to seismic events), some features are formed in weeks and months e.g. due to vulcanicity (volcanic cones such as ash or cinder cones), erosional activity (e.g. sand dunes by wind, gullies by storm rains etc.) while the evolution o f some landforms takes m il­ lions of years such as the formation of planation surfaces. Plate tectonics have demonstrated that earth movements leading to upliftment are not sud­ den and rapid rather they are slow and continuous. It may be concluded that, no doubt, the cli­ matic oscillations and tectonic activities since Terti­ ary and mainly during Quaternary have so greatly modified (Pleistocene glaciation) pre-existing m or­ phological features that they have lost their original characteristics at least in North A m erica and north­ ern Europe but many relic geomorphic features o f longer geological histories are indicative of their palaeo-genesis. ‘Indeed wherever geomorphic his­ tories are long there seems to be evidence that things were different in the past’ (C.D. Oilier, 1981). CO N CEPT 9

'Each clim atic type produces its ow n charac­ teristic assem blage o f landform s \ This concept is based on the basic tenet o f clim atic geom orphology based on the work o f Von Richthofen (in China), Passarge, Jenson, Walther, and Thorbecke (in Africa) and Sapper (in Central America and Malanesia) and advocated by J. Budel (1948, 1982), L.C. Peltier (1950), C. Troll (1958), W.F. Tanner (1961). P. Birot (1968). D.R. Stoddart

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G.H. Ashlay has forcefully pleaded for the very young nature of landforms at global level and maintained that ‘most of the word's scenery, its mountains, valleys, shores, lakes, rivers, waterfalls, cliffs and canyons are post-Miocene, that nearly all details have been carved since the emergence of man, and that few if any land surfaces to day have any close relation to pre-Miocene surfaces’ (G.H. Ashlay, 1931). It may be mentioned following C.D. Oilier that since major parts of N.America and northern Europe were affected by Pleistocene gla­ ciation and the impact of glaciation on landscapes was great and perceptible that most of the writers of geomorphology text books were guided by Pleistocene bias and subscribed to the above view points. But there are also many geomorphologists who do not subscribe to this view point because, according to

them, there are numerous relic (fossil) landforms which arc not the result o f present-day processes or to those o f Quaternary times, rather they are quite old.

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(1969), L. Wilson (1969, 1973),J. Tricart and A. Cailleaux (1972) etc. The concept envisages that geomorphic processes, which shape the landscapes, are determined and controlled by climate which thus produces distinctive landscapes through processes. The advocates o f climatic geomorphology have attempted to validate the influences o f climatic conditions on the evolution and characteristics of landforms on the basis of certain diagnostic landforms such as duricrusts (such as laterites, silcrete, calcrete etc.), inselbergs, pedim ents, tors etc.

The climatic geomorphologists (Budel, Peltier, Tricart and Cailleux) have divided the world into definite morphogenetic (climatogenetic) regions on the basis o f dom inant weathering and erosional processes generated by a particular suite o f climatic parameters. This concept is further elaborated in chapter 4 (clim atic geom orphology) o f this book separately in o r d e r to in c lu d e all a s p e c t s o f c l im a t ic geomorphology and morphogenetic regions.

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It is argued that climatic parameters control landscape development directly and indirectly. Cer­ tain climatic parameters such as temperature and

and erosional processes while indirect influence o f climate on landforms is through vegetation and soils.

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THEORIES OF LANDFORM DEVELOPMENT

L a c k o f c o m m o n l y a c c e p ta b le th e o ry ; s ig n if ic a n c e a n d g o a l s o f g e o m o r p h i c th e o rie s ; h is to ric a l p e r s p e c tiv e ; b a s e s a n d ty p e s o f g e o m o r p h i c th e o rie s (teleo lo g ical theory, im m a n e n t th e o ry , h is to ric a l th e o r y , ta x o n o m ic th e o ry , fu n ctio n al theory, realist th e o ry , c o n v e n t i o n ­ a lis t t h e o r y ) ; m a jo r g e o m o rp h ic th eories o f G. K. G ilb e rt, W .M . D a v is , W . P e n c k , L. C. K ing , J. T. H ack, M . M o r is a w a an d S. A . S c h u m m ; g e o m o r p h i c th e o rie s in In d ian co ntext.

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CHAPTER 3

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3 THEORIES OF LANDFORM DEVELOPMENT This chapter deals with a few aspects of geomorphic theories viz. lack of commonly accept­ able general theory, significance and goals of geom orphic theories, historical perspective of geomorphic theories, bases and types of geomorphic theories and evaluation of important theories.

explain the landscapes of the earth's surface in all environments on the basis of a single theory. The conceptual vacuum created by the rejection o f Davisian cyclic model of landforms could not be filled up as yet inspite of postulation of non-cyclic model of landform development (dynamic equilibrium theory).

3.1 LA C K O F COMMONLY A CC EP TA B LE THEORY

Question arises as to why no such common theory could be postulated which can be acceptable to majority of geomorphologists and can be applied in different environmental conditions. C.G. Higgins has opined that ‘it would seem that one reason we lack an acceptable theory of landscape development is that there is as much diversity of opinion about structure, process and form as there is diversity among structure, process and landforms themselves.’ It is, thus, obvious that there is spatial and temporal variation in the factors controlling the genesis and development o f landforms e.g. geologic structure, tectonic events, climatic elements, geomorphic proc­ esses, vegetal covers, pedological characteristics and human interference with physical environment through his economic activities, and landscapes are more complex than simple. In spite of the fact that complexity of geomorphic evolution is more com ­ mon than simplicity, landform development has been related to single causative factor by individual geomorphologist. According to C.G. Higgins the controversy regarding the theories of landform de­ velopment has surfaced because of the fact that the theories have been oversimplified. He further cat-

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The crux o f the problems of landform evolu­ tion as to whether there is sequential change in landform development with the march of time (cy­ clic evolution of landform s, time-dependent series of landform development), or landform develop­ ment is time-independent and there is dynamic equi­ librium (tim e-independent series of landform de­ v e lo p m e n t or n o n -c y c lic d ev elo p m en t of landform), or each geomorphic process produces its own characteristic assemblage of landforms (process-geom orphology), or geological structure is the most dominant control factor in the evolution of landforms (structural or geological geomorphology ), or each climatic type produces its own charac­ teristic assemblage of landforms (climate-processfonm approach, clim atic geomorphology), or tec­ tonics play important role in the evolution of landforms (tectonic geom orphology or tectono-geomorphic concept), or episodic erosion model is most appro­ priate to explain fluvially originated landforms etc. still remains unresolved because of the fact that the postulator has always attempted to describe and

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OriOMORFHOIXKJY

cgorically stated that ‘there m ay be no definitive theory or geom orphic system that can fit all lan d scap es.’ S. Schumm (1975) has also corrobo­ rated the idea o f Higgins as he has aptly remarked, ‘most models o f geomorphic evolution arc oversim ­ plified and therefore they arc unsatisfactory for short-term interpretation of landform changc. T here­ fore, a very complcx denudational history of a landscape may be gcomorphologically norm al.’

more than one theories may be applicable in a region having uniform environmental conditions e.g. paral­ lel retreat and slope decline may be applicable side by side. For example, the hillslope having sandstone capping above weak shales in Bhander plateau (M.F*,) near Maihar is characterized by all the four elements of ideal hillslope profile (e.g. summital convexity, free face, rectilincarity and basal concavity) and is *undergoing the process of parallel retreat of free facc clement and slope replacem ent at the basal segment (Penckian model o f parallel retreat and slope re­ placement) while the con vexo-cogcave hills, girdling the Bhander plateau (fig. 3.8), which have lost sand­ stone capping because o f prolonged backwasting and parallel retreat, are undergoing the process of activc downwasting and slope dcclinc (e.g. Sharda Pole hill very close to Sharda T em ple hill (fig. 3.8), popularly known as Maihardevi hill, is experiencing the process o f slope decline Davisian model of slope decline.

It may be pointed out that majority o f theo­ rists have postulated their respective geomorphic theories on the basis o f limited study of landforms in a small area and thus the results so derived may not be universal and may not be acceptable to all. It may not be out ot context to emphasize that there is so much diversity, variability and complexity in the landform characteristics and their mode of forma­ tion and their controlling factors (as mentioned above) that the problems of landscape development in all parts of the earth's surface and in all environ­ mental conditions cannot be solved on the basis of a single geomorphic theory rather these can be tackled on the basis of composite or multiple theories. Thus, according to C.G. Higgins, ‘we need multiple theo­ ries or different theories for different purposes........ as scientists we may all be seeking a correct or complete rational answer to landform origins, but if the natural world is irrational, no internally com ­ plete and substantive theory or system would work.’

It may be concluded that the most compelling reason for the lack o f com m only acceptable general geomorphic theory has been the lack o f proper and meaningful investigation o f processes and landforms and establishment and explanation of relationships between geom orphic processes and landforms in different physiographic regions in correct perspec­ tive. Many of the geom orphologists have related the present-day geom orphic features Df the earth’s sur­ face to the geom orphic processes operating pres­ ently whereas many o f these landforms are relic features and the result o f past processes (older than Quaternary.

Further, all of the geomorphic theories, pos­ tulated so far, lack in elastacity and broader perspec­ tives and are unable to accommodate all aspects and view points related to genesis and development of landforms in different environmental conditions in­ volving a host o f landform controlling parameters. But it may also be pointed out that because of complexity in landforms and parameters controlling their evolution no single theory can incorporate all aspects o f landform development. It is also not desirable that wc should seek solution of all prob­ lems of landform development from a single theory or geomorphic system. In fact, wc need multiple solutions instead of single solution of landformrelated problem. For example, the evolution and development o f hillslope in varied environmental conditions may be explained separately involving alternative theories e.g. slope decline theory, paral­ lel retreat theory, slope replacement theory etc. Even

3.2 S IG N IF IC A N C E AND G O A L S M ORPHIC T H E O R IE S

OF GEO ­

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In any branch o f science a theory plays an important role for the developm ent of new concepts and approaches to the study of scientific problems and hence the formulation of theories is necessary for the furthcrencc o f scientific knowledge. Thus, general theory is also required in geomorphology for the understanding o f mode o f formation and devel­ opment o f landforms. The main role o f a geomorphic theory is to in teg rate th ree m a jo r aspccts of geom orphology e.g. decription (descriptive aspeci). classification (taxanomic aspect) and genesis and explanation (genetic/evolution aspect) of landforms in different environmental conditions. A geomorphic

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t h e o r ie s o f l a n d f o r m d e v e l o p m e n t

theory may be formulated on the basis of empirical generalization, deductions or on the basis of inter­ pretation of observed facts related to landforms and related geomorphic processes, or on models. Only that theory becomes most significant and commonly acceptable which is most general, simple and elastic so that it can accommodate and explain nearly all aspects of landforms e.g. right from their mode of genesis through development to the present form. In fact, the main problem of landform study (genesis and development) may be conveniently and logically, if not unambiguously, tackled if three lines of geomorphic inquiry viz. precise way of description of landforms. their classification and mode of genesis and evolution through time and space and process-form relationship together with the mode of operation of processes are taken into account because ‘a future for geomorphic theory seems assured by the needs of geologists for a sound basis for historical interpretation of landscape, of environmentalists and planner for a sound basis for predicting man’s effects on the landscapes and of the science itself for a means of maintaining communi­ cation between its perspective, genetic-historical and process-oriented linesof inquiry’ (Higgins, 1975). A c c o r d in g to C.G. H iggin s (1975) a geomorphic theory must seek the solution of the following three lines of inquiry related to landforms and landscapes— (i) How the landforms can best be described ? (ii) How these have been formed and how these have changed through time ? (iii) Which processes have formed them and how these processes operate ? It means a sound and forceful geomorphic theory must be competent enough to decribe the landforms, to explain the mode of formation and historical evolution o f landforms and to identify and reveal the mode of operation of geomorphic proc­ esses. According to C.G. Higgins an ideal geomorphic theory must include the following properties (i) Simple and easily understandable terms should be used to describe the landforms.

changes. 3.3 GEOM ORPHIC T H E O R IE S : H ISTO RICA L P ER SP EC T IV E

Though a well organized and general theory of landscape development was propounded by W .M. Davis in 1889 (com plete cycle o f river life) and 1899 (geographical cycle) but a few theories and concepts related to genesis, evolution and decay of geomorphic features appeared before Davis e.g. concept o f catastrophism and James Hutton s con ­ cept of uniform itarianism . In fact, the formulation of real geomorphic theory began with G.K. Gilbert though he did not admit himself to be called as a theorist rather he preferred to be an ‘investigator* and postulated a set of principles based on broad generalization regarding the genesis and develop­ ment of landforms in different parts o f the U.S.A. e.g. law of uniform slope, law o f structure, law of divides or law of increasing acclivity, law of ten­ dency of equality of action, law of interdependence of parts etc. The first real and general geomorphic theory was postulated by W.M. Davis in the form of ‘geo­ graphical cycle’ in 1899. In the beginning Davis formulated his model of geographical cycle for the explanation of landscape development in humid temperate regions of the world but later on he ap­ plied hiscyclic model fortheexplanation of landforms in arid regions (arid cycle o f erosion, 1903, 1905, 1930), glaciated regions (glacial cycle o f erosion, 1900, 1906), coastal regions (1912, also by D.W. Johnson in 1919)etc. D avis’ cyclic model became so popular that it was applied to explain nearly all of the landscapes produced by different geomorphic proc­ esses by geomorphologists all over the world even in Germany where his model was severely criticised and most of the geomorphologists pleaded for out­ right rejection of Davisian model. Karst cycle of erosion by Beede (1911) and Cvijic (1948) and periglacial cycle of erosion by L.C. Peltier (1950, a German geomorphologist) etc. are such examples of application of Davisian cyclic model. It may be pointed out that universal applica­ tion of Davisian model (e.g. fluvial cycle o f erosion, marine cycle of erosion, karst cyclc o f erosion, arid cycle oferosion, glacial cycle of erosion and periglacial

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(ii) Theory should be based on contemporary geological and geomorphic ideologies and thoughts.

(iii) Theory should present bases for histori­ cal interpretation and future prediction for landform

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tutes to fill the conceptual vacuum created by the rejection o f Davisian model o f cyclic evolution of landforms.

cycle o f erosion) w eakened the theory to such an extent that not only the model was severely criticised and modifications were suggested but siren was raised for the total rejection o f the model. Subse­ q u e n tly , W . P e n c k p o s tu la te d his m o d e l o f ‘geom orphic sy stem ’ or ‘m orphological an aly­ sis’— ‘m orp h ologisch c a n a ly se’ in 1924 (posthu­ mous publication o f his work) wherein he rejected D avis’ evolutionary model involving sequential changes in landforms and pleaded for time-independent developm ent o f landfom is (dynamic equi­ librium model). C.H. Crickmay's ‘panplanation cy cle’ (1933) and ‘concept o f unequal a cclivity’ (1975), L.C. King's ‘pediplanation cy cle’ (1948), ‘h illslop e cy cle’ (1953), ‘river cy cle’ (1951) and ‘landscape cy cle’ (1962), J.C. Pugh's ‘savanna cycle o f erosion ’ (1966), S. A. Schumm's ‘episodic erosion m od el’ (1975) etc. came as a result of modifications in Davisian model o f geographical cycle.

3.3

B A S E S A N D T Y P E S O F G E O M O R P H IC T H E O R IES

If we look into the history o f geom orphic thoughts for the last two hundred years, it appears that the bases o f geom orphic theories have been greatly influenced by the contem porary geological, scientific and philosophical concepts and ideologies such as teleological, im m anent, historical, taxo­ nomic, functional, realist, conventionalist etc. con­ cepts and view points w hich b eca m e bases of geomorphic theories in historical perspective. R.J. Chorley (1978) has elaborated the bases o f geomorphic theories in historical perspective and has also Out­ lined the c h a ra c te ristic s o f d if fe r e n t ty p e s o f geomorphic theories. (1) T E L E O L O G IC A L TH EO R Y

The teleological base o f geom orph ic theory in the beginning o f the dev elopm en t o f geom orphic thoughts was influenced by religious orthodoxy wherein all the natural events were taken as the result of God's creation. ‘In som e senses it m ight be argued until the later part o f the eighteenth century the true object o f geom orphological study was not the landform itself but the m ind o f the A lm ighty, o f which the landform was held to be an outw ard and visible m a n i f e s t i n ’ (R. J. Chorley, 1978). T hus, it is obvious that landforms were co nsidered as G o d ’s creation. Theory o f catastrophism , w hich envisaged quick and sudden origin and evolution o f all anim ate and inanimate objects in a very short period o f time, may be cited as a typical exam ple o f teleological geom orphic theories. It m ay be m entioned that quick and widespread events o f larger m agnitude, both in temporal and spatial contexts (like valcanic erup­ tions, seismic events etc.) formed the basis o f tele­ ological geom orphic theories. Even the earth's age was calculated to be only a few thousand years. Events o f sm aller m agnitude (both in spatial and temporal context) were ignored. T he concept of sudden change and evolution also sw ep t the biolo­ gists and naturalists (e.g. Cuvier) w ho believed in abrupt evolution and destruction o f all the Jiving organisms. R.J. Chorley has aptly rem arked ( l 978) that ‘the decline o f old teleology was due to break­

Geomorphic theory was given a new turn and direction in the decades. 1930-40 and 1940-50 when Krumbein and R.E. Horton (1932 and 1945) intro­ duced quantitative techniques in the interpretation of geomorphic processes and landforms resulting therefrom. The introduction of quantification in geom orphology was further strengthened by A.N. Strahler (1950, 1952 and 1958). It may be m en­ tioned that after 1950 geomorphologists were least interested in the formulation or search of geomorphic theories as they became more interested in the study o f geomorphic processes (mode of operation) through field instrumentation and experimentation in the laboratories and interpretation of landforms result­ ing from these processes. This is the reason that L.C. King's popular work ‘Canons of Landscape Devel­ o p m e n t’ (1953) and models such as ‘landscape cy­ c le ’, ‘epigene cy cle’ and ‘pediplanation cycle’ could not draw proper attention rather went unnoticed by the geomorphologists.

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The forceful rejection o f Davisian evolution­ ary model (cyclic evolution) o f landscape develop­ ment resulted in the postulation o f ‘dynam ic eq u i­ librium th eory’ (A.N. Strahler, 1950, 1952, J.T. Hack, 1960, 1965, 1975, R.J. Chorley, 1962). The ‘geom orphic threshold th eory’, ‘tectono-landform th eory’ (M. Morisawa, 1975) and ‘episodic erosion th eory’ (S.A. Schum m , 1975) appeared as substi­

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THEORIES OF LANDFORM DEVELOPMENT

singular past events and description o f landforms in evolutionary manner. The main goal of geomorphic theories is retrodiction or reconstruction o f past events and not prediction of future events and changes in landforms and processes. Thus, historical theories are essentially based on the ‘law o f evolution’ or ‘law of historical succession’. Models o f cycle of erosion, denudation chronology and tectonic theory fall under the category of historical theories. Scien­ tifically speaking, these theories are not considered as scientific theories because these are based on singular events whereas scientific laws are not based on individual events rather these are based on a host of events and their recurrence whereas history is based on unique events and non-repeatable proc­ esses. Davis' ‘geographical cycle’ is considered to be the first successful attempt for the formulation o f theoretical model in geomorphology. This model aimed at the genetic classification and description o f landforms on the basis of regional spatial and geo­ logical temporal scales. The model o f denudation chronology was based on the ‘concept o f historical succession9. Though both the models (cycles of erosion in the USA and denudation chronology in U.K.) were initially framed separately but later on they merged together. The model of denudation chronology aims at the reconstruction of successive stages of the earth's history. Though the main goal of study is landforms but in reality it remained to be the study of geological history of a given region. It is argued that Davisian model of geographical cycle begins on ttte basis of initial conclusion drawn from the study of maps of the region concerned and then attempts to validate the initial conclusion on the basis of logical arguments and ‘carefully selected field observation’ which may justify the initial con­ clusion.

down in confidence regarding the magnitude and frequency of events which it presupposed. It was natural that it should be replaced by a causal basis of theory founded upon events of smaller magnitude both in space and time.’ (2) IMMANENT THEORY

The significance of events of smaller magni­ tude in both space and time, inherent features of endogenetic and cxogenetic processes, interpreta­ tion of landform characteristics on the basis of their inherent features and causal basis formed the bases of immanent theories which became dominant dur­ ing eighteenth and nineteenth centuries as a conse­ quence of rejection of teleological theories. Theory of uniformitarianism of James Hutton and John Playfair is a typical example of immanent geomorphic theories. They believed that spatial patterns of ero­ sion and deposition were auto-correlated. Thus, sci­ entists began to conceptualize inherent relationships between erosion and deposition, upliftment and sub­ sidence, form and process. The further manifestion of immanent theories in the nineteenth century was the development of ideas regarding relationship be­ tween landform and geology and between rocks and relief. J.P. Lesley, W. Smith and J.W. Powell studied the relationships between geology and landforms in much detail and postulated that there was clearcut expression of structure in landforms. It may be pointed out that intimate relationship between lithology and structure and landforms was so deeply conceived that it was not needed to study the causal relationships between rocks and reliefs ‘in terms of detailed studies of the manner by which certain differences in rock types support the recurring dif­ ferences observed to exist in terrain’ (R.J. Chorley, 1978). At a later date detailed studies of lithology and structure at smaller spatial scale revealed re­ markable variations in geological structure and thus immanent theory was further modified and strength­ ened. The micro-level studies and results coming therefrom convinced the en vestigators that very close relationship between rocks and relief was possible only at a larger spatial scale and no profound rela­ tionship between these variables couid be possible at smaller spatial scale.

Though tectonic theory of W. Penck is more or less theoretically similar to denudation chronol­ ogy but it could not acclaim as much popularity as was in the case of the latter because of ‘language, political and personal considerations on the one hand, and less technical assumptions on the other’ (R.J. Chorley, 1978).

(3) HISTORICAL THEORY

The historical theories started losing their ground and popularity after 1950 because these involved very long temporal (geological time scale

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The base of historical geomorphic theories has been the historical succession of individual or

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involving hundreds o f m illions o f years) and very large spatial scales. ‘They (historical theories) broke down because their time scales were so large and unsignposted that they becam e the playground for unbridled and untestable speculation. T he field b e ­ cam e dominated by the spinners o f ingenious his­ torical sagas, following themes that were traditional both in development and outcom e’ (R.J. Chorley, 1978). (4) TAXONOMIC THEORY

The availability of huge dataset regarding landforms after 1890 necessitated the classification of these data and landform assemblages resulting in the g row th o f reg io n al ta x o n o m ic s tu d ie s in g e o m o r p h o l o g y . L ik e h u m a n g e o g r a p h y , geomorphology was also armed with dualism wherein ‘the theoretical binality of taxonomy has caused it to assume the gloss of more challenging theory and thus in geomorphology we find historical/cyclic, fu n ctional/clim atic and in teractive/ecological developments of regional taxonomy, not to mention the social/utilitarian ones upon which present land classifications rest’ (R.J. Chorley, 1978). The base of taxonomic theories was provided by two major geomorphic concepts of clim atic geom orphology and m orphological geom orphology which devel­ oped in the beginning of the 20th century mainly in Germany and France. Considering the paramount influence of climatic parameters mainly humidity (precipitation) and temperature on geomorphological processes and landforms resulting therefrom the concept of m orphogenetic/m orphoclim atic region was developed and the division o f the globe into morphogenetic regions (by J. Budel, 1948, L.C. Peltier, 1950, W.F. Tanner, 1961, D.R. Stoddart, 1969, L. Wilson, 1969, J. Tricart and A. Cailleux, 1972 etc.) became the major manifestion o f taxo­ nomic theory. (5) FUNCTIONAL THEORY

(6) R E A L IS T T H E O R Y

Realist theory, in fact, is the extended and modified form o f functional theory. T he basis of realist theory is the study o f the structure (geomaterials) of which the landform s have been form ed and the

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The main basis o f functional theories is func­ tional relationships between forms (landforms) and processes i.e. cause and effect relationship. The major methodological shift in geomorphology after Second World W ar was characterized by the appear­ a n ce o f ‘n ew g e o m o r p h o lo g y ’ , ‘ s c ie n t if ic geom orphology,’ and ‘qu antitative geom orphology’ as a consequence o f application o f statistical

and m athem atical m e th o d s to the study o f landform s and processes. T h e p rim ary goal o f the em erg e n c e o f functional theory was to relate m o rp h o lo g ic a l forms to their controlling factors. It m a y be m e n tio n e d that a few geo m o rp h o lo g ists (e.g. G .K . G ilb e rt) used functional basis for the in terp reta tio n o f landform s and processes and their in terrela tio n sh ip s even b e ­ fo re th e f o r m a l e m e r g e n c e o f q u a n t i t a t i v e geom orphology i.e. before the classical w o rk o f R.E. Horton in 1945 w ho e m p h a siz e d the stu d y o f rela­ tionship betw een erosional la n d fo rm s and gross hydrological transfers and the d etaile d study of erosional processes but he could not s u ccee d in developing ‘a genetic m odel for the d e v e lo p m e n t of large-scale drainage n e tw o r k ’. T h e e m e rg e n c e of ‘classic f u n c tio n a l s c ie n c e ’ in the d e c a d e 1950-60 augm ented the study o f m e so -s c a le la n d fo rm s which were taken as the function o f g e o m o rp h ic processes. Further, the relationship betw een fo rm s and proc­ esses was: substantiated w ith the h elp o f statistical correlation techniques. T h e study o f fu n ctio n al rela­ tionship betw een the fo rm s (la n d fo rm s) o f m edium tosmall spatial scale involving rapid tem poral changes and geom orphic p rocesses and o th e r la n d fo rm c o n ­ trolling factors becam e the focal th e m e o f functional theory but the required in fo rm atio n o f rap id te m p o ­ ral changc to validate functional re la tio n sh ip s w'as not forthcoming. T h u s, the fun ction al theory d e­ pended on the co m p cten c e o f statistical and m ath­ ematical m ethods. T h e functional theory faces a form idable problem o f re la tin g the present-d ay landforms to the present processes. It m ay be m en­ tioned that most o f the la n d fo rm s o f the earth's surface are considered to be relict an d the landform assem blages are e x am p les.o f ‘p a lim p se st top ogra­ phy . The real functional rela tio n sh ip betw een forms and processes m ay be estab lish ed o nly w hen the rate of changes o f form s and the rate o f operation of processes is properly u nd ersto od. T h is necessitates m easurem ent o f the rate o f o p eratio n o f processes in the field so that ord ered inform ation m ay be avail­ able but the absence o f su ch d ata b ecam e major im pedim ent in the validation o f functional theory.

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physical and chemical processes which are respon­ sible for the developm ent and sustenance o f external form (o f landforms). In other words, the study of the detailed causal mechanisms and materials of landforms on one hand, and the study of their (of processes and materials) interrelationships forms the basis of real­ ist theory. Thus, realist theory emphasizes the de­ tailed and minute investigations of physical and chemical mechanisms operating in the geomaterials and geological structure within the external forms because these mechanisms are responsible for the creation, changes and maintenance of geomorphic features. The realization of the importance of the aforesaid theme blossomed in the form of the emer­ gence o f realist theory and a significant shift in m ethodology o f geomorphic investigation appeared after 1960 wherein micro-scale process study was preferred to meso-scale form study. Though the seed o f ‘p ro c e s s r e a lis m ’ was sown by G.K. Gilbert (1909, 1914), A.K. Sundborg (1956), R.E. Horton (1945), S.A. Schumm (1956) etc. but this concept blossomed with the work o f A.E. Scheidegger (1961) and G.H. Dury (1972). It may be mentioned that a few geom orphologists became so much engrossed with ‘process realism ’ that they concentrated on the study o f the mechanisms o f physical and chemical weathering processes at very micro-spatial and tem­ poral scales. Here, the geomorphologists face two major problem s viz. (i) the study of physical and chemical processes at very micro-spatial and tempo­ ral scales requires specially trained geoscientists in general and biochemists in particular and this may not be possible for the geomorphologists, and (ii) the results draw n through the investigation of processes at micro-scales may not be applicable for the gener­ alization o f mechanisms o f processes at meso-scale.

vation and accumulation of data regarding landforms and related geomorphic processes. It may be empha­ sized that neither theory can be formulated without observation and aquisition o f data nor external real­ ity may be properly understood without theory. The study o f gully erosion and management in Deoghat area of Allahabad district (U.P., India) at microspatial (about 56,000 m2 area) and temporal scales (1991— 1994) by Savindra Singh and Alok Dubey (1996) is suitable example o f such approach as they have studied the causal mechanisms o f soil erosion and gully development in man-impacted (cultivated) gully basins and have suggested management of fragile gully basins. 3.4 MAJOR GEOM ORPHIC T H EO R IES

Various theories of landform development have been formulated by different geomorphologists from time to time on the basis of contemporary thoughts prevalent in the field of science o f landforms (geomorphology). It may be pointed out that most of the geomorphic theories revolved around two basic concepts of landform development e.g., ‘sequential change of landform through time’ (i.e. progressive and irreversible change involving positive feedback mechanism) and ‘compensatory change or oscilla­ tory change’ (involving steady state and equilibrium and governed by negative feedback mechanism). The significant geomorphic theories include those of G.K. Gilbert, W.M. Davis, W. Penck, J.T. Hack, L.C. King, Marie Morisawa, S.A. Schumm etc. 1. Geomorphic Theory of G.K. Gilbert

It may be pointed out at the very outset that Grove Karl Gilbert did not propound any definite theory of landform development. He did not prefer to be called as theorist rather he opted to be an investigator. According to him theorists are seldom able to prove their theories while investigators are always in search of collecting information and data, through field observation and instrumentation, about landform characteristics and processes which shape the landforms. Tentative theories of landform devel­ opment are seldom proved on the basis o f field data. This is the reason that Gilbert devoted most of his time in the investigation of landforms and landform making processes in different parts o f the U.S.A. (e.g. Great Basin, Bonneville Lake, artesian wells o f Great Plains, Alaska, Basin Range, Henry M oun­

(7) CONVENTIONALIST TH EO RY

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Conventionalist theory is, in fact, admixture o f different geom orphic theories. The study of geom orphic processes and forms at micro-spatial and temporal scales (base o f realist theory) leading to human welfare and blending o f utilitarian consid­ erations form the base o f conventionalist theory. The philosophical base o f such theory is the concept that no appreciable distinction may be made between theory and observation because theory is constructed on the basis o f observation. In other words, the construction o f geom orphic theory precedes obser­

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Though Gilbert did not specifically claim to have framed any definite geomorphic theory but on the basis of his writings and interpretation of landforms and processes his geomorphic theory may be stated as follows— 4Landscapes remain in equilibrium condition, their history/ is rhythm ic punctuated by oscillatoty changes and their fo rm s are punctuated by frictional, rhythms arising out o f the m echanism o f driving and resisting forces. ’ According to Gilbert the identification and quantification o f fric tio n a l rhythm s (processes) and determ ination o f their ( o f processes) dynam ic com ­ petition is the m ajor geom orphic problem and the m ain task before the geom orphologists is to solve this problem . The geomorphic principles of G.K. Gilbert revolve around three major components of his pos­ tulates viz. ‘concept o f quantification’, ‘concept o f tim e’ and ‘concept o f equilibrium ’. Gilbert used scientific methods for interpre­ tation o f geom orphic processes and landforms re­ sulting therefrom wherein he gave more emphasis to ‘q u antity’ in place of ‘q u ality’ and applied the laws of thermodynamics to the analysis o f geological processes. According to first law o f thermodynam­ ics in any system o f constant mass, energy is neither created nor destroyed but total energy remains con­ stant and it can be transferred from one type to another type (the law is known as conservation of energy) while the second law o f thermodynamics

states that ‘as time passes and the energy within the system becom es m ore equally distributed the en­ tropy (measure o f order or disorder) increases until, at the state o f m axim um entropy, all parts o f the closed system have the sam e energy level’ (R.J. Chorley et. at, 1985). In other w ords, with the passage o f time a system tends to achieve m inimum energy and m axim um entropy (m ax im um disorder). Gilbert took n ature in the p resen t ten se i.e. he was more interested in the present forms and processes and their future trends (prediction) rather than in the reconstruction o f past events and forms (retrodiction). His concept o f nature was based on two fundamental concepts o f natural philosophy i.e. (i concept o f rhythm ic tim e, and (ii) co n cep t o f equilibrium . G ilb e rt's understanding o f ‘tim e ’ was quite different from geologists’ concept o f time. A ccord­ ing to him geologic time is rhythm ic. ‘A ny event (of the earth) represents a plexus o f particular rhythm. The motion o f the earth is the basic rh y th m ’ which affects climate which in turn affects and controls processes which create different suites o f landforms. It may be mentioned that motion o f the earth, which is responsible for the genesis o f seasons and cli­ mates, includes rotation and revolution o f the earth. Gilbert attempted to differentiate the traditional con­ cept of evolution (involving continual grow th or decay on the basis o f basic tenet o f progressive evolutionary change o f landform s) from non-evolutionary concept involving equilibrium model. He ciiticised and rejected the evolutionary concept of geologists involving continuous progressive change in landforms through time and advocated the con­ cept o f time-independent model o f landform devel­ opment involving dynam ic equilibrium and steady state. His concept o f eq u ilib riu m envisages that in the final form of any functional system ‘the sum of the forces acting on the final form equalled zero.* This is also known as the prin ciple o f least force* The forces in question are o f two types, i.e. driving force and resisting force. He explained his model o f equilibrium with specific examples which were based on his own field studies. First, he applied the concept o f equilibrium for the explanation o f the formation o f loccoliths resulting from vulcanicity. The forma­ tion and rise o f laccolith depends on the competence

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tain, California, Sierra Mt. etc.) but did not prostulate any com m on theory regarding the evolution and development o f landforms, rather he postulated a set o f principles regarding different geomorphic fea­ tures viz. ‘law o f uniform slo p e’, ‘law o f stru c­ tu re’, ‘law o f d ivid e’ (law o f increasing acclivity), ‘law o f tendency to eq u ality’, ‘dynam ic equilib­ riu m ’, ‘law o f interdependence o f p arts’ etc. In fact, Gilbert was ahead o f his time as he propounded such advanced concepts as ‘steady states’ ‘graded curve and profile of equilibrium,’ ‘dynamic equilib­ riu m ’ etc. in the beginning of the 20th century which became the base of the ruling theory o f landform development (e.g. dynamic equilibrium theory in­ volving time-independent development o f landforms) and became the pivot of drastic methodological shift in the post-second world war geomorphology.

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THEORIES OF LANDFORM DEVELOPMENT

of driving force (rise o f m agm a) and resisting force (overlying pressure o f superincum bent load). The formation and growth of loccoliths continue so long as the driving force o f rising m agm a is not countered by resisting force (acting downward) of equal m ag­ nitude. In other words, so long as driving force exceeds the resisting force, m agm a continues to rise upward and loccoliths register continuous growth but when the driving force is balanced by resisting force, the state of equilibrium sets in and the growth of laccoliths becomes static. Thus, the principle of least work becomes operative wherein the sum of driving and resisting forces becomes zero. Gilbert also applied this principle of least force leading to establishment of equilibrium condi­ tion in the case o f river to elucidate profile of equilibrium. The downstream flow of river water (river discharge) is guided by the force of gravity wherein the potential energy is converted into ki­ netic energy. The driving force in the case of a river (say energy o f the river system) is provided by its flow velocity while the resistance is offered by the bed-load and lithology of river valley. More pre­ cisely, the friction to flow velocity is offered by the materials of the valley. So long as the system energy say driving force (flow velocity) equals the resisting force say frictional force, the state of equilibrium is established and this condition prevails till the equilib­ rium condition is maintained and thus the principle of least force works. The long profile of a river which has attained the equilibrium state is called profile of equilibrium (i.e. equilibrium of actions) and such river (in the state o f equilibrium) is called graded river. It may be mentioned that Gilbert applied the concept of ‘grad e’ to all of the landforms and processes which he studied in the field e.g. ‘graded h each ’ in the case of Bonneville Lake, ‘graded hillslope’ in the case of Sierra mountain etc. Thus, Gilbert propounded that ‘the landscape is the result o f two competing tendencies i.e. ten­ dency towards variability ( when driving force ex­ ceeds resisting fo rce) and tendency towards uni­ fo rm ity (when driving fo rce equals resistingforce).'

2. Geomorphic Theory of Davis

The general theory of landform development of Davis is not the ‘geographical cycle’ as many of the geomorphologists believe. His theory m a y b e expressed as follows— "There are sequential changes in landforms through time (passing through youth, m ature and old stages) and these sequential changes are d i­ rected towards well defined end product-developm ent o f peneplain. ” The basic goal of Davisian model of geo­ graphical cycle and general theory of landform de­ velopment was to provide basis for a systematic description and genetic classification of landforms. The reference system of Davisian general theory of landform development is ‘that landform s change in an orderly manner as processes operate through time such that under uniform external environm en­ tal conditions an orderly sequence o f landform d e ­ velops” (R.C. Palmquist, 1975). Various models were developed on the basis of this reference system e.g. normal cycle of erosion, arid cycle of erosion, glacial cycle of erosion, marine cycle of erosion etc. Thus, ‘geographical cycle’ is one o f the several possible models based on Davis’ reference system of landform development. Davis postulated his concept o f ‘geographi­ cal cycle’ popularly known as ‘cycle o f erosion9 in 1899 to present a genetic classification and system­ atic description of landforms. His ‘geographical cycle' has been defined in the following manner. ‘Geographical cycle is a period o f time during which an uplifted landmass undergoes its transfor­ mation by the process of landsculpture ending into low featureless plain or peneplain (Davis called peneplane).’ According to Davis three factors viz. struc­ ture, process and time play important roles in the

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W illia m M o rris D av is, an A m eric an geomorphologist, was the first geomorphologist to present a general theory of landform development. Infact, his theory is the outcome of a set ol theories

and models presented by him from time to time e.g. (i) ‘com plete cycle o f river I if e \ propounded in his essay on “The Rivers and Valleys of Pennsylvania’ in 1889, (ii) ‘geographical cycle’ in 1899, (iii) ‘slope evolution’ etc. He postulated the cyclic con­ cept of progressive development o f erosional stream valleys under the concept o f ‘complete cycle of river-life’, while through ‘geographical cycle’ he described the sequential development of landforms through time.

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(5) Erosion docs not start until the upliftment is complete. In other words, upliftment and erosion do not go hand in hand, Thi* assumption of Davis bccarnc the focal point o f severe attacks by the critic* of the cyclic concept. Davis has described his model o f geographi­ cal cycle through a graph (fig. 3 . 1).

origin and development of landfonns of a particular place. These three factors are called as ‘Trio o f D avis’ and his concept is expressed as follows— ‘Landscape is a function of structure, process and tim e’ (also called as stages by the followers of Davis). Structure means lithological (rock types) and structural characteristics (folding, faulting, joints etc.) of rocks. Tim e was not only used in temporal context by Davis but it was also used as a process itself leading to an irreversible progression of change of landforms. Process means the agents of denuda­ tion including both, weathering and erosion (run­ ning water in the case of geographical cycle).

The c y c le of erosion begins with the upliftment o f landmass. There is a rapid rate o f short-period upliftment of landmass o f hom ogeneous structure. This phase o f upliftment is not included in the cyclic time as this phase is, in fact, the preparatory stage of the cycle of erosion. Fig. 3 . 1 represents the model of geographical cycle wherein UC (upper curve) and LC (lower curve) denote the hill-tops or crests of water divides (absolute relief from mean sea-level) and valley floors (lowest reliefs from mean sealevel) respectively. The horizontal line denotes time whereas vertical axis depicts altitude from sea-level, AC represents maximum absolute relief whereas BC denotes initial average relief. Initial relief is defined as difference between upper curve (summits o f wa­ ter divides) and lower curve ( valley floors) o f a landmass. In other words, relief is defined as the difference between the highest and the lowest points of a landmass. A DG line denotes ba.se level of erosion which represents sea-level. No river can erode its valley beyond base level (below sca-lcvcl Thus, base level represents the limit o f maximum vertical erosion (valley deepening) by the rivers. The upliftment of the landmass stops after p o im C (fig. 3.1) as the phase o f upliftment is complete. Now erosion starts and the whole cycle passes through the following three stages—

The basic prem ises of Davisian model of ‘geographical cycle’ included the following assump­ tions made by Davis. (1) Landforms are the evolved products of the in te r a c tio n s o f e n d o g e n e tic (diastrophic) forces originating from within the earth and the external or exogenetic forces originating from the atmosphere (denudational processes, agents of weath­ e rin g and e r o s io n - r iv e rs , w ind, groundwater, sea waves, glaciers and periglacial processes). (2) The Evolution of landform takes place in an orderly manner in such a way that a systematic sequence of landforms is de­ veloped through time in response to an environmental change. (3) Streams erode their valleys rapidly down­ ward until the graded condition is achieved. (4) There is a short-period rapid rate of up­ liftment in land mass. It may be pointed out that Davis also described slower rates o f upliftment if so desired.

(1) Y outhful S ta g e — Erosion starts after the completion of the upliftment o f the landmass.

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Fig. 1 1 : Graphical presentation of geographical cycle presented by W.M Davis.

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T h e top-surfaces or the summits of the water divides are not affected by erosion because the rivers are small and widely spaced. Small rivers and short tributaries are engaged in headward erosion due to which they extend their length. The process is called stream len gth en in g (increase in the lengths of the rivers). B ecause o f steep slope and steep channel gradient rivers actively deepen their valleys through vertical erosion aided by po th ole drilling and thus „ there is gradual increase in the depth of river valleys. This process is called valley deepening. I he valleys become deep and narrow characterized by steep valley side slopes of convex plan. The youthful stage is characterized by rapid rate of vertical erosion and valley deepening because fi) the channel gradient is very steep, (ii) steep channel gradient increases the velocity and kinetic energy of the river flow, (iii) increased channel gradient and flow velocity in­ creases the transporting capacity of the rivers, (iv) increased transporting capacity of the rivers allow them to carry big boulders of high calibre (more angular boulders) which help in valley incision (val­ ley deepening through vertical erosion) through pothole drilling. The lower curve (LC, valley floor) falls rapidly because of valley deepening but the upper curve (UC. summits of water divides or interstrcam areas) remain almost parallel to the hori­ zontal axis (AD, in fig. 3.1) because the summits or upper parts of the landmass are not affected by erosion. Thus, relative relief continues to increase till the end o f youthful stage when ultim ate m axi­ m u m relief (EF, in fig. 3.1) is attained. In nutshell, the youthful stage is characterized by the following characteristic features. (i) Absolute height remains constant (CF is parallel to the horizontal axis) because of insignificant lateral erosion. (ii) Upper curve (UC) representing summits of water divides is not affected by ero­ sion. (iii) Lower curve (LC) falls rapidly because of rapid rate of valley deepening through vertical erosion. (iv) Relief (relative) continues to increase. (v)' Valleys are of V shape characterized by convex valley side slopes. (vi) Overall valley form is gorge or canyon.

ally diminish with march of time and these practically disappear by the end of late youth. The main river is graded. (2) M a tu r e Stage— The early mature stage is heralded by marked lateral erosion and well inte­ grated drainage network. The graded conditions spread over larger area and most of the tributaries are graded to base level of erosion. Vertical erosion or valley deepening is remarkably reduced. The sum ­ mits of water divides arc also eroded and hence there is marked fall in upper curve (UC) i.e. there is marked lowering of absolute relief. Thus, absolute relief and relative relief, both decrease. The lateral erosion leads to valley widening which transforms the V shaped valleys o f the youthful stage into wide valleys with uniform or rectilinear valley sides. The marked reduction in valley deepening (vertical ero­ sion or valley incision) is because o f substantial decrease in channel gradients, flow velocity and transporting capacity of the rivers. (3) Old Stage— Old stage is characterized by almost total absence of valley incision but lateral erosion and valley widening is still active process. Water divides arc more rapidly eroded. In fact, water div id es are reduced in d im e n s io n by both, downwasting and backwasting. Thus, upper curve falls more rapidly, meaning thereby there is rapid rate of decrease in absolute height. Relative or avail­ able relief also decreases sharply because of active lateral erosion but no vertical erosion. Near absence of valley deepening is due to extremely low channel gradient and remarkably reduced kinetic energy and maximum entropy. The valleys become almost flat with concave valley side slopes. The entire land­ scape is dominated by graded valley-sides and di­ vide crests, broad, open and gently sloping valleys having extensive flood plains, well developed me­ anders, residual convexo-concave m onadno ck san d extensive undulating plain of extremelyMow relief. Thus, the entire landscape is transformed into peneplain. As revealed by Fig. 3.1 the duration of old stage is many times as long as youth and maturity combined together. Evaluation of the Davisian Model of Landform Development

Davisian model of landform development involving progressive changes in landforms through time and his concept o f ‘geographical cycle’ re­

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(vii) Long profiles of the rivers are character­ ized by rapids and water falls which gradu­

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(6) His model is capable o f both predictions and historical interpretation o f landform evolution (retrodictions).

ceived world wide recognition and the geomorphologists readily applied his model in their geomorphological investigations. The academic intoxica­ tion of Davis’ model o f cycle of erosion continued from its inception in 1899 to 1950 when the model had to face serious challenges though hi';, model was being criticised from the very beginning of its pos­ tulation. S. Judson (1975) while commenting on Davis' geographical cycle remarked, “His grasp of time, space and change; his com m and of detail; and his ability to order his information and frame his arguments remind us again that we arc in the pres­ ence o f a giant” . C. G. H iggins (1975) admitted that “ Davis system came to dominate both teaching and research in the descriptive and genetic-historical aspects o f geomorphology. Its continued validity is attested in part by continuing objections to it by recent critics such as R.C. Flemal (1971) and C.R. Twidale (1975), that such an obviously flawed doc­ trine could have enjoyed such prolonged popularity among large segment of the geomorphic community suggests that there must be compelling reasons for its appeal” (Charles G. Higgins. 1975).

NEGATIVE ASPECTS OF DAVIS' MODEL

(1) Davis' concept o f upliftment is not acceptable. He has described rapid rate o f upliftment of short duration but as evidenced by plate tec­ tonics upliftment is exceedingly a show and long continued process. (2) D av is' c o n c e p t o f r e la tio n s h ip b etw ee n upliftment and erosion is erroneous. A ccord­ ing to him no erosion can start unless upliftment is complete. Can erosion wait for the com ple­ tion o f upliftment ? It is a natural process that as the land rises, erosion begins. Davis has answered this question. He adm itted that he deliberately excluded erosion from the phase of upliftment because o f tw o reasons- (i) to make the model sim p le,a n d (ii) erosion is insignificant during the phase o f upliftment. (3) The Davisian model requires a long period of crustal stability for the com pletion o f cycle of erosion but such eventless long period is tectonically not possible as is evidenced by plate tectonics according to w hich plates are always in motion and the crust is very often affected by tectonic events. Davis has also offered explanation to this objection. Accord­ ing to him, if crustal stability for desired period is not possible, the cycle o f erosion is inter­ rupted and fresh cycle o f erosion may start.

POSITIVE A SP EC T S O F DAVIS' MODEL

(1) Davis' model of geographical cycle is highly simple and applicable. (2) He presented his model in a very lucid, com ­ pelling and disarming style using very simple but expressive language. Commenting on the language of Davis used in his model Bryan remarked, “Davis' rhetorical style is just ad­ mired and several generations of readers be­ came slightly bemused by long, though mild intoxication of the limpid prose of Davis' re­ markable essay.”

(4) Walther Penck objected to over em phasis of time in Davis' model. In fact, Davisian model envisages ‘tim e-d ep en d en tseries’ o f landform development whereas Penck pleaded for ‘timeindependent serie s’ o f landforms. According to Penck landiorm s do not experience pro­ gressive and sequential changes through time. He, thus, pleaded for deletion o f ‘tim e’ (stage) from Davis' ‘trio ’ of ‘stru ctu re, process and tim e’. According to Penck “geom orphic forms are expressions o f the phase and rate of upliftment in relation to the rate o f degrada­ tion” (Von Engcln, 1942).

(3) Davis based his model on detailed and careful field observations. (4) Davis' model came as a general theory of landform development after a long gap after Hutton's ‘cyclic nature of the earth history.’ (5) This model synthesized the current geological thoughts. In other words, Davis incorporated the concept of ‘base level’ and genetic classi­ fication o f river valleys, the concept of ‘graded stream s’ of G.K. Gilbert and French engi­ neers’ conccpt o f ‘profile of equilibrium’ in his model.

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(5) A.N. Strahler, J.T. H ack and R.J. Chorley and several others have rejected the Davisian con­ ccpt o f ‘historical ev o lu tio n ’ o f landforms. They have forwarded the dyn am ic equilib­ rium theory for the explanation o f landform

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THEORIES OF LANDFORM DEVELOPMENT

(6) Though Davis has attempted to include struc­ ture, process and time in his model but he overemphasized time. His interpretation of geomorphic processes was entirely based on empirical observation rather than on field in­ strumentation and measurement. Though Davis decribed the structural control on landforms but he failed to build any model of lithological adjustment of landforms. (7) Davis attempted to explain the concept of grade in terms of ability to work (erosion and deposition) and the work that needs to be done. It is evident from the essays of W.M. Davis that in the initial stage o f landform develop­ ment (in terms of cycle o f erosion) the avail­ able energy is more than needed to transport the eroded sediment. Thus, the river spends additional available energy to erode its valley. As the river valley is deepened the sediment supply (the work needed to be done increases) for transportation increases but available en­ ergy decreases. Ultimately, required energy and available energy become equal and a con­ dition ofequilibrium isattained. Butthe critics maintain that the concept o f balance between available energy and the work to be done has not been properly explained by Davis. It is apparent from the writings of Davis that the work to be d o n e’ refers to transportation of debris by the rivers and energy is spent in two ways e.g. in transportation o f debris and in valley deepening. Such division of expendi­ ture of energy is not justified. Thus, there are two shortcomings in this concept viz. (i) ero­ sion in itself depends on the mobility of sediments and erosion is never effective in the abscnce o f sediments, (ii) such condition when the whole energy is spent in transporting the sediments and erosion becomes totally absent is practically not possible.

It may be concluded in the words of C.G. Higgins (1975) that ‘if the desire for a cyclic, time-

dependent model stems from an unacknowledged fundamental postulate that the history of the earth is itself cyclic, then no non-cyclic theory o f landscape development can win general acceptance until this postulate is unearthed, examined and possibly re­ jected*.

3. Geomorphic Model of Penck W. Penck is perhaps the most misunderstood geomorphologist of the world. It is not yet sure whether he used the word ‘cycle’ or not in his model of landform development. Penck's views could not be known in true sense and could not be interpreted in right perspective because of (i) his incomplete work due to his untimely death, (ii) his obscure composition in difficult German language, (iii) illdefined terminology, (i v) misleading review by W.M. Davis and (v) some contradictory ideas. His work was posthumously published in the form o f ‘Die morphologische Analyse’ in 1924. It may be pointed out that German scientist Walther Penck pleaded for the rejection o f Davisian model of geographical cycle based on time-depend­ ent series of landform development and presented his own model o f ‘m orphological sy stem ’ or ‘m o r ­ phological analysis’ for the explanation o f land­ scape development. The m ain goal of Penck's model of morphological system was to find out the mode o f development and causes o f crustal movement on the basis of exogenetic processes and m orphological characteristics. The reference system o f Penck's model is that the characteristics of landforms o f a given region are related to the tectonic activity of that region. The landforms, thus, reflect the ratio between the intensity of endogenetic processes (i.e. rate o f upliftment) and the magnitude o f displace­ ment of materials by exogenetic processes (the rate of erosion and removal of materials).

According to Penck landform development should be interpreted by means of ratios between diastrophic processes (endogenetic, or rate of uplift) and erosional processes (exogenetic, or rate of ver­ tical incision). Following arc the basic premises of Penckian model of landscape development— (1) The morphological characteristics of any region of the earth's surface is the result of competi­ tion between crustal movement and denudational processes.

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development. It may be pointed out that noncyclic concept of ‘dynamic equilibrium* as valid substitute o f Davis' cyclic concept of landform development and other so-called ‘open sy stem ’ and non-cyclic models of landform development could not arouse any e n th u sia s m am ong the m o d e rn geomorphologists.

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GEOMORPHOLOGY

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(2) Landscape development is time-independ­ ent. (3) Tectonic m ovem ents can be explained and their causal factors may be ascertained on the basis of morphological characteristics. (4) The shape of the hillslope depends on relative rates of valley incision by rivers and re­ moval o f debris from hillslope. (5) There are three crustal states e.g. (i) state o f crustal stability when there is no active displace­ ment, (ii) state o f initial domed uplift in a limited area followed by widespread uplift and (iii) state of extensive crustal upliftment. (6) There are three states of adjustment be­ tween crustal m ovem ent and valley deepening viz. (i) if crustal upliftment remains constant for longer period of time, the vertical erosion by the river is such that there is balance between the rate of upliftment and erosion, (ii) if the rate of uplift exceeds the rate of valley deepening, then the channel gradient con­ tinues to increase till the rate of valley deepening matches with the rate of upliftment and the state of equilibrium is attained when both become equal, and (iii) if the rate of valley deepening exceeds the rate of crustal upliftment, then the channel gradient is lowered to such an extent that the rates of upliftment and erosion become equal and the state o f equilib­ rium is attained. (7) Upliftment and erosion are always co­ existent. Penck is supposed to have deliberately avoided the use of stage concept in his model of landscape development either to undermine the cy­ clic concept of W.M. Davis or to present a new model. According to O.D. Von Engeln (1960) “Penck found escape from the concept of cyclic change marked by the stages youth, maturity and old age’1. In the place of ‘stage’ he used the term entwickelung meaning thereby ‘development’. Thus, in the place o f youth, mature and old stages he used the terms aufsteigende entwickelung (waxing or accelerated rate o f development), gleichformige entwickelung (uniform rate of development) and absteigende entwickelung (waning or decelerating rate of devel­ opment).

Contrary to the concept o f W .M . Davis, ‘that landscape is a function o f structure, process and time (stage)’, Walther Penck postulated that, ‘geomorphic forms are an expression o f the phase and rate of uplift in relation to the rate o f degradation. It is assumed that interaction between the two factors, uplift and degradation, is continuous. T he landforms observed at any given site give expression to the relation between the two factors (uplift and degrada­ tion) that has been or is in effect, and not to a stage in a progressive sequence” (O.D. Von Engeln, 1960, pp. 261-62). The landscape developm ent (we may say the cycle of erosion) begins with the upliftm ent of primarumpf (initial landscape with low height and relief) representing an initial featureless broad land surface. In other w ords, p rim a r u m p f is initial geomorphic unit for the beginning o f the develop­ ment of all sorts o f landforms. Penck is supposed to have assu m ed v ary in g rates o f u p liftm e n t of prim arum pf for the developm ent o f landforms. In the beginning the uplift is characterized by exceed­ ingly slow upheaval of long duration and thereafter the rate o f uplift is accelerated and ultimately it stops after passing through the intermediate phases of uniform and declerating rates o f upheaval. In fact, ‘the most tectonic m ovem ents began and ended slowly, and that the com m on pattern o f such move­ ments involved a slow initial uplift, an accelerated uplift, a deceleration in uplift and, finally, quies­ cence’ (R.J. Chorley, et al., 1985, p. 28). The initial uplift begins with regional updoming and the landform development passes through the following three phases.

(1) Aufsteigende Entwickelung means the phase o f waxing (accelerating) rate o f landform development. Initially, the land surface rises slowly but after some time the rate o f upliftment is acceler­

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Penck used the term prim aru m p f to repre­ sent the characteristic lanscape before upliftment. Primarumpf is, in fact, initial surface or primary

peneplain representing either new ly em erged sur­ face from below sea level or a ‘fastenbene’ or ‘pen ep lain ’ type of land surface converted into fea­ tureless landmass by uplift. A ccording to Von Engeln (1942) the “prim aru m p f is a prim ary peneplain, one which could, in either case, exhibit truncated beds and structures, and yet need n ever have had a greater altitude or a higher re lie f’. In other words, primarumpf is the initial landscape with ev iden ces o f erosion but with low altitude.

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THEORIES OF LANDFORM DEVELOPMENT

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ated. Because o f upliftment and consequent increase in channel gradient, flow velocity and kinetic energy and of course increase in discharge (not due to uplift) the rivers continue to degrade their valleys with accelerated rate of downcutting (valley deepening or incision) but the rate o f upliftment far exceeds the rate of valley deepening (say degradation of uplifted landmass). Continuous active downcutting and val­ ley deepening results in the formation of deep and narrow V -shaped valleys. As the rate of uplift (aufsteigende entwickelung) continues to increase the V-shaped valleys are further deepened and sharp­ ened. Since valley deepening does not keep pace with the upliftment of landmass, the absolute height continues to increase. In other words, the altitudes of divide summits as well as the altitudes of valley bottoms continue to increase as the rate of upliftment far exceeds the rate o f vertical erosion (fig. 3.2 ) but the relative or available reliefs continue to increase due to everincreasing rate o f vertical erosion or valley deepening. Thus, both maximum altitude (absolute height from sea level) and maximum relief

o f U p lift

(relative) increase (1 in fig. 3.2). The slopes o f valley sides are convex in plan. The valley side slopes are continuously steep­ ened due to continued valley deepening. The radius of convexity o f .slopes is reduced with passage o f time due to parallel retreat o f the steeper slope segments. With the passage of time and more accel­ erated uplift and degradation the primary peneplain or say primarumpf is surrounded by a series of benches called as piedm ont treppen. Each o f such benches develops as a piedmont flat, called in G er­ man as piedm ontflache on the slowly rising m ar­ gins o f the dome. (2) G leichform ige E n tw ick elu n g means uniform development of landforms. This phase may be divided into 3 subphases on the basis o f rate o f uplift and degradation (2 in fig. 3.2). P hase (a) is characterized by still accelerated rate o f uplift. A b ­ solute height still increases because the rate o f ero­ sion is still less than the rate o f upliftment. Altitudes of both summits of water divides and valley floors

Curve No F u rth er Uplift

i Roschung o r G ravity Slope Ilaldcnhag o r W ash Slope

Case Level

A ltitude

Insell>erg

Fig. 3.2 : Graphic presentation o f Penck's model o f landform development.

due to matching o f upliftment by the lowering of divide Summit due to denudation. It means that upliftment still continues. Relative relief also re­ mains constant because the rate o f erosion o f divide summits matches with the rate o f valley deepening while both are uplifted uniformly. The slopes of valley sides are still straight as in phase 2 a because of parallel retreat. This phase is, thus, characterized by constant absolute and relative reliefs and thus uniform developm ent o f landform s. P h a s e (c)Upliftment of the land stops com pletely. A bsolute

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continue to increase but at relatively lower rate than in the phase o f aufsteigende entwickelung. M axi­ mum altitude (absolute relief) is attained but relative relief remains constant because the rate of valley deepening equals the rate of lowering of divide summits. The valley sides are characterized by straight slopes (2a in fig. 3.2). This phase is called the phase o f uniform development probably because of uni­ form rate of valley deepening and lowering of divide summits. P h a s e (b)-Altitude (absolute relief) nei­ ther increases nor decreases i.e. remains constant

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GEOMORPHOLOGY

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EVALUATION OF PENCK'S MODEL

reliefs or altitudes o f summit divides start decreas­ ing because of absence o f upliftment but continued erosion of summits o f divides. Relative reliefs also remain constant bccausc the rate o f the lowering of divide summits equals the rate o f valley deepening. Thus, this subphasc is also characterized by uniform development o f landscape.

The Penck’s model o f landscape develop­ ment. as pointed out in the beginning, could not be correctly interpreted because o f its publication in obscure G erm an language and wrong interpretation of his ideas by English translators. P enck’s morpho­ logical system was severely criticised in the USA in the same way as the ‘geographical c y c le ’ was criti­ (3) A b steig en d e E n tw ic k e lu n g means w an­ cised in G erm any. P en ck ’s concepts o f parallel re­ ing development o f landscape during which the treat o f slope and continued crustal movements landscape is progressively dominated by the proc­ became the most sensitive points o f attacks by Ameri­ ess of lateral erosion and consequent valley w iden­ can geologists. It m ay be pointed out that earlier ing and marked decrease in the rate o f valley deep­ translation o f Penck s w ork in E nglish revealed that ening through vertical downcutting. This phase is Penck believed in parallel retreat o f slopes but sub­ marked by progressive decline o f landforms. A bso­ sequent English translations sho w ed that Penck be­ lute relief (altitude from sea level) decreases re­ lieved in slope replacem ent w herein each upper markably because o f total absence of upliftment but slope unit o f hillslope and valley sides w as consid­ continued downwasting o f divide summits. Relative ered to he replaced by low er slope unit o f gentler relief also decreases because the divide summits are slope. It may be, thus, forw arded that m ost o f the continuously eroded down and lowered in height criticisms o f Penck's m orphological system came while downcutting of valley floor decreases remark­ out o f the faulty interpretations o f his views. Some ably due to decrease in channel gradient and kinetic o f the American critics stooped do w n to such an energy. Parallel retreat o f valley side slopes still extent that they rem arked that ‘his p eculiar notions continues. Nov/ the valley side slope consists o f two owed to his incomplete recovery from a head wound segments. The uppermost segment maintains its suffered in World W a r I ’ (quoted by C.G. Higgins, steep angle inspite of continuous lowering of ridge 1975). His concept o f long con tin ued upliftm ent and crests. T his slope is called g r a v it y slo p e or tectonic speculations could not find any support but b o sch un gen . The lower segment o f the valley sides his concepts of slope d ev elo p m en t and weathering is called wash slope or h a ld e n h a n g . Haldenhang, processes are definitely o f m uch geom orphological composed o f talus materials o f lower inclination, is significance. formed at the base o f the valley sides due to rapid 4. Geomorphic Model of L.C. King parallel retreat o f gravity slope or boschungen and The geom orphic theory or very com m only consequent elimination of much of the convex wax­ known as geom orphic system o f L.C. King co m ­ ing slopes. Divide summits are continuously low­ prises a set of cyclic m odels such as the lan d scap e ered by the intersection o f the retreating boschungen cycle, the epigene cycle, the p ed ip la n a tio n cycle, o f adjoining valleys. Thus, the intersection of hillslope cycle etc. essentially based on the land­ boschungen and haldenhang produces sharp knick scape characteristics o f arid, sem i-arid and savanna (break in slope). Haldenhang or wash slope contin­ regions of South Africa as studied by him. ues to expand at the cost of upper gravity slopes. In the advanced stage o f the phase o f absteigende entwickelung the gravity slopes or boschungen are reduced to steep-sided conical residuals called inselbergs (fig. 3.2). Eventually, inselbergs arc also consum ed and the whole landscape is dominated by a series o f concavc wash slopes or haldenhang. Such extensive surface produced at the end o f absteigende entwickelung is called ‘endrumpf% which may be considered equivalent to D avis’ peneplain.

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The reference system o f K ing's m odel is that 'there is uniform d evelo p m en t o f la n d fo rm s in varying environm ental co n d itio n s a n d th ere is insignifi­ cant influence o f clim a tic ch a n g es in the develop­ m ent o fflu v ia lly o rig in a ted la n d fo rm s. M a jo r land­ scapes in a ll the co n tinents have been evo lved by rhythm ic g lo b a l tectonic events. There is continuous m igration (retreat) o f h illslope a n d such retreat is alw ays in the fo r m o f p a ra lle l retreat. ‘

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THEORIES OF LANDFORM DEVELOPMENT

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Each cycle begins with rapid rate o f upliftment followed by long period o f crustal (tectonic) stabil­ ity. Thus, King's concept o f upliftment and crustal stability is similar to the concept o f Davis. It may be pointed out that cycle of pediplanation begins with the upliftment o f previously form ed pediplains and not of any structural unit. T he pediplanation cycle passes through the stages o f youth, mature and old as in the Davisian cycle o f erosion.

As stated above King formulated his model (theory) on the basis o f information of landform characteristics derived through his personal studies of landscape scenery of South Africa having arid, semi-arid and savanna environment and then as­ serted that his model may be practicable in other parts of the globe. According to L.C. King an ideal hillslope profile consists of all the four elements of slope viz. summit, scarp, debris slope and pediments and such hillslopes develop in all regions and in all climates where there is sufficient relief and fluvial process is dominant denudational agent.

The stage o f youth is characterized by initia­ tion of rapid rate o f active dow n cutting of valleys by the rivers consequent upon upliftment. Thus, the long profile of the rivers is punctuated by a series o f nick points which move upstream. The valleys are so deepened that they assume the form o f gorges and canyons. With the march o f time active dow n cutting of valleys is slowed down and as a consequence o f which the valley side slopes are characterized by constant slope angles. The form o f valley side slope is controlled by physical processes operating on the slopes and lithologica! characteristics. ‘Eventually, downcutting will become less active, and small pediments will begin to appear in the valley bottoms. These will become more extended as interfluve and upland areas are consumed by scarp retreat’ (R.J. Small, 1970). By the late youth most o f the interfluves are narrowed down due to scarp retreat and are converted to steep sided hills which are called as inselbergs. The rounded inselbergs are called as bornhardts and castle koppies.

King, through his extensive field observa­ tion, identified ‘remarkable surfaces of planation, surmounted by isolated hills (inselbergs) and piles o f rock boulders (castle koppies), that are such an obvious feature o f the landscape in arid, semi-arid and savanna parts o f A frica’ (R.J. Small, 1970). Thus, King propounded an entirely new ‘cyclic m odel o f p ed ip lan ation ’ (known as pediplanation cycle) in 1948 to account for the unique landscapes as referred to above as he was convinced that Davisian model o f arid cycle o f erosion was not competent to explain these landscapes. It may be mentioned that King claimed to have propounded his geomorphic system as entirely different from Davisian cyclic model and based on some assumptions of Penckian model but in fact King's model is nearer to Davisian model than the Penckian model. After extensive study of South African land­ scape scenery King was convinced that the African landscape consisted of three basic elements e.g. (i) rock p e d im e n ts flanking river valleys and having concave slope varying in angle from 1.5° to 7° cut into solid rocks, and (ii) scarps having steep slopes bounding the uplands and varying in angle from 15° to 30° and experiencing parallel retreat due to backwasting by weathering and rainwash. (iii) The third element com prises steep sided residual hills known as inselbergs (bornhardts) which vary in size and shape. The size o f inselbergs is determined by the magnitude of erosion, less eroded inselbergs are large in size (e.g. mesa) while intensely eroded ones are small in size (e.g. buttes). The shape of these inselbergs depends on the nature of underlying struc­

The beginning of m ature stage is heralded by the absence of active valley deepening and initiation of lateral erosion. There is backw ard retreat o f valley side slope because of valley widening and hence valley sides are distanced from the channel but there is no significant change in the angle o f valley side slope. Extensive pediments varying in slope angles from 5° to 10° are formed at the base o f valley side slope. The pediments are o f concave slope plan. Continuous erosion and w eathering results in pro­ gressive decrease in the num ber 9f inselbergs. M any o f the inselbergs are so greatly w eathered that they are converted to castle koppies. G radually, m any o f the inselbergs and castle koppies finally disappear while there is continuous extension o f pedim ents consequent upon gradual parallel retreat o f scarps (upper segm ent o f valley side slope). Eventually, many pediments coalesce to form extensive flat

ture.

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The cycle o f pediplanation is performed by twin processes viz. scarp retreat and pedimentation.

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GEOMORPHOLOGY M

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(pediplanation cy cle) both the m odels are compatible to som e extent as boch envisage cyclic develop­ ment o f landscape w herein cy cle o f erosion begun with rapid rate o f upliftment o f short period followed by long period o f crustal stability (tectonic stability or tectonic inactivity). Eventually, the landmass is eroded dow n to peneplain (D a v is) and pediplain (King). Both the landscapes (peneplain and pediplain> have com m on sim ilarity in that both have antique characteristics, extensive areas and subdued reliefs. Both the models are based on the assumption of completion o f all the three stages (youth, mature and old) of the cycle. Besides these sim ilarities, both the models also differ from each o th e r viz. Davis peneplain is formed due to do w n w astin g w hile King's pediplain is formed due to c o alescen ce and integra­ tion o f several pedim ents w hich are form ed due to parallel scarp retreat. D av is’peneplain, once formed, does not experience further d e v elo p m en t (growth) until it is,reuplifted. W hen uplifted, new’ cycle of erosion is initiated and the rivers are rejuvenated. On the other hand. K ing’s pediplain. once formed, fur­ ther grows headward. New scarp is initiated at the far end o f the previously fo rm ed p ed ip lain which is progressively consum ed by the retreat o f new scarp and thus second pediplain is form ed w h ile the former pediplain experiences decrease in its e x t e n t The process co n tin u es and a series o f intersecting pediplains are formed which extend headw ard. Thus. King's pediplains, so form ed, are an alo g o u s to W. Penck’s p ied m on t trep p en .

surface termed by King as pediplain which is char­ acterized by uneven surface with low reliefs and subdued intersecting concave surfaces. The pediplain surface is still characterized by the presence of a few remnants o f inselbergs and mounds. By old stage m o st o f the residual hills (inselbergs) disappear. ‘The whole landscape will now be dom inated by low-angled pediments; the multi-concave surface is the ultimate form (pediplain) o f the cycle, the pediplain its e lf (R .J. Small, 1970). King has also postulated the concept of an­ tique pediplanation. According to King the rem ­ nants o f original pediplains developed during each cycle are preserved and exist on all summits. ‘Par­ ticularly where formed in resistant rocks, pediplains and pediplain remnants are believed to achieve great antiquity, so much so that the highest pediplain remnants are believed by King to have formed be­ fore the break-up o f the southern hemisphere conti­ nental plates in the Jurassic’ (R.J. Chorley et. al, 1985). King has identified a few antique pediplanation surfaces in Africa, S. A merica and Australia viz. (i) African G ondwana pediplain (formed in Jurassic period) of 1300 m height having its counterpan at the elevation of 7 0 0 -1000m in Brazil ; (ii) African pediplain (formed in Creataceous period) at two elevations i.e. 600-800m (in the coastal areas o f Africa) and 1000-1600m (in the interior of South Africa) which is comparable to Australian pediplain at the elevation o f 400-500m. Regarding the development of hillslope King has opined that the form of migrating or retreating (parallel retreat) slope is controlled by the processes operating on them. The summit o f hillslope is con­ vex and summital convexity results from the process o f soil creep. Scarp slope (free face element) is carved out o f rock outcrops and is characterized by parallel retreat due to backwasting under the influ­ ence o f rock fall, landslides and gullying. Scarp is the most active element o f hillslope. Debris slope is formed by the debris com ing from upslope and the gradient is determined by the angle o f repose of debris while the pediment, forming the lowermost segment o f the hillslope, is formed due to erosion of solid rocks by turbulent sheet flood.

A few' o f the assu m ptions o f K ing's model are controvercial e.g. (i) K ing’s m odel is based on Afri­ can experience but ‘it is not su rp risin g to find that King has gone to apply his con cept not only to the African landscape, but also to the regions which today experience clim atic con dition s quite different from those o f Africa w hich exhibit ‘p en ep lain s', not readily accounted tor by the D avisian theory* (RJ. Small, 1970). (ii) K in g s assertion that there is uni­ form developm ent of landscapes in different envi­ ronmental conditions is doubtful, (iii) ‘Despite the existense of these extensive surfaces (pediplain sur­ faces) ol low relief separated by cliff-lik e escarp­ m en ts in the trop ics, the co n cep t o f antique pediplanation must rem ain questionable, if Q*dy because ot vast periods o f time involved and our lack ot knowledge regarding the nature and rapidity of

Evaluation

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If wc com pare the geom orphic models o f W.M. Davis (geographical cycle) and L.C. King

J

i

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erosional processes in subhumid environment’ (R.J. Chorley, et. al, 1985).

differences in the rocks and the processes acting on them’.

It may be pointed out that King's geomorphic theory could not receive as much support and recog­ nition as it deserved because of the fact that his ‘canons of landscape developm ent’ came at the time (1953) when most of the geomorphologists were least interested in geomorphic theories as they were busy in quantifying the landforms and processes on the basis o f information and data obtained through field instrumentation and laboratory experimenta­ tion at much shorter temporal and smaller spatial scales.

The goal o f the theory of Hack is to explain the landscapes of any region of the earth s surface on the basis o f present denudational processes operat­ ing therein and to demonstrate lithological adjust­ ment to landforms (for which he presented examples from the Shenandoah valley of the Applachians, USA). The reference system o f Hackian model is that ‘geomorphic system is an open system which always tends towards steady state while his m odel may be stated as 'the shape o f the landform s reflects the balance between the resistance o f the underlying m aterials to erosion and the erosive energy o f the active processes. ’

5. Geomorphic Model of J.T. Hack

J. T. Hack, an American geomorphologist, is a supporter and advocate of dynam ic equilibrium theory of landscape development, which implies a delicate condition o f energy balance and envisages that ‘so long as the factors controlling landscape development and denudational processes and en­ ergy in the open geomorphic system remain con­ stant, there is no appreciable change (evolution) in landforms through tim e’. In fact, Hack's geomorphic model is a serious attempt to fill the conceptual vacuum created by the criticism and rejection of Davisian evolutionary model of geographical cycle and Penck's ‘m orphological system ’. According to Hack multilevel landscape (polycyclic relief) can­ not be explained on the basis of multiple erosion cycles as m aintained by W.M. Davis and his follow­ ers, albit these landscapes can be explained on the basis of dynam ic equilibrium theory. He further admitted that ‘eq u ilib riu m co n cep t’ is not in itself a m o d el’ rather it is a reality in nature. Hack’s geomorphic model is exclusively based on the con­ cept o f open system but minute analysis of Hackian model also reveals clear glimpse of evolutionary model in it. The basic tenet o f Hack's model is that (as referred to above) geom orphic system is an open system and so long as energy remains constant in the geomorphic system, landscapes remain in the condi­ tion o f steady slate though there is lowering in the landscape by denudational processes. It is, thus, o b v io u s that Hack's model envisages time inde­ pendent or timeless developm ent of landscapes. Besides, Hack also invoked a model of lithological ad justm ent to lan d form s as he stated that topo­ graphic forms and processes are closely related to

The basic p rem ise o f Hackian model o f land­ scape development is that ‘the landscape a nd the processes that fo rm it are p a rt o f an open system which is in steady steady o f b a la n c e ' (Hack^ i960). Hack further conceived the following reference sys­ tems on the basis of his basic assumptions— (i) T h e r e is balance between denudational processes and rock resistance’. (ii) ‘There is uniform rate o f dow nwasting in all components o f landscapes.’ (iii) ‘Differences and characteristics o f form are explicable in terms of spatial relations in which geologic patterns are primary consideration’ (Hack, 1960). (iv) The processes (denudational) which o p ­ erate today have carved out the landscapes of the earth's surface. (v) ‘T h e re is lith o lo g i c a d j u s t m e n t to landforms’.

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Though J.T. Hack did not construct evolu­ tionary model of landscape developm ent directly but he did opi ne *that evolution is also a tact o f nature and that the inheritance o f form is always a possibil­ ity’ (Hack, 1960). Though he did not build a model of progressive changes in landform s through time with changing environmental conditions but he opined that ‘landforms do experience changes w ith chang­ ing equilibrium conditions but these changes are not like Davisian evolutionary changes.

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remains stable for long geological period (stable base level) then the landmass is eroded down and lowered to base level o f erosion and thus the changes in landform s from initial stage to the final stage occur in evolutionary sequ en ce (like D avisian model o f cycle o f erosion). A cco rd in g to H ack in the case o f stable base level ‘an orderly netw ork o f ridges and ravines’ is produced in the final ph ase o f landscape development. Thus, there is gradual and sequential lowering in reliefs w hen base level o f erosion is

Hack postulated the concept o f variations in landscapes in relation to varying conditions of bal­ ance between rates o f upliftment and erosion viz.— (i) The rate o f upliftment is balanced with the rate o f erosion. If there is rapid rate o f upliftment and erosion, there is produced high reliefs. This condi­ tion is m aintained so long as the higher rate of upliftm ent and erosion remains constant. (ii) So long as the rate o f upliftment increases, the relief also increases so that rate of erosion matches the increasing rate of upliftment.

stable. (ii) If the base level o f erosion rises because o f positive change in sea-level then the lo w e r segment o f the rivers is subm erged due to transg ressio n o f sea water on coastal land but there is n o appreciable effect o f base level chang e (positive) on the up­ stream segm ent o f the streams. T h e p ositiv e change in base level also leads to low ering o f relief. Hack maintains that the long profiles o f rivers and their normal work w hich controls the d ev elop m en t o f valley side slopes are influenced and controlled by upstream conditions o f the drainage basin and not by the dow nstream conditions. Thus, H ack on the basis of this concept justified the validity o f R.E. Horton's scheme o f ordering o f stream s and stream segments. It may be mentioned that H orton (1942 and 1945) attempted to determ ine the hierarchy o f stream seg­ ments in the fluvially originated d rain ag e basins from upstream section (source tributary streams).

(iii) When the rate o f upliftment becomes zero i.e. when upliftment stops, then relief also declines, though ridge and ravine topography is still maintained. H ack has opined that if the diastrophic move­ ment is gradual and if it is balanced by the denudational processes (i.e. rates o f upliftment and erosion are equal) then landscape, while changing from one form to the other, remains in equilibrium condition. O n the other hand, if there is rapid rate o f diastrophic movement, then relict landforms are preserved until new equilibrium condition is not attained. R.C. Palmquist has rightly revealed inherent glimpse o f evolutionary model o f Davis in Hack's model— ‘H ack (1965) paraphrases Davis' ideal geo­ graphical cycle in terms o f the equilibrium concept and develops a similar evolutionary scheme. An initial disequilibrium stage (youth) of rapid stream incision is followed by an equilibrium stage (ma­ ture) wherein the rounded interfluves are lowered as potential energy decreases though they do not change in fo rm ’ (Palmquist, 1975). H a c k a lso d e v e l o p e d a ‘c o n tin u o u s dow n w astin g m o d e l’ which though envisages ten­ dency for dynam ic equilibrium but it is not neces­ sary that the dynamic equilibrium is in steady state. He him self admitted that ‘though there is possibility for steady state but it is not possible in reality.’ He further opined, ‘that evolutionary models can be conceived on the basis o f base level of erosion. In this context he considered three condi­ tions o f base level viz. (i) stable base level, (ii) positive (rise) change in base level and (iii) negative (fall) change in base level.

(iii) If there is low ering o f base level o f erosion (negative change) then there is rapid rate o f erosion in the dow nstream section (m ainly near the new base level i.e. m outh o f the river) o f the stream which influences larger areas o f the d rain age basin. New adjustm ent betw een erosion (rapid rate) and rock resistance is attained. H a c k a lso p r o p o u n d e d th e c o n c e p t o f lithological ad ju stm en t to landform s* ‘F o r exam ­ ple, it has been s u g g e s te d that, in the folded Applachians, the local relief and slo pe angles have been so adjusted that each m ajor geological outcrop yields an equal sedim ent load p er unit area (i.e. hard rocks-high, rugged and steep ; soft r o c k s — low, gently rolling and w ith low slo pes— Hack, I960)* (quoted by R.J. C horley et. al, 1985). R.J. Chorley c t al have rem arked that ‘although this is an attractive alternative explanation for geological limited 4cy* clic surfaces, but it is difficult to su ppo rt’ (RJ« Chorley et. al, 1985).

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In the case o f stable base level o f erosion he maintains that if any landmass is uplifted and then

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T h e advocates o f dynam ic equilibrium theory including J.T. H ack m aintain that the so-called peneplain and planation surfaces at different eleva­ tion levels (the outcom e o f rejuvenation and succes­ sive cycles of erosion as envisaged by Davis and his followers) are not the result o f completion o f succes­ sive cycles o f erosion bul they have been formed differently. T h e y argue, in any area of rocks which are reasonably uniform in terms of resistance, when the stream spacing (drainage density) is uniform, and where the slopes are at the same maximum angle, it is to be expected that the summits and the divide crests will all reach the same height and so give the impression of a former level surface which has, subsequent to its formation, been dissected by valleys. Hack has even gone so far as to propose that such a landscape, which he refers to as ‘ridge-andravine topography', is the normal expression of a condition ofdynamic equilibrium’ (R.J. Small, 1970).

6. Tectono-Geomorphic Model of M. Morisawa The appearance o f plate tectonic theory since 1960 has provided impetus to geomorphological i n v e s t ig a t io n s in n ew d i r e c t i o n s as som e geomorphologists have attempted to explain land­ scape development on the basis o f gradual and continuous tectonic movements as evidenced by plate movements and sea-floor spreading. American geom orphologist Marie M orisaw a's geomorphic model of landscape development (1975) is based on such premise. The following are basic premises o f Morisawa's ‘tectono-geom orphic model’— (1) Landforms are the result o f inequality o f force or inequality of resistance or o f both. (2) The variations in landforms are due to inequality of rates of operation o f exogenetic proc­ esses acting on different geomaterials o f the earth’s surface and inequality o f the rates o f endogenetic processes.

Evaluation

(3) Nature tends to attain balance/equilibrium between force (of processes) and resistance (o f geomaterials) but this situation (of balance) is not always possible because the earth is unstable and dynamic. Thus, the earth's surface is characterized by frequent changes and hence in stead o f static equilibrium there is tendency to equilibrium. D y­ namic earth system is characterized by isostatic feedback which affects upliftment and erosion, and deposition and subsidence i.e. upliftment is fol­ lowed by erosion and erosion is followed by deposi­ tion which is followed by subsidence which again leads to upliftment and thus the process continues. The isostatic feedback also affects the rates o f upliftment and erosion, and deposition and subsid­ ence.

The Hack's concept that ‘most of the land­ scapes are in uneasy dynamic equilibrium between available energy for work (erosion and transporta­ tion) and the work being done’ cannot be validated because if there is gradual and continuous lowering in regional elevation (and hence decline in energy available for denudational work) then no landscape of open system may remain in steady state. Simi­ larly, the concept o f Hack that landscapes are adapted/ adjusted to changing environmental conditions is doubtful because there are very little landscapes which have instantaneously adjusted/adapted to new environmental conditions. R.J. Rice ( 1977) has aptly remarked, ‘to an extent all landforms are prisoners of their own evolutionary history. A few of the assumptions or precepts of dynamic equilibrium theory are merely deductions which do not have ground support. For example, the fact that ‘there is perfect relationship between present-day processes and landforms' is not always true. A.L. Bloom (1978) has evaluated the Hackian model in right perspec­ tive— ‘If, however, tectonics and climatic changes invalidate the assumption ol initial upliit or other constructional processes followed by still stand and landscape evolution, then the dynamic equilibrium model, changing only from disequilibrium to equi­ librium, is most suitable as a basis for interpreting the present landscape1 (A.L. Bloom, 1978).

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(4) The present landforms are the result o f difference of ratios of the actions o f endogenetic and exogenetic processes. It may be mentioned that W. Penck also postulated identical concept (landforms reflect the ratio between the intensity o f endogenetic processes i.e. rate o f upliftment and the m agnitude o f displacement of materials by exogenetic processes i.e. rate of erosion and removal o f eroded materials). The ratio ot rates o f action by endogenetic and exogenetic processes varies temporally and spa­ tially. This aspect is responsible for temporal and spatial variations in landform characteristics. Thus, the landforms o f the earth's surface becom e com plex

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(1968). It m ay be pointed out that the ta id rate erosion is for those rivers w hich corns out (rtntt the

and hence it becom es difficult to understand the mode o f their genesis and development.

Himalayas. Based on aforesaid in f o r m a l/f t MorHawa hypothesised that ‘there h d ire c t p• o >

Instabi/rty

Episodic Erosion c

o “D D

T I M E Fig. 3 7 : Modified concept o f geomorphic cycle o f erosion. A - dotted line denotes progressive, lowering o f altitude as envisaged in Davis'm odel while solid lines indicate stepped features as suggested by Schumm. B - portion o f v a l le y floor C - Portion o f valley floor V F2 (as shown in B) which indicates dynamic equilibrium period between two periods o f instability o f shorter duration. After S. A. Schumm, 1975.

portion indicated by VF1 in fig. 3.7 A represents normal pattern o f valley floor o f river channel but when observed minutely at smaller spatial scale then it looks stepped as is evident from fig. 3.7B where real form of VF1 in fig. 3.7 A has been extended and projected. Normally, such stepped form o f valley floor is explained in terms o f influences o f external variables like upliftment, subsidence, climatic changes etc. but according to Schum m such stepped valley

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Schumm maintains that divide summits un­ d e r g o m o d e ra te c h a n g e s b e c a u se o f lim ited downwasting caused by surface runoff resulting from rainfall but downwasting is more or less uni­ form on all summits. The form of valley floor be­ comes stepped because o f reduction in valley floor but for shorter duration. It may be mentioned that the stepped form o f valley floor is because of sediment storage (deposition) and sediment flushing. The

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ates period o f valley deepening and the process is repeated over and again. It is, thus, evident that if the episodes o f erosion (period o f instability, of short duration) and deposition (period o f stability, of long duration) are repeated then there is no need of external variables to explain minor details o f land­ scapes like small terraces, alluvial fills, riffles and pools etc. because these features are the result of internal variables of the fluvial system. R.J. Chorley et. al (1985) have aptly remarked that ‘this dynamic metastable equilibrium model of episodic erosion shows, in addition, that many o f the details of the landscape (e.g. small terraces and recent alluvial fills) do not need to be explained by the influence of external variables because they develop as an inte­ gral part of system evolution’.

floor is not of external variables rather it is because of control of internal variables of the fluvial system. Such model is in fact representative of dyn am ic m etastable e q u ilib riu m model. It may be men­ tioned that in steady state equilibrium model (here is fluctuation around a stable average value whereas dynamic metastable equilibrium envisages ‘a condi­ tion o f oscillation about a mean value of form which trending through time and, at the same time, is subjected to step-like discontinuties as a threshold effect’ (R.J. Chorley et. al, 1985). According to Shumm there is possibility of influences of external variables on system equilibrium but in terms of denudation of landmass dynamic mestastable equi­ librium reflects reponses of inherent geomorphic thresholds of the fluvial system i.e. internal vari­ ables of the fluvial system influence and control dynamic metastable equilibrium. Forexample, depo­ sition of sediments in the valley floor upsets the said equilibrium state and introduces changes in the sys­ tem (e.g. increase in channel gradient due to sedi­ mentation) and when these changes exceed the criti­ cal geomorphic threshold, the eroding fluvial sys­ tem i.e. fluvially originated drainage basin is rejuve­ nated leading to accelerated rate of erosion (valley downcutting). Such situation of accelerated erosion is called p erio d o f episodic erosion. The period of episodic erosion, when it exceeds the geomorphic threshold, is succeeded by period of deposition. Thus, the bedrock valley floor of the river becomes step-like which denotes the period of instability (period of episodic erosion) and period of stability (period of dynamic metastable equilibrium). It may be pointed out that the period of instability/erosion is of short duration while the period of stability (dynamic metastable equilibrium or graded period) is o f longer duration. It may be clarified that the periods o f instability and stability are, in fact, peri­ ods of erosion and deposition respectively (fig. 3.7 C).

Schumm has also postulated the concept of several subcycles within a larger fluvial cycle. Ac­ cording to him the major cycle begins with denuda­ tion of uplifted landmass. In the initial stage maxi­ mum sediments are produced because of active vertical erosion (valley deepening) and the quantity and size of sediments decreases with time because of decrease in the rate and magnitude o f erosion due to lessening of channel gradient. Within major cycle second order cycles are initiated due to isostatic adjustment (upliftment) in the 1 st cycle and climatic changes. Within the second order cycles third order cycles are initiated when geomorphic thresholds in the fluvial systems are exceeded. The fourth order cycles are initiated due to complex geomorphic responses which are the result of changes in any one of the variables of the fluvial system e.g. tectonic events, isostatic adjustment (upliftment or subsid­ ence), climatic changes or geomorphic thresholds. The fouth order cycles o f smaller magnitude are initiated as a result of adjustment to changes in the 1st, 2nd and 3rd order cycles. The final or 5th order cycles are initiated due to seasonality o f hydrologic events or large floods.

Schumm has further stated that during peri­ ods o f stability there may be changes in the channel pattern because of changes in the nature of sediments passing through the channel, i.e. straight channel courses may be transformed to sinuous and mean­ dering courses. Again, the sinuous or meandering course of the river may be straightened during exten­ sive floods. The straightened and thus shortened river course again stimulates erosion and thus initi­

Evaluation

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The Schumm's model o f landform develop­ ment is, in fact, modified form o f Davisian model of geographical cycle which envisaged progressive changes in landforms through time. Schumm has successfully attempted to remove the major draw­ backs of Davis’ decay model and has tried to blend the cyclic model with equilibrium model. His model

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A

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THEORIES OF LANDFORM DEVELOPMENT

is nearer to m ore reality than Davisian model. He has also attempted to explain minor landscape details mainly in the valley floors which were obscured in Davis' m odel. But the concept of numerous subscycles within a m ajor or say super cycle in a fluvial system is difficult to digest but the effort of S.A. Schumm is commendable. There is a need o f blending o f decay and equilibrium models to build a more flexible model as R.J. Chorley et. al (1985) have also opined, ‘more than this, modern studies of thresh­ olds and complex response have suggested how the Davisian cyclic decay model and the steady state model o f Gilbert may be effectively, combined into a united vision of landform evolution.’ 8. Geomorphic Theories : In Indian Context

Now, the author presents geomorphic prob­ lems of a typical nature from the sub-humid tropical environm ent of India for critical evaluation of the landscape developm ent of the region which may

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lead us to corroborate the concept o f composite theory o f landscape development. Bhander plateau (24° 3’ 29" N— 24° 39' 1” N lat. and 80° 16’ 30" E— 80° 53’ 15” E long.), located between Panna plateau in the northwest and Rewa plateau in the east, is characterized by Vindhyan sandstones, shales and limestones generally lying in a horizontal manner with alternating bands o f hard and soft rocks. It registers an ascent o f about 350m above the general surrounding surface o f lower uplands and is drained by the feeders o f the Tons, the Satna and the Ken rivers. Mean annual rainfall is 1137mm and mean monthly m axim um temperatures of January and June are 30.5°C and 45.3°C respec­ tively whereas mean monthly temperatures o f corre­ sponding months are 20.4°C and 23.1°C respec­ tively. Hilly tract of the plateau has mixed vegeta­ tion of open and dense forests whereas low er up ­ lands have scattered bushes.

Fig. 3 .8 : Bhander Plateau, M.P., India (after Savindra Singh, 1974).

lower and rolling upland developed over V indhyan basement which has been m oderately incised by shallow valleys, the depth o f which m atches with the thickness of alluvia (4m to 18m). This low er upland is dotted with flat-topped hills, the exam ples o f

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A well-marked zonation of three distinct topo­ graphic features (fig. 3.8) from the higher plateau to the outer margins upto the river valleys is identified on three sides (fig. 3.8) viz. north, west and north­ east— (i) at the outer margins, there is significantly

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OliOMOimiOLOOY

m e sa s and buttes (K u sh la hill, Siiuliirin paluir* Shankargarh hill, Lai pahar, Pithaurahad liill, Nurduliit pahar, D hark ana pahar, Patna hill, liandhnurn hill, Satani hill, M utw ari hill etc.). These accordant re­ sidual hills having sandstone capping above and the alternate hands ol sandstones and shales helow are flat-topped m esas and huttes having vertical sleep scarps o f free-face element and rectilineal flanks of 30° to 40° slope helow and join the billowing surface form ed at their base which seldom exceeds 3 to 4 degrees in slope; (ii)T h e second ring o f topographic features incorporates numerous cinhaymcnts and indentations which girdle the plateau proper from three sides and indicate massive breaching o f the plateau rims. The most outstanding feature o f this zone isa c re n u la te d line o f imposing and precipitous scarps ; (iii) The third zone includes the top-surface of the central plateau and lacks in pronounced reliefs cxccpt som e convexo-concave low hills having lim ­ ited flat tops but in majority o f the cases they have round tops, some long and narrow ridges, knolls and irregular and asymm etrical valleys. The major river courses have graded profiles over the higher plateau and lower uplands but arc punctuated by sudden falls when they descend through the precipitous scarps. The existence o f numerous waterfalls along the rims o f the escarpm ent ranging between 10m and 60m makes the riddle o f the geom orphic history o f the region moc complex.

The region appears to be in equilibrium stage as there is gradual parallel retreat o f scarps and thus there is no significant chang e in landscape. The hack wasting is the most dom inant process. Various detached hills projecting above the general rolling surface of lower uplands are the left-over remnants of the recession o f the escarpm ents and thus the surrounding lower flat and rolling uplands arc not the outcome o f lateral planation by the rivers rather they are the results o f parallel retreat o f the scarps. This explanation, no doubt, goes in favour o f equi­ librium model but the existence of Sharda Pole hill (488m), only a km away from the precipitous Naktara escarpm ent, exhibits an exam ple o f dow nwasting and reduction o f relief because the recession of scarps (of sandstone capping) is com plete, the sand­ stone capping has been stripped o ff and the weaker shales have been exposed. T hus, the absence o f hard and resistant lithologic elem ent (sandstones) has effected d ow nw asting in D avisian style o f lowering of reliefs. During the sam e erosional history o f the region, Sharda Pole hill has un dergo ne the reduction of relief of at least 72m w h ereas the tops o f central plateau and flat-topped m esas (ranging in height between 500m and 58 0 m ) are least affected by dow nw asting though they have undergone parallel retreat and the process is still continuing. Such conditions again support D avisian model of land­ scape evolution and taboo H ack's equilibrium model and co rrob orates the slope rep lacem en t model of

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The entire Bhandcr plateau is a maturely dissected plateau but the existence of waterfalls cannot be accom m odated in Davisian model of g eo ­ graphical cycle. The heights o f the scattered hills standing over the lower uplands (fig. 3.8 : block diagram ) equal the central plateau surface in height (accordant level) and the exposed rock beds over the escarpm ents and these hills show perfect parallel­ ism. Such conditions do not support any upliftment, a necessary requirement for rejuvenation and nick points as required in Davisian model o f landscape development. Further, dow nw asting seem s to be ineffective in this region. This problem can be, for the time being, solved if we look at the locations and nature o f these waterfalls. There arc two distinct locations o f waterfalls viz., (i) the steepest and highest waterfalls (upto 60m ) are located along the rim s o f the plateau generally at the heads o f the cmbayrncnts and small tributaries ; (ii) the second

line ol waterlalhi in located further inland over the higher plateau and in n ge in height from lOrn to30m and are characterized by deep, long and narrow gorges helow their banc*, ft may be pointed out that the find category ol falls is, in fact, head* of em baym enls or scarp heads where water falls down the vertical walls only when there is rain, otherwise they remain dry during rnoM period o f the year. Thus, these waterfalls are not true falls signifying heads or rejuvenation rather they arc structural in character, liven this is accepted, the coexistence of the drainage net with graded profile of equilibrium over the higher plateau and lower uplands, signifi­ cant breaks in slope in their middle courses and above all steep slopes having frcc-facc elem ent of scarp faces in no case can be explained on the basis of Davisian model and thus his model miserably fails in the present case.

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t h e o r ie s o f l a n d f o r m d e v e l o p m e n t

Penck due to backw asting and hillslope cycle model o f L.C. King. Thus, both the exam ples o f steady state and o f no substantial changc in the landscape on the one hand and effective low ering o f relief and progres­ sive change (from free face, rcctilincar hillslope to convexo-concave slope and reduction of height from 560m to 488m ) on the other hand within a distance of one km, over a region o f uniform structure and same geological history, having no trace o f any fossil landform apparently different from the present ones, no subaerial processes in the past history of the

Fig. 3.9:

geomorphological evolution o f the region at least since Cretaceous period etc. nullify the need and desirability and even the authenticity o f a single theory o f landscape developm ent all over the globe. I f wc p r o c e e d f u r t h e r e a s t w a r d a n d northeastward from Bhander Plateau (say towards R cw a p la te a u ) ‘t e c to n o -g e o m o r p h ic m o d e l ’ (Morisawa, 1974 and 1975) becom es valid in ex­ plaining the landscape characteristics. The northern rim o f Rewa Plateau (fig. 3.9) overlooking transYamuna plain ascends slowly from 160m to 200m and then is characterized by an abrupt, vertical and

Part o f Rewa scarps with indentation, valley embayments, nicks and waterfalls (after Savindra Singh, 1974)

em bayments similar to Bhander escarpm ents but o f lower heights. The T on s river, the upper course o f

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precipitous escarpment from 200m to 260 or 280m and is highly c ren u laied and indented having

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(very slow) due to plate tectonics is equalled by degradation and the scarps are experiencing parallel retreat maintaining their original character (free face above and middle rectilinear segm ent together with lower segments of concave elem ent below) but it should be remembered that equilibrium stage is not static as the earth is so dynamic.

which is graded over the eastern lower upland of Bhander plateau, abruptly descends through a steep vertical waterfall of 70m height carvcd out in hori­ zontal but massive V indhyan sandstones (24°47' N and 81°r56" E) and after draining for a distance of about 6 km downstream in a narrow, deep and vertical gorge (valley walls rise upto 60m from the river bed) receives the Bihar river which makes the m ost outstanding Chachai Falls o f 127m hardly 1.5 km upstream from its confluence with the Tons river and the gorge (1.5 km long) is very massive and has been carved out of horizontal massive beds of Vindhyan sandstones. Further eastward, the Mahanadi, a tribu­ tary of the Tons, makes a 98m falls at Kevati (only 9km cast of Sirmaur market) and drains through a straight but narrow and deep gorge having a vertical valley-side wall o f 80m for a distance of 4 km and thence the gorge widens out further downstream. Further in the east and northeast there is a line of waterfalls ranging between 20m and 145m in height.

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The above discussion and observation of Palmquist (that ‘only two premises are necessary to produce a reference system which allow s both for landform evolution and dynam ic equilibrium, (i) geomorphic systems arc multivariate open systems which tend towards a steady state equilibrium and (ii) the mass of rock existing above base level con­ stitutes an external variable to which the system is in constant disequilibrium’, Palmquist, 1975, p. 159) warrant the necessity of multiple theories. Thus, it facilitates us to conclude that the landscapes are complex rather than simple and these should be studied with no bias of a particular theory or model but should be viewed with open mind taking into Such conditions (nick points in the long pro­ account the consideration of adjustment of landforms files of major rivers of 5th to 8th order) indicate to lithology, geologic history of the region, tectonic rejuvenation of northern rim of the Deccan Fore­ activity and magnitude of denudational processes land. The subduction of Indian plate beneath Asiatic plate culminated in the Himalayan orogency and and above al I minute observation of landforms in the jerks caused by the Himalayan upliftment intro­ field and laboratory. Thus, the composite approach duced rebound impact on northern rims of the Deccan envisages detailed objective description of landforms Foreland which was responsible for relative uplift of through field observation and morphometric details, the latter in relation to the trans-Yamuna plain. This their classification into gcnetic/non-gcnetic catego­ activity accelerated the rate of denudational proc­ ries and their explanation highlighting their devel­ esses and caused disequilibrium of action. It is to be opment whether they may be the result of progres­ noted that landscape is the outcome of the relation­ sive change through time (as is the case of Sharda ship between the rates of intensity of tectonic force Pole hill, referred to above), or they may be the and denudational processes and between the force of . outcome of the balance between continuing uplift resistance of materials and energy. Whenever there and erosion (as is the case of the northern rim of is difference between these tw-o, disequlibrium re­ Deccan Foreland) as a case of open-system steadysults and when these two equal, equilibrium condi­ state model of landform development or they may be tion is maintained. If M orisawa’s statement is fol­ the product of interaction between diastrophic ac­ lowed, ‘denudational and tectonic forces in Japan tivity and climate or they may be due to parallel and in the Himalayas have reached an equilibrium of action at present’ (Morisawa, 1975, p. 211), equilibretreat etc. A combination of m ore than one possi­ •ium model w'orks in this case as the rate of uplift bilities may be possible in a single region.

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CLIMATIC GEOMORPHOLOGY AND MORPHOGENETIC REGIONS D iagnostic landforms ; geomorphological processes and climatic con­ trol ; direct control of clim ate; indirect climatic control; climatic changes and landforms ; morphogenetic regions.

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4 CLIMATIC GEOMORPHOLOGY AMD MORPHOGENETIC REGIONS The concept o f climatic geomorphology envis­ ages that each clim atic type produces its own characteristic assemblages of landforms and set of geomorphic processes which shape them. Though the concept o f climatic geomorphology found grcund in Germany and France by the end of the 19th century based on the works o f scientific explorers like Yon Richtchofen in China, Passarge, lessen, Walther, and Thorebecke in Africa, and Sapper in central A merica and M alanesia but certain funda­ mental problems regarding this concept could not be solved as yet. Even W.M. Davis recognized humid temperate region as ‘n o r m a l ’ for landscape develop­ ment but ‘he treated the landforms of non-temperate climatic regions as deviants from the ‘normal’ scheme’ (D.R. Stoddart, 1969). The German scientists, who were convinced about the imposing influences of climate on geomorphic processes and landforms resulting therefrom, propounded that in Germany each climatic region was characterized by distinc­ tiv e a s s e m b la g e o f la n d fo rm s w hile French geoscientists identified climate as a major control­ ling factor o f landscape development.

D.R. Stoddart (1969), L. Wilson (1969, 1973), J. Tricart and A. Cailleux (1972) etc. The advocates of climatic geomorphology argue that the rate of oper­ ation of weathering and erosional processes, vegeta­ tion type, surface runoff, nature and rate of erosion and mechanisms of landform genesis and develop­ ment differ considerably from one climatic region to the other but it may be pointed out that they could not be able to present convincing evidences in support of their arguments as yet. The concept of climatic geomorphology is based on the following three major themes (D.R. Stoddart, 1969)— (1) Landforms differ significantly in different climatic regions. (2) Spatial variations of landforms in differ­ ent climatic regions are because of spatial variations in climatic parameters (e.g. temperature, humidity, precipitation etc.) and their influences on weather­ ing, erosion and runoff. (3) Quaternary climatic changes could not obscure relationships between landforms and cli­ mates. In other words, there are certain diagnostic landform s which clearly dem onstrate climatelandforms relationships.

The concepts of climatic geomorphology and morpho-climatic / morphogenetic landscapes and regions were further enriched by the classical work of J. Budel (1948, 1982), L.C. Peltier (1950), C. Troll (1958), W.F. Tanner (1961), P. Birot (1968),

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(4) Besides, ‘not only do different levels of magnitude and frequency of processes have differ­

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GEOMORPHOLOGY

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than present climatic con ditio ns9 (D .R Stoddart,

ent geomorphic effects in different environments, but within a single environment different attributes of morphometry (e.g. hydraulic geomorphometry, slope forms and divide configuration) may them ­ selves be formed by processes of different magni­ tude and frequency’ (R. J. Chorley, ct. al, 1985). The above mentioned themes of climatic geomorphology need explanation separately.

1969). In se lb e rg s representing steep sided residual hills are considered to be the representative landforms of hot and arid and semi-arid clim ates and the end product o f arid cycle o f erosion but insclberg* have been found in different parts o f the world having different ‘climatic conditions, from hurnid subtropical in Georgia, N orth A m erica to humid tropical in the Guinea coastlands, south India, Bra­ zil, and to desert areas in western North America, M auretania, and south-w est A fric a ’ (D.R. Stoddart, 1969). It is argued that inselbergs are structurally controlled rather than clim atically controlled and most o f the present inselbergs w ere form ed before Quaternary epoch, ‘hence present clim ates are not necessarily those in which the inselbergs were formed’ (D.R. Stoddart). It may be possible that inselbergs might have been formed when the clim ate was arid or semi-arid which m ight have changed after their formation.

4.1 DIAGNOSTIC LANDFORMS

The advocates of climatic geomorphology have attempted to collect information about the characteristics of such landforms w'hich may be regarded as diagnostic landforms to determine climate-landforms relationships. Such typical diag­ nostic landforms are regarded as representatives of a particular climate. Climatogenetic or climatically controlled landforms are identified and differenti­ ated in two ways e.g. (i) general observation and acquaintance of whole landscape of each climatic region, and (ii) identification o f typical or distinctive landforms which represent the control of a particular climate. The typical landforms are, in fact, main tools o f climatic geomorphologists which help them in determining climate - landforms relationships in different climatic regions. Such distinctive landforms are designated as diagnostic landform s. The diag­ nostic landforms, identified by the climatic geomor­ phologists so far include inselbergs, duricrusts, ped­ iments, tors etc.

P ed im en ts, characterized by low-anglerockcut surfaces surrounding m ountains, are considered to be the representative landform s o f arid (desert) and semi-arid climates. P edim ents are also found in a variety of climatic conditions e.g. tropical wet and dry climate, subtropical and tem perate climate. A few geomorphologists (e.g. W. Penck) argue that pediments are structurally and tectonically rather than climatically controlled. L.C. King has opined that the process of pediplanation and pedimentation is universal and it occurs in all environm ental condi­ tions. In fact, ‘many arid zone pedim ents are clearly polycyclic, developed during the com plex sequence of Pleistocene pluvials (period o f prolonged rain­ fall) and interpluvials : many appear to be being distroyed under present climatic conditions, rather than being form ed’ (D.R. Stoddart, 1969).

D uricrusts are indurated hardened surfaces of different kinds such as laterites, silcretes, cal­ cretes, alcretes, ferricretes etc. depending on domi­ nance of constituent minerals. Normally, lateritic crusts are supposed to have been formed in hot and humid climate of tropical and subtropical areas and therefore these are indicative of hot and humid climates. Lateritic crusts are predominantly found in Chotanagpur highlands (Patlands of Ranchi and Palamau plateaus) of Bihar (India) and over many areas o f Decean plateau (e.g. Mahabaleshwar and Panchgani plateaus of Maharashtra). The presence o f lateritic crusts in certain parts of Europe (e.g. U.K., Germany etc.) clearly demonstrates the fact that these are not the result o f the present climate. ‘Such crusts are often interpreted as o f Tertiary age, or as having been under continuous formation since the end o f the Mesozoic. Exposers o f silcretes and calcretes similarly are often related to past rather

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T o rs, ‘one o f the m ost controvercial land­ forms, are piles o f broken and exposed masses of hard rocks particularly granites having a crown of rock-blocks ol different sizes on the tops and clitters (trains of blocks) on the sid e s ’ (Savindra Singh, 1977). Tors have been considered to be o f periglacial origin by J. Palmer and R.A. Neilson (1962), of fluvial origin (humid climate, deep chem ical weath­ ering and exhum ation ol rock debris by running water) by D.L. Linton (1955), w hereas L.C. King

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CLIMATIC g e o m o r p h o l o g y a n d m o r p h o g e n e t i c r e g io n s

has opined that tors are the result o f universal proc­ ess of pediplanation in different climatic conditions It may be pointed out that ‘various theories of torformation have been put forth but there is no una­ nimity among the exponents and it must not be as tors, as mentioned earlier, are not confined to a particular rock type and climate but a variety of rocks and climates claim their existence’ (Savindra Singh, 1977). In fact, the presence of tors right from Dartmoor of England through Nicaragua to India has complicated the problem o f origin of tors rather than solving it.

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physical weathering are considerably slowed down. Dense vegetation covering the valley sides and even reaching the valley floors discourages lateral ero­ sion by streams and thus the processes of valley widening becomes sluggish. Dense vegetation o f humid tropics also reduces surface runoff because a sizeable portion of rainfall is intercepted by forest conopy and thus rainwater reaches the ground sur­ face in the form of aerial stream lets through the leaves, twigs, branches and stems o f trees and thus allows more infiltration.

It may be concluded that the aforesaid repre­ sen tative/diagn ostic la n d fo rm s are older than Pleistocene climatic changes, so they are definitely not related to present climates where they are found. It may be pointed out that climatic relation of landforms at least in glacial, periglacial and desert cli­ mates are undoubtedly confirmed but more mor­ phometric evidences are needed to establish close relationship between climate and landforms in other climatic regions. ‘This is not to deny that climati­ cally conrolled landform differences exist, though morphometric confirmation o f this is scanty; but it is to assert that the climatic inputs and geomorphic outputs in denudation system are so litle known that one cannot be inferred from the other’ (D.R. Stoddart, 1969).

Annual R a in fa ll(in c h e s ) 70 60 50 U0 30 20 10

Chemical

4.2 GEOMORPHIC PROCESSES AND CLIMATIC CONTROLS It is an established fact that different pro­ cesses work in different climatic regions and with climatic variations there is also variability in the nature and mode of influences of climatic parame­ ters which affect denudational (weathering and ero­ sion) processes. Tem perature and humidity have emerged as the most significant climatic parameters of the control of geomorphological processes in different climatic regions. High mean annual tempera­ ture and rainfall (and hence perennial humid condi­ tion with high temperature throughout the year) favour deep chemical weathering in humid tropics, but the presence of gullies on steep slopes and canyons in the same humid tropics presents a geo­ morphic riddle. Besides, vegetation also plays im­ portant role in controlling geomorphic processes in tropical humid areas, because the combination of high mean annual temperature and rainfall favour dense vegetation even on steeper slopes with the result the processes o f soil erosion, sheetwash and

weathering

80 70 60 50 hd e a r ‘r t h e V O ,C a n ic e r u P t i o n s a r eo ut a s s o c i a t e d w i t h w e a k e r z o n e s o f t h e e a r t h surface r e p r e se n te d b y m o u n t a in b u ild in g a t th e d e s tr u c tiv e or c o n v e r g e n t p la t e m a r g in s a n d fr a c tu r e z o n e s r e n r e se n te d b y c o n s t r u c t iv e o r d iv e r g e n t p la te b o u n d a ­ r ie s a t t h e s p l i t t i n g z o n e s o f m i d - o c e a n i c r i d g e s a n d th e z o n e s o f t r a n s f o r m s e r v a tiv e v u lc a n ic it y

p la te

fa u lts r e p r e se n te d b v co n

b o u n d a r ie s .

(v u lc a n is m ) a n d

The

m e c h a n is m

of

v o l c a n i c e r u p tio n s is

c lo s e ly a s s o c ia t e d w it h s e v e r a l in t e r c o n n e c t e d p r o c ­ esses su ch

a s ( i ) g r a d u a l in c r e a s e o f te m p e r a tu r e

with in c re a s in g d e p th a t th e ra te o f 1°C p er 32 m due to heat g e n e ra te d fro m th e d isin te g ra tio n o f rad io a c­ tive e le m e n ts d e e p w ith in th e earth , (ii) o rigin o f m agm a b e c a u s e o f lo w e rin g o t m e ltin g p o in t caused by r e d u c t i o n in th e p r e s s u r e o f o v e r ly in g su p erin cu m b en t lo a d d u e to fra c tu re cau sed by sp lit­ ting o f p la te s a n d th e ir m o v e m e n t in o p p o site d irec­ tion. (iii) o rig in o f g a s e s an d v a p o u r d u e to heating of w ater w h ic h re a c h e s u n d e rg ro u n d th ro u g h p erco ­ lation o f ra in w a te r a n d m e it-w a te r (w ater derived through the m e ltin g o f ice an d sn o w ), (iv) the ascent of m agm a fo rc e d by e n o rm o u s v o lu m e o f gases and vapour an d (v ) fin a lly th e o c c u rre n c e o f volcanic eruptions o f e ith e r v io le n t e x p lo siv e cen tral type or quiet fissu re ty p e d e p e n d in g up o n the intensity o f gases and v a p o u r an d th e n a tu re o f cru stal surface. T h e o r y o f p l a t e te c to n ic s now very well explains th e m e c h a n is m o f v u lc an ism and volcanic eruptions. In fact, v o lc a n ic e ru p tio n s are very closely

12.5 : Illustration o f constructive (divergent) and destructive (convergent) plate boundaries and their relationship with vulcanicity. https://telegram.me/UPSC_CivilServiceBooks

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It is apparent from the above d iscussion that the m id-oceanic ridges, rep resen tin g splitting zones, are associated w ith active volcanoes w herein the supply o f lav a com es from the upper m antle ju st below the ridge because o f differen tial m elting o f the rocks into th oleiitic b asa lts. Since there is constant supply o f basaltic lavas from below the m id-oceanic ridges and hence the v o lcan o es are active near the ridges but the supply o f lavas d ecreases w ith increas­ ing distance from the m id -o cean ic ridges and there­ fore the volcanoes becom e inactive, d orm ant and extinct d epending on th eir distances from the source o f lava supply, e.g. m id -o cean ic ridges. This fact has been validated on the basis o f the study o f the b asaltic floor o f the A tlantic O cean and the lavas of several Islands. It has been found that the islands n earer to the m id -A tlan tic R idge have younger lavas w hereas the islands aw ay from the ridge have older lavas. F or exam ple, the lavas o f A zores islands situated on eith er side o f the m id-A tlantic Ridge are 4 -m illion y ear old w hereas the lavas o f Cape V erde Island, located far aw ay from the said ridge, are 120m illion year old. D estru ctive or con vergent plate b ou nd a­ ries are associated w ith explosive type o f volcanic eruptions. W hen tw o convergent plates collide along B en io ff zon e (subduction zone), co m p aratively heavier plate m argin (boundary) is subducted be­ neath com paratively lighter plate boundary. The subducted plate m argin, after reaching a depth o f 100 km or m ore in the upper m antle, is m elted and thus m agm a is form ed. T his m agm a is forced to ascend by the enorm ous volum e o f accum ulated explosive gases and thus m agm a appears as violent volcanic eruption on the earth's surface. Such type o f volcanic eruption is very com m on along the d estru c­ tive or co n v erg en t plate boundaries w hich rep resen t the volcanoes o f the circu m -P a cific b elt and the m id -con tin en tal belt. T he volcanoes o f the island arcs and festoons (o ff the east co ast o f A sia) are caused due to subductio n o f oceanic cru st (p late) say Pacific plate below the co n tin en tal plate, say A siatic plate near Japan T rench. 12.8 HAZARDOUS EFFECTS OF VOLCANIC ERUPTIONS

(1) H uge vo lu m es o f h o t and liq u id lavas m oving at co n sid erab ly fast sp eed (reco rd ed speed is 48 km per hour) bury h um an stru ctu re s, kill people and anim als, destro y ag ricu ltu ral farm s and pas­ tures, plug rivers and lakes, b u m an d d estro y forest etc. The great eru p tio n o f M t. L o a on H aaw aii poured out such a huge vo lu m e o f lav as th a t these covered a distance o f 53 km dow n the slo p e. E n o r­ m ous Laki lava flow o f 1783 A .D . tra v e lle d a d is­ tance of 350 km eng u lfin g tw o c h u rch es, 15 a g ric u l­ tural farm s and k illin g 24 p er c e n t o f th e total population o f Iceland. T he cases o f M t. P elee e ru p ­ tion o f 1902 in M artinique Islan d (in C a rib b e a n S ea) (total death 28,000) and St. H elen s eru p tio n o f 1980 (W ashington, U SA ) are rep resen tativ e e x a m p le s o f dam ages done by lav a m o v em en t. T h e th ic k c o v ers o f green and dense fo rests on th e flan k s o f M t. St. H elens w ere com p letely d e stro y e d d u e to sev ere forest fires kindled by h o t lav as. (2) F allo u t o f im m en se q u an tity o f v o lc an ic m aterials including frag m en tal m a teria ls (p y ro clastic m aterials), dusts and ashes, sm o k es etc. c o v e rs la rg e ground su rface and th u s d estro y s cro p s, v e g e ta tio n and buildings, d isru p ts an d d iv e rts n a tu ra l d ra in a g e system s, creates h ealth h azard s d u e to p o iso n o u s gases em itted d u rin g the eru p tio n , and c a u s e s k ille r acid rains. (3) A ll ty p es o f v o lc an ic e ru p tio n s , if n o t predicted w ell in ad v an c e, c a u se s tre m e n d o u s lo sses to p recio u s h um an lives. S u d d en e m p tio n o f v io le n t and ex p lo siv e type th ro u g h c en tral p ip e d o e s n o t give any tim e to h u m an b ein g s to e v a c u a te th e m ­ selves and th u s to save th e m se lv e s fro m th e c lu tc h e s o f d eath lo o m in g larg e o v er th em . S u d d en e m p tio n o f M t. P elee on the Islan d o f M a rtin iq u e , W e st In d ie s in the C arib b ean S ea, on M ay 8, 1902 d e stro y e d th e w hole o f St. P ierre tow n and k illed all the 2 8 ,0 0 0 in h ab itan ts leav in g b eh in d o n ly tw o su rv iv o rs to m ourn the sad d e m ise o f th e ir b re th re n . T h e heavy rain fa ll, asso c ia te d w ith v o lc a n ic e ru p tio n s , m ixin g w ith fallin g v o lc a n ic d u sts an d ga ses ca u ses e n o r­ m o u s m u d flo w o r ‘la h a r * on th e s te e p slo p es o f https://telegram.me/UPSC_CivilServiceBooks

V olcanic eru p tio n s cau se heavy d am ag e to h u m an lives and pro p erty th ro u g h ad v an cin g hot lavas and fallo u t o f vo lcan ic m a terials; d estru ctio n

to hum an structures such as b u ild in g s, factories, roads, rails, airp o rts, dam s and reserv o irs through hot lavas and fires cau sed by h o t lav as; flo o ds in the rivers and clim atic ch an g es. A few o f the severe dam ages w ro u g h t by v o lcan ic eru p tio n s m ay be sum m arized as given b elo w —

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VULCANICITY AND LANDFORMS

(5 ) V o lca n ic eruptions also ch an ge the radia­ tion balance o f the earth and the atm osphere and thus help in causing clim atic ch an ges. Greater con cen tra­ tion o f volcan ic dusts and ash es in the sky red u ces the am ount o f insolation reaching the earth s su rface (4 ) E arthquakes ca u sed b efore and after the as they scatter and reflect som e am ount o f in co m in g volcanic eruptions gen erate d e s tr u c ti v e tsu n a m is shortwave solar radiation. D u st v e ils , on the other seism ic w a v es w h ic h create m o st d estructive and hand, do not hinder in the lo ss o f heat o f the eart s disastrous sea w a v e s ca u sin g innum erable deaths o f surface through ou tgoin g lo n g w a v e terrestrial ra hum an b ein gs in the a ffe c te d co a sta l areas. O n ly the diation. The ejection o f nearly 2 0 cu b ic k ilo m etres exam ple o f K rakatoa in 1883 w o u ld be su fficien t o f fragm ental m aterials, dusts and a sh es u p to e enough to d em on stra te the d isastrous im pact o f height o f 23 km in the sky during the v i o le n t eru p tion tsunam is w h ich gen erated en orm ou s sea w a v es o f 30 o f Krakatoa volcano on A u gu st 2 7 , 1 8 8 3 , fo rm e a to 40 m h eig h t w h ic h k illed 3 6 ,0 0 0 p eop le in the thick dust veil in the stratosphere w h ich c a u sed a coastal areas o f Java and Sum atra.

volcanic c o n e s w h ich c a u ses sudden deaths o f human b ein gs. For ex a m p le , great m ud flo w created on the steep slo p es o f K elu t v o lc a n o in Japan in the year 1919 killed 5 ,5 0 0 p eo p le.

C uldera w ith C in d e r Volcanic Neck with Rnclianf inf> Dikes

C one E ro d e d L acco lith

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nroduced during volcanic activities. fig. 12.6: Different types o ,f .landforms proauc

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geom orphology

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m a tio n o f c in d e r c o n e s is in itia te d d u e to accu m u la­ tio n o f fin e r p a rtic le s a ro u n d v o lc a n ic v e n t in the form o f tin y m o u n d , say ‘a n t m o u n t’ w h ich varies (6) A g ro u p o f scien tists b eliev es th a t v o l­ in h e ig h t fro m a few c e n tim e tre s to a few m etres in canic eru p tio n s and fallo u t o f d u sts and ash es cau se the b eg in n in g . T h e size o f th e c o n e g rad u ally in­ m ass ex tin ctio n o f a few sp ecies o f an im als. B ased cre a se s d u e to c o n tin u o u s a c c u m u la tio n o f volcanic on this h y p o th e sis th e m ass ex tin ctio n o f d in o sau rs m aterials m in u s la v as. S o m e tim e s , th e ra te o f grow th ab o u t 60 m illio n y ears ago has been rela ted to o f the co n e is so h ig h th a t it g a in s h e ig h t o f 100 m or increased w o rld -w id e v o lcan ic activity. A cid rains m o re w ith in a w eek . T h e s lo p e s o f c in d e r cones acco m p an ied by v o lcan ic eru p tio n s cau se largeran g e b etw ee n 30° an d 45°. L a rg e r p a rtic le s are • scale d estru ctio n o f p la n ts and anim als. arran g ed n ear th e c ra te rs a n d re s t a t th e a n g le be­ tw een 40° and 45° an d th e fin e r p a rtic le s a re d epos- * 12.9TOPOGRAPHY PRODUCED BY VULCANICITY ited at the o u te r m a rg in s o f th e c o n e s . S in c e such N u m e ro u s ty p es o f lan d fo rm s are created due co n es are fo rm ed o f u n c o n s o lid a te d la rg e r p article s to co o lin g an d so lid ificatio n o f m ag m as b elow the and are seld o m c o m p a c te d by la v a s a n d h e n c e they earth 's su rface and lav as at the earth 's surface and are p erm eab le to w ater. du e to acc u m u la tio n o f frag m en tal m aterials, dusts and ashes w ith lav as such as d ifferen t types o f Such co n es are on an a v e ra g e le ss s u s c e p itb le v o lcan ic co n es. T h e cones and craters are n ot alw ays to ero sio n and h e n ce th ey m a in ta in th e ir o rig in a l p erm a n e n t lan d fo rm s b ecau se they are ch an g ed and form s fo r h u n d red s o f y ears p ro v id e d th a t th e y a re m o d ified d u rin g every su ccessiv e eruption. E x p lo ­ n ot d estro y ed by en su in g v io le n t e x p lo s io n . T h e sive ty p e o f volcan ic eru p tio n s helps in the fo rm a­ v o lcanic co n es o f M t. Jo ru llo o f M e x ic o , M t. Iz a lc o tio n o f several types o f volcanic cones w hereas o f San S alv ad o r, M t. C a m ig u in o f L u z o n Is la n d o f fissu re flow s resu lt in the fo rm ation o f lava plateaus P h illip p in es etc. are ty p ic a l e x a m p le s o f c in d e r c o n e s and lav a plains due to accu m u latio n o f th ick layers (fig. 12.7(1). o f basaltic lavas over ex ten siv e areas. T he to p o ­ (ii) C o m p o site c o n e s a re th e h ig h e s t o f all graphic features produced by the entire process o f volcan ic cones. T h e se are fo rm e d d u e to a c c u m u la ­ vulcanicity are grouped into tw o broad categ o ries tion o f d iffe re n t la y ers o f v a rio u s v o lc a n ic m a te ria ls viz. (1) extru sive to p o g ra p h y and (ii) in tru sive and h en ce th e se are a lso c a lle d as s tr a to -c o n e s (fig. top ograp h y. Fig. 12.6 depicts m ajo r ch aracteristic 12.7(2). In fact, th e se c o n e s a re fo rm e d d u e to volcanic landform s. d ep o sitio n o f a lte rn a te la y e rs o f la v a ai^d fra g m e n ta l (1) E xtru sive V olcan ic T op ograp h y (p h y ro cla stic) m a te ria ls w h e re in la v a a c ts as c e ­ (i) F rom exp losive type o f eru p tion s m en tin g m a teria ls fo r th e c o m p a c tio n o f fra g m e n ta l (a) E levated form s, e.g. volcanic cones m aterials. T h e co n e b e c o m e s c o m p a ra tiv e ly r e s is t­ (b) D epressed form s, e.g. craters and an t to ero sio n if it is c o ated by th ic k la y e r o f la v a . O n calderas the o th er h an d , if th e o u te r la y e r is c o m p o s e d o f (ii) F rom fissu re eru p tion s frag m en tal m a te ria ls, the c o m p o s ite c o n e is s u b ­ je c te d to sev ere ero sio n . M o s t o f th e h ig h e s t s y m ­ (a) L av a p lateau s and dom es (b) L av a plains m etrical and e x te n siv e v o lc a n ic c o n e s o f th e w o rld co m e u n d er th is c a te g o ry e.g . M t. S h a s ta , M t. R a n ie r, (2) In tru siv e V o lca n ic T o p o g ra p h y

global d ecre ase o f so lar ra d ia tio n re c e iv e d at the earth 's su rface by 10 to 20 p er cent.

M t. H o o d (U S A ), M t. M a y o n o f P h illip p in e s , M t. (i) in tru siv e lav a d o m es, (ii) b ath o lith s, (iii) F u z iy a m a o t Ja p a n , M t. C o to p a x i o f E c u a d o r etc. lacco lith s, (iv) p h aco lith s, (vi) lo p o lith s, (vi) sills, (vii) d ik es, (viii) v o lcan ic p lu g s and sto ck s etc. (iii) P a r a s ite c o n e s- S e v e ra l b ra n c h e s o f pipes c o m e o u t fro m th e m a in c e n tra l p ip e o f th e v o lc a n o w h en the v o lc a n ic c o n e s are e n o rm o u s ly e n la rg e d . VOLCANIC CONES L av a s an d o th e r v o lc a n ic m a te ria ls c o m e o u t from (i) C in d e r o r a sh c o n e s are u su ally o f low th e se m in o r p ip e s a n d th e se m a te ria ls a re d e p o s ite d height and are form ed o f volcanic d u sts an d ashes and a ro u n d n e w ly fo rm e d v e n ts lo c a te d o n th e o u te r p y ro c la stic m a tte r (fra g m e n ta l m a te ria ls). T h e fo r­ su rfa c e o f th e m a in c o n e a n d th u s s e v e ra l s m a lle r https://telegram.me/UPSC_CivilServiceBooks

Elevated Forms

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yyiXANICTTY AND LANDFORMS cones are form ed on m ajo r cone (fig. 12.7(3)). T hese cones are called p arasite cones b ecau se the supply o f lava for these cones com es from the m ain pipe. T hese cones are also know n as a d v en tiv e or lateral con es. S hastina cone is a p arasite co n e o f M t. S h asta o f the U SA . (iv) B asic lava con e is fo rm ed o f lig h t an d less viscous lava w ith less q u an tity o f silica. In fact, w hen the lava co m in g o ut o f fissu se flow is d e fic ie n t in silica and is ch aracterized by h ig h d eg ree o f fluidity, it cools and so lid ifies afte r sp read in g o v er larger area. Thus, a long co n e w ith sig n ifican tly low h eig h t is form ed. Such cones are also c a lle d as sh ield cones because o f th eir sh ap es re se m b lin g a sh ield . Since these cones are co m p o sed o f b a sa ltic la v as, they are also called as b asic la v a co n es. T h e se are also know n as H aw an a ty p e o f co n es (fig. 12.7(4)). (v) A cid la v a co n es are fo rm ed w h e re the lavas com ing out o f v o lcan ic e ru p tio n s are h ig h ly viscous and rich in silica co n ten t. In fact, such viscous lavas have very low m o b ility an d h e n ce th e y are im m ediately cooled and so lid ified a fte r th e ir appearance on the earth's su rface. T h u s, h ig h c o n e s of steep slopes are form ed. S u ch co n es are very o fte n know n as S tro m b o lia n ty p e o f co n es (fig. 12.7(5). (vi) L ava d om es are in fa c t sim ila r to sh ield cones in one w ay or the other. L av a d o m e s d iffe r from shield cones as reg ard s th eir size. A c tu a lly , lava dom es are larg er and m o re ex te n siv e in size th a n the shield cones. T h ese are fo rm ed d u e to a c c u m u la ­ tion o f so lid ified lavas aro u n d the v o lc a n ic ven ts. B ased on the m o d e o f o rigin and the p la c e o f fo rm a ­ tion lava dom es are d iv id ed into 3 c a te g o rie s e.g. (A ) p lu g d om e (fo rm ed o f lav as d u e to fillin g o f v o l­ canic vents), (B ) en d o g en o u s d o m e (fo rm e d o f silica rich v isco u s lavas) an d (c) e x o g e n o u s d o m e (form ed o f s ilica-d eficien t la v a w ith h ig h d e g re e o f fluidity). (vii) L ava p lu g s are fo rm e d d u e to p lu g g in g o f volcanic pipes and v en ts w h en v o lc a n o e s b e c o m e extinct. T h ese v ertical c o lu m n s o f s o lid ifie d la v a s ap p ear on the earth 's su rface w h en th e v o lc a n ic cones are ero d ed aw ay. T h e la v a -fille d v o lc a n ic piple is called as v o lc a n ic n e c k (fig . 1 2 .7 (6 )). G en ­ erally, volcan ic n eck s are c y lin d ric a l sh a p e d a n d m easu re 50 to 6 0 m in h e ig h t (a b o v e th e g ro u n d su rface) and 3 0 0 to 6 0 0 m in d ia m e te r. S o m e tim e s d ia trem e term is u sed to in d ic a te v o lc a n ic neck or

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% 12,7: Different types o f volcanic cones.

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GEOMORPHOLOGY 212

len t ex am p le o f a diatreme ex p o se d by the erosion o f its en clo sin g sed im en tary ro c k s’ (F. P ress and R.

pipe filled w ith breccia. ‘S h ip ro ck * w hich tow ers 515 m etres (1700 feet) ov er the su rrounding, flatlying sedim entary rocks o f N ew M exico, is an excel-

S iev er 1974) (fig. 12.8).

Fig. 12.8 : Shiprock (New Mexico, USA), an example o f diatreme or volcanic neck.

Depressed Forms

th eir size e.g. craters ran g e fro m sm all craterlets Craters— T he d ep ressio n fo rm ed at hthe av in g a d ia m ete r o f a few h u n d re d m e tre s to la rg e m outh o f a volcanic vent is called a crater o r a craters h av in g th e d ia m e te r o f a few k ilo m e tre s. T he volcanic mouth, w h ich is usually funnel shaped. c rater o f e x tin c t A n ia k c h a k v o lc a n o o f A la sk a h as a

(i)

T he slope o f the c ra te r d ep en d s upon th e vo lcan ic cone in w hich c ra te r is fo rm ed . N o rm ally , a c rater fo rm ed in a c in d e r co n e slo p es at the an g le b etw een 25° an d 30°. T h e size o f a c ra te r in c re ases w ith in c re ase an d e x p an sio n o f its co n e. A c ra te r m ay be d iffe re n tia te d fro m a c a ld e ra on the b asis o f size and m o d e o f fo rm atio n . A n av era g e c ra te r m e asu re s 300 m in d ia m e te r an d 3 0 0 m in d e p th b u t th e re is w ide ra n g e o f v aria tio n s in c ra te rs fro m th e sta n d p o in t o f

d ia m e te r o f 9 .6 km (6 m iles) a n d th e sid e w alls are 364 m to 91 2 m (1 2 0 0 to 3 0 0 0 fe e t) hig h . I f the Crater Lake o f th e state o f O re g o n (U S A ) is a c ­ ce p te d as a c ra te r, it b e c o m e s o n e o f th e m o st e x te n siv e c ra te rs o f th e w o rld , th o u g h m an y sc ie n ­ tists c o n s id e r it as an e x a m p le o f a ca ld e ra . W h en a c ra te r is filled w ith w a te r, it b e c o m e s a crater lake.

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W hen the crater o f v o lca n o b ecom es very ex ten siv e and if there are fe w eruptions o f very sm all

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213

VULCANICITY AND LANDFORM S

intensity a fte r long tim e, sev eral sm aller co n es are form ed w ithin the e x ten siv e o ld er c ra te r and thus several sm all-sized craters are fo rm ed at the m o u th o f each v o lcan ic v en t in sid e the e x ten siv e crater. Such craters or craterlets are called ‘nested cra­ ters’ or ‘craters within the crater’ o r ‘grouped craters’. S uch c ra te rs are fo rm ed only w hen the next eruption is sm a lle r in in ten sity than the p rev io u s one. T he c ra te rs fo rm ed at the m o u th o f v o lcan ic vents o f p arasite co n es d e v e lo p e d o v er an ex ten siv e volcanic co n e is ca lle d adventive crater. T h ree sm aller c ra te rs are fo u n d w ith in the e x ten siv e crater o f M t. T aal o f P h illip p in e s. S im ilarly , th ree and tw o craters are fo u n d w ith in th e craters o f V isu v iu s and E tna v o lcan o es.

T arso Y eg a (20 km x 14 k m ) in S h a ra (A fric a ), A so San (23 km x 14 k m ) in Ja p a n , A lb a n (11 km x 10 km ) in Italy , C ra te r L ak e (1 0 km x 10 k m ) in U S A , K rak ato a (7 km x 6 k m ) in In d o n e sia , K ila u e a (5 km x 3 k m ) in H aw aii etc. S m a lle r c a ld e ra s h o u se d in a big c a ld era are c a lle d nested calderas o r grouped

calderas (fig. 12.9). C aldera

(ii) Calderas— G e n e ra lly , en larg ed form crater is c a lle d ca ld e ra . T h e re are tw o p arallel c o n ­ cepts fo r th e o rig in o f ca ld e ra s. A cco rd in g to the first group o f sc ie n tists a c a ld e ra is an en larg ed form o f a cra te r and it is s u rro u n d e d by steep w alls from all sid es. T h e c a ld e ra is fo rm ed d u e to su b sid en ce o f a c rater. T h is c o n c e p t has been p ro p o u n d ed by the U .S. G e o lo g ic a l S u rv ey . It is b eliev ed acco rd in g to this c o n c e p t th a t A so c ra te r o f Jap an and C rater L ake o f the U S A are th e re su lt o f su b sid en ce. T he second g ro u p o f s c ie n tis ts has o p in e d th at the cald eras are fo rm ed d u e to v io le n t a n d ex p lo siv e eru p tio n s o f v o lcan o es.

of a

Fig. 12.9 : Exam ple o f nested cladera.

Intrusive Topography W hen g ases an d v a p o u r a re n o t v e ry m u c h strong d u rin g v o lc an ic a c tiv ity , th e a s c e n d in g m a g ­ m as do not eru p t as lav as ra th e r th e se are in tru d e d in viods b elow the cru stal su rfa c e a n d a fte r c o o lin g a n d so lid ificatio n a ssu m e s e v e ra l in te re s tin g fo rm s lik e

batholiths, laccoliths, phacoliths, lopoliths, sills and dykes. T h e se in tru siv e v o lc a n ic fo rm s a re seen only w hen th e s u p e rin c u m b e n t lo a d s o f o v e rly in g co u n try ro ck s are re m o v e d th ro u g h p ro lo n g e d e r o ­ sion. T h ese featu res h av e a lre a d y b e e n d is c u s s e d in the p rece d in g c h a p te r 8 on rocks.

D aly, the le a d in g a d v o c a te o f ‘eruption hy­

pothesis’ o f th e o rig in o f c a ld e ra s, b eliev es th at the to p o g rap h ic fe a tu re s fo rm e d by su b sid e n c e are ‘vol­ canic sinks.’ A c c o rd in g to th e a d v o c a te s o f this

Geysers

h y p o th esis if c a ld e ra s are fo rm ed d u e to su b sid en ce there sh o u ld n o t be any d e p o s it o f p y ro c la stic m a te ­

G ey ser, in fact, is a sp e c ia l ty p e o f hot spring w h ich sp o u ts h o t w a te r an d v a p o u r fro m tim e to

rials and v o lc a n ic a sh e s re la te d to a p a rtic u la r v o l­

tim e. T h e w o rd g e y s e r h as b e e n d e riv e d fro m an Icelan d ic w o rd ‘geysir’ w h ic h m e a n s gusher o r

canic co n e n e a r the c a ld e ra b u t e v id e n c e s h av e revealed th a t th e re m a in s o f v o lc a n ic m a te ria ls re ­

spouter. T h is w o rd w as u sed to in d ic a te the sp o u tin g w ater o f a h o t s p rin g o f Ic e la n d k n o w n as Great Geyser o r Gesir.

lated to a p a rtic u la r c o n e are fo u n d n o t o n ly n e a r the concerned c a ld e ra but a re a lso fo u n d s ev eral k ilo m e ­ tres aw ay from the c a ld e ra . F o r e x a m p le , v o lc an ic

G e y se r, re p re s e n tin g a m in o r form o f the b ro a d e r p ro c e ss o f v u lc a n ic ity , h as b een v a rio u sly

m aterials h av e been fo u n d at th e d is ta n c e o f 128 km from the c a ld e ra o f C ra te r L ak e (U S A ). T h e s ig n ifi­ cant c ald eras o f th e w o rld are (fig u re in th e b ra c k e ts denote dim ension in k ilo m e tre s)L a k e T o b a o fS u m a tra (50 km x 50 k m ) in S u m a tra , A ira (25 km x 24 km ) in Japan, L ak e K u tc h a io (2 6 km x 2 0 k m ) in Ja p a n , https://telegram.me/UPSC_CivilServiceBooks

d efin ed by the scien tists. F o re x a m p le , A rth u rH o le m s has d e fin e d g e y s e r in th e fo llo w in g manner-. “G e y ­ sers are h o t sp rin g s fro m w h ic h a co lu m n o f h o t w a te r an d steam is e x p lo siv e ly d isch arg e d a t in te r­

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G EO M O R PHOLOGY

214

vals, sp o u tin g in so m e cases to h eig h ts o f h u n d red s

p ie, G ra n d G e y s e r o f Ic e la n d s p o u ts w a te r for 30

o f fe e t.” A cco rd in g to P .G . W o rc e ste r “G e y se rs are

m in u te s in c o n tin u a tio n b e fo re th e n ex t interval

in te rm itte n t ho t sp rin g s th a t fro m tim e to tim e sp o u t

p e rio d sta rts) an d (iv ) f e e b le g e y s e r (w h erein the

steam and hot w a te r fro m th e ir c ra te rs .”

activ e p e rio d o f w a te r s p o u tin g is v ery sh o rt). C o n ­

"

tin u o u s ly a c tiv e g e y s e r s are, in fa c t, hot springs

T h e d iffe re n c e b etw ee n h o t sp rin g s and g e y ­

w hich spout w ater w ith o u t an y in terv al. T h e Excelsior

ser lies in the fact th a t th ere is co n tin u o u s sp o u tin g

G ey ser o f the Y ello w S to n e N a tio n a l P ark o f the

o f h o t w a te r fro m th e f o r m e r w h ile th e re is

U S A is th e e x am p le o f th is c a te g o ry .

in te rm itte n t(w ith in terv al) sp o u tin g o f w ater from the alter. A g ey ser sp o u ts w ater from a sm all and

T h ere is no certain o b s e rv a b le d istrib u tio n a l

n arrow vent w h ich is c o n n ec ted by a circu ito u s pipe

p attern o f g e y se rs o v er th e g lo b e as th ey are found

w ith the u n d erg ro u n d aquifers. T h is pipe is called as g e y s e r p ip e or g e y s e r tu b e . T he length o f g ey ser tube ran g es betw een 30 to 100 m at d ifferent places. T h e te m p eratu re o f w ater co m in g out o f a g ey ser

in alm o st all the c o n tin e n ts an d in a lm o s t all the clim atic zones. T he g ey sers o f the U S A , Ic e la n d and N ew Z ealan d are m o st w id ely stu d ie d g e y se rs. G e y ­ sers are found in g ro u p s in the Y ello w S to n e N a ­ tional Park (U SA ). A b o u t one h u n d re d g e y se rs h ave

ran g es betw een 75° to 90°C. G eysers are classified into tw o types viz. (i) pool type o f geyser and (ii) nozzle t]/pe o f geyser. W hen a geyser spouts w ater through an open and relatively large pool, it is called po o l ty p e o f g ey ser. Such geysers spout larger volum e o f w ater and vapour through long geyser tubes. N o deposits are possible around the geyser pools. N ozzle ty p e o f g ey sers spout w ater and vapour through a very small and constricted vent. Em itted m aterials are d ep o s­ ited around the geyser vents and thus g ey ser cones are form ed.

been nam ed and an o th er h u n d red g e y se rs are k n o w n to the scien tists. T h ere are fo u r m a jo r b a sin s o f g ey sers viz. (i) N o rris B asin, (ii) U p p e r L a k e B asin , (iii) L o w er L ake B asin and (iv ) H eart L ak e B asin. T he m ajo r g ey ser o f N ew Z e a la n d is lo c ated in the w estern region o f the n o rth ern Islan d w h ich is also dom in ated by v o lcan ic ac tiv itie s. T h e g e y se rs and hot springs are spread o v er an a re a o f 1786 km 2 (5000 square m iles) in Iceland. T h e m o st s ig n ific a n t g eyser o f Iceland is G ran d G ey ser.

Som e scientists do not agree to accept hot sp rin g s and g e y se rs as tw o sep arate fo rm s o f vulcanicity rather they believe that both are the sam e, the difference is only o f periodicity o f sp o u t­ ing o f water. Thus, they have grouped geysers into two categories viz. (1) no n -co n tin u o u s geysers or geysers w ith interm ittent spouting and (2) co n tin u ­ ously active geysers. The in term itten t geysers are further divided into (i) geysers o f equal intervals between two successive period o f spouting (w herein interval period betw een two successive active p eri­ ods of spouting is certain and fixed, such geysers are, thus, considered to be reliable as regards the p eriods of interval and spouting, exam ple, O ld F aithful G ey ­ ser of the Y ellow Stone N ational Park, U SA ), (ii) v a ria b le geysers (w herein the interval period b e ­ tw een tw o successive periods o f spouting is not certain), (iii) lo n g -p e rio d g ey sers (w herein the ac­ tive period o f spouting is longest o f all the geysers, ranging betw een a few m inutes to one hour, exam -

F um arole m ean s such a v en t th ro u g h w hich there is em ission of g ases and w a te r v ap o u r. It appears from a d istan t place th a t th ere is em issio n o f enorm ous volum e o f sm o k es from a p a rtic u la r c e n ­ tre. Thus, sm oke o r gas e m ittin g v en ts are called fum aroles. In fact, fu m aro les are d ire c tly lin k e d w ith volcanic activ ities. E m issio n o f g ases and v a p o u r

12.10 FUM AROLES

begins after the em issio n of v o lc an ic m a teria ls is term inated in an active v o lcan o . S o m e tim es the em ission o f gases and v ap o u r is c o n tin u o u s but in m ajority o f the cases em issio n o ccu rs a fte r intervals. It is believ ed that g ases and v a p o u r are g en era ted due to co o lin g and co n tractio n o f m a g m a afte r the term i­ nation o f the eru p tio n o f a v o lcano. T h e se gases and vap o u r ap p ear at the earth 's su rface th ro u g h a narrow and co n stricted pipe (tube). It m ay be po in ted out th at fu m aro les are the last sig n s o f th e activ en ess o f a volcano.

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N u m ero u s fu m aro les are fo u n d in groups near Katm ai volcano o f A laska (U SA ). H ere fum aroles

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are found in groups in ex ten siv e v a lley zone, w hich is called a v a lle y o f te n th o u sa n d s m o k e s ’ w hich

9 8 .4 to 9 8 .9 9 p ercen t o f the total g a ses em itted from fum aroles. Other g a ses in clu d e carbon d io x id e, h y ­

m eans fu m aroles appear from 10 ,0 0 0 vents the d i­

drochloric acid, hydrogen su lp h id e, nitrogen, som e o x y g en and am m onia. S o m e m inerals are a lso em it­

ameter o f w h ich is around 3 m etres. Here fum aroles

ted w ith g a s e s and v a p o u r fro m f u m a r o le s . Sulphur is the m ost im portant m ineral. F um aroles dom inated by sulphur are ca lled s o lfa ta r a or s u l­

are found along a linear fracture. Elsew here, fumaroles are found a lo n g the v o lca n ic craters. The tem pera­ ture o f vapour em itted from fum aroles is around 645°C . It m ay be m en tio n ed that vapour constitutes

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p h u r fu m a r o le s.

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:

MOUNTAIN BUILDING

216*246

In tr o d u c tio n ; c la s s ific a tio n o f m o u n ta in s ; b lo ck m o u n ta in s ; fo ld e d m o u n ta in s ; g e o s y n c lin e s ; th eo ries o f m o u n tain b u ild in g - g e o s y n c lin a l th e o r y o f K o b e r ; therm al co n tra ctio n th eory o f J e ffrey s ; slid in g c o n ti­ n e n t th e o r y o f D a ly ; therm al c o n v e c tio n current thery o f H o lm e s ; r a d ia c tiv ity th eo ry o f J o ly ; p late te c to n ic th eory.

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CHAPTER 13

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13 MOUNTAIN BUILDING

ta in o u s re d o n o f th e w e s te rn p a r t o f N o rth A m e ric a

13.1 INTRODUCTION M o u n ta in s ir e sig n ific a n t re lie f fe a tu re s o f the seco n d o rd er on th e earth s s u rta c e . A m o u n ta in m av have sev eral fo rm s viz. (1) m o u n ta in rid g e, iji) m o u n tain ran g e. viiO m o u n ta in ch ain , (iv ) m o u n tain system , (v) m o u n ta in g ro u p an d (v i) c o rd illera . A m o u n ta in rid g e is a sy stem o f lo n g , narro w and hig h hills. G en erally , th e slope o f one side o f a rid g e is steep w hile die o th e r side is o f m o d erate slope but a ridge m ay also h av e sy m m e trica l slo p es on b o th the sides. A m o u n ta in ra n g e is a sy stem o f m o u n tain s and hiTk h av in g several rid g es, p eaks and su m m its and valleys. In fact, a m o u n tain range stretch es in a lin ear m anner. In o th e r w o rd s, a m o u n tain range rep resen ts a long but narrow strip o f m o u n tain s and hills. A ll o f the hills o f a m o u n tain ran g e are o f the sam e age but there are stru ctu ral v aria tio n s in d iffe r­ ent m e m b ers o f the range. A m o u n ta in ch a in c o n ­ sists o f sev eral p arallel long and n arro w m o u n tain s

is th e b est e x a m p le o f a c o r d ille r a .

13.2 CLASSIFICATION O F MOUNTAINS 1. On the Basis of Height (i) lo w m o u n ta in s; h e ig h t ra n g e s b e tw e e n 7 0 0 to 1.00 m . (ii) rough m ountains; height-1000 m to 1.500 m (iii) rugged m ountains; h eig h t-1.500 to 2.0 0 0 m (iv) h ig h m o u n ta in s; h e ig h t a b o v e 2 .0 0 0 m

2. On the Basis of Location (i) C o n tin en ta l m o u n ta in s (a) coastal m o u n ta in s, e x a m p le s: A p p la c h ia n s, R ockies, A lpine m o u n tain ch ain s. W e ste rn a n d E a s te rn G h ats o f In d ia etc. (b) in la n d m o u n ta in s, e x a m p le s ; U ra l m o u n ­ tain s (R u ssia). V o sg e s and B la c k F o r e s t b lo c k m o u n ­ tains (E urope). H im alay as, A ra v a llis, S a tp u ra . M aik al, K aim u rs etc. (In d ia ), K u n lu n , T ie n s h a n , A lta i etc. (A sia) etc.

o f different periods. S om e tim es. the m o u n tain ranges are separated by flat upland or plateaus. A m o u n ta in sy ste m con sists o f different m ountain ranges o f the sam e period. D ifferen t m ountain ranges are sepa­

(ii) O c e a n ic m o u n ta in s -m o s t o f the o cea n m ountains are b elo w w ater su rface (b e lo w sea lev el). O cean ic m ountains are lo ca ted on continental sh elv es and ocean flo o rs. S o m e o c e a n ic m ountains are also w ell a b o v e the sea le v e l. If the h eigh t o f the m ountains is co n sid ered from the o c e a n ic flo o r and not from se a -le v e l, m any o f the o c e a n ic m ountains

rated by valleys. A m o u n ta in g ro u p co n sists o f several unsystem atic patterns o f different m ountain system s. C o r d ille r a co n sists o f several m ountain groups and system s. In fact, cordillera is a co m m u ­ nity o f m ountains having d ifferent ridges, ranges,

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m ountain ch ain s and m ountain system s. The m oun­

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m o u n t a in b u il d in g

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will becom e m u c h h ig h e r than the M ount E verest For ex am p le M a u n a K ea volcanic m ountain o f Hawaii Islan d is 4 2 0 0 m h ig h from the sea level but if its h eig h t is c o n sid e re d from th e sea bottom it height b eco m es 9 1 4 0 m w hich is h ig h er than ’the highest m o u n ta in , M o u n t E v erest (8848 m AMSL1

K ilam ean m ountains etc. (N orth A m erica), m oun­ tains o f Feno-Scandia, N orth-W est H ighlands and A nglesey etc. (Europe). (2) Caledonian mountains: mountains formed during Silurian and D evonian periods, exam ples : T aconic m ountains o f the A pplachian system , m oun­ tains of Scottland, Ireland and Scandinavia (E u­ rope), B razilid es o f S outh A m erica, A ravallis, M ahadeo, Satpura etc. o f India.

SLmilar,y’ th£ An,ilean Mountain

system is 3 0 0 0 m a b o v e sea-lev el b u t it is also 5400 m below se a -le v e l, an d th u s its total height from the oceanic flo o r b e c o m e s 8 4 0 0 m. M o st o f the oceanic m o u n ta in s a re v o l c a n i c m o u n ta in s .

(3) H e rc y r ;an m ountains: m ountains form ed during Perm ian and Perm ocarboniferous periods, exam ples: m ountains o f Iberian peninsula, Ireland, Spanish M esseta, B rittany of France, S outh W ales, Cornw all, M endips, Paris basin, B elgian coalfields, Rhine M ass, B ohem ian plateau, V osges and B lack F o re s t, p la te a u re g io n o f c e n tr a l F r a n c e , T h u ringenw ald, F ran k en w ald , H a rtz m o u n ta in , Donbas coalfield (all in E urope); V ariscan m o u n ­ tains o f A sia include A ltai, Sayan, B aikal A rcs, T ien Shan, Khingan, m ountains o f D zu n g arian b asin, Tarim basin, N anshan, A lai and T ran s A lai m o u n ­ tains o f A m ur basin, M o n golia and G obi etc; A u ­ stralian V ariscan m ountains include the scattered hills in the Eastern C ordillera, N ew E n g lan d o f N ew Southerw ales; N orth A m erican V ariscan m o u n tain s include A pplachians; S outh A m eric an V a riscan m ountains are A ustrian and S aalian fo ld s o f S an Juan and M endoza, m ountains o f Puna arc o f A tacam a, G ondw anides o f A rgentina etc.

3. On the B a sis of Mode of Origin (1) O rig in a l or tecton ic m ountains are caused due to t e c t o n ic f o r c e s e.g. c o m p r e s s iv e and tensile forces m o t o r e d b y e n d o g e n e t ic fo rces c o m in g from d e e p w i t h in th e e a rth . T h e s e m o u n ta in s are further di v id e d i n to 4 t y p e s o n th e b a s is o f o ro g en e tic forces r e s p o n s i b le f o r th e o r ig in o f a p a rtic u la r type o f m o u n ta in . (1) F o ld e d m o u n ta in s are fu rth er divided in to 3 s u b - t y p e s o n the b a s is o f their area. T h e se are o r ig i n a te d by c o m p r e s s i v e forces. (A ) y o u n g fo ld e d m o u n ta in s (B ) m a t u r e fo ld e d m o u n ta in s (C ) o ld fo ld e d m o u n ta in s (ii) B lo c k m o u n ta in s are originated by ten ­ sile f o r c e s l e a d i n g to th e fo rm a tio n o f rift valleys. T h e y a r e a ls o c a lle d as h o r s t m o u n ta in s .

(4) A lpine m ou n tain s : m o u n tain s fo rm ed during Teritary period, ex am ples: R o ck ies (N o rth A m erica), A ndes (S outh A m erica), A lp in e m o u n ­ tain system s o f E urope (m ain A lp s, C arp ath ian s, Pyrenees, B alkans, C au casu s, C an tab rian s, A pen­ nines, D inaric A lps etc.), A tlas m o u n tain s o f n o rth ­ w est A frica; H im alayas and m o u n tain s c o m in g out o f Pam ir K not o f A sia (T au ru s, P au n tic, Z agros, Elburz, K unlum etc.).

(iii) D o m e m o u n ta in s are o rig in ated by m a g m a t i c i n tr u s io n s a n d u p w a r p in g o f the crustal surface. E xam ples, normal domes, lava domes, batholithic dom es, laccolithic domes, salt domes etc. (iv ) M o u n ta in s o f a c c u m u la tio n s are form ed d u e to a c c u m u la tio n o f v o lc a n ic m ate ria ls. T hus, th e s e a re a ls o c a lle d as v o lc a n ic m o u n ta in s. D iffe r­ en t ty p e s o f v o lc a n ic c o n e s (e.g. cin d er cones, co m p o s­

Block Mountains

ite c o n e s , a c id la v a c o n e s , b a sic lava c o n e s etc.)

B lock m o untains, also know n as fau ltb lo ck m ou n tain s, are the resu lt o f fau ltin g cau sed by te n s ile a n d c o m p r e s s iv e f o r c e s m o to r e d by endogenetic forces co m in g from w ithin the earth. B lock m ountains represent the u p stan ding p arts of the ground betw een tw o faults o r on e ith er side of a rift valley or a graben. E ssen tially , b lo ck mountains are form ed due to faulting in the g ro u n d surface.

c o m e u n d e r th is c a te g o r y .

(2) C i r c u m - e ^ o s i o n a l o r re lic t m o u n ta in s : exam ples, V in d h y ach al ranges, A rav alh s, Satpura, E astern G h ats, W estern G h ats etc. (all from India).

4. On the basis of period of origin (1 ) P r e -C a m b r ia n m o u n ta in s : examples,

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L a u ren tia n m o u n ta in s, A lg o m a n m o u n ta in s,

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geom o rph

218

r e p r e s e n te d b y fa u lt s c a r p a n d o n e g e n tle side and(ii) lifted block m ountains r e p r e s e n t real horst and are c h a r a c te r iz e d by f la tte n e d s u m m i t o f tab u la r shape

Horst

and very steep side slo p e s re p re sen te d by tw o boundary fault s c a rp s . B lo c k m o u n ta in s a re a lso called as horst mountains (fig. 13.1).

If

B lo c k m o u n ta in s a re found in all the conti. n e n ts e.g. (i) y o u n g b lo c k m o u n ta in s a ro u n d Albert, W a r n e r an d K la m a th lak e s in the S te e n s M ountain D istrict o f S o u th e r n O r e g o n , W a s a t c h R a n g e in the U tah p r o v in c e etc. in the U S A , (ii) V o s g e s and Black F o re st m o u n ta in s b o r d e r in g the fa u lte d R h in e Rift va lle y in E u r o p e , (iii) S a lt R a n g e o f P a k ista n etc. S ierra N a v a d a m o u n ta in o f C a l if o r n ia ( U S A ) is c o n s id e r e d to be the m o s t e x te n s iv e b lo c k m ountain o f the w orld. T h is m o u n ta in e x te n d s fo r a length of 6 4 0 km ( 4 0 0 m ile s ) h a v in g a w id th o f 80 km (50 m iles) and the h e ig h t o f 2 ,4 0 0 to 3 ,6 6 0 m (8 ,0 0 0 to 12,000 feet). T h e r e is d if f e r e n c e o f o p i n io n s am o n g the sc ie n tists r e g a r d in g the o rig in o f b lo c k m o u n ­ tains. T h e re are tw o th eo ries for the o rig in o f these m o u n ta in s viz. ( 1) f a u lt th e o ry a n d (ii) ero sio n

th e o ry . F ault T heory M o st o f the g e o lo g is ts are o f the o p in io n that block m o u n ta in s are fo rm e d d u e to faulting. T h e structural pa tte rn s o f G re a t B asin R a n g e m o u n ta in s o f U tah p r o v in c e ( U S A ) w e re c lo se ly s tu d ie d by C la re n c e K ing and G .K . G ilb e r t w h o n a m e d these m o u n ta in s as f a u l t e d b l o c k s ( b e tw e e n 1870 and 1875 A .D .). S in c e then the m o u n ta in s f o rm e d d u e to larg e -sc a le fa u ltin g w e re n a m e d b lo c k m o u n ta in s. L a ter on G .D . L o u d e r b a c k o p in e d that B asin Range m o u n ta in s w e re fo rm e d d u e to f a u ltin g a n d tilting in the g ro u n d s u rfa c e . W .M .D a vis a lso a d v o c a te d for the fault th eo ry o f the o rig in o f b lo ck m ountains. B lo c k m o u n ta in s are fo rm e d in a n u m b e r o f ways.

C niock Mountain

mock

M o u n t a in

(i) B lo c k m o u n ta in s are f o rm e d due to up­ w ard m o v e m e n t o f m id d le b lo ck b e tw e e n tw o nor­ mal faults (fig. 13.1 ). T h e u p th ro w n block is also ca lled as horst. T h e s u m m ita l a rea o f such block m o u n ta in is o f Hat s u rf a c e but the side slopes are very steep.

Fig. 13.1 : A-lilock mountain form ed due to rise o f m id­ dle block, /?-form ation o f block mountain due to downward movement o f side blocks and Cform ation o f block mountain due to down­ ward movement o f middle block-due to rift vailey formation.

(ii) B lo c k m o u n ta in s m ay be fo rm e d when the side b locks o f tw o faults m o v e d o w n w a rd whereas the m id d le b lo ck re m a in s s ta b le at its place (fig1 3.1B). It is a p p a re n t that the m id d le b lock projects

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B lo c k m o u n ta in s are g e n e ra lly o f tw o basic types e.g. (i) tilted b lock m ou n tain s ha v in g o n e steep side

j ^

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m ou nta in b u il d in g

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above the surrounding surface because o f downward m ovem ent o f sid e b lo ck s. Su ch b lock m ountains are generally form ed ,n h igh plateaus or broad dom es

c u m bent folds caused by pow erful com pressive forces. ( 2 ) F o ld e d m o u n ta in s are c la s s ifie d in to (i) you n g fold ed m ou n tain s (w h ic h a re le a s t a ffe c te d * by d e n u d a tio n al p ro c e s s e s) a n d (ii) m atu re fo ld ed m ou n tain s. It m a y be p o in te d o u t th a t it is d iffic u lt to find true y o u n g fo ld e d m o u n ta in s b e c a u s e th e p ro ce ss o f m o u n ta in b u ild in g is e x c e e d in g ly slo w p ro ce ss and thus d e n u d a tio n a l p r o c e s s e s s a rt d e ­ n u d in g the m o u n ta in s rig h t fro m the b e g in l i n g o f their origin. M a tu re fo ld e d m o u n ta in s a re ch lra c te rized by m o n o c lin a l rid g e s a n d v a lle y s. T h is classifi­ cation is b a s e d on the a g e factor.

u J hi' ) ® lo c k m o u n ta in s m ay b e fo rm ed w hen the m id d le b lo c k b e tw e e n tw o n o rm al fau lts m oves dow nw ard. T h u s , th e s id e b lo c k s b e c o m e horsts and block m o u n t a i n s ( fig . 13 i q S u r h ♦ • • . . , ■, . f ^ u c n m o u n ta in s are a s s o c ia te d w i t h t h e f o r m a t i o n o f r ift v a lle y s

Erosion Theory J .F . S p u r r , o n th e b a s is o f d e ta i le d stu d y o f G reat B a s i n R a n g e m o u n t a i n s o f the U S A o p in e d that th e s e m o u n t a i n s w e r e n o t f o r m e d d u e t o ’fau ltin g and tiltin g , r a t h e r t h e y w e r e f o r m e d d u e to d if f e r e n ­ tial e r o s i o n . A c c o r d i n g to S p u r r th e m o u n ta in s , after their o r i g i n in M e s o z o i c e ra , w e r e s u b je c te d to intense e r o s i o n . C o n s e q u e n t l y , d iff e re n tia l e ro s io n re s u lte d i n t o t h e f o r m a t i o n of e x is t in g d e n u d e d G re a t B asin R a n g e m o u n t a i n s . It m a y be p o in te d out that e ro s io n t h e o r y o f t h e o r i g i n o f b l o c k m o u n ta in s is not a c c e p ta b le to m o s t o f th e s c ie n t is ts b e c a u s e they b e lie v e t h a t d e n u d a t i o n m a y m o d i f y m o u n ta ns but c a n n o t f o r m a m o u n t a i n . In fact, d e f o r m a t o r y p r o c ­ ess p la y m a j o r r o l e in th e o r i g i n o f b l o c k m o u n ta in s .

(3) O n the basis o f the p e r io d o f o r ig in f o ld e d m o u n ta in s are d iv id e d into (i) o ld fo ld ed m o u n ta in s a nd (ii) new fold ed m o u n ta in s. All th e o ld f o ld e d m o u n ta in s w e re o r ig in a te d b e fo r e T e r ti a r y p e r i o d . T h e folded m o u n ta in s o f C a l e d o n ia n a n d H e r c y n i a n m o u n ta in b u ild in g p e rio d s c o m e u n d e r th is c a t ­ egory. T h e s e m o u n ta in s h a v e b e e n so g r e a tly d e ­ n u ded that they h a v e n o w b e c o m e r e lic t -folded m o u n t a i n s , for e x a m p le , A r a v a llis , V i n d h y a c h a l etc. T he Alpi ne fo ld e d m o u n ta in s o f T ertiary' p e r i o d are g ro u p e d u n d e r the c a te g o r y o f n e w f o ld e d m o u n ­ tains, for e x a m p le , R o c k ie s , A n d e s , A lp s , H i m a l a ­ yas etc.

Folded Mountains F o l d e d m o u n t a i n s a re f o r m e d d u e to fo ld in g

C h a ra c te ris tic s o f F o ld e d M o u n ta in s (1) F o ld e d m o u n ta in s a re th e y o u n g e s t m o u n ­ tains on the e a rth 's s u rfa c e.

o f c ru s ta l r o c k s b y c o m p r e s s i v e fo rc e s g e n e ra te d by e n d o g e n e t ic f o r c e s c o m i n g f r o m w ith in the earth. T h e s e a re t h e h i g h e s t a n d m o s t e x te n s i v e m o u n ta in s o f the w o r l d a n d a r e f o u n d in all the c o n tin e n ts . T h e d is tr ib u tio n a l p a t t e r n o f f o l d e d m o u n t a i n s o v e r the g lo b e d e n o t e s th e f a c t t h a t th e y a re g e n e r a lly fo u n d a long th e m a r g i n s o f t h e c o n t i n e n t s e it h e r in n o r t south d i r e c t i o n o r e a s t - w e s t d i r e c ti o n . R o c k ie s , A n ­ des, A lp s , H i m a l a y a s , A t l a s e tc . a re th e e x a m p l e s o t fo ld e d m o u n t a i n s . F o l d e d m o u n t a i n s a re c assi le

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(2) T h e lith o lo g ic a l c h a r a c te r is t ic s o f f o l d e d m o u n ta in s rev e a l th a t th e s e h a v e b e e n f o r m e d d u e to fo ld in g o f s e d im e n ta ry r o c k s by s tr o n g c o m p r e s s i v e forces. T h e fo ss ils fo u n d in th e r o c k s o f f o ld e d m o u n ta in s d e n o te the fa c t th a t th e s e d i m e n t a r y r o c k s o f these m o u n ta in s w e re f o r m e d d u e to d e p o s i t i o n and c o n so lid a tio n o f s e d im e n ts in w a t e r b o d ie s m a in ly in o c e a n ic e n v i r o n m e n t b e c a u s e th e a r g i l l a c e o u s on v a rio u s b a s e s a f o ll o w s . r o c k s o f fo ld e d m o u n ta in s c o n t a i n m a r i n e fo s s ils . (1) F o l d e d m o u n t a i n s a r e d i v i d e d into 2 b ro a d (3) S e d i m e n ts a re f o u n d u p t o g r e a t e r d e p t h s , c a te g o rie s on t h e b a s i s oi t h e n a tu r e o o s. th o u s a n d s o f m e t r e s ( m o r e th a n 12 ,000 m e t r e s ) . S im p le fo ld e d m o u n t a i n s w i t h o p en 0 s u B a s e d on this fa c t s o m e s c ie n tis ts h a v e o p i n e d th a t m o u n ta in s a r e c haracterized by w e ll d e v e o p e sys the s e d im e n ts i n v o lv e d in th e f o r m a t i o n o f s e d i m e n ­ tern o f a n ti c li n e s and synclines w h e r e in o s a tary r o c k s o f f o ld e d m o u n t a i n s m i g h t h a v e b e e n a rra n g e d in w a v e - l i k e p a t t e r n . T h e s e m o u n ta in s h a v e d e p o s ite d in d e e p o c e a n i c a re a s b u t th e m a r i n e o pen a n d r e l a ti v e ly s i m p l e fols. (ii) C o m p ex o e fo ss ils f o u n d in th e r o c k s b e lo n g to s u c h m a r i n e o r ­ m o u n ta in s r e p r e s e n t v e r y c o m p l e x s tr u c tu r e o in^ g a n is m s w h i c h c a n s u r v i v e o n ly in s h a l l o w w ater or ten se ly c o m p r e s s e d f o ld s . S u c h c o m p l e x stru c ure s h a llo w sea. It m e a n s t h a t th e s e d i m e n t a r y rock s o f o f folds is c a l l e d ‘n a p p e ’ . In fac t, c o m p l e x fo ld fo ld e d m o u n ta in s w e r e d e p o s i t e d in sh allow seas. m ountains are f o r m e d due to the f o r m a ti o n o re

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O n an average, a g eo sy n clin e m eans a water d ep ressio n c h a ra c te riz e d by se d im e n ta tio n . It has now b een acc ep ted by m a jo rity o f th e g eo lo g ists and g e o g ra p h e rs th a t all th e m o u n ta in s h a v e co m e o u t o f the g eo sy n c lin e s an d th e ro c k s o f th e m ountains o rig in ated as se d im e n ts w ere d e p o s ite d and later on c o n s o lid a te d in s in k in g s e a s , n o w k n o w n as g eo sy n clin es. I f w e c o n s id e r th e h e ig h t and thick­ ness o f sed im en ts o f the y o u n g fo ld e d m o u n tain s of T ertiary p erio d (e.g. R o c k ie s, A n d e s, A lp s, H im ala­ yas etc.), then it ap p ea rs th at th e g e o s y n c lin e s should have been very d eep w a te r b o d ie s b u t th e m arine fossils found in the se d im en tary ro c k s o f th e se folded m o u n tain s b elo n g to the c a te g o ry o f m a rin e organ­ ism s o f sh allo w seas. It is, th u s, o b v io u s th at the g eo sy n clin es are sh allo w w a te r b o d ie s ch aracterized by grad u al sed im en tatio n an d su b sid e n c e . B ased on above facts g eo sy n clin es can n o w be d efin ed as

T h e sea bottom s w ere su b jected to co n tin u o u s su b ­ sidence due to gradual sed im en tatio n . T hus, the greater thickness o f sed im en ts co u ld be p o ssib le due to continuous sed im en tatio n and su b sid en ce and consequent co n so lid atio n o f sed im en ts d u e to ev er increasing su p erin cu m b en t load. (4) Folded m ountains extend for greater lengths but their w idths are far sm aller than th eir len g th s, F or ex am ple, the H im alay as ex ten d from w est to east for a length o f 2400 km (1 5 0 0 m iles) but th eir northsoutjf w idth is only 400 km (250 m iles). It m eans that folded m o u n tain s have been form ed in long, n arrow and shallow seas. S uch w ater bodies have been term ed g eo sy n clin es ^nd it has been estab lish ed that ‘o u t o f .geosyn clin es h ave co m e ou t th e m o u n ­ ta in s ’ or ‘g eo sy n clin es h ave b een crad les o f m o u n ­ t a in s / A ccording to P.G . W o rcester ‘all g reat folded m ountains stand on the sites o f fo rm er g eo sy n clin es’.

follow s-

(5) F olded m o u n tain s are g enerally round in arch shape having one side concave slope and the other side convex slope.

‘G eo sy n clin e s are lo n g b u t n arro w and shal­ low w ater d ep ressio n s c h a ra c te riz e d by sed im en ta­ tion and su b sid e n c e ’.

(6) F olded m o u n tain s are found along the m argins o f the contin en ts facing oceans. F or exam ple^R ockies and A ndes are located along the w est­ ern m argins o f N orth and S outh A m ericas resp ec­ tively and face Pacific O cean. T hey are lo cated in tw o directions e.g. n o rth-south (e.g. R ockies and A ndes) and w est-east d irectio n s (e.g. H im alayas). T he A lpine m ountains are lo cated along the southern m a rg in ! o f E urope facing M ed iterran ean sea. If we consider form er T ethys Sea, then the H im alayas w ere also located along the m argins o f the continent.

J.A . S teers (1 9 3 2 ) has ap tly rem ark ed , ‘the g eo sy n clin es h ave been long and rela tiv e ly narrow d ep ressio n s w h ich seem to have su b sid ed d u rin g the accu m u latio n o f sed im en ts in th e m .’ T he fo llo w in g are the g en eral ch aracteristics o f g eo sy n clin es. (1) G e o sy n clin e s are lo ng, narrow and shal­ low d ep ressio n s o f w ater. (2) T h ese are c h a ra c te riz e d by g rad u al sedi­ m en tatio n and su b sid en ce. (3) T he n atu re and p attern s o f geosynclines have not rem ain ed the sam e th ro u g h o u t geological h istory rath er th ese have w id ely c h an g e d . In fact, the location, shape, d im en sio n and ex ten t o f geosynclines have co n sid erab ly ch an g e d d u e to e a rth m ovem ents and geological p ro cess.

13.3 GEOSYNCLINES Meaning and C oncep t The geological history o f the continents and ocean basins denotes the fact that in the beginning our globe was characterized by two im portant fea­ tures viz. (i) rigid m asses and (ii) geosyn clin es. Rigid m asses representing the ancient nuclei o f the present continents, have rem ained stable for co n sid ­ erably longer periods o f time. T hese rigid m asses are supposed to have been surrounded by m obile zones o f w ater characterized by extensive sedim entation. These m obile zones o f w ater have been term ed ‘geosynclines’ w hich have now been converted by com pressive forces into folded m ountain ranges.

(4) G eo sy n clin e s are m o b ile z o n e s o f water. (5) G eo sy n clin es are g e n e ra lly b o rd ered by tw o rigid m asses w h ich are ca lle d fo rela n d s. E volution of th e C o n cep t

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T he co n cep t o f g e o sy n clin es w as given by Jam es Hall and D ana but the co n c e p t w as elaborated and further d ev elo p ed by H aug. J.A . S teers (1932) has rem arked, ‘w hile the th eo ry o f g eo sy n clin es is

M

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due to H au g , th e c o n c e p t o f id e a b e lo n g s to H all and D ana’. It is d e s ira b le to d is c u s s th e c o n c e p t o f geo sy n clin es d e v e lo p e d b y d iffe re n t e x p o n e n ts.

m o u n tain s. H e o p in e d th a t th e ro ck s o f fo ld e d m o u n ­ tain s w ere d e p o site d in sh allo w seas. A c c o rd in g to H all th e b ed s o f g e o sy n c lin e s are su b je c te d to su b ­ sid en ce d u e to c o n tin u o u s se d im e n ta tio n b u t the (1) Concept o f Hall and Dana- D a n a stu d ied d ep th o f w ater in th e g e o sy n c lin e s re m a in s th e sam e the folded m o u n tain s and p o stu lated th at the sedim ents (fig. 13.2). G e o sy n c lin e s are m u ch lo n g e r th a n th e ir of the ro c k s o f fo ld e d m o u n ta in s w ere o f m arin e origin. T h e s e ro c k s are d e p o s ite d in lo n g , narro w and sh allo w seas. D a n a n a m e d su ch w a te r b o d ies as geosynclines. H e d e f in e d , f o r th e f ir s t tim e , g eo sy n clin es as lo n g , n a rro w an d sh allo w and sin k ­ ing beds o f seas.

Fig. 13.2 : Sinking beds o f geosynclines due to sedimen­ tation and subsidence. H all e la b o ra te d th e co n ce p t o f geosynclines as ad v an c ed by D an a. H e p resen te d am ple evidences to show re la tio n sh ip b etw een geosynclines and folded

w idths. (2) Concept of E . Haug - ‘I f th e id g eo sy n clin es is d u e to H all an d D an a, the th e o ry o f th eir d ev elo p m en t is really d u e to H a u g ’. H e d e fin e d geo sy n clin es as long and d eep w a te r b o d ie s. A c ­ co rd in g to H au g ‘g eo sy n clin es are re la tiv e ly d e e p w ater areas and they are m u ch lo n g e r th a n th e y are w id e.’ He drew the p a la e o g e o g ra p h ic a l m a p s o f th e w orld and d ep icted long and n arro w o c e a n ic tra c ts to d em o n strate the facts th at th ese w a te r tra c ts w ere subsequently folded into m o u n tain ra n g e s (fig. 13.3). He further postulated th at the p o sitio n s o f th e p re se n tday m ountains w ere p rev io u sly o c c u p ie d by o c e a n ic tracts i.e. g eo sy n clin es. G e o sy n c lin e s e x iste d as m obile zones o f w ater b etw een rig id m a sse s. H e identified 5 m ajo r rig id m asses d u rin g M e s o z o ic e ra e.g. (i) N orth A tlan tic M ass, (ii) S in o -S ib e ria n M a s s, (iii) A frica-B razil M ass, (iv) A u stra lia -In d ia -M a d a gascar M ass and (v) P acific M ass. H e lo c a te d 4 geosynclines betw een th ese a n c ie n t rig id m a s s e s

North Atlantic Continent

180° Eq«»tor Pacific Continent

Ojy1 A frkun -Hrazlleun Continent

(Jeosyncline —

vr.A

AstraHnii-liulian M adagascar G

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Fig. 13-3 ; distribution o f rigid masses and geosynclines during Mesozoic era as depicted by £ H aug

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H im a lay as, this d ep ressio n w as la ter on filled with sed im en ts to form In d o -G an g etic P lain s), (iii) it may be alo n g th e m a rg in s o f th e co n tin en ts, (iv) it may be in front o f a riv er m o u th etc. A cco rd in g to E vans all the g eo sy n clin es irre sp e c tiv e o f th e ir v arying forms, sh ap es and lo catio n s are c h a ra c te riz e d by tw in proc­ esses o f sedim entation and su b sid en ce. G eosynclines, after long p erio d o f sed im en tatio n , are sq u eezed and folded into m o u n tain ran g es.

e.g. (i) R o ck ies g eo sy n clin e, (ii) U ral g eo sy n clin e, (iii) T eth y s g e o sy n clin e and (iv) C irc u m -P acific g eo sy n clin e. A cco rd in g to H au g th ere is sy stem a tic s e d i­ m e n tatio n in the g eo sy n c lin e s. T h e litto ral m arg in s o f the g e o sy n c lin e s are a ffected by tran sg ressio n al and re g re ssio n a l p h ases o f th e seas. T h e m arginal areas o f the g e o sy n c lin e s h av e sh allo w w ater w h ere­ in la rg e r se d im e n ts are d ep o site d w h ereas fin er se d im e n ts are d e p o s ite d in ce n tra l p arts o f the g eo sy n clin es. T h e sed im en ts are sq u eezed and folded in to m o u n ta in ran g es d u e to c o m p re ssiv e forces c o m in g fro m the m a rg in s o f the g eo sy n clin es. He h as fu rth e r re m a rk e d th a t it is n o t alw ay s necessary th a t all the g e o sy n c lin e s m ay pass th ro u g h the co m ­ p le te c y c le o f the p ro cesses o f sed im en tatio n , su b ­ sid en ce, c o m p re ssio n and fo lding o f sedim ents. Som e tim es, no m o u n tain s are form ed from the geosynclines in sp ite o f c o n tin u o u s sed im en tatio n for long d u ra­ tio n o f g e o lo g ical tim e.

(4) V iew s o f S ch u ch ert- H e attem p ted t classify g eo sy n clin es on the b asis o f th e ir character­ istics related to th e ir size, lo catio n , evolutionary history etc. H e has d iv id ed g e o sy n c lin e s into 3 categories, (i) M o n o g eo sy n clin e s are exceptionally long and narrow but sh allo w w a te r trac ts as con­ ceived by H all and D ana. T h e g e o sy n clin al beds are subjected to co n tin u o u s su b sid en ce d u e to gradual sedim entation and resu ltan t load. S u ch g eo sy n clin es are situated eith er w ithin a c o n tin en t o r along its borders. T hese are called m ono b ecau se they pass through only one cycle o f sed im en tatio n an d m o u n ­ tain building. A pplachian g eo sy n clin e is co n sid ered to be the best exam ple o f m o n o g e o sy n clin es. In place o f the A p plachians (U S A ) there ex isted a long and narrow A ppalachian g eo sy n clin e d u rin g preC am brian period. T he g eo sy n clin e w as b o rd ered by highland m ass know n as A p p lach ia in the east. A pplachian geosynclines were folded from O rdovician to P erm ian periods.

T h o u g h th e co n trib u tio n s o f H aug in this re g a rd are p ra isew o rth y as he d ev elo p ed the concept o f g e o sy n c lin e s b u t his th eo ry suffers from certain serio u s d ra w b a c k s an d co n fu sin g ideas about them . H is p a la e o g e o g ra p h ic a l m ap (fig. 13.3) o f M esozoic era d ep icted unbelievable larger extent o f rigid m asses (lan d area s) in co m p ariso n to g eo sy n clin es (oceanic areas). Q u estio n s arise, as to w h at h ap p en ed to such e x te n siv e land m asses afte r M eso zo ic era ? W here d id they d isa p p e a r ? H aug co u ld not ex p lain these and m any m o re Q uestio n s. H is g eo sy n clin es as very d eep o cean ic tracts are also not accep tab le because the m arin e fossils found in the fo ld ed m o untains belong to the g ro u p o f m arin e o rg an ism s o f shallow

(ii) P o ly g e o sy n c lin e s w ere long and wide w ater bodies. T hese w ere d efin itely b ro a d e r than the m o n o g eo sy n clin es. T h ese g eo sy n clin es ex isted for relativ ely lo n g er period than the m o n o g eo sy n clin es and these have p assed th ro u g h c o m p lex ev o lu tio n ­ ary h istories. T hese are c o n sid ere d to have experi­ seas. m ore than one phase o f o ro g e n e sis, conse­ (3) C o n c ep t o f J .W . E van s- A cco rd in g enced to q uently they ‘m ay have been d iv e rsifie d by the E vans the g eo sy n clin es are so varied th at it becom es p roduction o f one or m ore p arallel g ean ticlin es aris­ d ifficu lt to p resen t th e ir d efin ite form and location. ing from th eir floors in the sq u e e z in g p ro c e ss’. They T he beds o f g eo sy n clin es are su b jected to gradual o r ig i n a te d in p o s i t i o n s s i m i l a r to th o s e of subsidence b ecause o f sed im en tatio n . T h e form and m o n o g eo sy n clin es. R ocky and U ral geosynclines shape o f g eo sy n clin es ch an g e w ith ch an g in g en v i­ are q u o te d as th e r e p r e s e n ta tiv e e x a m p le s of p o ly g eo sy n clin es.

ronm ental conditions. A g eo sy n clin e m ay be narrow o r wide. It m ay be o f d ifferen t shapes. T h ere m ay be several alternative situ atio n s o f g eo sy n clin es e.g. (i) it m aybe betw een tw o land m asses (ex am p le, T eth y s geosyncline betw een L au rasia and G o n d w an alan d ), (ii) it m ay be in front o f a m o untain o r a plateau (for exam ple, resu ltan t long tren ch after the origin o f the https://telegram.me/UPSC_CivilServiceBooks

(iii) M e so g e o sy n c lin e s are very long, narrow and m o b ile o cean b asin s w h ich are bordered by c o n tin en ts from all sides. T h ey are characterized by g reat ab y ssal d ep th and long and co m p lex geologic^ histo ries. T h ese g e o sy n c lin e s pass through sever

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g e o s y n c lin a l p h a s e s e .g . p h a s e s o f s e d im e n ta tio n , s u b s id e n c e a n d fo ld in g . M e s o g e o s y n c lin e s are s im i­ la r to th e g e o s y n c lin e s c o n c e iv e d by H a u g . T e th y s g e o s y n c lin e is the. ty p ic a l e x a m p le o f su c h ty p e. M e d i t e r r a n e a n S e a is th e r e m n a n t o f T e th y s g e o s y n c lin e . T h is g e o s y n c lin e w a s fo ld e d in to A l­ p in e m o u n ta in s o t E u ro p e an d th e H im a la y a s o f A sia. T h e u n fo ld e d r e m a in in g p o rtio n o f T e th y s g e o s y n c lin e b e c a m e M e d ite r r a n e a n sea, an e x a m p le o f median mass o f K o b e r.

ty p ic a l e x a m p le s o f su ch g e o s y n c lin e . T h is con cep t o f H o lm e s h as been s e v e re ly c ritic is e d b e c a u se the tra n s fe r an d d is p la c e m e n t o f m a g m a s c a n n o t cause s u b sid e n c e to form g e o s y n c lin e s .

(ii) F o r m a tio n o f G e o sy n c lin e s d u e to M e ta ­ m o r p h is m - A c c o rd in g to H o lm e s th e ro c k s o f the lo w e r la y e r o f th e c ru s t, as re fe rre d to a b o v e , are m e ta m o rp h o s e d d u e to c o m p re s s io n c a u s e d by c o n ­ v e rg in g c o n v e c tiv e c u rre n ts. T h is m a ta m o rp h is m in c re a se s th e d e n sity o f ro c k s, w ith th e re s u lt the (5) Concept of Arthur H o lm e s -B e s id e s dlo e ­w er la y er o f th e c ru s t is s u b je c te d to s u b sid e n c e s c rib in g m a in c h a r a c te r is tic s o f g e o s y n c lin s , A. an d th u s a g e o s y n c lin e is fo rm e d . C a rib b e a n S ea, th e w estern M e d ite rra n e a n S e a an d B a n d a S e a h a v e H o lm e s h a s a ls o e la b o r a te d th e c a u s e s o f th e o rig in b e e n q u o te d as e x a m p le s o f th is c a t e g o r y o f o f d if f e r e n t ty p e s o f g e o s y n c lin e s . H e h as a lso d e ­ g e o sy n c lin e s. T h is c o n c e p t h a s b e e n re je c te d o n th e s c rib e d th e d e ta ile d p r o c e s s e s an d m e c h a n ism s o f g ro u n d th at c o m p re ss io n c a u s e d by c o n v e r g e n t c o n ­ s e d im e n ta t io n a n d s u b s id e n c e a n d c o n s e q u e n t v ectiv e c u rre n ts w o u ld n o t c a u s e m e ta m o rp h is m o ro g e n e s is . A c c o r d in g to h im no d o u b t s e d im e n ta ­ ra th e r it w o u ld ca u se m e ltin g o f ro c k s d u e to r e s u lt­ tio n le a d s to s u b s id e n c e b u t th is p ro c e ss can n o t an t h ig h te m p eratu re. a c c o u n t fo r th e g r e a te r th ic k n e s s o f se d im e n ts in g e o s y n c lin e s r a th e r e a r th m o v e m e n ts can cau se su b ­ (iii) F o rm a tio n o f G e o s y n c lin e s d u e to C o m ­ s id e n c e o f h ig h m a g n itu d e in th e g e o sy n c lin a l beds. p ressio n -S o m e g e o s y n c lin e s a re f o rm e d d u e to H e f u rth e r p o in te d o u t th a t th e p ro c e s s o f su b sid en ce c o m p re ssio n an d re s u lta n t s u b s id e n c e o f o u te r la y e r o f th e g e o s y n c lin a l b e d s w as n o t a su d d en p ro cess o f the c ru st c au sed by c o n v e rg e n t c o n v e c tiv e cu r­ r a th e r it w a s a g ra d u a l p ro c e s s . T h e d e p o sitio n o f rents. P ersian G u lf an d I n d o -G a n g e tic tro u g h a re s e d im e n ts u p to th e th ic k n e s s o t 12,160 m (4 0 ,0 0 0 c o n sid e re d to be ty p ical e x a m p le s o f th is g ro u p o f fe e t) in th e A p p la c h ia n g e o s y n c lin e c o u ld be p o s si­ g eo sy n clin es. b le d u rin g a lo n g p e rio d o f 3 ,0 0 0 .0 0 0 ,0 0 0 y ears from (i v) F o rm a tio n o f G e o s y n c lin e s d u e to T h in ­ C a m b ria n p e rio d to e a rly P e rm ia n p erio d at the rate n in g o f S ia lic L a y e r - A c c o rd in g to H o lm e s th e re o f o n e fo o t o f s e d im e n ta tio n ev e ry 7 ,5 0 0 years. m ay be tw o p o s sib ilitie s if a c o lu m n o f ris in g c o n ­ H o lm e s h a s id e n tif ie d 4 m a jo r ty p e s o f g eo sy n clin es v ectiv e c u rre n ts d iv e rg e s a fte r r e a c h in g th e lo w e r a n d h a s d e s c r ib e d th e m o d e o f th e ir o rig in sep arately lay er o f th e c ru s t in o p p o s ite d ire c tio n s , (i) T h e s ia lic as g iv e n b e lo w la y er is stre tc h e d a p art d u e to te n s ile fo rc e s e x e r te d (i) F o r m a t i o n o f G e o s y n c lin e s d u e to M i­ by d iv e rg in g c o n v ec tiv e c u rre n ts. T h is p ro c e s s c a u s e s g r a t io n o f M a g m a - A c c o rd in g to H o lm e s the cru st th in n in g o f sialic la y e r w h ic h re s u lts in th e c r e a tio n o f th e e a rth is c o m p o s e d of 3 sh ells o f ro ck s. Ju st o f a g e o sy n c lin e . T h e fo rm e r T e th y s g e o s y n c lin e is b e lo w th e o u te r th in s e d im e n ta ry la y e r lies (.) o u te r c o n sid e re d to hav e b een fo rm e d in th is m a n n e r, (ii) la y e r o f g ra n o d io rite ( th ic k n e s s , 10 to 12 k m ), fo l­ A lte rn a tiv e ly , the c o n tin e n ta l m a s s m a y be s e p a ­ lo w e d by (ii) an in te rm e d ia te la y e r ol a m p h ib o lite rated d u e to e n o rm o u s te n sile fo rc e g e n e r a te d by ( th ic k n e s s , 2 0 -2 5 k m ), an d ( i i i ) . lo w e r la y e r o d iv e rg e n t c o n v e c tiv e c u r r e n ts . F o r m e r U ra l e c lo g ite a n d s o m e p e rid o tite . H e h as fu rth er p o in ted g e o sy n c lin e is s u p p o se d to h a v e b e e n fo rm e d d u e to o u t th a t m ig ra tio n o f m a g m a s fro m th e in te rm e d ia te th is m e c h a n ism . la y er to n e ig h b o u rin g a re a s c a u s e s co lla p se and (6 ) V ie w s o f O th e r s - D u s t a r h a s c la s s i s u b s id e n c e o f u p p e r o r o u te r la y e r an d th u s is fo rm ed g e o s y n c lin e s in to 3 ty p e s on th e b a s is o f s tru c tu re o f a g e o s y n c lin e . It m a y be s u m m a ii/.e d that som e m o u n tain ran g es. g e o s y n c lin e s a rc fo rm e d d u e to d is p la c e m e n t ol lig h t m a g m a s and c o n s e q u e n t su b sid e n c e o f cru stal

(i) ln te r -c o n tin e n ta l g e o s y n c lin e s a re w ays situ ated betw een tw o c o n tin e n ta l o r la n d m a ss e s S c h u c h e rt's m e so g e o s y n c lin e is s im ila r to th is ty p e https://telegram.me/UPSC_CivilServiceBooks

s u rfa c e P re s e n t C o ra l S ea, T a s m a n S ea. A rafu ra S ea W e d d e ll S e a a n d R o ss S ea hav e been q u o te d as

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U ral G eo sy n c lin e is q u o te d as th e re p re se n ta tiv e ex am p le, (ii) C ir c u m -c o n tin e n ta l g e o sy n c lin e s are g en erally situ a ted alo n g the m a rg in s o f th e c o n ti­ nen ts. S ch u ch ert's m o n o g e o sy n c lin e is th e e x a m ­ ple. (iii) C ir c u m -o c e a n ic g e o sy n c lin e s are g e n e r­ ally fo u n d a lo n g the m a rg in a l areas o f th e o cean s w h ere c o n tin e n ta l m a rg in s m e e t w ith the o cea n ic m arg in s. S tille h as n a m e d su ch g e o sy n c lin e as m a r ­ g in a l g e o sy n c lin e w h ile o th e rs h av e called it sp ecia l ty p e o f g e o s y n c lin e o r u n iq u e g e o sy n c lin e . M o re ex te n siv e g e o sy n c lin e s h av e b een n am ed by S tille as o r th o g e o s y n c lin e s. S tille h as fu rth e r cla ssifie d the g e o sy n c lin e s on th e b asis o f in te rm itte n t v o lcan ic a c tiv ity d u rin g th e ir in fillin g in to (i) e u g eo sy n clin e s an d (ii) m io g e o s y n c lin e s . E u g e o sy n c lin e s h av e re la ­ tiv e ly h ig h a m o u n t o f v o lc an ic p ro d u cts (G reek p re fix eu m e a n s h ig h statu s o f ig n eo u s activ ity ) w h ile m io g e o s y n c lin e s h av e lo w v o lcan ic p ro d u cts (m io m e a n s low ).

Folded Ranges

Com ----------- V Landmass

Fig. 13.5 :Stage o f orogenesis : squeezing and folding o f geosynclinal sediments due to compressive forces; the whole o f geosyndinal sediments are folded when the compressive forces com­ ing from the sides o f geosyncline is enormous and acute.

Stages of Geosynclines T h e g e o sy n c lin a l h isto ry is d iv id ed into three sta g e s viz. (i) lith o g e n e sis (th e stag e o f crea tio n o f g e o sy n c lin e s, se d im e n ta tio n an d su b sid en ce o f the b ed s o f g e o sy n c lin e s, fig. 13.4), (ii) o r o g e n e sis (the stag e o f sq u e e z in g an d fo ld in g o f g e o sy n c lin a l sed im en ts into m o u n tain ran g es, figs. 13.5 and 13.6),

M arginal Ranges

M a rg in al R anges

Fig. 13.6 : Folding o f marginal sediments into marginal ranges and formation o f median mass when the compressive forces are moderate. tain s o f a c c u m u la tio n ) is m o re o r le s s w e ll u n d er­ sto o d b ut the p ro b le m o f th e o rig in o f fo ld e d m o u n ­ ta in s is very m u c h c o m p le x a n d c o m p lic a te d . D iffe r­ en t h y p g th e se s a n d th e o rie s h a v e b e e n p o stu la te d fro m tim e to tim e by v a rio u s s c ie n tis ts fo r th e e x p la­ n atio n o f th e o rig in o f fo ld e d m o u n ta in s b u t n o n e o f th e m c o u ld b e c o m e c o m m o n ly a c c e p ta b le to m a jo r­ ity o f the s c ie n tis ts . R e c e n tly , p la te te c to n ic theory h as, to la rg e r e x te n t, s o lv e d th e p ro b le m o f m o u n ­ tain b u ild in g at g lo b a l s c a le . T h e h y p o th e s e s and th e o rie s re la te d to m o u n ta in b u ild in g a re d iv id ed in to tw o g ro u p s , (i) th e o rie s b a s e d o n h o rizo n tal fo rces an d (ii) th e o rie s b a s e d o n v e rtic a l fo rces.

Fig. 13.4 : The stage o f lithogenesis : creation o f geosyncline followed by sedimentation and subsidence. an d (iii) g lip to g e n e s is (th e sta g e o f g rad u al rise o f m o u n ta in s , and th e ir d e n u d a tio n an d c o n s e q u e n t lo w e rin g o f th e ir h e ig h ts). T h e s e sta g e s w o u ld be e la b o ra te d d u rin g th e d is c u ssio n o f g e o sy n c lin a l th e o ry o f K o b er. 1 3 .4 THEORIES OF MOUNTAIN BUILDING

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T h e p ro c e s s o f the o rig in o f b lo ck m o u n ta in s , d o m e m o u n ta in s , an d v o lc a n ic m o u n ta in s (m o u n -

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(1) T he first group includes those theories which postulate the origin o f m o untains due to horizontal crustal m ov em en t and co n seq u en t c o n ­ traction and folding o f crustal surface into m o u n ­ tains. This group is fu rther sub d iv id ed into tw o subgroups e.g. (i) the group o f co n tractio n theories (i.e. horizontal m ov em en ts are caused due to co n ­ traction o f the earth b ecau se o f co o lin g ) and (ii) the group of drift theories (i.e. the h o rizontal m o v e­ m ents are caused due to continental disp lacem en t and drift). T herm al contraction theory o f Jeffreys and geosynclinal T heory o f K ober belong to the group o f co n tractio n theories w hereas C ontinental Drift th eo ries o f F.B . T aylor, and A.G. W egener, Therm al C o n v ectio n C u rren t T heory o f A. H olm es, Sliding C o n tin en ts T h eo ry o f D aly, R adioactivity Theory o f Joly and P late T ectonic Theory are in­ cluded in the g ro u p o f d rift theories. (2) The second group includes those th eo ries w hich are based on vertical m o v em en ts co m in g from w ithin the earth, e.g. U ndulation and O scillatio n Theory o f H arm on. Theories o f F.B . T ay lo r and A .G . W egener have already been d iscu ssed in ch ap ter 6 o f this book.

earth. He believes in the contraction history o f the earth. A ccording to J.A. Steers (1932) ‘K ober is definitely a contractionist, contraction providing the m otive force for the com pressive stress’. In other w ords, the force o f contraction generated due to cooling o f the earth causes horizontal m ovem ents o f the rigid m asses or forelands w hich squeeze, buckle and fold the sedim ents into m ountain ranges. B a se of the Theory A ccording to K ober there w ere m o b ile zones o f w ater in the places o f p resen t-d ay m o u n tain s. H e called m obile zo n es o f w ater as g eo sy n clin es or orogen (the place o f m ountain b u ild in g ). T h e se m obile zones o f g eo synclines w ere s u rr o u n d e d by rigid m asses w h ich w ere te rm ed by K o b e r as ‘k r a to g e n ’. The old rigid m asses in clu d ed C a n a d ia n Shield, B altic Shield or R ussian M assif, S ib e ria n Shield, C hinese M assif, P en in su la r In d ia, A fric a n Shield, B razilian M ass, A u stralian and A n ta rc tic rig id m asses. A cco rd in g to K o b e r m id -P a c ific geosyncline separated north and so u th P a c ific fo re ­ lands w hich w ere later on fo u n d ered to fo rm P a c ific O cean. Eight m o rphotectonic u n its can be id e n tifie d on the basis o f the descrip tio n o f the su rface fe a tu re s of the earth during M eso zo ic era as p re s e n te d by K ober e.g. (i) A frica to g eth er w ith so m e p a rts o f A tlantic and Indian O ceans, (ii) In d ian A u s tra lia n land m ass, (iii) E urasia, (iv) N o rth P a c ific c o n tin e n t, (v) South Pacific co n tin en t, (vi) S o u th A m e ric a a n d A ntarctica etc.

(1) G E O S Y N C L IN A L O R O G EN T H E O R Y O F K O B ER

O bjectives F am ous G erm an g eo lo g ist K ober has pre­ sented a d etailed and sy stem atic d escription o f the surface features o f the earth in his book ‘D e r B au d e r E r d e \ H is m ain o b jectiv e was to establish relationship betw een an cien t rigid m asses or tab le­ lands and m ore m o b ile zones or g eo sy n clin es, w hich he called ‘o r o g e n .’ K o b er not only attem p ted to explain the origin o f the m o u n tain s on the basis o f his geosynclinal theory but he also attem p ted to elab o ­ rate the various asp ects o f m o u n tain b u ild in g e.g. form ation o f m o u n tain s, th eir g eo lo g ical history and evolution and d ev elo p m en t. H e co n sid ered the old rigid m asses as the fo u n d atio n sto n es o f the p resent continents. A cco rd in g to him presen t co n tin en ts have grow n out o f rigid m asses. He d efin ed the process o f m ountain b u ild in g or o ro g en esis as that process w hich links rigid m asses w ith g eo sy n clin es. In other w ords, m o u n tain s are form ed from the geosynclines due to the im p acts o f rigid m asses.

K ober has id entified 6 m a jo r p e rio d s o f m o u n ­ tain building. T hree m ountain b u ild in g p e rio d s, a b o u t w hich very little is k n ow n, are re p o rte d to h a v e occurred during p re-C am b rian p erio d . P a la e o z o ic era saw tw o m ajo r m o u n tain b u ild in g p e rio d s - th e C aledonian o ro g en esis w as c o m p le te d by th e e n d o f S ilurian period and the V ariscan o ro g e n y w as c u lm i­ nated in P erm o -C arb o n ifero u s p erio d . T h e la s t (6 th ) orogenic activity k now n as A lp in e o ro g e n y w as com pleted d u rin g T ertiary ep o ch . K ober has op in ed th at m o u n ta in s a re fo rm e d out o f geosynclines. A ccording to K o b er g eo sy n clin es, the places o f m o u n tain fo rm atio n (k n o w n as o ro g e n ) are long and w ide w ater areas c h a ra c te riz e d by sed im en tatio n and su b sid en ce. A c c o rd in g .to J .A Steers (1932), ‘K o b er’s v iew s (on g e o s y n c lin e s a n d o ro g en esis) are, then, a c o m b in a tio n o f th e o ld g eo sy n clin al h y p o th esis o f H all a n d D a n a , which https://telegram.me/UPSC_CivilServiceBooks

O rogenetic Force K ober's g eosy n clin al theory is based on the forces o f co n tractio n p ro d u ced by the co o lin g o f the

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226.

GEOMORPHOLOGY

m asses or forelands are subjected to continuous erosion by fluVial processes and eroded materials are deposited in the geosynclines. This process of sedim ent deposition is called sedim entation. The everincreasing w eight o f sedim ents due to gradual sedim entation exerts enorm ous pressure on the beds o f g e o s y n c lin e s , w ith th e r e s u lt th e b ed s o f geosynclines are subjected to gradual subsidence. This process is know n as the process o f subsidence. These twin processes o f sedim entation and resultant subsidence result in the d eposition o f enorm ous volum e o f sedim ents and attain m en t o f great thick­ ness o f sedim ents in the g eosynclines.

was developed later by H aug, and his own views on orogenesis.’ M echanism of the Theory A ccording to K ober the w hole process o f mountain building passes thorugh three closely linked stages o f lithogenesis, orogenesis and gliptogenesis. The firststage is related to the creation of geosynclines due to the force o f contraction caused by cooling of the earth. This preparatory stage o f m ountain build­ ing is called lith ogen esis. The geosynclines are long and w ide m obile zones o f w ater w hich are bordered by rigid m asses, w hich have been nam ed by Kober as forelan d s or kratogen . T hese upstanding land

M a rg in a l R a n g es

M a rg in a l R a n g es

Fig. 13.7 : Illustration o f Kober's geosynclinal theory o f mountain building through a block diagram. m arginal sed im en ts o f the g e o sy n clin e are fo ld ed to form tw o m arg in al ran d k etten (m arg in al ran g es) and m id d le po rtio n o f the g eo sy n clin e rem a in s unaf­ fected by fo ld in g activ ity (th u s re m a in s unfolded). T h is u n f o ld e d m id d le p o i i i o n is c a lle d z w is c h e n g e b irg e (b e tw ix t-m o u n ta in s) o r m e d ia n m a s s (figs. 13.6 an d 13.7). A lte rn a tiv e ly , if the c o m p r e s s iv e f o r c e s a r e a c u te , th e w h o le o f g eo sy n clin al se d im e n ts are c o m p re sse d , squeezed, b u ck led and u ltim a te ly fo ld ed (fig . 13.5) and both the fo rela n d s are clo se te d . T h is p ro c e ss introduces co m p le x ity in th e m o u n ta in s b ec a u se acute com ­ p ressio n re su lts in the fo rm a tio n o f recu m b en t folds

T he S econ d S tage is related to m ountain b uilding and is called the sta g e o f o ro g en esis. B oth the forelands start to m ove to w ard s each other b e­ cause o f h o rizontal m o v em en ts cau sed by the force o f co n tractio n resu ltin g from the co o lin g o f the earth. T he co m p re ssiv e forces g en erated by the m o vem ent o f fo relan d s to g eth er cau se co n tractio n , squeezing and u ltim ately fo ld in g o f g eo sy n clin al sedim ents to form m o u n tain ranges. T h e parallel ranges form ed on eith e r sid e o f the g eo sy n clin e have been term ed by K o b er as ra n d k etten (m arg in al ranges) (figs. 13.6 and 13.7). A ccording to K ober folding o f entire sedim ents o f the g eo sy n clin e or part th e re o f d ep en d s upon the intensity o f co m p ressiv e forces. If the co m p re ssiv e forces are norm al and o f m o d e ra te in ten sity , on ly the

an d n ap p es.

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K o b er h a s a tte m p te d to e x p la in th e form s and stru c tu re s o f fo ld e d m o u n ta in s on th e b asis o f h is - .J

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typical m edian m ass. ‘R eally, K ober's typical “orogen” (g eo sy n clin es) w ell ex p lain s the orig in o f m o u n ­ tain s’. ‘T h e id ea o f m e d ian m ass o f K o b er fully explains the p ro c e ss o f m o u n tain b u ild in g ’. A cco rd ­ ing to K o b er the A lp in e m o u n tain ch ain s o f E urope can w ell be e x p la in e d on the basis o f m ed ian m asses. A ccording to him T eth y s g eo sy n clin e w as b ordered by E u ro p ean lan d m ass in the n orth and by A frican rigid m ass in the so u th . T h e sed im en ts o f T ethys gco sy n clin e w ere c o m p re sse d and folded due to m o v e m en t o f E u ro p e a n lan d m ass (fo relan d ) and A frican rig id m a ss (fo rela n d ) to g e th er in the form of

A lp in e m ountain system . A cco rd in g to K ober the A lpine m ountain chains w ere form ed because o f co m p ressiv e forces com ing from tw o sides (north and south). Betic C o rd illera, Pyrenees, Province ranges, A lps- proper, C arp ath ian s, B alkan m oun­ tains and C aucasus m o untains w ere form ed due to northw ard m ovem ent o f A frican foreland (fig. 13.8). On the other hand, A tlas m ountain (north-w est A frica), A p en n in es, D in a rid e s, H e lle n id e s and T aurides w ere form ed due to so u th w ard m o v em en t o f E uropean landm ass (fig. 13.8).

C a i p a tfiiu n s

Fig. 13.8 : The directions o f folding in Alpine mountains o f Europe. Arrows indicate directions (based on Kober). m ountain ranges take so u th erly tren d in th e fo rm o f B urm ese hills. A siatic A lp in e ran g es b eg in fro m A sia m inor and run upto S u n d a Isla n d in th e E a s t Indies. K ober has also ex p lain ed th e o rie n ta tio n o f thrust or com pression o f A siatic fo ld e d m o u n ta in s

7'he m e d ian m asses located in the A lpine m ountain sy stem very w ell ex p lain the m echanism o f m o u n tain b u ild in g . It is ap p are n t from fig. 13.8 that the d ire c tio n o f fo ld in g in the C arp p ath ian s and D inaric A lp s (D in a rid e s) is north and south resp ec­ tively, w hich m e a n s th a t H u n g arian m edian m ass is located b etw een tw o m o u n tain ranges having o p p o ­ site d irec tio n s o f fo ld in g . M ed iterran ean S ea is in fact an e x am p le o f m ed ian m ass betw een PyrenessProvence R an g es in the north and A tlas m ountains and their eastern e x te n sio n in the south. C o rsica and Sardinia are rem n a n ts o f th is m edian m ass. A natolian plateau b etw een P an tic and T au ru s ran g es is a n o th er exam ple o f m edian m ass. S im ilarly , further e a st­ ward, Iranian p lateau is a m edian m ass betw een Zagros and E lb u rz m o u n tain s. A lpine m o u n tain s fu rth er ex ten d into A sia w here m ountain ran g es fo llo w latitudinal directio n s e -g. w est-east o rien tatio n b u t th e latitu d in al pattern is broken in n o rth -eastern hill region o t In d ia w here

on the basis o f his f o r e la n d th e o r y . A sia tic fo ld ed m ountains including the H im a la y a w ere fo rm e d due to com pression and folding o f s e d im e n ts o f T e th y s

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geosyncline caused by the m o v e m e n t o f A n g a ra la n d and G ondw ana F o relan d s to g e th e r (fig. 13.9). T w o m arginal ranges (ran d k etten ) w ere fo rm ed on e ith e r side o f the g eo sy n clin e and u n fo ld ed m id d le p o rtio n rem ained as m edian m ass. A c c o rd in g to K o b e r A si­ atic A lpine folded m o u n tain s can be g ro u p e d in to tw o categ o ries on the b asis o f o rie n ta tio n o f fo ld s i.e. (i) the ranges, w hich w ere fo rm ed by th e n o rth w a rd co m p ressio n , in clu d e C a u c a su s, P an tic an d T a u ru s (o f T urkey), K unlun, Y an n an an d A n n a n ra n g e s , a n d (ii) the ranges, w h ich w ere fo rm e d b y th e s o u th w a rd

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GEOMORPHOLOGY

228

betan p lateau is a fine e x a m p le o f median mass b etw een K u n lu n -T ie n -S h a n and the Himalayas.

com pression, include Zagros and Elburz o f Iran, Oman ranges, H im alayas, B urm ese ranges etc. T i­

S F o ld e d M a r g in a l R a n g e s N M a rg in a l F o ld e d R anges K u n lu n M t

Til>etan P la te a u

H im a la y a

"G eosyncline \;°r f

M l Fig. 13.9 : Illustration o f Kober s median mass through Tibetan plateau between Kunlun and Himalaya. The median mass m ay be in various form s e.g. (i) in the form o f plateau (exam ples, T ibetan plateau betw een K unlun and H im alaya, Iranian p la­ teau betw een Z agros and E lburz, A natolian plateau betw een P antic and T aurus, B asin R ange betw een W asatch ranges and S eirra N av ad a in the U S A ) ; (i) in the form o f plain (exam ple, H ungarian plain betw een C arpathian s and D inaric A lps), and (iii) in the form o f seas (exam ples, M ed iterran ean S ea b e­ tw een A frican A tlas m o u n tain s and E u ro p ean A l­ p in e m o u n tain s, C arib b ean S ea b etw een the m o u n ­ tain ran g es o f m id d le A m erica and W est Indies).

Third Stage

the A lps, the H im a lay as, th e R o c k ie s and the A ndes can n o t be fo rm ed by the fo rce o f c o n tra c tio n gener­ ated by co o lin g o f th e earth . (2) A cco rd in g to S u e ss o n ly o n e side o f the g eo sy n clin e m o v es w h e re a s th e o th e r sid e rem ains stable. T h e m o v in g sid e h as b e e n te rm e d by S uess as backland w h ereas stab le sid e h a s b e e n c a lle d fore­ land. A cco rd in g to S u ess th e H im a la y a s w ere form ed due to so u th w ard m o v e m e n t o f A n g a ra la n d . The G o n d w a n alan d re m a in e d s ta tio n a ry . T h is o b serv a­ tion o f S u ess g a in e d m u c h fa v o u r p re v io u s ly but after the p o stu la tio n o f plate tectonic theory his

o f m o u n ta in b u ild in g is c h a r a c ­

view s h av e b e c o m e m e a n in g le s s a n d th e c o n c e p t of

t e r iz e d by g r a d u a l rise o f m o u n ta in s a n d th e ir d e n u ­

K ober, that b o th th e fo re la n d s r r o v e to g e th e r, has

d a tio n by flu v ia l a n d o t h e r p r o c e s s e s . C o n t i n u o u s

been

d e n u d a t i o n r e s u l ts in g r a d u a l l o w e r i n g o f the h e ig h t

v a lid a te d

b ecau se

a m p le

e v id e n c e s of

p alaeo m ag n etism an d s e a -flo o r sp re a d in g h av e shown

o f m o u n ta in s .

th at b o th A siatic a n d I n d ia n p la te s are m o v in g to­ w ard s e a c h o th e r.

Evaluation of the theory T h o u g h K o b e r's g e o sy n c lin a l th eo ry s a tis fa c ­ to rily e x p la in s a few a sp e c ts o f m o u n tain b u ild in g b u t the th e o ry su ffe rs from c e rta in w e a k n e sse s and la c u n a e .

(3 ) K o b e r's th e o ry so m e h o w e x p la in s the w e s t-e a st e x te n d in g m o u n ta in s b u t n o rth -s o u th ex­ te n d in g m o u n ta in s (R o c k ie s a n d A n d e s ) c a n n o t be e x p la in e d on th e b a s is o f th is th e o ry . In s p ite o f a few by in h e re n t lim ita tio n s a n d w e a k n e s s e s K o b e r is given c re d it fo r a d v a n c in g th e id e a o f th e fo rm a tio n o f m o u n ta in s fro m g e o s y n c lin a l s e d im e n ts

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(1 ) T h e fo rce o f c o n tra c tio n , as e n v isa g e d K o b e r, is n o t s u ffic ie n t to c a u se m o u n ta in b u ild in g . In fa c t, v ery e x te n s iv e an d g ig a n tic m o u n ta in s lik e

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b ecau se

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m o u n t a in b u il d in g

g eosyncline fo u n d b e rth in a lm o s t all th e su b se q u e n t theories e v en in p la te te c to n ic th eo ry .

trac tio n c au sed b y g rad u al c o o lin g o f th e ea rth d u e to loss o f h eat th ro u g h rad ia tio n fro m th e v ery b e g in ­ n in g o f its o rig in . H e h as m a th e m a tic a lly c a lc u la te d th e e x te n t o f c o n tra c tio n o n c o o lin g . A d e c re a se o f te m p eratu re u p to 400°C in th e 4 0 0 k m th ic k o u te r shell o f the earth w o u ld ca u se sh o rte n in g o f th e d ia m ete r o f the ea rth by 2 0 km a n d th e c irc u m fe r­ en ce by 130 km d u e to c o o lin g a n d re s u lta n t c o n tra c ­ tion. H e c a lc u la te d th e m a x im u m s h o rte n in g o f th e cru st d ue to c o n tra c tio n to b e 2 0 0 k m a n d th e r e d u c ­ tion in su rface a re a u p to 5 x 1 0 16 c m 2.

(2) THERMAL CONTRACTION THEORY OF JEFFREYS Objectives J e ffre y s, a s tro n g e x p o n e n t o f co n tra ctio n theory, p o stu la te d h is ‘thermal contraction theory’ to explain th e o rig in a n d e v o lu tio n o f m a jo r reliefs o f the earth 's s u rfa c e (c o n tin e n ts , o cea n b asin s, m o u n ­ tains, isla n d a rc s a n d fe s to o n s ) b u t h is m a jo r o b je c ­ tive w as to e x p la in th e o rig in and d istrib u tio n a l patterns o f m o u n ta in sy ste m s o f th e g lo b e. Jeffrey s was a c o n tra c tio n is t. H is th e o ry w as b a se d on m a th ­ em atical re a s o n in g . H e p o stu la te d h is co n tractio n theory b e c a u s e he c o u ld n o t fin d any stro n g reason in the c o n tin e n ta l d rift th e o ry w h ic h ad v o ca ted h o ri­ zontal m o v e m e n t o f th e c o n tin e n ts d ue to tid al force of the sun a n d th e m o o n an d th e g rav itatio n al force as e n v isa g e d by A .G . W e g e n e r.

A cco rd in g to Je ffre y s th e e a rth is c o m p o s e d o f several co n ce n tric sh ells (la y e rs). T h e c o o lin g a n d resu ltan t co n tra ctio n tak e p la c e la y e r a fte r la y e r b u t the co o lin g is effe c tiv e u p to th e d e p th o f o n ly 7 0 0 k m from the earth 's su rface. “T h e re g io n o f th e e a rth from the cen tre to so m ew h ere a b o u t 7 0 0 k ilo m e tre s from the su rface m ay h av e u n d e rg o n e n o a p p r e c i­ able ch an g e o f te m p e ra tu re , an d c o n s e q u e n tly n o m arked change in v o lu m e” (J.A . S teers, 1932). W ith in the zone o f 70 0 km from th e e a rth 's s u rfa c e e v e ry uper lay er has co o led e a rlie r an d m o re th a n th e la y e r im m ediately b elo w the u p p e r la y er. T h u s , e a c h u p ­ p er layer co n tra cted m o re th a n th e la y e r j u s t b e lo w it. F u rther, each u p p er la y e r c o n tin u e d to c o o l u n le s s o bstructed by th e im m e d ia te lo w e r la y e r. T h e o u te r layer began to cool first d u e to lo ss o f h e a t th ro u g h radiation. It m ay be p o in te d o u t th a t th e re is a lim it o f cooling b ey o n d w h ich no fu rth e r c o o lin g is p o s ­ sible. A fter m ax im u m c o o lin g a n d r e s u lta n t c o n tr a c ­ tion o f the uper la y e r lo w e r la y e r ju s t ly in g b e lo w th e upper lay er b eg in s to co o l a n d c o n tra c t, w ith th e resu lt alread y co o led an d c o n tra c te d u p p e r la y e r b eco m es too larg e to fit in w ith th e s till c o o lin g a n d co n tractin g lo w er lay er. T h e c o re o f th e e a rth is n o t affected by c o o lin g b e c a u se o f e x c e p tio n a lly h ig h tem p eratu re p re v a ilin g th e re . T h u s , th e c o re o b ­ structs the c o n tra c tio n o f th e la y e r ly in g a b o v e it. T h e co o lin g an d c o n tra c tin g la y e r ly in g b e lo w th e alread y c o o le d an d c o n tra c te d la y e r b e c o m e to o b ig to fit in w ith th e c o re o f th e e a rth . T h e re is s u c h a ay er etw ee n th e u p p e r a n d lo w e r la y e r w h e re co n tra ctio n is su ch th a t th e in te rm e d ia te la y e r c a n fit m w ith the lo w e r la y er. T h is la y e r is c a lle d level o f

Orogenetic Force J e ffre y s u sed th e fo rc e o f co n tra ctio n resu lt­ ing p artly fro m c o o lin g o f the earth due to loss o f heat th ro u g h ra d ia tio n fro m the earth 's su rface and partly fro m th e d e c re a s e o f the speed o f the earth's rotation. In fact, th e fo rc e s in v o k ed by Jeffrey s are divided in to tw o g ro u p s. (1 ) F o rce co m in g through the c o o lin g o f th e e a rth . T h e earth , afte r being form ed, sta rte d c o o lin g d u e to loss o f h eat through rad iatio n . T h is p ro c e s s re su lte d in the gradual d e ­ crease o f th e size o f th e earth d u e to co n tractio n on cooling. T h e re s u lta n t co n tra c tio n p ro v id ed adequate force (as b e lie v e d by Je ffre y s ) to form vario u s re lie f features in c lu d in g m o u n ta in s. (2) F o rce co m in g through d e c re a s e in th e sp eed o f earth s rotatio n . A bout 1600 m illio n y e a rs a g o th e earth co m p leted its one ro ta tio n in a b o u t 0 .8 4 h o u r w h ereas it p resen tly com pletes o n e ro ta tio n in a b o u t 2 4 h o u rs. T h e d e­ crease in th e ro ta tio n a l sp e e d ca u se d co n tra ctio n in the e q u ato rial c irc u m fe re n c e o f th e earth . It m ay he con clu d ed th a t th e fo rc e o f c o n tra c tio n w as d e rived through th e c o n tra c tio n o f th e earth d ue to (i) co o lin g o f the e a rth an d (ii) d u e to d e c re a se in th e speed o f earth's ro tatio n .

no strain.

•Jeffreys' th e o ry is b a se d essen tially on the history o f th e c o n tra c tio n o f the earth . A cco rd in g to Jeffreys th e e a rth b eg an to sh rin k b eca u se o f c o n ­

T h e la y e r ly in g o v e r th e le v e l o f n o s tra in is too b ig to fit w ith th e lo w e r la y e r a n d h e n c e th e u p p e r ay er has to co lla p se on th e lo w e r la y e r so th a t it c a n https://telegram.me/UPSC_CivilServiceBooks

Mechanism of the Theory

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230 GEOMORPHOLOGY f it w ith th e lo w e r la y e r. T h is p ro c e s s (c o lla p s e o f

re g io n s h a v in g h a rd an d le ss e la stic ro c k s are af­ fe c te d by te n sile fo rc e s an d th u s se v e ra l fau lts and fra c tu re s a rc fo rm e d b e c a u se su c h ro ck s are easily b ro k e n in to b lo c k s. It is, th u s, a p p a re n t th a t m oun­ tain b u ild in g is lo c a liz e d in c e rta in zo n es o f the g lo b e .

u p p e r la y e r on lo w e r la y e r ) re s u lts in th e d e c re a s e in th e ra d iu s o f th e e a rth w h ic h c a u s e s h o riz o n ta l c o m p re s s iv e s tre s s w h ic h le a d s to b u c k lin g and fo ld in g o f th e ro c k s o f u p p e r la y e r. T h u s , th e m o u n ­ ta in s a re fo rm e d . T h e lo w e r la y e r b e lo w th e lev el o f n o s tra in is to o s h o rt to fit w ith th e c o re o f th e e a rth a n d h e n c e th e lo w e r la y e r h a s to s tre tc h h o riz o n ta lly . T h is p ro c e s s im p lie s a la te ra l s p re a d in g an d th in n in g o u t o f th e m a te ria ls o f th e lo w e r la y e r b e lo w th e level

Direction o f the Force- A c c o rd in g to Jeffreys n o t all th e a re a s b elo w th e e a rth s u rfa c e are equally affe c te d by the m e c h a n ism o f c o o lin g a n d co n tra c­ tion. T h e c o o lin g p ro c e ss w as m o re a c tiv e b e lo w the o c e a n ic c ru s t th an th e c o n tin e n ta l c ru s t b e c a u se o f d is s im ila r s tru c tu re o f th e se tw o z o n e s. T h u s, the ro ck s b elo w th e o c e a n ic c ru s t e x p e rie n c e d m o re c o o lin g and c o n tra c tio n than th e ro c k s b e lo w the c o n tin en ta l cru st. T h u s, the fo rc e o f c o n tra c tio n is d ire c te d fro m th e o c ea n ic c ru st to w a rd s th e c o n ti­ n ental cru st. T h is m e c h a n ism re su lts in th e fo rm a ­ tion o f m o u n tain s alo n g th e c o n tin e n ta l m a rg in s p arallel to ,th e o cean s. R o ck ies an d A n d e s are the e x am p les o f such situ atio n .

o f n o s tra in . T h e s p re a d in g a n d th in n in g o f th e lo w er la y e r in tro d u c e s a s ta te o f s tre s s w h ic h c a u s e s fra c ­ tu re s a n d fis s u re s re s u ltin g in to b re a k in g o f ro ck s. T h is m e c h a n is m a llo w s fu rth e r c o lla p s e o f th e a l­ re a d y c o o le d o u te r la y e r a n d th u s a lre a d y fo rm ed m o u n ta in s a re s u b je c te d to fu rth e r rise in h eig h t. J e ffre y 's h a s a lso e x p la in e d v a rio u s asp ects o f m o u n ta in b u ild in g e.g . p e rio d o f m o u n ta in b u ild in g , z o n e s o f m o u n ta in b u ild in g , d ire c tio n o f m o u n tain s, e tc .

Period o f Mountain Building- A cco rd in g to

D ire c tio n o f M o u n ta in s - A cco rd ing to Jeffreys the co m p re ssiv e fo rce g en era ted by c o n tra c tio n o f the earth d u e to co o lin g w as d ire c te d fro m o cean ic areas to w ard s the co n tin e n ta l area s a lm o s t at right an g le and thus the m o u n tain ra n g e s w ere form ed p arallel to th e o cean ic areas. T h e la y o u t an d d irec­ tion o f the R o ck ies and A n d es m o u n ta in s are very w ell ex p la in e d on the b asis o f th is th e o ry because th ese m o u n tain s run n o rth to so u th a lo n g th e w estern m a rg in s o f N o rth and S o u th A m e ric a resp ectiv ely an d are p arallel to the P acific O c e a n b u t the w esteast e x ten t o f th e A lp s an d th e H im a la y a s can n o t be e x p la in e d on the b asis o f th is th e o ry .

J e ffre y s th e p ro c e ss o f a fo re sa id m e ch an ism o f m o u n ­ ta in b u ild in g is n o t a lw a y s a c tiv e th ro u g h o u t the g e o lo g ic a l p e rio d s ra th e r is c o n fin e d to c e rta in p e ri­ o d s o n ly . T h e re is c o n tin u o u s a c c u m u la tio n o f c o m p re s s iv e and te n sile fo rc e s re su ltin g from c o n ­ tra c tio n o f th e e a rth d u e to c o o lin g an d th is p ro cess c o n tin u e s until th e a c c u m u la te d fo rces ex c e e d the ro c k stre n g th . W h en , th is sta te (w h en a c c u m u la ted c o m p re s s iv e an d te n sile fo rc e s e x c e e d th e ro ck s tre n g th ) is re a c h e d , fo ld in g an d fau ltin g are in tro ­ d u c e d a n d th e p ro c e ss o f m o u n ta in b u ild in g sets in a n d th is p ro c e s s c o n tin u e s till th e c o m p re ssiv e and te n s ile fo rc e s a re s tro n g an d activ e. W h e n th ese fo rc e s b e c o m e w eak , m o u n ta in b u ild in g sto p s and th e p e rio d o f q u ie s c e n c e sets in. A g ain th e p ro c e ss o f a c c u m u la tio n o f c o m p re ss iv e and te n sile fo rces starts and th e n e x t p ro c e s s o f m o u n ta in b u ild in g b egins when th e s e fo rc e s a g a in b eco m e stro n g en o u g h to fold th e c ru s ta l ro ck s. T h u s , tw o p e rio d s o f m o u n tain building a re s e p a ra te d by a lo n g p erio d o f q u ie s ­

Evaluation of the Theory T h o u g h Je ffre y s h as a tte m p te d to ex p lain the o rig in an d e v o lu tio n o f su rface fe a tu re s o f the earth an d h as p re se n te d sev eral e v id e n c e s in su p p o rt o f his th erm al c o n tra c tio n th e o ry b u t h is th e o ry h as been sev erely .c ritic is e d and a tta c k e d on the follow ing g ro u n d s.

( I) T h e fo rce o f c o n tra c tio n re su ltin g from th c o o lin g o f the ea rth is not s u ffic ie n t en o u g h to a c c o u n t for th e o rig in an d e v o lu tio n o f m a j o r surface

cence. Zones o f M ountain Building- A c c o rd in g to J e ffre y s m o u n ta in b u ild in g d e p e n d s u p o n the n atu re a n d stre n g th o f ro c k s. T h e a re a s h a v in g so ft and e la s tic ro c k s are m o s t a ffe c te d by the p ro c e ss o f m o u n ta in b u ild in g as th e ro c k s are e a sily fo ld e d by c o m p re s s iv e fo rc e s c a u s e d by c o n tra c tio n b u t the

re lie fs o f the g lo b e. A H o lm e s h as re m a rk e d that ‘the c a lc u la te d re d u c tio n o f a re a (by J e ffre y s) is seri­ o u sly in d e fic it o f the a m o u n t to e x p la in m ountain

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b u ild in g .’

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231

(2) The concept o f cooling o f the earth in the system o f concentric shells (layers) is erroneous and is not acceptable.

processes o f m ountain building. H e attem pted to explain salient aspects o f folded m ountains e.g. origin, successive upheavals, distributional patterns and orientation and extent.

(3) The im pact o f decrease in the speed of rotation o f the earth on m ountain building is doubt­ ful. J.A. Steers (1932) has aptly rem arked, ‘It may, in fact, be safely concluded that w hatever effects the changing speed o f rotation in geological tim es may have had, it w as totally inadequate to influence m ountain building in any m arked w ay .’

Orogenetic force The main force im plied by Daly for the origin of the m ountains has been the force o f gravity. The w hole theory o f D aly is based on the nature and rate o f dow nw ard slide o f the continents fostered by gravitational force. ‘The key to the D aly's view s is the idea that there has been dow nhill sliding m ove­ m ent o f continental m asses. In other w ords, the controlling factor has been g rav ity ’ (J.A . S teers, 1932). Daly h im self proclaim ed th at his theory based on gravitational force w as co m p eten t to deal w ith all the problem s o f m ountain bu ild in g satisfacto rily .

(4) It is im p ro p er to believe that contraction would have been so im m ense about 200 m illion years ago so that it m ight have form ed such gigantic m ountains o f T ertiary period as the R ockies, the A ndes, the A lps, the H im layas etc. (5) A s per th erm al con tractio n theory o f Jeffreys the co n tin en ts and oceans should have been uniform ly d istrib u ted as the earth w as contracted from all sid es but p resen tly there is uneven d istribu­ tion o f co n tin en ts and oceans.

Axioms of the Theory Daly has assum ed certain ax io m s (se lf p ro v ed facts) in support o f his theory. If w e lo o k into the history it appears that ‘a m ajo r p art o f the th e o ry is based on self proved facts or a x io m s’ . It m ay b e pointed out that D aly did n ot elab o rate h is ax io m s. He adm itted h im se lf th at his th eo ry can w ell e x p la in the problem s o f o ro g en esis on th e fo rce o f g rav ity alo n e.’

(6) A cco rd in g to this theory the situation o f m ountains sh o u ld alw ay s be parallel to the oceans. The arran g em en t o f the R ockies and A ndes is ju s ti­ fied on the basis o f this th eory but the arrangem ent o f E uropean A lp in e m o u n tain s and the H im alayas cannot be ex p lain ed .

A cco rd in g to D aly a so lid c ru st w as fo rm e d ju s t after the o rigin o f the earth . H e n am ed th is so lid • crust as primitive crust. In early tim es th e re e x iste d a series o f an cien t rig id m a sses w h ic h w ere g e n e ra lly situ ated near th e p o les and a ro u n d th e e q u a to r. T h e s e rigid m asses h av e b een n am ed by D aly as polar and equatorial domes. T h u s, th e re w ere th re e b e lts o f rig id m asses e.g . (i) n o rth p o la r d o m e s, (ii) e q u a to ­ rial d o m es and (iii) so u th p o la r d o m e s. T h e s e th ree belts o f rig id m a sses w ere s e p a ra te d by d e p re sse d reg io n s w h ich w ere c a lle d by D a ly as midlatitude furrows an d primeval Pacific Ocean. T h e s e d e ­ p re sse d re g io n s w ere, in fact, o c e a n ic a re a s (o r say g e o sy n c lin e s) th e b e d s o f w h ic h w ere fo rm e d o f primitive crust w h ic h w as fo rm e d w ith th e o rig in o f th e earth .

(7) If we b eliev e in the co m p eten ce o f the force o f co n tra ctio n to form m o u n tain s it cannot produce g reat ran g es o f m o u n tain s as they are found at p resen t o v e r the g lo b e but it w ould p roduce a larger n u m b e r o f sm all p u ck ers or m in o r folds. (8) A c c o rd in g to this th eo ry there sh o u ld not be any d efin itiv e d istrib u tio n a l p attern o f m o u n tain s as they m ay be fo rm ed e v ery w h ere b eca u se all parts o f earth's c ru st e x p e rie n c e d co n tra ctio n b u t c o n trary to this m o u n tain s are fo u n d in certain p a tte rn s e.g. along the m a rg in s o f the c o n tin e n ts e x te n d in g eith er n o rth -so u th w ard o r w est-eastw ard .

(3) SUDING CONTINENT THEORY OF DALY

Objectives D aly p o stu la te d h is th e o ry o f sliding conti­ nents in his b o o k ‘Our Mobile Earth* in th e y e a r

T h e c ru st, a c c o rd in g to D aly , co m p o se d o f g ra n ite s, w as h e a v ie r th a n th e ro c k s o f su b stratu m b elo w th e c ru st. T h e c ru s t w as c o m p o se d o f h eav ier g ra n ite s w h ile th e s u b stra tu m w as fo rm ed o f lig h ter g la ssy b asalt. It m ay b e p o in te d o u t th a t this view o f D aly is iso sta tic a lly to ta lly w ro n g . H e fu rth er as­

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1926 to e x p la in the o rig in an d e v o lu tio n o f d iffe re n t relief featu res o f th e e a rth 's su rface. T h o u g h D aly attem pted to th ro w lig h t on m a jo r re lie fs o f th e g lo b e but his m ain o b je c tiv e w as to e x p la in th e c a u s e s an d

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su m ed th a t th e w a te r b o d ies o ccu p ied ab o u t h a lf o f th e g lo b e an d T eth y s g eo sy n clin e (n o rth ern m id ­ la titu d e fu rro w b etw een n o rth p o la r d o m e and e q u a ­ to rial d o m e ) ‘w as a m a rk e d featu re th ro u g h o u t m uch o f g eo lo g ical tim e .’ L an d m a sses (p o lar eq u ato rial d o m e s - rig id m a sse s) p ro je c te d ab o v e the w ater b o d ies an d th e p o la r an d eq u ato rial d o m es w ere slo p in g to w a rd s m id -la titu d e fu rro w s (w h ich w ere in fa c t g e o sy n c lin a l trac ts) an d th e P acific O cean.

sed im en ts, the size o f d o m es co n tin u ed to increase. It m ay be p o in ted o u t th at g rad u al in crease in the lateral p ressu re resu ltin g from co n tin u o u s dow n­ w ard m o v em en t o f g eo sy n clin al b ed s w as responsi­ ble fo r in crease in the size an d h e ig h t o f the continen­ tal dom es. T h e sed im en ts o f th e c o n tin e n ta l dom es be­ gan to ex p an d b eca u se o f in c re a se in th e size and h eig h t o f the d o m e s an d c o n se q u e n tly sedim ents o f the d o m es b eg an to lo se w e ig h t an d b e c a m e lighter in w eight. In o rd e r to c o m p e n sa te th e lo ss o f w eight o f sed im en ts o f the c o n tin e n ta l d o m e s th e re began u n d erg ro u n d flo w ag e o f d e n se m a te ria ls fro m below the o cean ic (g e o sy n c lin a l b ed s) b ed s to w ard s the co n tin en tal d o m es. B e c a u se o f th is p ro c e s s denser m aterials began to a c c u m u la te in th e co n tin en tal d o m es from b elo w . B e c a u se o f th e re p e titio n o f the above p ro cesses th e c o n tin e n ta l d o m e s co n tin u ed to gro w in size an d h e ig h t, ‘p ro b a b ly n o t as rapidly in the c en tre as to w a rd s th e ir p e rip h e rie s ’. T he increase in th e size o f d o m e s cau sed p ressu re on the crust u n d er the o c ea n ic beds (g e o sy n c lin a l beds). A s the size o f d o m e s co n tin u e d to e x p a n d , the re su lta n t p ressu re on o c e a n ic b ed s also c o n tin u e d to in crease. W h en the to le ra n c e lim it o f th e o c e a n ic cru st to

Mechanism of the Theory D aly h as b e lie v e d in the co llap se o f the p rim i­ tiv e c ru st b u t has no t e la b o rated the m ech an ism o f c o lla p se . It m a y be su rm ise d th at the p rim itiv e cru st w o u ld h av e b een p ro b ab ly b ad c o n d u cto r o f h eat and so th e su rfa c e te m p e ra tu re w o u ld have fallen soon to th a t o f the p re se n t tim e b u t th e loss o f h eat from the in te rio r into the e x te rio r p art co n tin u ed and hence th e in te rio r p art c o n tra cted aw ay from the o u ter shell o r crust. C o n seq u en tly , the o u te r cru st w o u ld have co llap sed on the still co n tra c tin g in terio r d u e to (i) the w eig h t o f the o cea n ic w ater, (ii) the w eig h t o f g eo sy n ciin al se d im e n ts an d (iii) g rav itatio n al fo rce o f the cen tre o f the earth . It m ay be p o in ted o u t th at the im p act o f g ra v ita tio n a l fo rce w as m o re u n d er the o cean ic c ru st than the co n tin e n ta l d o m es b ecau se the fo rm e r w as n eare r to the earth 's cen tre. It ap p ears (th o u g h not d e sc rib e d by D aly ) that the m id -la titu d i­ nal fu rro w s w ere fo rm ed as g eo sy n c lin e s due to c o lla p se o f o u te r c ru s t on th e c o n tra c tin g in te rio r o f th e e a rth an d d u e to th e g rav ita tio n a l force c o m in g fro m the c e n tre o f the earth . T h e se d im e n ts d e riv e d th ro u g h the ero sio n o f p o la r an d e q u a to ria l d o m e s (m o re p recise ly c o n ti­ n en tal d o m e s) w e re d e p o s ite d by th e riv ers in to the m id - la titu d in a l fu rr o w s a n d th e P a c ific O c e a n (g eosynclines). C o n tin u o u s sed im en tatio n and w eig h t o f the o c ea n ic w a te rs e x e rte d e n o rm o u s p re ssu re on the beds o f o c e a n s (g e o s y n c lin e s ) w ith th e re su lt their b eds w ere s u b je c te d to c o n tin u o u s su b sid e n c e . T h u s , d o w n w a r d p r e s s u r e o n th e o c e a n i c (g eosynclinal) b e d s d u e to c o n tin u o u s s e d im e n ta ­ tion and resu lta n t s u b s id e n c e o f g e o s y n c lin a l b ed s caused lateral p re s s u re on th e c o n tin e n ta l m a sse s, w ith the resu lt th e y w ere tra n s fo rm e d in to b ro a d co n tinental d o m e s k n o w n as p o la r an d e q u a to ria l

Folded Mi - -

om es. A s the o cea n ic b e d s w e re d e p re s s e d d o w n ­ Fig. 1 3 .1 0 : Illustration o f slid in g https://telegram.me/UPSC_CivilServiceBooks

w ard due to g rav ita tio n a l fo rc e o f th e e a rth 's c e n tre , an w eig h t o f o c e a n ic w a te r a n d g e o s y n c lin a l

Daly.

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c o n tin e n t

theory ofR-A-

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w ithstand the ev erin creasin g pressure w as crossed, the oceanic beds began to rupture and break. Thus, the support o f the co n tin en tal dom es w as rem oved due to rupture o f the ocean ic beds w hich introduced strong tensional m o v em en ts d ue to w hich larger blocks o f continen tal m ass began to slide tow ards the geosynclines. T he geo sy n clin al sedim ents were thus squeezed and fo ld ed due to com pressive force coming from the slid in g continental blocks (fig. 13.10) giving birth to folded m ountains.

building in a sim ple m anner yet it does not present coherent account o f the problem as the theory does not go into details and there is a w ide gap betw een theoretical and practical aspects o f the theory. M ajor part o f the theory is based on self proved facts (axiom s). The theory suffers from the follow ing lim itations. (1) The sliding continent theory presents er­ roneous concepts about the structure o f the interior o f the earth. His concept, th at the outer crust is denser than the substratum , is against the evidences of seism ology because it is now proven fact that the density increases with increasing dep th in the in te ­ rior o f the earth.

A ccording to D aly the broken continental blocks and parts o f o ceanic crust founder in the substratum b ecau se the density o f outer crust is more than th e s u b s tra tu m . Or: th e o th e r h an d , the geosynclinal sed im en ts do not founder in the sub­ stratum rath er th ese flo at on the substratum because these are less d en se than the substratum . Because of this fact geo sy n clin al sedim ents are m ore folded by the com pressive fo rces g enerated by sliding conti­ nental blocks, it is, thus, obvious that greater the am ount o f slip p in g o f co n tin en tal blocks, the more geosynclinal sed im en ts are squeezed and m ore and greater folded m o u n tain s are form ed. D aly has fur­ ther pointed out that the foundered continental blocks in the su b stratu m are m elted due to high tem perature and thus rise in the volum e o f m olten continental blocks cau ses fu rth er rise in the m ountains.

(2) D aly’s theory is based on sev eral guesses and surm ises. W hy did the earth ’s cru st b eco m e asym m etrical? W hy the co n tin en tal do m es w ere sloping towards m id-latitude furrow s (geosynclines)? How was the Pacific O cean form ed ? D aly d o es not offer any convincing ex p lanation to th ese an d m an y more questions. (3) This theory presents erro n eo u s view s ab o u t geosynclines because these are g en erally c o n sid e re d as long, narrow and relatively sh allo w d e p re ssio n s of water but D aly's geosynclines w ere in fa c t o c e a n s (e.g. m id-latitude furrow s and P acific O c e a n ). If these are accepted as g eo sy n clin es they w o u ld h a v e never been filled w ith sed im en ts and th u s no m o u n ­ tains could have been form ed.

T he sliding c o n tin en t theory o f Daly also well explains the d istrib u tio n a l patterns o f folded m oun­ tains e.g. n o rth -so u th and w est-east extents. A ccord­ ing to D aly fo ld ed m o u n tain s are form ed because ol squeezing and fo ld in g ol geosynclinal sedim ents by com pressive fo rces cau sed by sliding o f the conti­ nental blocks to w ard s the geosynclines. Thus, westeast ex ten d in g m o u n tain s (e.g. A lpine chains and the H im alayas) w ere fo rm ed due to sliding o f polar and equatorial dom es tow ards m id-latitude furrow (Tethys geosyncline) and n o rth -so u th extending m ountains (e.g. R ockies and A n d es) w ere form ed due to sliding of continental m asses to w ard s P acific O cean. Sim i­ larly, the island arcs and festoons parallel to the A siatic co ast w ere form ed due to sliding o f A siatic

(4) Daly has also p resen te d c o n fu s in g id e a s and erroneous co n cep ts ab o u t the m e c h a n is m o f m ountain building. In fact, this th e o ry d o e s n o t ca re for the extension and dep th o t o cea n s an d a m o u n t o f sedim ents d eposited in them b u t e x p e c ts m o u n ta in s from every ocean (g eo sy n clin e). (5) T he theory p ro v id es w ro n g v ie w s a b o u t the m echanism and p ro cess o f g rav ity . T h e th e o ry does not throw light on the te rm in a tio n o f p u llin g effects of gravity and the b e g in n in g o f th e ru p tu re o f the beds o f the g eo sy n clin es. T h u s, th e re is n o coherence b etw een d iffe re n t e v en ts re la te d to m o u n ­ tain building as en v isag e d by th e slid in g c o n tin e n t theory. In fact, the th eo ry p resen ts so m e p ie c e m e a l

m ass tow ards P acific O cean.

Evaluation of the Theory

analysis o f m o u n tain b u ild in g ra th e r th a n a c o m p le te or perfect p ersp ectiv e.

T h o u g h the ‘sliding co n tinent th e o ry ’ by Daly is based on w ell know n principle o f gravitational force and tries to explain the problem o f m ountain https://telegram.me/UPSC_CivilServiceBooks

(6) The theory to certain exten t b e lie v e s in such distribution o f land and sea (m id latitud e fur­

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p art o f the lo w er layer. C ru st and substratum are co m p o sed o f sial and sim a resp ectiv ely . G enerally, sial is ab sen t in the oceanic areas.

row s and P acific O cean an d p o la r and equatorial continental dom es) as to suit its ow n purpose. W ooldridge and M org an have aptly rem arked, ‘com plete rejectio n o f the id ea m ay be prem ature, b ut it is a fair co m m en t to say th at the cause o f prim ary “b u lg e s” w h ich start the slipping has in no sense been satisfacto rily in d ic a te d .’

T h e origin o f co n v ec tiv e cu rren ts w ithin the earth d epends on the p resen ce o f rad io a ctiv e ele­ m ents in the rocks. T h e d isin teg ratio n o f radioactive elem en ts g en erates h eat w h ich cau ses convective currents. A cco rd in g to H o lm es th e re is m axim um co n cen tratio n o f rad io activ e elem en ts in the crust but tem p eratu re is not so high b eca u se there is gradual loss o f h eat th ro u g h co n d u ctio n and radia­ tion from the u pper su rface at the rate o f 60 calories per square cen tim eter per year. ‘T h is is ap p ro x i­ m ately equal to the rad io activ e en erg y p ro d u ced by a layer 14 km thick o f granite, 16.5 km o f granodiorite, 52 km o f p lateau b asalt or g ab b ro and 6 0 km o f p erid o tite’ (J.A . Steers, 1932). A cco rd in g to H olm es the loss o f heat from the earth ’s su rface is co m p en ­ sated by the heat produced by a cru stal shell o f 60 km thickness. Thus, there is no acc u m u latio n o f addi­ tional heat in the earth's cru st in sp ite o f m axim um concentration o f radioactive elem en ts. On the other hand, though there is very low co n cen tratio n o f radioactive elem ents in the substratum but the gradual accum ulation o f heat p roduced by the rad io activ e elem ents causes convective cu rren ts. T h e c o n v e c ­ tive currents depend on tw o facto rs e.g. (i) th ick n ess o f the crust near the eq u ato r and p o les and (ii) uneven distrib u tio n o f rad io activ e elem en ts in the crust. A scending co n v ectiv e cu rren ts o rig in ate un­ der the crust near the eq u ato r b eca u se o f greater thickness o f crust w hereas d e scen d in g convection currents are o rig in ated u n d er the p o la r c ru st because o f its shallow depth. T he risin g c o n v ec tiv e currents originating from below the continental crust are more pow erful than the convective currents originating from below the oceanic crust because o f g reater concentra­ tion o f radioactive elem ents in the continental crust.

(4) THERMAL CONVECTION CURRENT THEORY OF HOLMES

Objectives A rth u r H olm es p o stu lated his therm al con­ v ectio n cu rren t theory in the year 1928-29 to explain the in tricate problem s o f the origin o f m ajor relief featu res o f the earth's surface. H o lm es’ m ajor objec­ tiv es w ere not co nfin ed to search the m echanism o f m o u n tain building b ased on sound scientific b ack ­ g round b u t w ere also directed to w ard s finding scien ­ tific explanation for the origin o f the continents and ocean basins in term s o f co n tin en tal d rift as he was opposed to the co n cep t o f perm anency o f the co n ti­ nents and ocean basins as en v isag ed by the ad v o ­ cates o f therm al contractio n o f the earth. W ooldridge and M organ have aptly rem arked, “T he only unify­ ing theory w hich show s a hopeful sings o f reconcil­ ing certain o f the d iv erg en t h ypotheses o f m ountain b uilding and co n tin en tal d rift is that due to H olm es' (S.W . W ooldridge and R .S. M organ, 1959). Orogenetic Force T he driving force o f m ou n tain building im ­ plied by A rthur H o lm es is p ro v id ed by therm al convection currents originating deep w ithin the earth. T he m ain source o f the origin o f co n v ectiv e currents is e x c e s s iv e h e a t in th e s u b s tr a tu m w h e re in d isin te g ra tio n o f rad io a ctiv e elem en ts generates h eat regularly. In fact, the w hole theory depends exclusively on the m ech an ism o f co n v ectiv e c u r­ rents.

Mechanism of the Theory Base of the Theory

C o n v ectiv e cu rren ts, th u s, are generated at som e p laces in th e su b stratu m . B ecau se o f differ­ ence o f tem perature gradient from the e q u a t o r (greater) to w ard s the p o les (lo w ) risin g c o n v ec tiv e currents are fo rm ed u n d er the eq u ato rial cru st w hile dow n­ w ard m o v in g (d escen d in g ) co n v ec tiv e currents are g en erated u n d er the p o lar cru st. T he convective cu rren ts o rig in atin g u n d er the co n tin en ta l crust are m ore p o w erfu l th an the c o n v ec tiv e cu rren ts origi­ https://telegram.me/UPSC_CivilServiceBooks

A ccording to H olm es the earth co n sists o f 3 zones or layers e.g. (i) u p p er lay er o f g ran o d io rite (10 to 12 km ), (ii) interm ediate lay er (20 to 25 km ) of am phibolite and (iii) low er layer o f eclo g ite. He has further grouped these three layers into tw o zones e.g. (i) crust consisting o f u pper and m iddle or interm ediate layers and cry stallin e u p p er p art o f lower layer and (ii) su b stra tu m rep resen tin g m olten

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m o u n t a in b u il d in g

nating u n d e r th e o c e a n ic cru st. I t m a y b e p o in te d o u t that th e c u rre n ts o rig in a tin g u n d e r th e e q u a to ria l crust m o v e to w a rd s th e p o le s i.e. to w a rd s n o rth an d south a n d th u s th e c ru s ts a r e c a rrie d aw ay w ith th e co n v ec tiv e c u rre n ts .

risin g c o n v e c tiv e c u rren ts d iv e rg e in o p p o s ite d ire c ­ tio n s, is stre tc h e d an d th in n e d d u e to te n sio n a l fo rces an d u ltim a te ly th e c ru s t is ru p tu re d an d b ro k e n in to tw o b lo c k s w h ich are c a rrie d aw ay by la te ra l d iv e r­ g en t c o n v e c tiv e c u rre n ts a n d th e o p e n in g b etw ee n tw o b lo ck s b eco m es seas. T h u s, d iv e rg e n t c o n v e c ­ tiv e c u rren ts c a u se c o n tin e n ta l d rift, (ii) W h ere tw o lateral c o n v e c tiv e c u rre n ts o rig in a tin g u n d e r th e c o n tin en ta l and o c ea n ic c ru sts c o n v e rg e (fig . 13.11).

T h e re a re tw o s itu a tio n s o f ris in g c o n v e c tiv e cu rrents w h e n th e y r e a c h th e lo w e r lim it o f th e cru stal m a s s e s , (i) T h e c ru s ta l m a ss, w h e re tw o

C o n tin e n t

C o n tin e n ta l S h e lf

C o n t in e n t

Sea

tt tt

A scending Current Fig. 13.11 : Illustration o f rising and descending convective currents under the crust. A-D ivergent convective currents cause opening o f the landm ass and creation o f oceans. B-convergent convective currents cause closin g o f la n d m asses a n d oceans a n d create mountains. c o m p re ss iv e fo rc e is g e n e r a te d w h ic h c a u s e s s u b sid ­ ence in th e c r u s ta l z o n e s g iv in g b irth to g e o s y n c lin e s and c lo s in g o f s e a . It is a p p a r e n t th a t d iv e rg e n t c o n n e c tiv e c u r r e n ts m o v e th e c ru s ta l b lo c k s aw ay in o p p o site d ir e c tio n s a n d th u s c re a te s e a s a n d o cea n s w hile c o n v e r g e n t c o n v e c tiv e c u rre n ts b rin g c ru sta l

to H o lm e s th e e q u a to ria l c r u s t w a s s tr e tc h e d a n d ru p tu re d d u e to d iv e rg e n c e o f r is in g c o n v e c tiv e c u rre n ts w h ic h c a rrie d th e r u p tu r e d c r u s ta l b lo c k s to w a rd s th e n o rth a n d s o u th a n d T e th y s S e a w a s fo rm ed . T h is p h a s e is c a lle d ‘O p en in g o f T eth y s’ . A g ain tw o sets o f c o n v e rg e n t o r d o w n w a rd m o v in g (d e s c e n d in g ) c u rre n ts b r o u g h t L a u r a s ia a n d G o n d w a n a la n d to g e th e r a n d th u s T e th y s w a s c o m ­ p re s s e d an d fo ld e d in to A lp in e m o u n ta in s . T h is p h a se is c a lle d ‘C losing o f T eth y s’.

blocks to g e th e r a n d th u s fo rm m o u n ta in s . T h e c o n v e c tiv e c u r r e n ts a re d iv id e d in to tw o g roups o n th e b a s is o f th e ir lo c a tio n a l a s p e c t e.g . (i) convective cu rren ts o f risin g colum ns an d (ii) convective cu rren ts o f fa llin g colum ns. T h e ris in g c o n v e c tiv e c u r r e n ts a f te r r e a c h in g th e lo w e r lim it o f the c ru s t d iv e rg e in o p p o s ite d ire c tio n s . T h is o u t­ w ard o r d iv e rg e n t m o v e m e n t in tro d u c e s te n sio n a l

‘T h e c o n v e c tiv e m e c h a n is m is n o t a s te a d y p ro c e s s b u t a p e rio d ic o n e , w h ic h w a x e s a n d w a n e s a n d th e n b e g in s a g a in w ith a d if f e r e n t a r r a n g e m e n t o f c e n tr e ’ (A . H o lm e s 1 9 5 2 ). I t m e a n s th a t th e c o n v e c tiv e c u rre n ts o r ig in a te a t s e v e r a l c e n tr e s w h ic h a re n o t p e rm a n e n t. G e o s y n c lin e s a re f o r m e d d u e to s u b s id e n c e o f c ru s ta l b lo c k s m a in ly c o n t in e n ­ tal s h e lv e s d u e to c o m p r e s s iv e f o r c e g e n e r a te d by https://telegram.me/UPSC_CivilServiceBooks

force d u e to w h ic h th e c r u s t is s tr e tc h e d , th in n e d an d u ltim a tely b r o k e n a n d th e b ro k e n c ru s ta l b lo c k s are m o v ed a p p a rt. T h e w id e o p e n a re a b e tw e e n tw o d riftin g c ru s ta l b lo c k s in o p p o s ite d ir e c tio n s is fille d w ith w a te r a n d th u s a n o c e a n is fo rm e d . A c c o rd in g

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convergent convectiv e cu rren ts m oving laterally to ­ g ether under continental and o ceanic crusts. In other w ords, w hen the co n tin en tal and oceanic crusts m ove together and co n v erg e u nder the continental shelves, they descen d d o w n w ard and thus cause im m ense com pression due to w hich the crust is subjected to subsid en ce to form geosyncline. The

F irs t S tag e

£ Geosyncline

S eco n d S tag e

g eosynclines w hich are alw ays located above the convective currents o f rising colum ns under conti­ nental and oceanic crusts bring m aterials in the d escending convective currents o f falling column. C ontinuous com pression and sedim entation causes gradual subsidence o f geosynclines. H olm es has described a cyclic pattern o f therm al convective currents which includes the origin o f convective currents, form ation o f g eo sy n clin es, sedim entation and orogenesis and further rise in the m ountains. A ccording to H olm es the cyclic pattern o f convec­ tive currents and related m ountain b uilding pass through three phases or stages (fig. 13. 12).

£.

F ir s t S tag e : The first stage is o f the longest duration during w hich convective currents are o rig i­ nated in the substratum . The rising con vective c u r­ rents o f two centres converge under the continental shelves and thus form geosynclines due to c o m p re s­ sion com ing from the co n vergence o f tw o sets o f lateral currents. G eosynclines are su b jected to co n ­ tinuous sed im en tatio n and s u b sid e n c e . A s the sedim ents are pressed d o w nw ard into g eosynclines, these go further dow nw ard and are in tensely heated and m etam orphosed. M etam o rp h ism o f sedim ents causes rise in their density w hich fu rth er causes dow nw ard m ovem ent o f the m etam o rp h o sed m ate­ rials. Thus, the falling colum n o f d o w n w ard m oving convective currents is the co lu m n o f increasing den sity . A m p h ib o lite s are m e ta m o rp h o se d into eclogites. A portion o f heat is sp en t during the process o f m etam orphism and h en ce the heat does not accum ulate to g reater ex ten t. T h e first stage, characterized by high velo city co n v ec tiv e currents, is in fact the prep arato ry stage o f m o u n tain building w hich is m arked by the creatio n o f geosynclines, sedim entation and su b sid en ce o f m aterials partly, caused by co m p ressio n resu ltin g from convergence o f co n v ectiv e cu rren ts and partly by increase in the density o f m aterials due to m etam o rp h ism .

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t:

"S eco n d S ta g e : T he seco n d stage is marked by phenom enal in crease in the velo city o f convec­ tive currents but this stage is relativ ely o f short duration. T he m ain cause fo r the phenom enal in­ crease in the velocity o f co n v ec tiv e currents is the dow n w ard m o v em en t o f cold m aterials in the falling colum n and upw ard m o v em en t (rise) o f hot materi­ als in the risin g co lu m n o f co n v ectiv e currents. In c r e a s e d p r e s s u r e d u e to m e ta m o rp h is m o f https://telegram.me/UPSC_CivilServiceBooks

Fig. 13.12 : Illustration o f successive stages o f thermal convective currents under the crust and moun­ tain building.

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m o u n t a in b u il d in g

g eo m aterials in th e fa llin g co lu m n o f d esc e n d in g cu rrents in c re a s e s th e v e lo c ity o f d o w n w a rd m o v in g co n v ec tiv e c u rre n ts . T h e h ig h v elo c ity c o n v e rg e n t co n v ec tiv e c u rre n ts b u c k le g e o sy n c lin a l sed im en ts and thus in itia te th e p ro c e s s o f m o u n ta in b u ild in g (fig. 13.12). T h is s ta g e , th u s, is c a lle d th e sta g e o f o ro g en esis.

on th erm al co n v ec tiv e cu rren ts. T h e theory w as criticised , at the tim e o f its p o stu latio n in 1928-29, on the fo llo w in g grounds. (1) C o n v ectiv e cu rren t th eo ry , no doubt, is a lead in g th eo ry in a new d irectio n b u t w hole o f the th eory d ep en d s on such facto rs ab o u t w hich very little is know n. R ising and fallin g co lu m n s are doubtful p h en o m en a and th e re fo re d o u b tfu l stag e can nev er be taken fo r the ex p lan atio n o f n atu ral p h en o m en a.

T h ir d S t a g e : T h e t h i r d s t a g e is c h a r a c te r iz e d by w a n i n g p h a s e o f t h e r m a l c o n v e c t i v e c u r r e n ts d u e to i n c o m i n g h o t m a t e r i a l s in t h e f a l l in g c o l u m n a n d u p w a r d m o v e m e n t ( r i s e ) o f c o l d e r m a t e r i a l s in the risin g c o l u m n . G r a d u a l l y , t h e r is i n g c o l u m n b e ­

(2) T he w h o le m e ch an ism o f c o n v e c tiv e c u r­ rents d ep en d s on the h eat g e n e ra te d b y ra d io a c tiv e elem en ts in the su b stratu m (now m a n tle ) b u t sev eral scien tists have raised d o u b t ab o u t th e a v a ila b ility o f req u ired am o u n t o f h eat g e n e ra te d by ra d io a c tiv e elem en ts. If h e a t5 thus, is in su ffic ie n t, c o n v e c tiv e cu rren ts m ay not be g en era ted an d , th e re fo re , th e w hole m ech an ism and w o rk in g o f th e th e o ry w o u ld not be p ossible. It m ay be fu rth e r p o in te d o u t th a t th e rising cu rren ts p ass on th e ir h e a t in to th e c ru s t through co n d u ctio n . T h is p ro c e ss a lso c a u s e s lo s s o f heat w hich m ay w eak en th e c u rre n ts.

c o m e s a c o l d c o l u m n i.e. c o l d m a t e r i a l s a re a c c u m u ­ lated a t t h e c e n t r e o f t h e o r i g i n o f r is in g ( u p w a r d m o v in g ) c o n v e c t i v e c u r r e n t s d u e to w h i c h th e s e c u r r e n ts c e a s e t o o p e r a t e a n d th e w h o l e m e c h a n i s m o f c o n v e c t i v e c u r r e n t s c o m e s to a n e n d . T h e t e r m i ­ n a tio n o f t h e m e c h a n i s m o f c o n v e c t i v e c u rr e n ts y ie ld s s e v e r a l r e s u l t s e .g . (i) T h e m a t e r i a l s o f the f a llin g c o l u m n s t a r t r i s i n g b e c a u s e o f d e c r e a s e in the p r e s s u r e a t t h e t o p o f t h e f a l l in g c o l u m n d u e to the e n d o f d e p o s i t i o n o f m a t e r i a l s . T h i s m e c h a n is m c a u s e s f u r t h e r r i s e in th e m o u n t a i n s , (ii) T h e d e ­ p r e s s e d a n d s u b s i d e d h e a v i e r m a t e r i a l s in the fallin g c o l u m n o f d e s c e n d i n g c o n v e c t i v e c u r r e n ts start r is ­ ing d u e to d e c r e a s e in th e w e i g h t a n d p r e s s u r e at the

(3) T he h o rizo n tal flo w o f th e rm a l c o n v e c tiv e cu rren ts u nder the co n tin e n ta l an d o c e a n ic c ru s ts is also a d o u b tfu l p h e n o m e n a b e c a u se o f la c k o f r e ­ q uired am o u n t o f h e a t to d riv e th e se c u rre n ts . I f h o rizontal flow o f c o n v e rg e n t m o v e m e n t o f c o n v e c ­ tive cu rren ts is n o t p o ssib le , th e n th e fa llin g c o lu m n w ould not exist and hen ce m o u n tain c a n n o t b e fo rm e d .

top o f t h e f a l l i n g c o l u m n , (iii) E c l o g it e , w h i c h w as d e p r e s s e d d o w n w a r d , g e t s m e l t e d d u e to i m m e n s e h e a t a n d t h u s it e x p a n d s . T h i s e x p a n s i o n in the v o l u m e o f m o l t e n e c l o g i t e c a u s e s f u r t h e r ris e in the

(4) T h e m e ta m o rp h ism o f a m p h ib o lite s in to eclo g ites and re su lta n t d o w n w a rd m o v e m e n t o f r e la ­ tively d en ser e c lo g ite s is a lso a d o u b tfu l p h e n o m ­ e n o n . E v e n w e a c c e p t th e m e t a m o r p h i s m o f am p h ib o lites in to e c lo g ite s b u t th e r e s u lta n t in c re a s e in d en sity from 3.0 to 3.4 w o u ld n o t b e e n o u g h to

m o u n t a i n s . T h i s s t a g e is k n o w n as the s t a g e o f

g lip to g e n e s is . I t is, t h u s , a p p a r e n t th a t th e th e r m a l c o n v e c t i v e c u r r e n t s o f H o l m e s e x p l a i n s all th e th re e s ta g e s o f m o u n t a i n

b u i l d i n g e .g . l i t h o g e n e s i s ,

o r o g e n e s is a n d g l i p t o g e n e s i s . G r i g g s t h r o g h h i s e x p e r i m e n t s h a s v a lid a te d

d ep ress and sin k e c lo g ite s in th e fa llin g c o lu m n . I f d esired sin k in g o f e c lo g ite s is n o t p o s s ib le , th e re w o u ld not be p ro p e r a c c o m m o d a tio n o f m e te ria ls b ro u g h t by the h o riz o n ta l c o n v e rg e n t c o n v e c tiv e c u rre n ts in to th e fa llin g c o lu m n . I f th is is so , th e w hole fallin g c o lu m n w o u ld be fille d w ith e c lo g ite s and the n ext stag e o f the m e c h a n is m o f th e c o n v e c ­ tive cu rre n ts w o u ld n ot w o rk . It is, th u s , a rg u e d th a t

the m e c h a n i s m o f c o n v e c t i v e c u r r e n t s a n d c o n s e quent m o u n ta in b u ild in g .

Evaluation of the T h e o ry C o m m e n t i n g o n H o l m e s 1t h e r m a l c o n v e c u v c c u r r e n t t h e o r y J .A . S t e e r s ( 1 9 3 2 ) h a s r e m a r k e d

The

t h e o r y is i n t e r e s t i n g , b u t it d e p e n d s u p o n s u c h f a c ­ to rs a b o u t w h i c h lit tl e is k n o w n ' . It m a y b e p o i n te d

the th e o ry d o e s not m a k e p ro p e r p ro v is io n f o r th e a c c o m m o d a tio n o f a d d itio n a l m a te ria ls .

o u t t h a t t h i s c o m m e n t o f S t e e r s a b o u t 6 8 y e a r s a g o is n ot v alid to d a y as th e re are am p le c o n v in c in g s e e n •r*a v v h irh v a l i d a t e th e m e c h a n i s m o t

(5 ) A c c o rd in g to th is th e o ry c o n v e c tiv e c u r ­ ren ts are o rig in a te d a t few c e n tr e s o n ly u n d e r th e https://telegram.me/UPSC_CivilServiceBooks

oolwTCtive c u rre n ts o rig in atin g from w ithin the m antle.

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traction o f the substratum o f the earth resulting into transgressional and regressional phases o f the seas (geosynclines). T he expansion and contraction of the substratum are based on the m echanism o f heat generated by radioactive elem ents o f the rocks. It m ay be pointed out that the theory o f A. H olm es and Joly are based on radioactive elem ents but they sought their help differen tly e.g. H olm es used radio­ active elem ents to explain the origin o f therm al convective currents in the sub stratu m w hile Joly used them to explain the m elting and resolidification o f the substratum . H e also im plied tid al fo rce and fric tio n to explain continental drift.

continental and oceanic crusts but question arises, why are they not originated at all places ? If this so happens, the horizontal m ovem ent o f these currents w ould not be possible. The w hole o f the continents w ould be divided into several blocks as the rising convection currents originating from num erous cen ­ tres w ould break the crusts and w ould give birth to volcanic eruptions o f various sorts. This observation has been now validated on the basis o f plate tectonics as rising convective currents diverge under the midoceanic ridges and thus the plate is ruptured and two plates m ove in opposite directions due to divergent convective currents and fissure flow s o f lavas occur along the m id-ocean ic ridges representing the rup­ ture zone. It m ay be concluded that the idea o f therm al convective currents conceived by A. H olm es about 71 years ago (in 1928-29) proved its w orth in 1960s w hen the scientists w ere looking forw ard to search such a force w hich can explain the m ovem ent o f plates. N ow , the process o f m o u n tain ’building can be very satisfactorily explained on the basis of co n ­ vective currents though not in the way as conceived by A. H olm es in 1928-29 but on the lines o f plate tectonics.

B a se of the Theory

The w hole m echanism o f Jo ly 's theory is based on the presence o f radioactive elem ents o f the rocks o f the earth. In ord er to explain various aspects o f the m echanism o f rad io activ e elem en ts Joly has described first the structure o f the earth. A ccording to him continents are m ade o f lig h ter sialic m aterials the density o f w hich is 2.67 w h ile the o cean ic beds are form ed o f heav ier m aterials o f sim a h aving average density o f 3.0. T hus, the cru st has been assum ed to have been co m p o sed o f sial and s u b stra ­ tum o f basalt (sim a). B esides a few ex ce p tio n s, sial is not found in oceanic beds.

(5) RADIOACTIVITY THEORY OF JOLY O bjectives

Joly postulated his theory based on rad io ac­ tivity o f certain rad io activ e m inerals in the year 1925 in his book, ‘S urface H istory o f the E a rth ’ to account fo r the origin and evo lu tio n o f su rface features o f the earth. H is theory is also know n as ‘th e r m a l cycle th e o r y ’ or ‘th e o ry o f th e s u r f a c e o f th e e a r t h ’. T h o u g h the m ain o b jectiv e o f Joly's theory w as to p resen t a d e ta ile d acc o u n t o f the therm al history o f the earth and m ath em atical ex p lan atio n o f the stru c­ ture o f the in te rio r o f the earth but he also attem p ted to e x p la in the p ro b lem s o f m o u n tain b u ild in g and the co n tin en tal drift. In fact, ‘J o ly ’s view s on the earth 's surface history are based on such reasonable prem ises, and are so sim p le in th e ir c o n ce p tio n , that they have m et w ith a g reat deal o f fa v o u r’ (J.A . S teers, 1932). W hile co m m e n tin g on Jo ly 's th eo ry o f rad io a ctiv ity he has rem a rk ed , ‘It sh o u ld not be acc ep ted as p ro v ed , but reta in ed as an h y p o th e sis w h ich p ro b ­ ab ly co n ta in s a certain ele m e n ts o f tru th ’.

A ccording to Joly the ro ck s o f th e ea rth c o n ­ tain rad io activ e e lem en ts b u t th e ir d istrib u tio n is not uniform in all zo n es o f the earth . R ad io a c tiv e e le­ m ents are found in ab u n d an c e in sialic zo n e or the co n tin en tal ro ck s b u t the ro ck s o f sim a fo rm in g the oceanic crusts are less rad io activ e. C o n tin u o u s break­ dow n o f certain ra d io a ctiv e ele m e n ts lik e uranium , thorium etc. g en era tes heat. It m ay b e p o in ted out that the actual rate o f heat p ro d u c tio n by rad io activ e elem en ts is e x ce ed in g ly sm all b u t it b e c o m e s suffi­ cien t en o u g h to p ro d u ce a p p re c ia b le re su lt after long p erio d o f ac c u m u la tio n . T h o u g h th e production o f heat is c o m p a ra tiv e ly h ig h e r in th e continental c ru st b eca u se o f m o re ra d io a c tiv e e le m e n ts than the o cea n ic b ed s b u t th e re is no la rg e -sc a le accu m u la­ tion o f heat in th e c o n tin e n ta l c ru s t d u e to continuous lo ss o f h eat th ro u g h ra d ia tio n . M echanism of the Theory A c c o rd in g to Jo ly th e d isin te g ra tio n o f rad io ­ activ e e le m e n ts o f sialic o r c o n tin e n ta l ro ck s pro­ d u c e s h e a t b u t it d o e s n o t a c c u m u la te in the contir

Orogenetic Force

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T h e d riv in g force o f m o u n ta in b u ild in g as in v o k e d by Jo ly is p ro v id e d by e x p a n sio n and c o n ­

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m ou ntain b u il d in g

239 nfftts o r sial b e c a u se th e to tal loss o f h eat th ro u g h radiation fro m th e sia lic c ru s t is m o re th an the total heat p ro d u c e d by th e ra d io a c tiv e e le m e n ts. H e has further p o in te d o u t th at te m p e ra tu re in c re ases w ith increasing d e p th . A fte r d e ta ile d m a th e m a tic a l c a lc u ­ lation Jo ly e s tim a te d th e a m o u n t o f te m p e ra tu re at the depth o f 3 0 k m to b e 1050°C. H e estim a te d the m axim um th ic k n e s s o f sial to b e 3 0 km . A cco rd in g to him th e re is n o tra n s fe r o f h eat fro m sim a to overlying sia l. H e h a s a lso e stim a te d th e am o u n t o f te m p eratu re a t th e o u te r lim it o f sim a u n d er the co n tin en ts to b e 1050°C . T h e c o n d itio n s u n d er the oceans a te la th e r d ilte r e n t. S in c e th e re is no sial in the o c e a n ic b e d s , so th e h e a t p ro d u c e d by rad io activ e elem en ts, th o u g h v e iy sm a ll, is lost to the oceanic w ater th ro u g h c o n d u c tio n but su ch situ atio n does not e x ist a t g re a te r d e p th s in th e su b stratu m (sim a) u n d er the o c e a n s . T e m p e ra tu re in c re ases w ith in­ crea sin g d e p th in th e s u b stra tu m (sim a) u nder the o cean s b e c a u s e o f a c c u m u la tio n o f h e a t p ro d u ced by ra d io a c tiv e e le m e n ts . T h is m e c h a n ism cau ses te m ­ p eratu re g r a d ie n t a t g re a t d ep th in sim a (su b stra­ tum ). T h e te m p e ra tu r e b e c o m e s eq u al to the m elting p o in t o f b a s a lt. T h e re is no tra n s fe r o f h eat from the lo w er p art o f s im a to the u p p e r part o f sim a so there is a c c u m u la tio n o f h e a t in the lo w er lay ers o f sim a b e n e a th th e o c e a n s . T h e m e ltin g p o in t is 1150°C w h ereas th e te m p e ra tu re at th e top ot su b stratu m (sim a ) is 1050°C . I f th e te m p e ra tu re o f the su b stra­ tum rise s to 1150°C it w o u ld attain its m eltin g point but the s u b s tr a tu m w o u ld still rem a in in solid state u n less re q u ire d a m o u n t o f l a te n t h e a t o f fu s io n is pro v id ed . J o ly h as c a lc u la te d that the req u ired am ount o f h eat to liq u e fy th e s u b stra tu m w o u ld be available in 33 0 0 0 0 0 0 to 5 6 .0 0 0 .0 0 0 y ears. If su ch condi-

level o f ocean ic w ater rises d u e to sin k in g o f sialic or con tin en tal m asses into liq u id sim a. T his m e ch a­ nism cau ses exten sio n o f ocean ic w ater o v e r the con tin en tal m argins. T h is p ro cess o f ex p an sio n o f o ceanic w aters and th eir en cro a ch m en t on co n tin en ­ tal m argins is called transgression of sea and th e c o n c e r n e d s ta g e is k n o w n as th e p h a se o f transgressional sea. (5) T ran sg ressio n o f se a re ­ sults in sed im en tatio n on the su b m e rg e d c o n tin en ta l m argins. T hus, this th eory o f rad io a c tiv ity a cc o u n ts fo r the origin o f g eo sy n clin es d ue to s u b m e rg e n c e o f continental m arg in s d u rin g tra n sg re ssio n a l p h a s e o f “sea. (6) The co n d itio n s u n d er th e o c e a n s are d iffe re n t b ecause there is ab sen ce o f sial. T h e in c re a s e in th e radius and the circ u m feren c e o f th e g lo b e d u e to m elting o f sim a p ro d u ces ten sio n in th e o c e a n ic b e d s w hich cau ses cracks and fau lts. M o lte n m a te ria ls o r m olten basalts co m e u p w ard th ro u g h th e se c ra c k s and faults. T h ese m o lten b asalts are th e n s o lid ifie d and thus oceanic islan d s are fo rm ed . T h e r a d io a c tiv ­ ity theory, thus, ex p lain s th e isla n d s o f th e P a c ific and other oceans. (7) C o n tin e n ta l m a ss e s e a s ily flo a t over m olten sim a, c o n s e q u e n tly th e y are m o re a f ­ fected by tidal force w h ich c a u se s w e s tw a rd m o v e ­ m ent o f the co n tin en ts. It is in th is w ay th a t th e radio activ ity th eo ry also d e s c rib e s th e p ro c e s s o f con tin en tal drift. (8) C o n tin e n ta l d rift c h a n g e s th e position o f the c o n tin e n ts an d th e o c e a n s a s th e fo rm er occupy th e p o sitio n s o f th e la tte r. T h is p r o c ­ e s s a llo w s th e e s c a p e o f h e a t a n d t h u s th e tran sg ressio n al p h ase co m es to an e n d . P e r io d o f R e g r e s s io n a l S e a - T h e p h a s e o f reg ressio n al sea is c h a ra c te riz e d by th e f o llo w in g ev en ts - (1) T h e te m p e ra tu re o f th e s u b s tra tu m d e ­ creases b ecau se ot lo ss o f h e a t d u e to c o n tin e n ta l drift. T h u s, the c o o lin g o f the s u b s tra tu m re s u lts in the re so lid ific a tio n o f m o lte n s u b s tra tu m . T h e c o o l­ ing o f the su b stra tu m b e g in s fro m its u p p e r la y e r a n d co n tin u es d o w n w a rd and u ltim a te ly th e w h o le o f t h e su b stratu m b e c o m e s so lid o n c o o lin g . (2 ) T h e d e n ­ sity o f the s u b stra tu m , w h ic h w a s re la tiv e ly d e ­ crea sed d u rin g its m o lten sta g e , a g a i n i n c r e a s e s t o reg ain its p re v io u s v alu e. (3 ) T h e r a d i u s a n d t h e circ u m fe re n c e o f the g lo b e , w h ic h w e r e i n c r e a s e d due to m e ltin g o f th e s u b stra tu m , a r e a g a i n s h o r t e n e d to th e ir p re v io u s p o sitio n , w i t h t h e r e s u l t t h e c o n t i ­ nents, w h ich w ere ra ise d re la tiv e t o t h e c e n t r e o f t h e g lobe, are a g ain b ro u g h t to th e ir p re v io u s p o s i t i o n s

tions b e c o m e p o s s ib le i.e. if th e su b stra tu m reach es the m o lten c o n d itio n , s e v e ra l ch a n g e s take p lace in the e a rth ’s s tru c tu re . P e r io d o f T r a n s g r e s s io n a l S ea - S everal in ­

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te re stin g e v e n ts ta k e p la c e w h en the su b stratu m reac h es the m o lte n c o n d itio n d u e to acc u m u ation ot g reater a m o u n t o f h e a t p ro d u c e d by the b reak ow n o f ra d io a c tiv e elem en ts. (1 ) T h e e x p a n sio n o f sim a d u e to m e ltin g c a u se s in c re ase in the rad iu s ot the g lo b e. (2 ) C o n tin e n ta l m asses or sialic m asses are ra ise d r e la tiv e to the cen tre o f th e g lo b e. (3) I he d e n sity o f s im a d e c re a se s d u e to m e ltin g and hence sia lic m a ss e s b e g in to sin k in m o lten sim a. (4) The

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geom orphology

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(5) R elative increase in th e den sity o f the sub stratu m due to reso lid ificatio n cau ses co n tractio n o f the oceanic beds w hich resu lts in the w ithdraw al o f oceanic w aters from the co n tin en tal m argins. T his is called th e p h a se o f reg r essio n a l sea. B ecause o f the w ithdraw al o f ocean ic w ater p rev io u sly su b m erg ed c o n t in e n ta l m a r g in s ( d u r in g th e phase o f tran sg ressio n al sea) rise up w ard and the d ep o sited sed im en ts are ex p o sed above the w ater level. (6) It m ay be rem e m b ered that the o ceanic beds w ere su b jected to m ax im u m ex p an sio n d u rin g the p erio d o f tran sg ressio n al ph ase d u e to m eltin g o f the su b ­ stratu m . S im ilarly , the o cean ic beds are also su b ­ je c te d to m a x im u m co n tractio n d u rin g the p erio d o f reg ressio n al sea due to reso lid ificatio n o f m olten su b stratu m . T h u s, co n tra ctin g beds o f tw o oceans e x e rt lateral co m p ressio n on the sed im en ts d e p o s­ ited on the co n tin en tal m arg in s (g eo sy n clin es), c o n ­ seq u en tly the sed im en ts d e p o sited d u rin g the perio d o f tran sg ressio n al sea are sq u eezed , b uckled and fo ld ed and th u s m o u n tain s are form ed.

tain bu ild in g is altern ated by the p eriod o f quies­ cence.

Evaluation of the Theory T h o u g h the rad io a ctiv ity th eo ry o f Joly based on scien tific facts and m a th em a tic al calcu latio n was w idely ap p reciated by sev eral scie n tists b u t sim ulta­ n eously it w as also sev erely criticized . A few critics o f the theory do n o t g ran t th e o re tic al statu s to the v iew s o f Joly rath e r they ta k e h is v iew s as m erely d escrip tiv e acco u n ts o f th e e a rth 's in terio r. In fact, the theory o f Jo ly is a w ell d ev e lo p e d geom orphic story o f the earth rath e r than a th eo ry . J.A . S teers (19 3 2 ) has rem ark ed that ‘th e th e o ry is, a t first sight, co n v in cin g and it certain ly d o es g iv e a d e q u a te ex ­ plan atio n s o f m any featu res o f the e a rth 's s u rfa c e ’. T h e fo llo w in g sh o rtco m in g s h av e been p o in te d ou t by the critics o f the theo ry . (1) T he theory is b a se d on ra d io a c tiv e e le ­ m ents o f the ro ck s o f th e e a rth at d iffe re n t d ep th s about w hich very little is k n o w n . T h u s, th e fo rc e o f ex p an sio n and co n tra ctio n o f th e s u b stra tu m (sim a) du e to m eltin g an d c o o lin g re sp e c tiv e ly b a s e d on rad io activ e elem en ts is d o u b tfu l an d p e rh a p s is no t en o u g h to form m o u n tain s.

Joly has d e sc rib e d tw o p arallel p ro cesses o f m o u n tain b u ild in g , (i) T h e sed im en ts d ep o sited in the sh allo w seas o f the co n tin en tal m arg in s are sq u eeze d and fo ld ed d u e to lateral co m p ressio n cau sed by tw o co n tra c tin g o cean ic beds, (ii) V ertical force is produced durin g the process o f resolidification o f th e su b stratu m . T h is v ertical force raises the w h o le m o u n ta in sy stem fo rm ed d u rin g the first p ro c ­ ess. It is o b v io u s th a t a cc o rd in g to this theory m o u n ­ ta in s are alw a y s fo rm ed alo n g the m arg in s o f the c o n tin e n ts fa c in g o cea n s. T h e in ten sity o f lateral p re s s u re an d c o n s e q u e n t m a g n itu d e o f fo ld in g d e ­ p e n d on the a m o u n t o f c o n tra c tio n o f o cea n ic b eds. It m ay be arg u e d th a t la rg e o cea n s w o u ld p ro d u ce m o re p o w e rfu l lateral c o m p re ssio n and h en ce g re a t­ est m o u n ta in w o u ld face la rg e st o cean . T o som e e x te n t th is s ta te m e n t is tru e as the R o ck ies and the A n d es m o u n ta in s face th e P ac ific O cean .

(2) Jeffrey s d id n o t a g re e w ith th e 3 0 -k m th ick n ess o f the co n tin e n ta l m a ss e s as e n v isa g e d by Joly. A cco rd in g to Je ffre y s th e th ic k n e s s o f the co n tin en ta l c ru st m ay n o t b e m o re th a n 16 k m . I f the th ick n ess o f th e c o n tin e n ta l c ru s t is a c c e p te d to b e 16 km then the w h o le m e c h a n is m o f J o ly 's th e o ry w ould co m e to a g rin d in g h alt as re q u ire d a m o u n t o f h e a t of 1 150°C w o u ld not be p o s sib le at th e d e p th o f 16km. (3) Jo ly 's c o n c e p t o f c y c lic n a tu re o f m oun­ tain b u ild in g has b een d is p u te d by s o m e c ritic s. The th eo ry e n v isa g e s u n ifo rm p e rio d s o f q u ie sc e n c e be­ tw een tw o p e rio d s o f m o u n ta in b u ild in g bu t this c o n c e p t h as also b e e n d is p u te d . J.A . S tee rs has c o m m e n te d th a t “ In s h o rt, th e v ery e s se n c e o f the th e o ry , th e a p ro x im a te ly e q u a lly s p a c e d recurrence o t s im ila r c o n d itio n s , s e e m s to be o n e o f its m ain d ra w b a c k s .” H e h a s fu rth e r re m a rk e d th at “there se e m s to b e little d o u b t th a t m o u n ta in building p e rio d s h a v e b e e n re c u r r e n t to s o m e e x te n t, b u t it is v ery d o u b tfu l if th ey h a v e b e e n so re g u la r as Joly's th e o ry w o u ld m a k e th e m ” (J.A . S te e rs , 1932).

Jo ly a lso e x p la in s th e p erio d o f q u ie sc e n c e b etw ee n tw o p e rio d s o f m o u n ta in b u ild in g . T h e total p erio d o f tw o so lid p h a se s o f th e s u b stra tu m (so lid p h ase, m olten p h ase an d re s o lid ific a tio n p h ase o f th e su b stra tu m ) is c alled o n e r e v o lu tio n w h erein th e m e ltin g o f su b stra tu m (s im a ) la k es to tal tim e p erio d o f 3 3 ,0 0 0 ,0 0 0 to 5 6 ,0 0 0 ,0 0 0 y ears. It m ay be, th u s, in ferre d th at th e p ro c e ss o f m o u n tin g b u ild in g o c ­ c u rs in cy clic m a n n e r w h e re in th e p e rio d o f m o u n https://telegram.me/UPSC_CivilServiceBooks

(4 ) T h is th e o ry e n v is a g e s tw o fa c ts about m o u n ta in b u ild in g , (i) ‘T h e g r e a te st m o u n ta in s m ust

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face the greatest ocean ic beds. ’ (ii) B oth the m argins 0f the continent m u st hav e m o u n tain s o f the sam e period and both th e m arg in s should be regular. The first fact is v alid ated to som e ex ten t but the second fact is not validated .

Base of the Theory

T he rigid lithospheric slabs or rigid and solid land m asses having a thickness o f about 100 km com posed o f earth ’s cru st and som e portion o f upper m antle are technically called ‘p la tes’ . T he term ‘p la te ’ was first used by C anadian g eologist J.T . (5) This th e o ry p resen ts erro n eo u s concept W ilson in 1965. T he w hole m ech an ism o f the ev o ­ about g eosynclines. A s p er this theory g eosynclines lution, nature and m otion and resu ltan t reactions o f are alw ays fo rm ed d u e to su b m erg en ce o f co n tin en ­ plates is called ‘plate tecton ics’. Plate tectonic theory, tal m argins due to tra n sg re ssio n o f seas. It m eans that a great scientific ach iev em en t o f the d ecad e o f geosynclines sh o u ld alw ay s be lo cated around the 1960s, is based on tw o m ajo r scien tific ev id en ces continents. O n th e o th e r h an d , it has been generally e.g. (i) ev idences o f p alaeo m ag n etism an d (ii) e v i­ accepted that g e o sy n c lin e s are long, narrow and dences o f sea-floor spreading. S ix m a jo r and 20 shallow w ater b o d ie s w h ich are ch aracterized by m inor plates have been id en tified so far (e.g. E u ra ­ continuous se d im e n ta tio n and su b sid en ce but Joly's sian plate, In d ian -A u stralian p late, A m eric an p late, geosynclines re c e iv e sed im en ts but do not undergo Pacific plate, A frican plate an d A n ta rc tic p late). the process o f su b sid en ce. W ith o u t subsidence the M cK enzie and P ark er d isc u sse d in d etail the enorm ous th ic k n e ss o f sed im en ts o f the present m echanism o f plate m o tio n s on the b asis o f E u le r s A lpine m o u n ta in s c a n n o t be exp lain ed . geom etrical theorem in 1967. H ary H ess (1 9 6 0 ) elaborated the m ech an ism o f p la te m o v e m e n t on th e basis o f the ev id en ces o f sea -flo o r sp re a d in g . W .J. M organ and Le P ichon e la b o ra te d th e v a rio u s a s ­ pects o f p late tecto n ics in 1968.

(6) PLATE TECTONIC THEORY

Objectives P late te cto n ic th eo ry is a com prehensive theory w hich o ffers e x p la n a tio n s fo r v arious re lie f features and te cto n ic e v e n ts viz. m o u n tain building, folding and fau ltin g , c o n tin e n ta l drift, vu lcan icity , seism ic events (e a rth q u a k e s ) etc. T h e theory belongs to a host o f s c ie n tists o f d iffe re n t d iscip lin es. Plate tec­ tonic th e o ry is, in fact, the o u tco m e o f com bined efforts o f m a n y sc ie n tists o f d iffere n t co untries w ork­ ing to g e th e r and se p a ra te ly . T h e th eo ry cam e into light in th e 1960s. It e n v isa g e s the fo rm atio n o f

T hree types o f p la te b o u n d a rie s (see fig . 6.7 in chapter 6 on the o rig in o f c o n tin e n ts a n d o c e a n b a sin s) have been id e n tified e.g. (i) d e s tru c tiv e p la te boundaries, (ii) c o n stru ctiv e p la te b o u n d a rie s a n d (iii) co n serv ativ e plate b o u n d a rie s. (1) C on stru ctive p la te b o u n d a r ie s also c a lle d as ‘d iv er g en t p la te b o u n d a r y ’ o r ‘a c c r e tin g p la te b o u n d a ry ’ rep resen t zo n es o f d iv e rg e n c e a lo n g the m id -o cean ic rid g es and are c h a ra c te riz e d b y c o n ­

m o u n tain s d u e to c o llisio n o f plate bo u n d aries.

tin u o u s ad d itio n (a c c re tio n ) o f m a te ria ls as there is co n stan t u p w ellin g o f m o lte n m a te ria ls (b a s a ltic

Orogenetic force T h e o ro g e n e tic fo rce to form m o u n tain s is

lavas) from b elo w the m id -o c e a n ic rid g e s . T h ese

p ro v id ed by th e c o m p re s s iv e fo rces cau sed by the

b asaltic lav as are c o o le d an d s o lid ifie d a n d a re a d d e d

collision o f tw o c o n v e rg e n t p la tes alo n g the d e stru c ­

to the trailin g m a rg in s o t th e d iv e rg e n t p la te s an d

tive p la te b o u n d a rie s. T h e rm a l co n v e c tiv e cu rren ts

thus new o ce a n ic c ru s t is c o n tin u o u s ly fo rm e d . In

o rig in atin g in th e m a n tle h av e been acc ep ted as the co m p eten t fo rce fo r th e m o v e m e n t o f p lates. The plates m o v e in d iffe re n t d ire c tio n s rela tiv e to each

fact, o cean ic p la te s sp lit a p p a rt a lo n g the m id -o c e ­ anic rid g es an d m o v e in o p p o s ite d ire c tio n s (se e fig . 6 .7 ) and th u s tra n s fo rm fa u lts are fo rm e d .

o th er u n d er th e im p a c t o f th e rm al c o n v e c tiv e c u r­

(2) Destructive plate boundaries a lso know n as ‘consum ing plate b ou nd aries’ or ‘con vergent plate boundaries’ are th ose w here tw o p lates c o l­ lide against each other and the lead in g e d g e o f o n e plate having relatively lighter m aterial o verrid es th e other plate and the overridden plate boundary o f

rents. P late m o v e m e n ts tak e p la ce in acc o rd an ce w ith th e E u le r 's g e o m e tr ic a l th eo rem w h ich e n v is ­ ages th e m o v e m e n t o f p la tes in the lorrn of sim p le ro tatio n alo n g a p o le o f ro tatio n (see fig. 6 .1 0 in

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GEOMORPHOLCXr cJgl A sia. T h e fo ld m o u n ta in ra n g e s o f isla n d arcs and re la tiv e ly d e n s e r m a te ria l is su b d u cted o r th ru st into festo o n s ‘form w h ere a sectio n o f th e o cean f lo o r# ® th e u p p e r m a n tle an d th u s a p a rt o f th e c ru st is lo st in su b d u cted in th e o cea n b a sin aw ay fro m a continent th e m a n tle (see fig. 6.8 in c h a p te r 6). T h is m e c h a ­ i.e. w h ere o cean flo o r c ru s t is o n e ith e r side o f the n ism re su lts in c o n s ta n t lo ss o f cru stal m aterials. c o n v e rg e n t p la te b o u n d a ry ’ ( M J . B rad sh aw et a l - f t . (3 ) C o n s e r v a tiv e p la te b o u n d a r ie s a lso 1978). k n o w n as ‘sh e a r p la te b o u n d a r ie s ’ are th o se w here T h e b e st e x a m p le o f th e fo rm a tio n o f m oun­ tw o p la te s slip p a st eac h o th e r w ith o u t any co llisio n tain s d u e to c o llisio n o f tw o o c e a n ic p la tes is the a lo n g th e tra n sfo rm fa u lt an d thus cru st is n eith er situ atio n o f Ja p a n e se isla n d arc. M o u n ta in s o f Japan c re a te d n o r d e stro y e d . ran g e in h e ig h t fro m 3 0 0 0 m to 4 0 0 0 m A M S L . It Mechanism of the Theory m ay be p o in ted o u t th a t all th e m o u n ta in s o f Japan A c c o rd in g to p late te cto n ic th eo ry m o u n tain s are o f v o lc an ic o rig in . T h o u g h J a p a n e s e m ountains a re fo rm e d d u e to co llisio n o f tw o co n v erg e n t plates. ex h ib it a n u m b e r o f c h a ra c te ris tic fe a tu re s o f folded M o u n ta in s are alw ay s fo rm ed alo n g the d estru ctiv e m o u n tain s b u t th ey can n o lo n g e r by re g a rd e d as fold p la te b o u n d arie s. It is o b v io u s th a t the p ro cess o f m o u n tain s lik e th e A lp s an d th e H im a la y a s . H onshu m o u n ta in b u ild in g is asso ciated w ith d estru ctiv e Islan d rep resen ts th e m o st c h a ra c te ris tic e x am p le of p la te b o u n d arie s o f tw o co n v erg e n t plates. T h e plate the situ a tio n o f th e c o n v e rg e n c e o f tw o oceanic te c to n ic theory envisag es the fo rm atio n o f m o u n ­ p lates. ta in s d u e to co m p ressio n o f sed im en ts cau sed by the H o n sh u is b o rd e re d b y J a p a n T re n c h in the co llisio n o f tw o co n v erg e n t p late bo u n d aries. T w o east and Jap an S ea in th e w est. T h e w e s te rn p art o f p lates m oving to g eth er u n d er the im p act o f therm al the islan d is m o re fre q u e n te d b y v o lc a n ic activ ities co n v ec tiv e currents co llid e ag ain st each o th er and than the eastern p art. T h e is la n d is c h a ra c te riz e d by the p late boundary h av in g relativ ely d en ser m a teri­ tw o belts o f m e ta m o rp h ic ro c k s on e ith e r sid e. It is als is su b d u cted u n d er the o th er p late boun d ary o f believ ed th at th e Jap an T re n c h w as fo rm e d d u e to relativ ely lig h ter m aterials. T his su b d u ctio n zone is su b d u ctio n o f P acific O ce a n ic p la te u n d e r th e o c e ­ also called B e n io ff zo n e . T he su b d u ctio n o f plate anic cru st to th e e a st o f Jap an . A c c o rd in g to p la te b oundary cau ses lateral co m p re ssiv e force w hich tecto n ic th eo ry th e s u b d u c te d p o rtio n o f p la te a fte r ultim ately sq u eezes and folds the sed im en ts and reach in g a d ep th o f 100 km o r m o re s ta rts m e ltin g m aterials o f the m a rg in s o f the p lates and thus due to h ig h te m p e ra tu re p re v a ilin g in th e u p p e r m ountains are form ed. T h e su b d u cted p art o f the m an tle. T h e m a g m a, th u s fo rm e d , a s c e n d s an d ap ­ plate after reac h in g a d ep th o f 100 km o r m o re in the p ears as v o lc an ic e ru p tio n a b o u t 2 0 0 k m aw ay from m antle is liq u efied and thus ex p an d s in vo lu m e th e o cea n ic tren c h . S in c e J a p a n is v e ry c lo s e to the because o f co n v ersio n o f th e p o rtio n o f p la te into m agm a. T his ex p an sio n o f m o lten m a te ria ls cau ses Jap an T re n c h an d h e n c e w e s te rn p a r t o f Ja p a n is further rise in the m o u n tain s. m o re fre q u e n te d by v o lc a n ic a c tiv itie s . T h is p rocess is still c o n tin u in g as th e P a c ific p la te is b e in g con­

T he co n v erg e n ce an d c o n se q u e n t c o llisio n o f plate b o undaries o ccu rs in th ree situ a tio n s viz. (i) collision o f tw o o cea n ic p lates, (ii) c o llisio n o f tw o continental p lates an d (iii) co llisio n o f o c e a n ic continental plates.

tin u o u sly s u b d u c te d u n d e r th e o c e a n ic c ru s t along th e Ja p a n T re n c h (fig . 1 3 .1 3 ). T h e eru p tio n s o f v o lc a n o in th e m o n th o f J u n e , 1991 in Ja p a n after a

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d o rm a n t p e rio d o f a b o u t 2 0 0 y e a rs a n d the eruption (1) C o n v e rg en ce (C o llisio n ) o f T w o O coefM ­ t.P in a tu b o o n J u n e 9 , 1991 in M a n ila , Phillippines, an ic P lates- The co llisio n o f tw o ocean ic plates and v a lid a te th e a u th e n tic ity o f th is th e o ry o f p la te tec­ subduction o f the boundary o f the plate o f relatively to n ic s. T h e v o lc a n ic e ru p tio n s ca u sed b y su bduction denser materials results in the form ation o f the fold o f o c e a n ic p la te s u n d e r the o cea n ic crust o ff the mountain ranges o f island arcs and festo o n s, for J a p a n e s e c o a s t r e s u lte d in to c o n tin u o u s acc u m u la­ exam ple, island arcs and festo o n s form ed by Japa­ tio n o f v o lc a n ic ro c k s and co n seq u en t in crease in the nese islands, Phillippines etc. around the w estern h eigh t o f islan d arc and thus the form ation o f vol­ margin o f the P acific O cean o f f the east co a st o f ca n ic m o u n ta in s co u ld b e p o ssib le .

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MOUNTAIN b u i l d i n g

243

U

Cretaceous belt

nf

Metamorphic belt

Flysch

jopan

wedge

fr e nc h

Rapid

sedimentation

Greatest volunta of q u a t e r n a r y volcanoes

High t e m p e r a t u r e , h i g h pressure /ik .

fclueschisthi gh p re ssure, low temperature

k Me t a mor ph i c be 11

Magma g e ne r at e d from pl ate

.— ••Surface er os io n, tr ansport 1

b e l o w 100 Km

R ising magma

Fig. 13.13 : Formation o f island arcs and mountain building on the basis o f plate tectonics (Reproduced from M.J. Bradshaw et. al, 1978, this diagram o f Dewey and Bird was reproduced in Cox, 1973). posed o f continental crusts co llid e ag ain st each other, the continental plate having relativ ely d e n se r m a te ­ rials is subducted under the o th er co n tin e n ta l p late having com paratively lighter m aterials than the form er. T he resultant lateral co m p ressio n sq u eezes and fo ld s the sedim ents deposited on eith er side o f the c o n ti­ nental plate m argins and the sed im en ts o f the geosynclines lying betw een tw o c o n v e rg e n t c o n ti­ nental plates and thus form s g ig a n tic fo ld ed m o u n ­ tains e.g. the A lps and the H im alay as.

(2 ) C o n v e r g e n c e (C o llisio n ) o f C ontin en tal and O c e a n ic P la te s- T h e co llisio n o f continental and o c e a n ic c o n v e rg e n t p la te s resu lts in the fo rm a­ tion o f c o rd ille ra ty p e o f fo ld ed m o u n tain s e.g. the w estern c o rd ille ra o f N o rth A m e ric a (in clu d in g the R o ck ies). W h e n o n e c o n tin e n ta l and the o th er o ce­ anic p la te s c o llid e d u e to th e ir co n v erg e n ce along subduction o r B e n io ff zone, the oceanic plate boundary b eing h e a v ie r d u e to c o m p a ra tiv e ly d e n se r m aterials is su b d u c te d b e lo w th e c o n tin e n ta l p late boundary. T he se d im en ts d e p o s ite d on the co n tin en tal m argins are sq u e e z e d a n d fo ld ed d u e to co m p re ssiv e forces cau sed by th e su b d u c tio n o f o c e a n ic p late (see fig. 6.8 in c h a p te r 6). T h e R o c k ie s an d th e A n d es m o u n ­ tains w ere fo rm ed d u e to su b d u ctio n o f the P acific o cean p la te u n d e r th e A m eric an co n tin en ta l plate

T he origin o f the A lp in e m o u n ta in s o f E u ro p e and A sia are w ell ex p lain ed on the b asis o f this m echanism (collision o f tw o c o n v e rg e n t co n tin en tal plate b o undaries) o f plate tecto n ics. T h ere e x isted a long T ethys g eo sy n clin e b etw een E u rasia n p late in the north and A frica-In d ian p late in the south d u rin g M esozoic era. T he g eo sy n clin al sed im en ts o f T eth y s sea were squeezed and folded into A lp in e-H im alay an m ountain ch ain s due to lateral c o m p re ssiv e forces

(fig. 13.14).

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(3 ) C o n v e r g e n c e (C o llisio n ) o f tw o C o n ti­ n en ta l P la te s- W h en tw o co n v e rg e n t plates c o m ­

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geomorphology

244

Continental Shelf

Continental rise

V olcan ics

Seo level o w lavas

Blueschisf metamorphism

Continental crust

Beginning of

TT T T T T TTTT

Pl a t e s u b d u c t e d

uplift

f ronj

G r a v i t y slide

ife V o l c o n o e s

Fig. 13.14 : Origin o f Cordillera type o f folded mountains on the basis o f plate tectonic theory 1. Subduction o f oceanic plate below continental plate upto 100 km depth leads to eruption o f submarine volcanic ocks (magma) and deformation o f plate margins; 2. Rising magma generates heat, sediments o f plate margins are folded and uplifted due to expansion o f dome and intrusion o f gabbro/diorites; 3. With the growth o f mobile orogenetic core plate deformation increases induced by high temperature/high pressure conditions which leads to subsidence o f continental shelf gravity slides occur due to continuance o f uplift; 4. Metamorphosed sheets are thrust towards continental parts due to mobile core. Granites come upward and are emplaced in the mobile core. Continental margins are compressed and folded due to tangential force generated by exfxmding mobile core (After M.J Bradshaw et. al, 1978). caused by the convergence and collision o f Eurasia and African- Indian continental plates during Cenozoic era. It m a y be pointed out that the formation o f Alpine-Himalayan

mountain chains could be possible due to continued collision ol continental plates and consequent orogenesis along several subduction zones for long period o f time. https://telegram.me/UPSC_CivilServiceBooks

>

Granites M o l a sse

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MOUNTAIN b u i l d i n g

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A b o u t 7 0 -6 5 m illio n y e a rs ag o th e re w a s an extensive g e o sy n c lin e , k n o w n as T e th y s g eo sy n clin e, in the place o f th e H im a la y a s . T e th y s g e o sy n c lin e was b o rd ered b y A s ia tic p la te in th e n o rth an d In d ian plate in th e so u th . T e th y s g e o s y n c lin e b eg a n to contract in s iz e d u e to m o v e m e n t o f In d ia n and A siatic p la te s to g e th e r. A b o u t 6 0 -3 0 m illio n y ears ago the In d ia n p la te c a m e v ery c lo se to A siatic p late. The In d ian p la te b e g a n to a c tiv e ly su b d u c t u n d e r the A siatic p la te . T h e c o n v e rg e n c e an d co llisio n o f

A siatic and Indian p lates an d c o n seq u en t subduction o f In d ian p late u n d er th e fo rm er cau sed lateral co m ­ p ressio n d u e to w h ich the sed im en ts o f T ethys g eo sy n clin e w ere sq u eezed an d fo ld ed into three p a ra lle l c h a in s o f th e H im a la y a s a b o u t 3 0 -2 0 m illio n y ears ago. It h as b een e stim ated th a t the c ru s t h as b een s h o rte n e d by 5 0 0 km b e tw e e n A siatic and Indian p lates d u e to co n v erg e n ce o f tw o p lates an d su b d u ctio n o f In d ian p la te (fig. 13.15).

Fig. 13.15 : Origin o f the Him alayas on the basis o f plate tectonic theory. 1. Movement o f Asiatic and Indian plates towards each other about 70-65 million years B. P .; 2. Collission o f Indian and Asiatic plate and subduction o f Indian plate under Asiatic plate about 60-30 million years B.P. ; 3. Beginning o f orogenesis o f the Himalayas due to thrusting resulting fro m subduction o f Indian plate under Asiatic plate; 4. Present position. and sea-flo o r sp read in g . P late te c to n ic th e o ry a lso satisfacto rily ex p lain s th e cy c lic p a tte rn o f m o u n ta in building.

A lp in e m o u n ta in s o f E u ro p e w ere fo rm ed due to c o n v e rg e n c e a n d c o llis io n o f E u ro p e a n an d A fri­ can p la te s. S in c e th e c o llis io n o f th e se tw o c o n tin e n ­ tal p la te s w a s v e ry c o m p le x a n d h e n c e th e stru ctu re o f the E u ro p e a n A lp in e m o u n ta in s is also very c o m ­ plex. T h e A fric a n p la te is still m o v in g n o rth w a rd and is b e in g su b d u c te d u n d e r E u ro p e a n p la te to th e so u th o f A eg e a n arc. S im ila rly , In d ia n p la te is also bein g c o n tin u o u sly s u b d u c te d u n d e r A sia tic p late.

It m ay be p o in ted o u t th a t 4 m a jo r p e rio d s o f m o u n tain b u ild in g h av e b een id e n tifie d e.g. (i) p re C am b rian o ro g en y , (ii) C a le d o n ia n o ro g e n y , (iii) H ercy n ian o ro g en y an d (iv ) T e rtia ry o ro g e n y . A ll the earlier th eo ries o f m o u n tain b u ild in g , as d is ­ cu ssed in the p rece d in g p ag es, s u ffe r fro m a c o m ­ m on d efec t th at th ey , so m e h o w , d o a tte m p t to ex p lain the o rig in o f the fo ld ed m o u n ta in s o f T e rti­ ary p erio d but they d o -n o t th ro w an y lig h t o n th e m o u n tain s o ld er than T ertiary p erio d . It m a y b e m en tio n ed that p lates are a lw ay s in m o tio n d u e to w hich so m e tim es all th e lan d m a sse s u n ite to g e th e r to form Pangaea an d ag ain b reak up a n d m o v e a w a v

T h e o v e rw h e lm in g m a jo rity o f the sc ie n tists all o v e r th e w o rld is o f th e v iew th at p la te te cto n ic th e o ry h a s a lm o s t so lv e d th e p ro b le m o f the o rig in o f c o n tin e n ts a n d o c e a n b a sin s an d o f m o u n tain b u ild ­ ing. In fa c t, th e c o n tin e n ta l d rift h as now b eco m e a reality on th e b a s is o f e v id e n c e s o f p a la e o m a g n e tism

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Evaluation of the Theory

iv.> y https://telegram.me/UPSC_CivilServiceBooks

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GEOMORPHOLOGY

246

tains w ere fo rm ed . A b o u t 3 0 0 m illio n y ears ago the A tla n tic O cean w as c o m p le te ly c lo se d an d the o ro g en esis o f th e A p p lach ian m o u n ta in s w as co m ­ p leted d u rin g P erm ian p erio d . A t th e sam e tim e H ercy n ian m o u n tain s o f E u ro p e w ere fo rm e d (see fig u re 6.12 in c h a p te r 6). A b o u t 2 0 0 m illio n years ago all the c o n tin e n ts w ere ag ain u n ite d to form P an g aea II. A b o u t 150 m illio n y e a rs ag o P an g aea w as again d isru p te d an d th e A tla n tic o cea n w as reo p en ed . T he A lp in e m o u n ta in s w ere fo rm e d due to plate m o v e m en ts d u rin g T e rtia ry p e rio d .

relative to each o th er and new distrib u tio n al pattern o f continents and ocean b asin s is evolved. T he p ast history o f the earth up to 200 m illion years has been r e c o n s tr u c te d on th e b a s is o f e v id e n c e s o f palaeom agnetism . A b o u t 20 0 m illion years before present all the co n tin en ts w ere united to g eth er in the form o f P an gaea II (a su p er co ntinent). It is b elieved that before the situation o f P an g aea II, the co n tin en ts w ere separated from each other. T h ese co n tin en ts m ig h t have m oved relativ e to each other in such a w ay th at they m igh t have been united to g eth er to form a super continent. It is b eliev ed that the c o n ti­ nents m oved together due to plate m otions and w ere united together in the form o f P a n g a ea I durin g Pre C am brian period, ab o u t 700 m illion years ago. A bout 600-500 m illion years ago P an g aea I w as disrupted. A bout 460 m illion y ears ago the A tlantic O cean began to close dow n d u e to co n v erg en ce o f A m eri­ can and E urasian plates and the C aledonian m o u n ­

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T he only p o in t o f a rg u m e n t a n d q u e s tio n is related to the c o m p e te n t fo rce re s p o n s ib le fo r the m o v em en t o f p lates and d riftin g o f c o n tin e n ts . M o st o f the scien tists still rely on th e th e rm a l c o n v e c tiv e currents o rig in atin g fro m th e m a n tle as th e p ro b a b le ad eq u ate fo rce to m o v e th e p la te s (c o n tin e n ts ) in d ifferen t d irec tio n s re la tiv e to e a c h o th e r.

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WEATHERING AND MASSMOVEMENT M e a n i n g a n d c o n c e p t ; c o n tr o llin g f a c to r s o f w e a th e r in g ; ty p e s o f w e a t h e r i n g p r o c e s s e s ; p h y s ic a l w e a th e r in g ; c h e m ic a l w e a th e r in g ; b i o t i c w e a t h e r i n g ; b io c h e m ic a l w e a th e r in g ; g e o m o r p h i c im p o r ta n c e o f w e a t h e r i n g ; m a s s m o v e m e n t a n d m a s s w a s tin g - m e a n in g a n d c o n c e p t ; c l a s s i f i c a t i o n o f m a s s m o v e m e n ts ; f a c to r s o f m a s s m o v e m e n t s ; s lid e s ; f a l l s ; f lo w s ; c r e e p .

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C H A PT E 14

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14 WEATHERING AND MASS MOVEMENT

14.1 MEANING AND CONCEPT The p ro cess o f d isin teg ratio n and d eco m p o si­ tion o f rocks in situ is g e n e ra lly called w eathering. It m eans w eath erin g is a static pro cess. A cco rd in g to C.D. O ilier (1 9 6 9 ) “ w e a th e rin g is the breakdow n and alteratio n o f m in e ra ls n e a r th e earth 's surface to pro d u cts th a t are m o re in eq u ilib riu m w ith new ly im posed p h y sic o -c h e m ic a l c o n d itio n s.” A ccord in g to P. R eich e (1 9 5 0 ) “ w e a th e rin g is the resp o n se o f m in erals w h ic h w e re in e q u ilib riu m w ith in the lith o sp h ere to c o n d itio n s a t o r n ear its co n tact w ith the a tm o sp h e re , th e h y d ro sp h e re , and p erh ap s still m ore im p o rta n tly , th e b is o p h e re .” It m ay be p o in ted out th a t ro c k s are n e v e r in p e rm a n e n t eq u ilib riu m ra th e r th e y a re in e q u ilib riu m o n ly m o m en tarily and th u s W .D . K e lle r (1 9 5 7 ) h as p le a d e d fo r th e d eletio n o f ‘w h ic h w e re in e q u ilib riu m ’ fro m R eich e's ab o v e d e fin itio n o f w e a th e rin g . B .B . P o ly n o v (1 9 3 7 ) has

It appears from th e ab o v e d e fin itio n s th a t w eathering is essen tially th e b reak d o w n o f ro c k s due to chem ical and m ech an ical p ro c e sse s a t th e ir places. T he defin itio n o f w eath e rin g by B .W . S p a rk s highlights the above facts. A cco rd in g to h im , “ w e a th ­ ering m ay be d efin ed as the m e c h a n ic a l fra c tu rin g o r chem ical d eco m p o sitio n o f ro ck s by n atu ra l a g e n ts at the surface o f th e e a rth .” It is ob v io u s th at w e a th e rin g in v o lv e s tw o types o f changes in the ro ck s e.g. (i) p h y sic a l o r m ech a n ica l ch an g es, w h e re in ro ck s are d is in te ­ gra ted th ro u g h te m p e ra tu re c h a n g e s (h e a t fa c to r), fro st-actio n (fro st facto r), b io lo g ic a l a c tiv itie s (b i­ otic facto r), and w in d a c tio n s; (ii) c h e m ic a l c h a n g e s w h erein ro ck s are d e c o m p o se d th ro u g h sta tic w a te r, ox y g en , carb o n d io x id e a n d b io lo g ic a l a c tiv itie s . S eco n d ly , th e b re a k d o w n o f ro c k s o c c u rs a t th e p la c e o f ro ck s (in situ ). T h ird ly , th e re is n o la rg e -sc a le tra n sp o rt o f w e a th e re d m a te ria ls e x c e p t m a s s m o v e ­

very p re c ise ly d e fin e d w e a th e rin g as “th e c h a n g e o f ro ck s fro m th e m a ss iv e to th e c la stic s ta te .”

m e n t or m a ss tra n s lo c a tio n o f w e a th e re d m a te ria ls (ro c k -w a ste s) d o w n th e slo p e u n d e r th e fo rc e o f g rav ity . W e a th e rin g , th u s, m a y b e d e fin e d so a s to in c lu d e all a s p e c ts o f th e m e c h a n is m o f b re a k d o w n o f ro c k s as fo llo w s.

A rth u r H o lm e s h a s p re s e n te d m o re e la b o ra te d efin itio n o f w e a th e rin g w h ic h a lso in c lu d e s the p ro cesses o f w e a th e rin g . A c c o rd in g to h im “ w e a th ­ ering is th e to tal e ffe c t o f all th e v a rio u s su b a e ria l p ro cesses th a t c o o p e ra te in b rin g in g a b o u t th e d eca y

“W e a th e rin g re fe rs to th e b re a k d o w n o r d is in ­

and d isin teg ratio n o f ro c k s, p ro v id e d th a t no la rg e scale tran sp o rt o f th e lo o s e n e d p ro d u c ts is in v o lv e d . T he w ork o f ra in w a sh a n d w in d , w h ic h is,e sse n tia lly e ro sio n al, is th u s e x c lu d e d ” (A . H o lm e s, 1952).

te g ra tio n a n d d e c o m p o s itio n o f r o c k s in s itu th ro u g h m e c h a n ic a l an d c h e m ic a l c h a n g e s in th e ro c k s a n d

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th e ir m in e ra ls e ffe c te d b y w a te r, te m p e ra tu re , w in d , d iffe re n t a tm o s p h e ric g a s e s a n d o rg a n is m s p ro v id e d

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GEOM ORPHOLOGY

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g en tle and m o d e ra te g ro u n d slo p e are le ss affected by m e ch an ic al d isin te g ra tio n .

th a t there is no larg e-scale tran sp o rt o f w eath e red products by d en u d atio n al p ro cesses e x ce p t m ass m ovem ent o f ro ck w astes (w eath ered p ro d u cts) dow n the slope u n d er the im p act o f g rav ity S a v in d r a S ingh.

3. Climatic Variations C lim ate is c o n sid e re d to b e very im portant facto r o f all ty p e s o f w eath e rin g . C lim a tic geom or­ p h o lo g ists are o f the v iew th at eac h clim atic type p ro d u ces d e fin ite c o n d itio n s fo r a p a rtic u la r type o f w eath erin g . F o r ex am p le, c h em ica l w eath erin g is m o re d o m in an t in h u m id tro p ical areas becau se o f m o re av ailab le w a te r and h ig h te m p eratu re. B ecause o f ab u n d an ce o f m o istu re an d h ig h tem p eratu re le a c h in g p ro c e s s and so lu tio n o f ro ck s are m ore effectiv e in the h u m id tro p ics. M e c h a n ic a l w eath e r­ ing is less effectiv e. O n the o th e r h an d , m e ch an ic al d isin teg ratio n o f rick s is m o re d o m in a n t in the tro p i­ cal and sem i-arid reg io n s. R o ck s are w eak en ed due to altern ate ex p an sio n on h eatin g d u rin g d ay tim e and co n tractio n on relativ e co o lin g d u rin g n ig h ts because o f diu rn al ch an g e o f te m p eratu re. It m ay be pointed out th at lim esto n es are v ery w e a k ro c k s in hum id clim atic reg io n s b u t they are re la tiv e ly m ore resistan t to w eath erin g and e ro sio n in h o t d esert clim ate. T he ro ck s in dry te m p e ra te c lim ates are m ore su scep tib le to m e ch an ic al w 'eathering than chem ical w eath e rin g b eca u se alte rn a te ex p an sio n and co n tractio n o f crack s, fra c tu re s an d jo in ts of ro ck s d ue to altern ate freeze and th aw o f w ater accu m u lated in th ese crac k s an d fra c tu re s w eaken the rocks. R o ck s are le ast a ffe c te d by m echanical d isin teg ratio n in co ld clim a te b u t c h e m ic a l d ecom ­ p o sitio n o f ro ck s m ay b e e ffe c tiv e p ro v id e d that the g ro u n d su rface is n o t c o v e re d by ice c o v e r for longer d u ratio n in a y ear. B o th , m e c h a n ic a l an d chem ical w eath erin g ce a se w h en th e g ro u n d su rface is cov­ ered by p e rm a n e n t ice sh eets. N o t o n ly th is, seasonal v aria tio n s in c lim a te o f a re g io n g en e ra te different

14.2 CONTROLLING FACTORS OF WEATHER­ ING T h e n atu re and m a g n itu d e o f w eath erin g d if­ fers fro m p la ce to p la ce an d reg io n to reg io n . W eath ­ ering o f ro ck s is a ffected and co n tro lled by the agents o f w eath e rin g , lith o lo g ical and stru ctu ral ch a ra c te ristic s o f ro ck s, h eig h t and slope factors. B esides, clim a tic co n d itio n s, to p o g rap h y and re ­ liefs, flo ra and m ic ro fa u n a also affect d iffere n t p ro c­ esses o f w eath e rin g to g reater extent. F o r exam ple, d isin te g ra tio n o f ro ck s is m o re effectiv e in h o t and dry reg io n and in th e reg io n s w h ere fro st action is m o re d o m in a n t w h ile ch em ical d eco m p o sitio n is m o re p re v a le n t in h o t and h u m id and tem perate h u m id reg io n s.

1. Composition and Structure of Rocks S in ce w eath e rin g in v o lv es d isin teg ratio n and d e c o m p o sitio n o f ro ck s and h en ce m ineral co m p o si­ tion, jo in t p attern s, lay erin g system , faulting, fo ld ­ ing etc. larg ely affe c t the n atu re and in ten sity o f w eath erin g . F o r ex am p le, carb o n a te ro ck s (e.g. c a l­ cium c arb o n a te, m a g n esiu m carb o n a te etc.) h aving m o re so lu b le m in erals are easily affected by c h e m i­ cal w eath erin g . W ell jo in te d ro ck s are m ore su b ­ je c te d to m e ch an ic al d isin teg ratio n . R o ck s h av in g vertical strata are easily lo o sen ed and bro k en dow n d u e to te m p e ra tu re ch an g es, fro st actio n , w ater and w in d actio n s. O n th e o th e r han d , the ro ck s h av in g h o riz o n ta l bed s are m o re co m p a c t and are less a f­ fe c te d by th e m e c h a n ism s o f d isin teg ratio n and d e c o m p o sitio n .

co n d itio n s fo r w eath e rin g . F o r e x a m p le , in m onsoon clim ate ro ck s are su b je c te d to m e ch an ic al disinte­ g ratio n ^during h o t an d dry su m m e r m o n th s whereas

2. Nature of Ground Slope G ro u n d slo p e c o n tro ls m e ch an ic al d isin te g ra ­ tio n o f ro c k s an d m a ss m o v e m e n t o f w eath ered p ro d u c ts d o w n th e slo p e. T h e ro ck s in the reg io n s o f s te e p h ills lo p e are ea sily d isin te g ra te d d u e to m e ­ c h a n ic a l w e a th e rin g an d th e w e a th e rin g m aterials a re in sta n ta n e o u s ly m o v e d d o w n the h illslo p e in the fo rm o f ro c k fa ll, d e b ris fall an d slid e, ta lu s c ree p etc. In s ta n ta n e o u s re m o v a l o f w eath e rin g p ro d u cts a l­ lo w s c o n tin u o u s e x p o s u re o f ro ck s to atm o sp h eric c o n d itio n s fo r fu rth e r w e a th e rin g . T h e reg io n s o f

ch em ical an d b io c h e m ic a l w e a th e rin g is m ore dom i­ n an t d u rin g w et m o n so o n m o n th s.

4. Floral Effects

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T h e n atu re o f w e a th e rin g is largely deter­ m in ed by the p re se n c e o r ab se n c e o f vegetations in a p a rtic u la r reg io n . It m ay be p o in te d o u t that vege­ tatio n is p a rtly a fa c to r o f w e a th e rin g and partly a p ro te c to r o f ro ck s. In fact, v eg e ta tio n s b ind the rocks

'*?

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WEATHERING AND M ASS M OVEM ENT

(iii) B lock disintegration due to frost

through their network o f roots and thus protect them from weathering and erosion but the sam e tim e the penetration o f roots w eakens the rocks by breaking them into several blocks. D en se vegetations protect the ground surface from the direct im pact o f sun rays. The m icro-organism s associated with the roots o f plants and trees encourage decom position and disintegration o f rocks through p h ysico-biochem ical weathering.

(iv) Exfoliation or onion weathering due to temperature and wind 2. C h em ical w ea th erin g (i) O xidation (ii) C arb o n atio n (iii) Solution (iv) H ydration

14.3 Types of Weathering Processes

(v) C helation

G e n e ra lly , w eath e rin g p ro cesses are co n v en ­ iently divided in to p h y sical, ch em ical and b io c h em i­ cal p ro cesses b u t th e se are so in tim ately interrelated that it is p ra c tic a lly d iffic u lt to isolate o ne process from th e o th er. In fact, “no ch em ical w eath erin g takes place w ithout the production o f physical stresses; d isin te g ra tio n o f rock by th erm al ex p an sio n p ro b ­ ably d o es n o t o ccu r in the ab sen ce o f the chem ical p ro cess a sso c ia te d w ith the p resen ce o f w ater; in the co u n try o f ev en sp arsest v eg etatio n chem ical w eath ­ erin g is re p la c e d in part by bio ch em ical (process)" (R .J. C h o rle y , et al. 1985). Inspite o f this lim itation one has to d iv id e w eath e rin g into physical w eath er­ ing, c h e m ic a l w e a th e rin g and b io ch em ical w eath er­ ing on the b a sis o f d o m in a n t agent o f w eathering and w eath e rin g p ro c e ss. T h e w ea th erin g agen ts are divided in to 3 ty p e s as follow s.

(vi) H ydrolysis

3. B iotic w ea th erin g a n d b io ch em ica l w e a th ­ ering (i) Plant w eathering (ii) A nim al w eathering (iii) B iochem ical w eath erin g (iv) A nthropogenic w eath erin g Physical W eathering

The physical or m ech an ical w e a th e rin g leads to fragm entation and b reak d o w n o f ro ck m a sse s in to big blocks and b o ulders, c o b b les and p eb b les, san d s and silts and feld sp ar and m ica m in erals are c h e m i­ cally decom posed and clay is form ed. P hysical w eath ­ ering m ay be d efin ed as the d isin te g ra tio n o f ro ck s due to tem p eratu re v ariatio n s, fro st a c tio n , w in d action and u n lo ad in g o f co n fin in g s u p e rin c u m b e n t pressure. T h o u g h te m p eratu re v aria tio n is a key factor in physical w eath erin g b u t p re ssu re release, freeze and thaw o f w ater and g rav ity a lso play major roles.

1. P h y sic a l o r m ech a n ica l w ea th erin g agents (i) M o istu re and w ater (ii) F ro st (iii) In so la tio n (tem p eratu re ) (iv) W in d 2. C h e m ic a l w e a th e r in g agen ts

1. B lo ck d isin te g r a tio n due to tem peratu ch a n g e- Temperature changes have been reported to have great im pact upon m any rocks but there are also som e rocks w hich are least affected by temperature changes such as clastic sedim entary rocks (e.g. shales and sandstones) b ecau se the particles are separated by thin cem enting lam inae o f silica . On the other hand, crystalline rocks, like granites, are m ore af­ fected by temperature changes as particles are c lo sely associated w ith each other and th ese particles e x ­ pand and contract w ith increase and d ecrease o f temperature resp ectively. It has been exp erim en tally dem onstrated that if the tem perature o f granite rocks is increased by 6 5 .5°C, the rock contracts by 2 .5 4 cm per 3 0 .4 8 m distance. Contrary to this B lack Welder

(i) O x y g en (ii) C a rb o n d io x id e (iii) H y d ro g en 3. B io lo g ica l w e a th e r in g a g en ts (i) V egetation (ii) A n im a ls, m a in ly m icro -o rg an ism s T h u s, w e a th e r in g p r o c e s s e s or sim p ly weatherings are d ivid ed , on the basis o f w eathering agents, into 3 m ajor types. 1. P h y sica l o r m ech a n ica l w ea th e r in g (i) B lock disintegration due to temperature (ii) Granular disintegration due to tempera­

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ture

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C&GMORPHGIjOGY

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.

•’

G rig g s’ex p e rim e n ts in v o lv in g p u rely th erm al changes e q u iv a le n t to d iu rn a l c h a n g e o f 11 OPC o v e r 2 4 4 years c o u ld n o t p ro d u c e an y c h a n g e in th e ro c k strength an d th u s c o u ld n o t c a u s e a n y d is in te g ra tio n o f rodks. In a n o th e r e x p e rim e n t G rig g s u s e d w a te r s p r i n k l e r to co o l h ig h ly h e a te d ro c k s in ste a d o f re d u c in g the te m p e ra tu re . T h e re s u lt w a s im m in e n t as th e rocks d e v e lo p e d c ra c k s a n d s u rfa c e s p a llin g . T h is process w o rk s w h en th e re is s u d d e n lig h t s h o w e rs in th e hot d e s e rt areas. T h e h ig h ly h e a te d ro c k s w h e n stru c k by su d d e n d riz z le s d e v e lo p n u m e ro u s c ra c k s. T h e re p ­ e titio n o f th is m e c h a n is m c a u s e s s p a llin g a n d g ra n u ­ lar d is in te g ra tio n o f ro c k s.

in 1925 found no im p a c t o f te m p e ra tu re c h a n g e on g ran ite w hen he d ro p p ed g ran itic b lo c k s in to h o t oil at the te m p eratu re o f 200°C . It m ay be p o in te d o u t that co n tra stin g resu lts h av e b een re p o rte d a b o u t th e im pact o f te m p eratu re c h a n g e s on th e ro ck s. T he p ro d u cts o f w e a th e rin g in h o t d e s e rt area s are d iffere n t from th o se o f m o re h u m id a re a s as th ey are co arser an d d e fic ie n t in clay an d o rg a n ic m a tte r. G enerally, it is a c c e p te d th a t th e b a re ro c k s u rfa c e s are heated d u rin g d ay tim e d u e to w h ic h th e ir o u te r layers ex p an d . D u rin g n ig h ts th e ro c k s are c o o le d d ue to re la tiv e d e c re a se in te m p e ra tu re w h ic h lead s to c o n tra c tio n in th e o u te r la y e r o f th e ro ck s. T h u s, the re p e titio n o f e x p a n s io n an d c o n tra c tio n o f o u te r ro ck la y ers d u e to d iu rn a l ra n g e o f te m p e ra tu re in the h o t d e s e rt a re a s c a u s e s te n sio n a n d stre s s e s w h ich in tro d u c e p a ra lle l jo in ts in th e ro c k s. T h e ro ck s, th e n , are d is in te g ra te d a lo n g th e se jo in ts an d b ro k en b ig b lo c k s o f ro c k s are d is lo d g e d fro m the m a in ro ck m a ss a n d fall d o w n th e slo p e u n d e r th e im p a c t o f g rav ity . T h is p ro c e s s o f p h y sic a l w e a th e rin g is called b lo c k d is in te g r a tio n . It m a y be p o in te d o u t th at b lo c k d is in te g ra tio n s h o u ld n o t be c o n sid e re d as the re s u lt o f o n ly te m p e ra tu re c h a n g e s, ra th e r u n lo a d in g o f s u p e rin c u m b e n t lo a d o r re le a se o f c o n fin in g p re s ­ su re a lso h e lp s in th is p ro c e ss.

4. B lo c k d is in te g r a tio n d u e to fr o s t- D is te g ra tio n o f ro c k s in to la rg e siz e b lo c k s d u e to fre e z e an d th aw o f w a te r is o f c o m m o n o c c u rre n c e in th e te m p e ra te an d co ld c lim a tic re g io n s. In fa c t, th is p ro cess is m o re a c tiv e in th o se a re a s w h ic h a re v ery o ften c h a ra c te riz e d by a lte rn a te p ro c e s s o f fre e z in g and th a w in g o f w a te r m a in ly d u rin g n ig h t a n d d ay resp ectiv ely . F ro st a c tio n w e a k e n s th e ro c k s in tw o w ays e.g. (i) d u e to fre e z e an d th a w o f w a te r b e tw e e n the p article s o f th e ro c k s a n d (ii) d u e to fre e z e a n d th aw o f w a te r in th e c re v ic e s a n d p o re s p a c e s . T h e m o re c o m p a c t an d h ig h ly c o n s o lid a te d ro c k s , lik e g ran ites, are le a st a ffe c te d by f re e z e -th a w a c tio n s w h ile less c o m p a c t a n d lo o s e ly c o n s o lid a te d ro c k s are m o re a ffe c te d by fro s t a c tio n s , f o r e x a m p le , se d im e n ta ry ro c k s b e in g m o re p o ro u s a re h ig h ly su sc e p tib le to th e m e c h a n is m o f w e a th e rin g . W a te r p re se n t b e tw e e n th e p a rtic le s o f p o ro u s ro c k s fre e z e s d u rin g n ig h t d u e to fall o f te m p e ra tu re b e lo w fre e z ­ ing p o in t an d th u s e x p a n d s d u e to in c re a s e in its v o lu m e by a b o u t 10 p e r c e n t a n d th a w s during d ay tim e d u e to re la tiv e in c re a s e in th e te m p e ra tu re and h e n c e it c o n tra c ts in v o lu m e b y 10 p e r c e n t. T h is d iu rn a l fre e z e an d th a w c y c le c a u s e s a lte rn a te ex ­ p a n sio n an d c o n tra c tio n w h ic h in tro d u c e te n sio n an d s tre sse s d u e to w h ic h ro c k s a re d is in te g ra te d in to s m a lle r p a rtic le s. T h is p ro c e s s , k n o w n a s g r a n u la r d is in te g r a tio n d u e to fr o s t a c tio n , is an e x c e e d ­ in g ly slo w p ro c e s s an d ro c k s are le a st a ffe c te d by

2. G r a n u la r d is in te g r a tio n d u e to te m ­ p e r a tu r e c h a n g e s - T h e c o a rs e -g ra in e d ro ck s are m o re a ffe c te d by s h a tte rin g p ro c e ss in th o se h ot d e se rts w h ic h are c h a ra c te riz e d by h ig h ra n g e o f d aily te m p e ra tu re . I f th e ro c k s are c o a rs e -g ra in e d an d are o f d iffe re n t c o lo u rs, th e y a b so rb in so la tio n d iffe re n tly . T h u s, th e d iffe re n t p a rts o f th e sam e ro ck m a s s r e c e iv e a n d a b s o r b d if f e r e n t a m o u n t o f in s o la tio n , c o n s e q u e n tly th e d iffe re n t p a rts o f the ro c k s a re a ffe c te d by d iffe re n tia l e x p a n sio n an d c o n tra c tio n w h ic h c a u s e s tre s s e s w ith in th e ro ck s d u e to w h ic h th e y a re d is in te g ra te d in to sm a lle r p a rtic le s. S u c h ty p e o f s h a tte rin g o f ro c k s is c a lle d g ra n u la r d is in te g ra tio n w h ic h is m o re a c tiv e in h o t d e s e rt area s. 3. S h a tte r in g d u e to ra in s h o w e r a n d h ea tT h e o u te r s h e lls o f th e ro c k s a re s h a tte re d d u e to su d d e n lig h t s h o w e rs in h o t c lim a tic re g io n s m a in ly in h o t d e s e rt a re a s. G rig g s h as re m a rk e d a fte r e x p e ri­ m e n ts th a t sm all c ra c k s a re d e v e lo p e d a t th e o u te r su rfa c e o f th e h ig h ly h e a te d ro c k s w h e n lig h t d r iz ­ z le s su d d e n ly s trik e th e m . It m a y b e m e n tio n e d th a t

th is p ro c e ss.

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A lte rn a tiv e ly , a lte rn a te e x p a n s io n an d c o n ­ tra c tio n o f c re v ic e s , p o re s a n d c ra c k s in th e ro ck s d u e to d iu rn a l fre e z e a n d th a w o f w a te r c a u s e s b lo c k d is in te g ra tio n o f ro c k s w h e re in ro c k s a re b ro k en d o w n in to la rg e r b lo c k s w h ic h a re d is lo d g e d from

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fected by sheeting* s p a ttin g an d b lo c k d isin teg ra­ tion due to reduction in die co n fin in g pressure b e ­ cause o f rem oval o f su p erin cu m b en t lo ad by u plift

the main rock m ass. W hen such process operates over the hillslopes o f w ell jo in ted m assive rocks, the dislodged rock blocks tum ble dow n the slope in the form o f rockslides and rock falls and collect at the base o f the hillslopes.

and erosion. S h e e tin g refers to the d ev elo p m en t o f c ra c k s and fractures parallel to the gro u n d su rface cau sed by rem oval o f su p erin cu m b en t lo ad resu ltin g in to reduction o f co nfining pressure. S uch p arallel crac k s and fractures are d ev elo p ed in m assiv e ro ck s such as granites and other igneous in tru siv es, q u artz ite s an d thickly bedded sandstones b ecau se o f ex p a n sio n o f rocks consequent upon u n lo ad in g o f su p erin cu m b en t load. R.H. Jahns (1943) h as en u m erated sev en p ro c ­ esses w hich cause sh eetin g in the ro ck s-

The disintegration o f rocks due to diurnal freeze-thaw cycles in the p eriglacial areas is called frost w eath erin g o r con gelifraction w hich form s very interesting lan d fo rm s like frostriven polygons. 5. E x fo lia tio n d u e to tem p eratu re and wind- E x fo liatio n w eathering, also know n as onion w eath erin g, refers to peelin g o ff concentric shells of rocks d ue to co m b in ed actions o f heat and w ind in hot arid and sem i-arid regions and m onsoon lands. E xfoliation is m ore com m on over crystalline rocks. The outer shells o f rocks b eco m e loose due to alter­ nate expansion and co n tractio n due to high tem pera­ ture d uring day tim e and com paratively low tem ­ perature d u rin g night respectively and these loos­ ened shells are rem oved (peeled off) by strong winds. D ifferen tial h eatin g o f outer and low er shells of a rock m ass cau ses fla k in g . T he solar radiation p en­ etrates u p to a few cen tim etres only in the rocks having low th erm al co n d u ctiv ity . Thus, the outer shells o f su ch ro ck s ex p an d m ore than the shells lying ju s t b elo w . T h is d ifferen tial expansion o f rock shells cau ses flak in g w h erein the thin rock sheets are detached from the ro ck m ass. T hese detach ed rock sheets are la te r on rem o v ed by strong w inds. Thus, sheets a fte r sh e e ts o f ro ck s are p eeled o ff and the rocks c o n tin u e to be bare. M any o f the granitic batholiths, w h ich are e x p o sed above the ground surface, are b e in g c o n tin u o u sly affected by ex fo lia­ tion w eath e rin g . K a n k e D o m e n ear R anchi city exhibits a fin e e x a m p le o f su ch w eath e rin g process.

(i) T ensional or co n tractio n al strain s set u p during cooling o f an igneous m ass, (ii) Local or regional c o m p re ssio n a l stresses due to tectonic m ovem ents. (iii) Insolation, w ith atten d an t d aily and s e c u ­ lar tem perature changes. (iv) Progressive h y d ratio n and fo rm atio n o f chem ical alteration products in su scep tib le m in e r­ als, (v) M echanical action o f fire, frost, and v e g ­ etation, (vi) D im inution o f p rim ary co n fin in g p re s ­ sure by rem oval o f su p erin cu m b en t lo ad, and (vii) C o m b in atio n s o f the ab o v e cau ses. C a m b e r in g p ro c e s s refers to fra c tu rin g o f brittle sandstone beds alo n g v ertical jo in ts d u e to expansion caused by u n lo ad in g o f su p e rin c u m b e n t load and co n seq u en t release o f co n fin in g p ressu re. G.W . B ain (1 9 3 1 ) has rep o rted the c a se o f fly in g ro ck -sh eets or sp alls k now n as ro c k b u rsts in lim e ­ stone qu arries d u e to sp o n tan eo u s m e c h a n ic a l ro c k ex p an sio n cau sed by u n lo a d in g o f s u p e rin c u m b e n t load. “A s new faces are cu t in the w alls o f (lim e ­ sto n e) q u arries the d en se lim e sto n e e x p a n d s, p ro ­ d u cin g crack s p arallel to th e su rface. In so m e in ­ stan ces q u arrie s h ad to b e clo sed d o w n b e c a u se o f the d a n g e r o f fly in g ro c k sh e e ts o r sp a lls” (C .D . O ilier, 1969).

6. D is in te g r a tio n a n d e x fo lia tio n d u e to u n load in g- T h e ro c k s, w h ich are b u ried u n d er th ick covers o f o v e rly in g ro ck s, are d isin te g ra te d w hen they are e x p o se d to th e su rfa c e d u e to rem o v al o f superin cu m b en t lo a d a n d c o n s e q u e n t rele ase o f c o n ­ fining p ressu re. T h e re m o v a l o f su p e rin c u m b e n t load, very p re c ise ly k n o w n as u n lo a d in g , m ay be effected th ro u g h g ra d u a l d e n u d a tio n o f o v erly in g rocks. In fact, th e b u rrie d ro ck s, w h en re lie v e d o f the confining p re ssu re d u e to u n lo a d in g , d e v e lo p crack s and jo in ts a n d u ltim a te ly b re a k u p a lo n g th e se crac k s and jo in ts. G ra n ite s, m a ss iv e sa n d sto n e s, m a ssiv e arkose, c o n g lo m e ra te s a n d lim e sto n e s a re m o re a f­

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T h e p ro c e ss o f s p a llin g re fe rs to th e d e v e lo p ­ m en t o f p laty ro c k fra g m e n ts, lo z e n g e sh a p e d o r ir r e g u l a r , in th e r o c k s d u e to u n l o a d i n g o f s u p e rin c u m b e n t load.

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GEOMORFHOLOQY 252

d io x id e ) c o n te n t o f w a te r a n d p H o f th e s o lu tio n W h en ra in w a te r m ix e s w ith a tm o s p h e ric C O j it b e c o m e s ac tiv e s o lv e n t a n d w h e n it c o m e s in c o n ta c t w ith the c a rb o n a te ro c k s , su c h as lim e s to n e s and d o lo m ite s, it d is s o lv e s th e ro c k s th ro u g h a set o f c h em ica l re a c tio n s o c c u rrin g th ro u g h v a rio u s stages. T h e v a rio u s sta g e s o f th e c h e m is try o f lim estone so lu tio n m ay be p re s e n te d in a sim p lifie d form as

7. O th er typ es o f p h y sica l w ea th erin g -B o u ld er clea v in g refers to b reak in g an d sp littin g o f boulders o f g ranites and b asalts and co m p lex b o u l­ ders due to th erm a l e x p a n sio n . A n o th e r ty p e o f in solation w ea th erin g is ‘d ir t c r a c k in g ’ w h erein the boulders c o n tain in g ‘d ir t’ are fractu red an d sp lit due to therm al ex p an sio n and co n tra ctio n . F ire, m ainly b ru sh fire, also cau ses in so latio n w eath e rin g due to therm al ex p an sio n and co n tractio n o f ro ck s w hich cau se ex fo liatio n and th u s n u m ero u s sp alls and flakes o f rocks are p ro d u ced . S la k in g w e a th e r ­ in g refers to the d isin teg ratio n o f ro ck s d ue to alter­ nate w ettin g and d ry in g o f ro ck s w herein c o n seq u en t ex p an sio n and co n tractio n o f ro ck shells resu lt in the d isag g reg atio n o f rocks. D isag g reg atio n o f rocks due to grow th o f salt cry stals from so lu tio n is called sa lt w ea th erin g w h ich gen erally occu rs in h ot arid areas. It m ay also be im p o rtan t in the ro ck s o f coastal areas.

fo llo w s. A c c o rd in g to R .M . G a rre ls ( I 9 6 0 ) th e re are sev en v aria b les w h ic h c o n tro l th e e q u ilib ria in v o lv ed in the so lu tio n o f lim e sto n e s(i) P artial p ressu re o f C 0 2 (ii) [H 2C 0 3] ... c a rb o n ic acid (iii) (iv)

C hem ical W eathering D eco m p o sitio n and d isin teg ratio n o f rocks due to ch em ical reac tio n s is called chem ical w eath ­ ering w herein the m in erals o f the rocks w eather aw ay. W ater v apo u r and w ater are the m ed ia w hich activate several types o f chem ical reactio n s w ithin the rocks. P ure w ater, d istilled w ater, is ch em ically inert b u t w hen it m ixes w ith th e atm o sp h eric gases, m ainly w ith C 0 2, it b eco m es p o ten t solvent. O x id a ­ tion , ca rb o n a tio n , so lu tio n , h y d ra tio n , ch ela tio n , h y d ro ly sis, b ase ex ch a n g e etc. are the im p o rtan t ch em ical reactio n s w h ich cau se v ario u s chem ical ch an g es in the m in erals o f rocks w hich u ltim ately lead to d eco m p o sitio n and d isin teg ratio n o f rocks.

[ h C O ^] ...b ic a rb o n a te ion C O 3 ] .. .carb o n ate an io n

(v) [H+] ... h y d ro g en ion (vi) [O H ']...h y d ro x y l ion (vii) [C a2+] .. .calciu m catio n C alciu m h y d ro x id e, C a (O H )2, is fo rm e d d u e to reaction o f calciu m o x id es (C aO ) w ith w a te r (H 20 ) in the follow ing m a n n er o f re v e rsib le e x o th e rm ic reactionC aO + H 20 = C a (O H )2 ..............eq. 14.1 C alciu m c a rb o n a te s, C a C O v is fo rm e d d u e to reactio n s o f calc iu m h y d ro x id e (C a (O H )2) w ith carb o n d io x id e (CO.,) in th e fo llo w in g m a n n e r o f rev ersib le ex o th erm ic re a c tio n s-

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1. S o lu tio n - S o lu tio n is co n sid ered to be the C a (O H )2 + C 0 2 C a C 0 3 + H 20 . . .eq 14.2 first step in the ch em ical d eco m p o sitio n and d isin te ­ C arb o n ic acid (H 2C O ?) is fo rm e d w hen CO^ is gratio n o f rocks. S o lu tio n refers to the d isso lu tio n o f d isso lv ed in w ater so lu b le p article s and m in erals from the rocks w ith the help o f w ater in m o tio n b u t a thin film o f w ater C 0 2 + H 20 H 2C 0 3 ..............eq. 14.3 aro u n d a solid p article also leads to ch em ical d isso ­ C arb o n ic a cid is also d is s o c ia te d in to p o sitiv e lution. S o lu tio n o f ro ck s d ep en d s on the n atu re o f h y d ro g en iop an d n e g a tiv e b ic a rb o n a te ion ro ck s, so lu b ility o f ro ck s o r so lid s and the ratio H 2C 0 3 H+ + H C O 3- .............eq. 14.4 betw een the volum es o f so lv en t (w ater) and the solids. C o m m on salts are m o st so lu b le w hereas C alciu m c a rb o n a te (lim e sto n e s) d isso c ia te s to lim ­ ca rb o n a te rocks (lim esto n es-ca lciu m carb o n ates, ited e x ten t in p u re w ater into a m e ta l catio n (C a2+) d o lo m ite s-m ag n esiu m carb o n a tes etc.) are o f m o d ­ and carb o n a te an io n ( C 0 32-) d u rin g th e pro cess o f erate so lu b ility . d isso lu tio n in th e fo llo w in g m a n n e r L im esto n es are m o re su scep tib le to so lu tio n p ro cess w h ich d ep en d s on tem p eratu re, C 0 2 (carb o n C a C 0 3 — d-issociali°" > C a 2+ + C O ? " ...... eq. 14.5

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253

WEATHERING AND MASS MOVEMENT

c o n ta in iro n in fe rro u s s ta te (F e ) e .g . m a jo r ironsulphide (pyrite, F eS ,), iron c arb o n a te (sid e n te , F e C 0 3), and various iron silicates.

It m ay be po in ted o ut that lim estone can be dissolved in w ater only w hen it is transform ed into calcium bicarbonate, C a ( H C 0 3)2, w ith help o f car­ bonic acid (H 2C 0 3) as follow s— C a C 0 3 + H 20 + C 0 2

W hen w ater m ixed w ith atm o sp h eric o x y g en com es in co n tact w ith iron bearin g rocks, the iro n oxidizes to form ferrous oxides (F eO ). F u rth er o x i­ dation o f ferrous oxides produces ferric oxides (Fe20 3) or ferric h ydroxides (F e(O H )2). T h e o x id atio n o f iron-bearing rocks rep ro d u ces ru sts in th e fo llo w in g

C a (H C 0 3)2 ...e q 14.6 or

C a C 0 3 + H 2C O 3 —> C a ( H C 0 3)2 T he carb o n ate ion ( C 0 32 * ) can react w ith w ater w hen it accep ts proton from acid and then the carbonate becom es base (w hich accepts the proton). The u ltim ate reactio n s yield hydroxyl ions and thus the calciu m carb o n a te becom es an alkaline sub­ stance as follow s-

m annerFe + H20 + 0 2 -> F e20 3.3H 20 . . . : . . e q 14.8 (rust) The rusting o f rocks w eak en s th em an d u lti­ m ately the rocks are d isin teg rated . T h e ferric ox id es and ferric hydroxides give red and y ello w c o lo u rs to m any rocks and soils. T he o x id atio n o f iro n -ric h V indhyan sandstones o f the K aim u r R an g es and R ew a scarps (M .P.) has help ed in the b lo c k d is in te ­ gration of m assively bedded and w ell jo in te d s a n d ­ stone capping.

C 0 32 - + H 20 — h-ydrolysis > H C O -3+ O H -...e q 14.7 T h e actual q u antity o f lim estone dissolved in w ater d ep en d s on tem p eratu re, C 0 2 content o f w a­ ter, p artial p ressu re o f C 0 2, pH o f the solution and k in etics o f reactio n s. T he solubility o f C 0 2 is di­ rectly re la te d to p ressu re (partial pressure) and is in v e rsely rela ted to tem p eratu re. In o ther w ords, the s o lu b ility o f C 0 2 increases and d ecreases with in­ crease and d e c re a se o f partial p ressure w hereas it (so lu b ility o f C 0 2) in creases w ith decrease in the te m p eratu re an d v ice versa. On the other hand, the so lu b ility o f so lid s (say lim esto n es) is directly re­ lated to te m p e ra tu re i.e. total solution o f lim estone in creases w ith in c re ase in tem p eratu re and vice versa. M ore and m o re lim esto n e s can be d isso lv ed in w ater eith er by in c re a sin g the tem p eratu re or C 0 2 content o f w ater o r by d e c re a sin g th e pH o f the solution. The solution o f lim e sto e n s and d o lo m ites gives birth to very in te re s tin g la n d sc a p e s kn o w n as k a rst to p o g ­ rap h y c h a ra c te riz e d by v ario u s so lu tio n holes (sink holes, s w a llo w h o le s, u v alas, d o lin es and polje) and various ty p e s o f c a v e s an d g alleries.

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3. C a r b o n a tio n - ‘C arb o n atio n is the reac tio n o f carbonate or b icarb o n ate ions w ith m in e ra ls ’. T he process o f carbonation is also k now n as ‘solution* w herein atm ospheric carbon d io x id e after m ix in g with w ater form s carbonic acid (H 2C O ?, see eq u atio n 14.3) w hich after reacting w ith carb o n a te ro ck s, say lim estones ( C a C 0 3), fo rm s calciu m b ic a rb o n a te (C a (H C 0 3)2) (see eq u atio n 14.6) w h ich is easily dissolved in w ater. T he m ech an ism o f so lu tio n o f carbonate rocks has already been d iscu ssed above. The rain w ater hav in g d isso lv ed carb o n d io x id e (C o 2 aq) p ercolates th ro u g h the d iffe re n t h o rizo n s o f the soils to reach u n d erly in g lim esto n es. T h u s, m o re and m o re o rg a n ic c a rb o n d io x id e is d is s o lv e d in g ro u n d w ater w hich then b eco m es a m o re active so lvent b ecau se d isso lu tio n o f m o re carb o n d ioxide 2. O x id a tio n - T h e ch em ical process pro o f d u ces m ore carb o n ic acid s w h ich d isso lv e m ore carb o n ate ro ck s after tran sfo rm in g calciu m carb o n ­ oxidation sim p ly m e a n s a reac tio n o f atm o sp h eric ates into calciu m b icarb o n ates. oxygen to fo rm o x id e s. W h e n w a te r is m ix ed w ith oxygen its re a c tio n w ith th e m in erals o f the rocks 4. H y d r a tio n - T h e p ro cess o f hy d ratio n is form s h y d ro x id e . In o th e r w o rd s, the atm o sp h eric related to the ad d itio n o f w ater to th e m in erals. T he oxygen a fte r re a c tin g w ith th e ro ck s p ro d u ces sev ­ ro ck s after h av in g a b so rb ed w ater u n d erg o th e p ro c ­ ess o f p o sitiv e ch an g e o f th e ir v o lu m e. In o th er eral ty p es o f o x id e s, iro n o x id e b ein g the m o st w o rd s, the v o lu m e o f th e h y d rate d ro ck s (ro ck s im portant, w h ich w e a k e n s th e ro ck s to d isin teg rate. w hich have ab so rb ed w ater) in c re ases rem a rk ab ly . T he o x id a tio n o f m in e ra ls o f th e ro ck s by g aseo u s S om e tim es, th e in c re ased v o lu m e b eco m es ab o u t oxygen b e c o m e s p o s sib le w h en o x y g en is d isso lv ed tw ice th e o rig in al volu m e. T h u s, th e in c re a se in th e in w ater. M o st o f th e iro n b e a rin g ro c k s co m m o n ly

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GEOM ORPHOLOGY

254

carb o n m o le c u le s” . In fact, th e w o rd ‘c h e la te ’ m eans a c o -o rd in a tio n c o m p o u n d in w h ich a cen tral m e ta l­ lic ion is atta c h e d to an o rg a n ic m o le c u le at tw o or m o re p o sitio n s. In o th e r w o rd s, c h e la tio n m eans ‘h o ld in g o f an io n , u su a lly a m e ta l, w ith in a ring stru c tu re o f o rg a n ic o rig in ’ (C .D . O ilier, 1969). W e m ay safely say th a t c h e la tio n is a fo rm o f chem ical w eath e rin g b y p la n ts. P la n ts e x tra c t m in e ra ls o r say n u trien ts from th e so ils w ith th e re s u lt m in eral lat­ tic e s a re d isru p te d a n d c ry sta l la ttic e s are fra g ­ m e n t e d a n d t h u s m in e ra l w e a th e rin g ta k es p la c e a t a m u c h f a s te r rate .

volum e o f rock* due to in crease in the v olu m e o f m inerals cause* strc**es and *train* in th e m ineral* o f the rocks w hich u ltim ately lead to p h y sical d is in ­ tegration o f rocks. “H y dration is an cx o th erm ic reaction, and in vol ve* a co n sid era b le v o lu m e ch an g e w hich m ay be important in p h y sical w eath e rin g exfoliation and g ran u la r d isin teg ratio n . H y d ratio n prepares m ineral su rfaces for fu rth er alteratio n by oxidation and carb o n atio n , and en ab les the tran sfer o f ions to take place w ith g reater e a s e ” (C.D. O ilier, 1969j. T he process o f h y d ratio n c h a n g e s f e ld s p a r m inerals into kaolim te clays, the process being k n o w n as ‘k a o lin iz a tio n ’

T h e p ro d u c ts o f c h e m ic a l w e a th e rin g are 5. H y d ro ly s is - “ H y d r o l y s i s is a c h e m iccala l s s e d u n d e r t h r e e c a te g o r i e s . reaction betw een m in eral a n d w a te r, th a t is b e tw e e n (i) S o lu tes o f so d iu m , p o ta ssiu m , ca lc iu m , hydrogen (H ) ions o r h y d r o x y l f O H ; io n s, a n d the m a g n e s i u m etc. p ro d u c e d by th e p ro c e ss o f c a rb o n a ­ ions o f the m in e ra l” (C.D. O lie r , 1969;. In fact, the tion o r s o lu tio n o f carb o n a te ro ck s, w h ich a re b ro u g h t h y d ro ly sis is th a t p r o c e s s w h e r e in b o th th e m in e r a ls to the lak e s and seas and are re p re c ip ita te d to fo rm o f the ro ck s a n d w a t e r m o l e c u l e s d e c o m p o s e and limestones!, d o lo m ite s and o th e r c a rb o n a te ro c k s . reac t in su ch a w a y t h a t n e w m in e r a l c o m p o u n d s are (ii) C la y s , d eriv e d fro m th e w e a th e rin g o f formed. S ilic a te m in e r a l s a re m o s t a ff e c te d by h y ­ fe ld s p a r an d fe rro m a g n e sia n m in e ra ls , fo rm d ro ly sis. T h is r e a c ti o n s ta rts i m m e d i a te l y w h e n a a r g illa c e o u s sed im en tary ro c k s lik e sh a le s. m in eral c o m e s in c o n t a c t w ith w a te r. T h e h y d r o ly s is ( i i i ) M in e r a l r e s i d u a l s , s u c h a s s i l i c a , o f m a g n esiu m s ilic a te m in e r a l s ( M g , S i O j in c o n ta c t u n w e a t h e r e d f e l d s p a r an d m ic a a n d o th e r h e a v y v»,ui 4 i o n i z e d w a t e r m o le c u le s (4 H20 —* 4H^ + m in e r a ls , f o rm c la stic se d im e n ta ry ro c k s s u c h as 4 C H ' ) ta k es p la c e in th e f o l l o w i n g m a n n e r s a n d s to n e s . M g2SK)4 + 4H ~~ 40H ~ 2M g~ + 40H"

Biotic Weathering

+ H 4S 1O 4 /'silica a c id in so lu tio n )

P la n ts a n d a n i m a l s in c lu d in g m a n la rg e ly c o n tr o l th e b r e a k d o w n o f ro c k s . It m a y b e p o in te d o u t th a t in all t y p e s o f w e a th e rin g in all c lim a tic r e g i o n s b io tic c o m m u n itie s p la y s o m e ro le s in o n e w a y o r the o th e r . T h is is w h y B .B . P o ly n o v (1 9 3 7 ) b e li e v e d th a t c o m p le te ly ste r ile w e a th e r in g w as im p o s s ib le . It m ay be m e n tio n e d th a t it d o e s n o t m e a n th a t b io tic c o m m u n itie s a lw a y s in d u lg e in

H y d r o l y s i s o f p o t a s s i u m f e ld s p a r (o r th o c la s e , 2 K A IS L O ,/ w ith c a r b o n i c a c id (H CO .) in w a t e r is

p e rh a p s th e m o s t c o m m o n t y p e o f c h e m i c a l w e a t h ­ e r i n g p r o c e s s w h e r e in t h e e n d p r o d u c t o f t h e r e a c tio n o f p o ta ss iu m f e l d s p a r w ith c a r b o n i c a c id in w a te r is p o ta ssiu m an d b i c a r b o n a t e io n s in s o lu tio n . T h e m e c h a n ism o f th e h y d r o l y s i s o f p o t a s s i u m f e ld s p a r

d estru c tiv e w o rk by d is in te g ra tin g a n d d e c o m p o s ­ ing the ro ck s b u t th e b u rro w in g a n im a ls d e fin ite ly help in the tra n s fe r o f so ils fro m lo w e r to u p p e r and u p p er to lo w e r h o riz o n s a n d th u s th e m ix in g o f , g e o m a te ria ls a c tiv a te s w e a th e rin g . T h o u g h v eg etatio n s p ro tec t the ro ck s by b in d in g th e m through th e ir ro o ts b u t d iffe re n t ty p e s o f ac id s (e.g . hum ic acid s, b a c te ria l a c id s, m ic ro flo ra l a c id s e tc .) pro­ d u c e d by th e m fa c ilita te b io c h e m ic a l w e a th e r in g . R e c e n tly , m a n h a s b e c o m e th e m o s t p o w e rfu l w eath ­ e rin g a g e n t b e c a u se o f th e d e v e lo p m e n t o f m o d e m te c h n o lo g ie s. B io tic w e a th e rin g , th u s, is d iv id ed

is g i v e n b e l o w 2 K A J 3 i/> 2 * W^COv hk 9 H 2O —yAl2S i205fG H j4 + o rth o c la te +• c a rb o n ic + w ater k ao lin ite, a clay ac id m in eral 4H 4S 1O 4 * 2 K* + 2 H C O , silic ic acid m to k ttk m

4 potassium and bicarbonate joins m solution

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& C fcd a tk a s- A cco rd in g to D .S . Lehm an 0 9 6 3 ) “c b e to ik m is a co m p lex organic p rocess by w k fc k m eta llic catio n s are incorporated in to hydro­

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hum us co n ten t, in creased m o istu re d u e to lo w ra te o evap o ratio n , in creased c o n te n t o f o rg a n ic C O r lo w tem p eratu re, all o f w h ich activ a te c h em ica l w e a th e r­ 1. F au n a l W ea th erin g - T he b u rro w in g an i­ ing. It m ay also be m e n tio n e d th a t v e g e ta tio n s a lso mals, w orm s and o th e r O rganism s help in gradual pro tect the ro ck s and so ils fro m w e a th e rin g p ro c ­ breakdow n o f rocks o r frag m en ts thereof. B u rro w ­ esses. ing anim als in clu d e g o p h ers, p rairie do g s, foxes, 3. A n th ro p o g en ic W e a th e r in g - M an b e in g a rabbits, ja c k a ls , term ites, rats etc. w hich d u g out biological agent accelerates and d ecelerates the natural burrow s and tu n n e ls in the ro ck s and u n co n so lid ated rates o f w eath erin g by m an y fo ld s. T h e ‘e c o n o m ic geom aterials as th e ir liv in g p laces (hom es). By d o ­ and tech n o lo g ica l m a n ’ la sh ed w ith m o d e rn te c h ­ ing so they w e a th e r th e ro ck s and g eo m aterials to n o logies has b eco m e th e m o s t p o w e rfu l w e a th e rin g great extent. S m all o rg an ism s play m ore im p o rtan t and erosion agent. M in in g a c tiv itie s fo r e x tra c tio n o f roles in ro ck an d soil w eath erin g . T h ese o rg an ism s m inerals, b lastin g o f h ills an d rid g e s by d y n a m ite s repeatedly m ix up th e soil m a teria ls and th u s alw ays for road and dam c o n stru c tio n an d m in e ra l e x tra c ­ expose fresh m a te ria ls to w eath e rin g ag en ts. T hey tion, q u arry in g fo r in d u strial (lim e s to n e s fo r c e ­ also h elp in m o v in g the o rg an ic m a tter do w n w ard m ent) and b u ild in g m aterial etc. re s u lt in su c h a fa s t into the soil p ro file s and th u s ex ten d the w eath erin g rate o f d isin teg ratio n o f g e o m a te ria ls (ro c k s ) th a t at g re a te r d e p th s w h ich o th e rw ise w ould have not this m ay be acc o m p lish e d by n atu ra l w e a th e rin g been p o ssib le . p ro cesses in th o u san d s to m illio n s o f y e a rs. M a n It is b e lie v e d th at th ere are about 1,50,000 accelerates the rate o f w e a th e rin g on h ills lo p e s b y c re a tu re s, b ig and sm all, in one acre o f land and these m o d ify in g the g ro u n d su rface th ro u g h d e fo re s ta tio n o rg a n ism s b rin g a b o u t 15 to n n es o f soils at the w hich red u ces th e m e ch an ic al re in fo rc e m e n t a n d su rface fro m b e lo w ev ery y ear. A cco rd in g to the co h esio n o f u n co n so lid a te d g e o m a te ria ls a n d th u s e stim ate o f C h a rle s D a rw in th e soil o rg an ism s bring in creases slope in sta b ility w h ic h c a u s e s slo p e fa il­ about 2 5 .4 th o u s a n d k ilo g ra m s o f soil at the surface ures and m ass m o v e m e n t o f m a te ria ls d o w n th e every y e a r in th e E n g lis h g ard en s. T erm ite s play slope in the form o f la n d slid e s, s lu m p in g an d d e b ris very im p o rta n t ro le in s o rtin g an d rearran g in g the fall and slides. soil m a te ria ls in th e u p p e r h o riz o n s o f soil p ro files in into 3 types e.g. (i) fau n a] w ea th erin g , (ii) floral w eatheirng and (iii) a n th ro p o g en ic w eath erin g.

tropical re g io n s . T e rm ita ria are the e v id e n c e s o f soil w eath e rin g by te rm ite s. A c c o rd in g to P o n o m arev a (1950) e a rth w o rm s b u rro w to ab o u t 1.5 m an d pass 10 to n s p e r a c re p e r y e a r as a m e an and 2 0 tons p er acre p e r y e a r a s a m a x im u m o f soil m a teria ls. R a b ­ bits, p ra irie d o g s etc. d e s tro y th e soil stru c tu re and they o b s tru c t th e le a c h in g a n d o th e r h o riz o n fo rm in g processes by c o n s ta n tly re m ix in g th e soil m a teria ls. 2.

Biochemical Weathering B io c h em ical w e a th e rin g re fe rs to d e c o m p o ­ sitio n and d isin te g ra tio n o f ro c k s d u e to o rg a n ic m a teria ls o f b o th flo ra an d fa u n a . A c o m p le x set o f d iffe re n t b io c h e m ic a l p ro c e sse s su c h as c a tio n r o o t ex c h a n g e , c h e la tio n , s o lu tio n by ro o t e x u d a te s an d p ro d u ctio n o f d iffe re n t k in d s o f o rg a n ic a c id s such as h u m ic acid s, b a te ria l a c id s, m ic ro fa u n a l a c id s etc.

Floral Weathering- W e a th e rin g o f ro ck s

p ro d u c e d by o rg a n ic m a te ria ls h e lp in th e d e c o m p o ­

by v e g e ta tio n s ta k e s p la c e in tw o w ay s viz. (i) physical w e a th e rin g a n d (ii) c h e m ic a l w e a th e rin g w hich is c a lle d as biochemical weathering, w h ich will he d is c u s s e d u n d e r s e p a ra te h e a d in g . It m ay be pointed o u t th a t flo ra l w e a th e rin g d o e s n o t ta k e p la ce independently ra th e r it h e lp s th e p h y sical a n d chem ical processes o f w e a th e rin g . L a rg e r p la n ts a ffe c t an d control w e a th e rin g in a n u m b e r o f w a y s, (i) C ra c k s are w id en ed by ro o t p e n e tra tio n a n d c o n s e q u e n t ro o t pressure, (ii) D e n s e v e g e ta tio n c o v e r g e n e ra te s d is ­ tinct m ic ro c lim a te a t th e g r o u n d s u rfa c e . T h e soil atm osphere is la rg e ly a ffe c te d by ro o t re s p ira tio n ,

sitio n an d d is in te g ra tio n o f ro c k s a n d so ils.

Humic acids a c tiv a te chelation a n d h e lp in th e d e c o m p o sitio n o f s ilic a te m in e ra ls . Fulvic acids, h u m ic ac id s d e riv e d fro m p e a t, p la y im p o r ta n t ro le in d e c o m p o sin g ro c k m in e ra ls . B a c te ria l a c id s , in ­ c lu d in g la c tic , a c e tic , o x a lic a n d g lu c o n ic , a tta c k a w id e ra n g e o f ro c k m in e ra ls im p o rta n t b e in g m a g n e ­ siu m c a rb o n a te , c a lc iu m a n d m a g n e s iu m s ilic a te s , fe ld s p a r an d k a o lin ite s . B a c te ria l a c id s a ls o p r o d u c e s u lp h id e s, o x id iz e iro n a n d h e lp in th e s o lu tio n o f

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s ilic a w h e n th e ro c k s a re c o n s ta n tly s u b m e r g e d

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256

alo n g co astal zo n es o b ta in th e se w eath e red m ateri­ als and m o v e th em to o th e r p laces. T h e ra p id rate o f w eath erin g d u e to m ass fellin g o f tree s (d efo resta­ tio n ) has a cc elera ted the rate o f ero sio n o f n ude rocks o f the hill ran g es w ith th e re s u lt m o st o f th e rivers C h em otrop h ic b a cteria m a n u fa ctu re su l­ h ave b eco m e o v erlo a d e d an d slu g g ish b ecau se m il­ phides and rem ove silica in the tropical soils and lions o f to n n es o f ero d e d s e d im e n ts are reac h in g the help in the carb o n ate m in eralizatio n in cav es. The m a jo r riv ers ev ery y ear. F o r e x a m p le , G arh w al and colonization by b lu e-g reen algae form s d esert v a r­ K um aun H im a la y a s an d o th e r p a rts o f th e H im a la­ n ish and m o b ilizes ferro u s irons and h elp in the yas h av e been e x te n siv e ly d e fo re s te d an d thus the co n cen tratio n o f ox id es on ro ck su rfaces. M icro ­ w eath ered ro ck s h av e in c re a se d th e rate o f flu v ial organism s also form v a rn ish in cru sta tio n s ori rocks. ero sio n , c o n seq u en tly m o st o f th e H im a la y a n riv ers L ich en s in tro d u ce the alteratio n o f m in erals co m p o ­ lik e the Y am u n a, the G an g a, the G h a g h ra , th e K osi sition o f ro ck s b oth m e ch an ic ally and ch em ically . etc. h ave b eco m e o v e rlo a d e d in th e p la in s d u e to T he o rg an ic carb o n d io x id e p ro d u ced by plants supply o f h u g e v o lu m e o f se d im e n ts e v e ry y ear. T h is a cc elera tes th e rate o f carb o n a tio n on carb o n atep ro cess has cau sed rap id rate o f s ilta tio n o f riv e r ro ck s e.g. lim e sto n e s and d o lo m ites. beds o f m a jo r allu v ial riv ers o f n o rth In d ia an d th e 1 4.4 GEOMORPHIC IMPORTANCE OF WEATH­ resu ltan t siltatio n h as in c re a se d th e fre q u e n c y a n d ERING d im en sio n o f re c u rrin g flo o d s. 1. P r o d u c tio n o f ro ck w a stes—-Rocks are 3. L o w erin g o f su rfa ce— C o n tin u o u s re m o v a l d is in te g ra te d and d e c o m p o se d and u ltim ately are and tran sfer o f w e a th e re d m a te ria ls th ro u g h d iffe r­ b ro k en d o w n in to sm a lle r p ie ces d ue to the o p eratio n en t p ro cesses o f m a ss tra n s lo c a tio n o f ro c k w a s te s o f d iffe re n t w ea th e rin g , ch em ica l w eath e rin g , b io tic su ch as la n d slid e s, d e b ris slid e s, ro c k fa ll, ro c k s lid e s, w e a th e rin g an d b io c h e m ic a l w eath e rin g . T h u s, d if­ talu s cree p etc. a n d by th e a g e n ts o f e ro s io n c a u s e s fe re n t w e a th e rin g p ro c e sse s p ro d u ce im m en se v o l­ g rad u al lo w e rin g o f th e h e ig h t o f th e a ffe c te d area. u m e o f ro c k w a ste s o r w e a th e re d m a teria ls. T h ese 4. E v o lu tio n of la n d fo r m s a n d th e ir m o d i­ w eath e red m a te ria ls ly in g o v e r the u n w e a th e re d fic a tio n s — D iffe re n tia l w e a th e rin g h e lp s in th e e v o ­ fresh ro ck s are c a lle d re g o lith s. T h e d ep th o f w e a th ­ lu tio n o f d iffe re n t ty p e s o f la n d fo rm s . W e a th e rin g ered rock s from the g ro u n d su rface to the un w eath ered p la y s im p o rta n t ro le in th e d e v e lo p m e n t o f sto n e fresh ro c k s is c a lle d w e a th e r in g z o n e . T h e d e p th o f la ttic e (in h o t d e s e rts ), to rs, b u tte s , ta lu s c o n e s , ta lu s w e a th e rin g zo n es v arie s fro m p la ce to p la c e and fan s, sa n d sto n e a n v ils etc. It m a y b e p o in te d o u t th at fro m re g io n to re g io n d e p e n d in g m a in ly on th e d ep th w e a th e rin g an d e ro s io n go h a n d in h a n d a n d th u s it o f w a te r ta b le o f g ro u n d w a te r an d th e d u ra tio n o f is n o t w ise to s e p a ra te th e in s e p a ra b le , so it is w e a th e rin g . T h e w e a th e re d m a te ria ls are v ery im ­ d iffic u lt to a s c e rta in th e q u a n tu m o f w o rk d o n e by p o rta n t e c o n o m ic a lly b e c a u se th e y h e lp in th e p ro c ­ w e a th e rin g an d e ro s io n in th e d e v e lo p m e n t o f a ess o f so il fo rm a tio n , th e y e x p o s e m in e ra ls etc. p a rtic u la r ty p e o f la n d fo rm . W ea th e rin g g e n e ra te s m a ss m o v e m e n t o f ro c k w a ste s 14.5 MASS MOVEMENT (MASS WASTING) d o w n th e h ills lo p e a n d th u s c a u s e s d a m a g e to h u m a n T h e stu d y o f m a ss m o v e m e n t o f ro c k w a s te s se ttle m e n ts in th e fo o th ill z o n e s, c a u s e s o b s tru c tio n s in v o lv e s th e a n a ly s is o f m e a n in g a n d c o n c e p t, c la s ­ in th e riv e r flo w a n d th u s fo rm s la k e s (by d a m m in g sific a tio n , c a u s e s , a n d g e o m o rp h ic s ig n ific a n c e o f th e riv e rs th ro u g h d e b ris fall). m a ss m o v e m e n t o r m a ss tra n slo c a tio n o f ro c k w astes. Under w ater (perpetual w aterlogging). M ic r o f a u n a l a c id s such as oxalic and citric acids are p ro d u ced by fungi and lichens. T h ese acids w eath er silicate m in ­ erals and clays.

2. Weathering helps erosional processes—

1. Meaning and Concept

W eathering lo o se n s the rock s by d isin tegratin g and d eco m p o sin g them and thus p aves the w ay for ero ­

D isin tegrated and fragm en ted rock m aterials

sion al p ro cesses to operate ea sily . D ifferen t agen ts

du e to m ech a n ism o f w ea th erin g p r o c e sse s (m e­ ch an ical, ch em ica l, b iotic and b io ch em ica l) are called

o f erosion lik e running w ater (rivers) in hum id

rockwastes. G en era lly , m o v e m e n t o f ro ck w a ste en b lo ck d o w n the h ills lo p e i s c a l l e d m a s s m o v em en t of

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g la ciers in co ld regio n s and sea w a v e s op eratin g

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of rock m aterials through different w eathering p roc­ esses, and enblock dow nslope transport o f w eath­ ered rock debris by gravity force w ithout any m e­ dium o f transport (e.g. running w ater, w ind, sea waves, glacier etc.) except som e lubricating role o f water or ice. The rock debris com ing through m a s s . m ovem ent are deposited at the foot-hill zone as scree or talus. The deposit o f large boulders in conical shape is called talus cone. It is, thus, appar­ ent that the most significant stim ulating factor o f

rockwaste or sim ply m ass m ovem ent. ‘M ass m ove­ ment is the detachem nt and dow nslope transport of soil and rock m aterial under the influence of gravity. The sliding or flow ing o f these m aterials is due to their position and to gravitational forces, but mass movement is accelerated by presence o f water, ice and air. This definition o f m ass m ovem ent permits consideration o f the m ovem ent o f earth m aterials at all scales and at all rate s’ (R.J. Chorley, et. al, 1985). It is evident from the above definition that mass movement includes both, detachm ent o f rock m ate­ rials and their dow nslope transport enblock. ‘The collective term for gravitational or dow nslope m ove­ ments of w eathered rock debris is m ass-wasting. The term im plies that gravity is the sole important force and that no transporting m edium such as wind, flowing w ater, ice or m olten lava is involved. A l­ though flow ing w ater is excluded from the process by definition, w ater nevertheless plays an im portant role in m ass-w asting by over steepening slopes through surface erosion at their bases and by generating seepage forces through groundw ater flow ’ (A.L. Bloom, 1978).

mass movem ents is gravity force. 2. Classification of M ass M ovem ents A wide range o f variations in term s o f rate, direction and type o f m ovem ents is noted in m ass movements in different places having varying en v i­ ronmental conditions. It is generally believed that mass m ovem ent of rock w astes occurs suddenly and instantaneously and hence all m ass m ovem ents c an ­ not be witnessed by man. But in reality m ass m o v e­ ments have long preparatory period and there are certain precursor events which herald the occur­ rence of mas m ovem ents but these are generally unnoticed. It may be m entioned that m ost of m ass movements occur in m ountainous areas and hence it is not possible to notice the precursor events such as restlessness of animals, deserting o f hives by bees etc. ‘Hence, if a landslide com es as a surprise to eyew itnesses, it would be m ore accurate to say that the observers failed to delect the phenom ena w hich preceded the slide’ (R.J. C horley et. al, 1985). M ass movem ents are generally classified on the basis o f causative factors e.g. rate o f m ovem ent, direction o f m ovem ent, type of m ovem ent, lubricating substance e.g. water, ice etc.

If w e look at the aforesaid two definitions of R.J. C horley et. al and A.L. Bloom it appears that the term m ass m ovem en t is m ore sound and appropri­ ate than m ass w asting to describe enblock downslope transport o f w eathered m aterials ranging from very fine (soils) to very coarse and large sized rock m aterials (boulders). In fact, the definition by R.J. Chorley and others is com prehensive one because it includes both the aspects of m ass m ovem ents, viz. detachm ent o f rock m aterials and their dow nslope transport w hereas B loom 's m ass w asting describes only the process o f dow nslope transport o f w eath­ ered rock debris.

The direction o f m ass m ovem ent o f rockw aste down the slope may be (i) vertical, (ii) lateral, and (iii) diagonal. B ased on direction m ass m ovem ent may be divided into vertical m ovem ent, lateral m ove­ m ent and diagonal m ovem ent o f rockw aste. V erti­ cal m ass m ovem ent is further divided into (a) rockfall, (b) collapse earthfall. L ateral m ass m ovem en t in­ cludes (a) block slide, (b) spread, (c) cam bering, (d) sackung etc. D iagon al m ass m o v em en t is divided into (a) soil creep, (b) rockcreep, (c) talus creep, (d)

E m phasising the significance o f tectonics in mass w asting (m ay be ro ck disintegration) and m ass m ovem ents R.J. C horley and others (1985) have rem arked that, ‘the relation betw een m ass w asting and tectonics is a relativ ely clear one. W here rocks are shattered and re lie f is high, this is w here m ass m ovem ent is com m on and, in fact, the denudation o f high m ountains m ay ........ be the result o f m ass w asting rath er than fluvial or glacial p ro cess’. It is, thus, ev id en t that m ass m ovem ent o f rock w astes includes the m echanism s o f detachm ent

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rockslide, (e) debris slide, (f) slum p, (g) d ebris flow ,

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wasting phenom ena on the basis o f direction o f m ovem ent, type o f m ovem ent and presence o f transporting agent as given below —

(h) m ud flow, (i) solifluction, (j) avalanche etc. R .J. Chorley et. al (1985) have presented exhaustive classification o f m ass m ovem ent-m ass Table 14.1: Mass Movement-Mass Wasting Phenomena

D iagonal

Lateral

Vertical

Direction o f m ovem ent Type o f m ovem ent

Fall

Subsidence

Slide

Spread

C reep

S lide

F low

Presence o f trans­

No

No

M inor in

M oderate

M inor

M in o r to

M ajor

basal layer

in basal

or on sliding

layer

porting agent

m o d erate

surface Types o f mass

R ockfall, collapse

m ovem ent

earthfall, settlem ent

B lock slide

Spread

soil creep, ro ck slide

soli­

cam bering

rock creep, debris

fluction m u d flo w

topple

talus creep slide,soil

slip, slump ro c k g la­ cier, ro ck av alan ch e Source : R.J. C horley et. al, 1985. B ased on the rate o f m ovem ent and w ater content m ass m ovem ents are classified in 3 types— (1) L arg e-scale rapid slide o f rock w aste. W ater is needed as lu b ricatin g ag en t for such type o f m ass m ovem ent. L an d slid e is the typical exam ple o f this type

(b)

debris slides

(c)

debris fall

(d)

rock fall

(e)

rock slides

2. Slow m o v em en t (flow age o r slid e)

(2) S low flo w ag e o f rock w aste and w eath­ ered d ebris. Partial satu ratio n o f rock d ebris is re ­ q u ired fo r such m ass m o v em en t and hence m oderate q u a n tity o f w ater is n eed ed as lu b ricatin g and stim u ­ la tin g agent. R o ck creep , soil creep, so liflu ctio n etc. are ty p ical ex am p les o f this type.

(little w ater is required) (i) R ock creep (ii) S oil creep (iii) S o liflu c tio n

(3) R a p id flo w ag e o f w eath ered debris. S u ffi­ c ie n t q u a n tity o f w ater is n eed ed as lu bricant. E arth flow , m u d flo w etc. are re p re se n ta tiv e o f this type o f m a ss m o v e m en t.

3. R apid m o v em en t (flo w a g e o r slid e) (enough w ater is req u ired ) (i) E arth flow

A g e n e ra liz e d c la ssific a tio n o f m ass m o v e ­

(ii) M u d flo w

m e n t o f ro c k w astes is p re se n te d as fo llo w s.

(iii) S h eetw a sh

Table 14.2 : Classification of mass movement On the basis o f direction and type o f m ove­

1. Very rapid movement (n o w a te r is req u ired )

m ent the fo llo w in g types o f m ass m ovem en t o f rock

(i) Landslides

w aste m ay be indentified (sim p lified sch em e o f R.J.

slu m p

C horley et. al, 1985)—

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(a)

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K ilf t ' wxP-

W EATH ER IN G A N D M A S S M O V E M E N T

Table 14.3 : Classification of Mass Movement

1. V ertical M o v em en t (A ) F all (o f earth m aterials from very steep slopes like steep scarps and cliffs) O n th e b a sis o f m a teria ls (a)

roc kfalls

(b)

ea rth fa lls (o f allu v ia, soils, colluvia)

(c)

d eb risfa ll (soils, alluvia, colluvia, vegetation and hum an stru ctu res)

(d) to p p le (ro tatio n al fall o f rock slabs, or o f earth en m aterial) (B) S u b sid e n c e ( o f th e g ro u n d su rface) S in k in g (a) (b ) 2. L a tera l M o v e m e n t (A ) S lid e s

co lla p se (o f ro o fs o f underground caves or cavities o r lav a tu b es) se ttle m e n t (co llap se o f ground surface due to w ith d raw al o f w ater, cru d e oil etc.)

(m o v e m e n t o f m aterials along a horizontal fracture o r interface b etw een tw o ro c k s tra ta

(slid in g )

o f v a ry in g resistan ce e.g. sandstone-shales or lim esto n es-sh ales) (a)

b lo c k slid e (d ow nslope m ov em en t o f a single large b lo ck o f m a ssiv e ro c k on (b lo ck g lid e)

such a surface w hich has been lu b ricated by w ater)

(B) S p r e a d in g (lateral disp lacem en t o f a series o f rock blocks (m ultiple blocks) or m u d b lo ck do w n slo p e) (a) (b)

ca m b e r in g (d rap in g o f sedim entary units) s a c k u n g (lateral spreading aw ay from anticlinal crests)

3. D ia g o n a l M o v e m e n t (A ) C r e e p in g (d o w n slo p e m o v e m e n t o f earth en m aterials at slow velo city ) (a)

s o il cre ep (m o v em en t o f m o isten ed soils d o w n slo p e)

(b )

r o c k cre ep (m o v em en t o f ro ck upon rock)

(c)

ta lu s cre ep (rearran g em en t o f scree and d o w n slo p e m o v e m en t)

(B) S lid e (rapid rate o f downslope movement of large quantities of debris of varying sizes) (on the basis of materials) (a)

r o c k slid es

(b )

d e b r is slid es

(c) (d )

so il cre ep s lu m p in g (m o v e m e n t o f fin e m aterials alo n g a cu rv ed p la n e)

(C ) F lo w s (d o m in a n t ro le o f w a te r , d o w n slo p e tran sp o rt o f w a ter-so ak ed fin e d e b ris) (a )

e a r th flo w

(b)

d e b r is f lo w

(c )

m u d flo w (ii) debris slum p (iii) earth slum p S lid e s (t>) (i) rock slid es (ii) debris slid es (iii) earth slid es 3. Toppli s (i) rock top p les (ii) debris top p les (iii) earth top p les

Table 14.4: Sim plified classification of m ass movement

(Land S lid es) 1. Fall (a) R o c k fa ll (b) D e b r i s f a l l (c) E arth fall

2. Slides (a) Slum p rock slu m p https://telegram.me/UPSC_CivilServiceBooks

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260

4 Blow s (a) R o c k flow (b) SoU flow

(i) (ii)

5 L a tera l S Prea d s

sp read s d e b ris sp re a d s

(i>>)

debris flow

e arth sp read

earth flow

B

ROCK FALL

SOIL

SLUMP

FALL

B L O C K G L ID E

DEBRIS SLID E

SLUMP AND EARTH FLOW

Slu m p

Earth Flow

D ifferen t types o f m ass m ovem ents, rock fa ll (A), soil fa ll (B), slum p (C), block glide (D), D eb ris slid e (E )o n d s lu m p a n d earth flo w (F). Source : D.J. Varnes (1978), M .J. Selby (198%) a n d R.J. C horley et. a l (1985). https://telegram.me/UPSC_CivilServiceBooks

F ig 1 4 .1 :

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A

D E B R IS F L O W

B

D E B R IS A V A LA N C H E

D E B R IS TOPPLE

L A T E R A L S P R E A D IN G

Fig. 14.2 : D ifferent types o f mass movements : debris flow (A), debris avalanche (B), debris topple (C) and lateral spreading (D). Source : D.J. Varnes, (1978), M.J. Selby (1982) and R.J. Chorley et. al(1985). begin to m ove d ow n slop e and thus m ass m ovem en t o f weathered debris occurs. It is apparent that m ass m ovem ent may occur w hen either shearing forces increase or shearing resistance o f m aterials decreases. It may be pointed out that either o f the tw o p ro cesses (increase in stress and d ecrease in resistan ce o f m aterials to stress) m ay operate or b oth the p roc­ esses m ay operate together.

3. F a c to r s o f M a ss M o v em en t A n y sort o f m ass m ovem en t o f weathered debris w ith any rate w hether on h illslop e or valley side slo p e d ep en d s on the ratio betw een shearing forces (sim p ly know n as stress) and resistance o f m aterials to shearin g forces (i.e. shearing resistance o f m aterials) as fo llo w s — strength or shearing resistance of materials Fs = — -------------------------------- ;— — m agnitude of sheanng forces

B ased on this corollary D .J. V arn es (1 9 7 8 ) cla ssified the factors w h ich con trol m a ss m o v em en t

w here Fs = factor o f safety

o f rock w astes in tw o broad ca teg o r ies and m any

W hen the quotient o f shearing resistan ce o f m aterials (sim p ly strength o f m aterials) and m agn i­ tude o f shearing forces i.e. safety factor (F s) is less than 1.0 (i.e. w hen m agnitude o f shearing forces o f

su b categories— (1 ) factors w hich increase sh ear­ ing forces (shear stress) and (2 ) factors w hich

h illslop e or v alley sid e slo p e e x c e e d s the'Shearing resistance o f m aterials resting on slo p e s) m aterials

o f m ass m o v em en t h a v e b een p resen ted tin tab le

reduce resistance o f m aterials to shear stess. T h e sum m arized form o f V a m e's c la ssific a tio n o f factors

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262

5.

Table 14.5 : Factors of Mass Movement Category I

Factors w hich in crease th e sh ea r strength

(c) sw elling (hydration o f clay)

(External P rocesses) 1.

L ateral pressure (a) w ater in cracks (b) freezing o f w ater in cracks (d)

R em oval o f lateral su p p o rt (u n dercu ttin gsteep en in g o f slope)

(a)

stream erosion

C ategory II F actors w hich d ecrease (red u ce) th e sh ear strength o f m aterials

(b)

glacial erosion

1. W eath erin g a n d o th er p h y sico ch em ica l re­

(c)

m arin e erosion by sea w aves

(d)

w eathering

(a) softening o f fissu red clay s

(these factors lead to rem oval o f lat­ eral support)

(b) physical d isin teg ratio n o f gran u lar rocks (frost action, therm al ex p an ­ sion etc.)

(A ) (N atural)

(e)

action s

prev io u s rockfall or slide, subsid­ ence or faulting (these factors steepen the slope)

(c)

hydration o f clay m ineral cau sin g d ecrease in p articles co h esio n , sw e ll­ ing

(B ) (A n th ro p o g en ic factors)

2.

(a)

C o n stru ctio n o f quarries, pits, canals, roads

(d)

base ex ch an g e (ch an g es in physical p ro p erties)

(b)

alteratio n o f w ater levels in lakes and reserv o irs

(e)

d ry in g (d e sic c a tio n ) o f c la y s an d shales (rack in g , loss o f co h esio n )

(f)

rem oval o f c em en t by so lu tio n

S u rch a rg e (lo a d in g o f slo p e) (a)

3.

2.

( n a tu r a l) w e ig h t o f r a in , s n o w (anthropogenic) w ater from pipelines, sew ers, can als

C hanges in in terg ra n u la r fo rces d u e to w ater co n ten t (p o rew ater p ressu re)

(b)

acc u m u la tio n o f talus

(a)

satu ratio n

(c)

v eg etatio n , trees

(b)

so ften in g o f m a teria l

(d)

see p a g e p ressu re o f p erco latin g w a­ te r

(e)

(a n th ro p o g e n ic ) co n stru ctio n o f fill, w a ste p iles, b u ild in g s

3.

C h a n g es o f stru ctu re (a)

A ssuring o f sh ales an d co n so lid ated clay s

(b)

re m o u ld in g o f lo e ss, san d a n d sen si­ tiv e clay

T r a n s ito r y e a r th stress 4.

(e n d o g e n e tic p ro c e s s e s )

O rg a n ic

(a)

e a rth q u a k e s

(a) burrowing animals

(b )

v ib ra tio n s, b la stin g , traffic

(b)

(c)

swaying of trees in wind

Source : D.J. Vames, 1978, in R.J. Chorley et. al, 1985 Recently, man has emerged as a significant factor of mass wasting and mass movement in al­ most all of the environmental conditions. His activi­ ties (e.g. deforestation for commercial wood and increase in agricultural land; construction of roads, dams, reservoirs; urbanization on fragile hillslope, manipulation of rivers, coastal areas etc.) destabilize

R e m o v a l o f u n d e r ly in g s u p p o r t

(a) undercutting by rivers and waves (b) solution at depth, mining (anthropogenic) (c)

loss o f strength o f underlying sediments

(d)

squeezing out of underlying plastic sediments

d ecay o f ro o ts

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4.

m o bilization o f residual stress (pres­ sure release)

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rapid w eath erin g o f an u n d erly in g w eak ro c k su ch a s shale or m u d sto n e .’ T h e freq u en cy o f ro c k fa lls dep en d s on certain e n v iro n m en ta l co n d itio n s su ch as arid ity /h u m id ity facto r,lith o lo g ical a n d stru ctu ra l ch aracteristics o f ro ck s, n atu re o f slo p e an d v e g e ta ­ tion etc. In hum id areas ro ck falls a re very co m m o n features but in h ot arid areas they are o f very rare occurrence. D e b ris fa ll in v o lv es rap id rate o f fall o f w eathered ro ck m a terials (w h ich are fin e r than the m aterials in v o lv ed in ro ck fall) d o w n slo p e (it m ay be h illslo p e or steep v alley sid e slo p e o f stre a m s) from g reat height. T h e fallen m a te ria ls c o lle c t at th e foot-hill or c liff b ase and form sm all m o u n d s an d ridges. E a rth fa ll in v o lv es d o w n slo p e m o v e m e n t o f finer m aterials than d eb ris fall.

hillslopes as w ell as valley side slopes and accelerate the process o f m ass w astin g and m ass m o v em en t and increase frequency and m agnitude o f different m echa­ nism s o f m ass m ov em en t. Increased defo restatio n , cultivation on cleared hillslope, co nstruction o f roads and reservoirs in the H im alay as have m ade the m ountain eco sy stem m o re frag ile and vuln erab le to increase freq u en cy and m a g n itu d e o f d iffere n t types o f m ass m o vem ent. Lan d slid es

It m ay be m e n tio n ed th a t g enerally all types o f m ass m o v e m e n ts o f ro ck w astes in clu d in g soils and ice are co lle c tiv e ly called as lan d slid es w hich are v ario u sly classified on d iffe re n t b ases i.e. d irec­ tion o f m o v e m en t, ty p e and rate o f m ov em en t, n a­ ture o f m a teria ls, p resen ce or ab sen ce o f lu b rican ts etc. (tab les 14.1 to 14.4). O n an average, lan d slid es (d o w n slo p e m o v e m e n t o f d iffere n t types o f debris e n b lo ck ) are d iv id ed into five m ajo r categ o ries e.g. fa ll, slid e, to p p le , flo w s and la tera l sp rea d s. On the basis o f n atu re o f m a teria ls th ese are fu rth er su b d i­ vide into sev eral types (tab le 14.4).

S lid es

Slides, very often k n o w n as la n d slid e s a m o n g general public, are m o st s ig n ific a n t o f all ty p e s o f m ass m ovem ents. ‘M a ss-w a stin g w h erein a m a ss o f rock or w eath ered d eb ris m o v e s d o w n h ill a lo n g d iscrete sh ear su rfaces is d e fin e d as a s lid e ’ (A .L . B loom ). It m ay be p o in ted o u t th a t slid e s in v o lv e dow n slo p e d isp la cem e n t o f b o th ty p e s o f m a te ri­ als— w eath ered ro ck m a te ria ls an d so ils. ‘S lid e s in rock or soil are ch a ra c te riz e d by m o v e m e n t a b o v e a sharply d efin ed sh ear p lan e. In ro c k s su ch as sla te , schist, and m any sed im en tary fo rm a tio n s th e s h e a r plane fo llo w s a stru ctu ra l p la n e w ith in th e ro c k su c h as a p lan e o f fo liatio n or b e d d in g — a n d it is o fte n s tra ig h t’ (M .J. S elb y , 1982). S lid e s are p ro m o te d by a h o st o f co n tro llin g v a ria b le s su c h as n a tu re o f slo p es (v ertical an d c liff slo p e is e s s e n tia l fo r s lid e s), m o d erate lu b ricatio n by w a te r, e a rth tre m o rs , g ra v ­ ity, v ertical and steep ly in c lin e d ro c k b e d s, b a se rem o v al etc. S lid e s are m o re fre q u e n t in c e rta in lo c atio n s h av in g fa v o u ra b le c o n d itio n viz. (1 ) ste e p h illslo p e o r s te e p v a lle y sid e s o f s tre a m s, (2 ) fa u lt scarp s, (2) re ju v e n a te d flu v ia lly e ro d e d v a lle y s , (4 ) sea co asts, (5) a llu v ia l riv e r v a lle y s , (6 ) d e g ra d e d h ills and m o u n ta in s (d u e to d e fo re s ta tio n , ro a d c o n ­ stru c tio n , s e ttle m e n t e x p a n s io n e tc .).

Fails In s ta n ta n e o u s fall o f w eath ered ro ck m a teri­ als in c lu d in g la rg e b lo c k s from steep h illslo p es or earth en m a te ria ls fro m steep and cliffed v alley sid es o f stre a m s u n d e r th e in flu en ce o f g ra v ity is called fall. T h e siz e o f ro c k frag m e n ts d ep en d s on the size and p a tte rn o f ro c k jo in ts . T h is type o f m o v em en t in v o lv e s v e rtic a l d is p la c e m e n t o f m a teria ls w ith o u t w ater. T h e v e lo c ity o f fall is g re a te st o f all o th er types o f m a ss m o v e m e n t. A c c o rd in g to A .L . B lo o m , ‘fa ll is a d is tin c t la n d slid e p ro cess, b u t it is rarely in d e p e n d e n t o f s u b se q u e n t e v e n ts .’ O n th e b asis o f m a teria ls fall is s u b d iv id e d in to ro c k fa ll, d eb ris fa ll and e a r th fa ll. ‘R o c k fa lls are re la tiv e ly sm all la n d slid es confined to th e re m o v a l o f in d iv id u a l and su p erficial blocks fro m a c lif f b a s e ’ (M .J. S elb y , 1982). R o ck fall (fig. 14.1 A ) is fa c ilita te d by g ra n u la r an d b lo ck d isin teg ratio n o f ro c k s u n d e r th e p ro c e sse s o f m e ­ chanical w e a th e rin g a n d lim ite d a c tio n o f o x id a tio n in san d sto n es. A c c o rd in g to M .J. S elb y (1 9 8 2 ) ‘m o st rockfalls a re p ro m o te d by h y d ro fa c tu rin g , stress release, th e w e d g in g a c tio n o f tree ro o ts, an d o th e r w eathering p ro c e s s e s . ... a c o m m o n c a u s e o f ro c k falls is u n d e rc u ttin g o f a fa c e by stre a m s o r th e m o re

On the basis of nature of materials, direction and rate of movement (intensity) slides are divided into (1) slump (which is further divided into rock slump, debris slump and earth slump), (2) rock slides, (3) debris slide, and (4) earth slide.

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(1) Slumping involves intermittent sliding rock fragments, rock blocks or soils downslope

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along a curved plane caused by rotational m ovem ent (fig. 14.1C and 14.3) an d d isp laced blocks (w hether rock blocks or soil blocks) co v er very short distance. Slum p is prom oted by un d ercu ttin g o f slope base (w ith hillslope o r valley side slope o f stream s) by stream s, seaw aves (in case o f co ast land) and by

hum an activities (quarrying). In fact, ‘slump is the form o f slide m ost com m on in thick, hom ogeneous, cohesive m aterials such as clay. The surface of failure beneath a slum p block is spoon-shaped, con­ cave upw ard or o u tw ard ’ (A .L. Bloom , 1978, fig 14.3).

to e Fig. 14.3 : Slump and earthflow (After A.L. Bloom, 1978). S lu m p in g o f allu v ial d eposits o f valley sides o f a llu v ial riv ers o f no rth In d ia through u ndercutting o f v alley sid es by h y d rau lic action o f the stream s d u rin g w et m o n so o n p erio d is o f com m on o ccu r­ rence. S lu m p in g is co n su m in g a large ch u n k o f rich ag ric u ltu ra l lands ev ery y ear along the G ang a valley in U .P. an d B ihar. B ased on the nature o f m aterials in v o lv ed slu m p is su b d iv id e d into r o c k slu m p , d e b r is s lu m p a n d ea rth slu m p .

su rface’ (A .L. B loom , 1978). The C ross V entre Slide o f 1925 in W yom ing, U SA and T urtle M oun­ tain Slide o f 1903 in A lberta, C an ad a, are typical exam ples o f d ev astatin g landslides. T he very m as­ sive landslide (rock slide), w hich o ccurred in the north-w estern side o f N aini L ake (N ainital, U .P .) in 1884, w as so enorm ous that the debris filled a sizeable portion o f the lake.

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(3) D e b ris slid e is m ore ex ten siv e and occur (2 ) R o ck slid e (also k now n as ro ck g lid eator larg er scale than slum p but there is little am ount o f b lo ck g lid e ) is m o st sig n ific a n t o f all types o f slides w ater. D ebris slide is pro m o ted because o f tw o basic w h erein la rg e ro ck b lo ck s slid e d o w n the hillslope. factors— (1) saturation o f rocks due to w ater, and (2) ‘R o c k slid e s m ay b e v ery larg e and catastro p h ic in sudden d o w n slo p e m o v em en t o f unconsolidated m o u n tain re g io n s w h ere th e la rg e av ailab le re lie f m antle rock. T h e m aterials involved in debris slide p e rm its a c c e le ra tio n s o f ro ck d eb ris to v elo cities as is a m ix tu re o f soils and rock fragm ents (boulders). g re a t as th o se o f ro c k falls an d ro ck a v a la n c h e s’ T he d ebris co llects at the foot-hill or the b ase o f the (M .J. S elb y , 1982). R o c k slid e s in v o lv e rap id m o v e ­ valleys and fo rm s in terestin g m o rphological fea­ m e n t o f m a te ria ls d o w n slo p e . S o m e tim es, the v e ­ tures. lo c ity is so h ig h a n d m a ss o f m a te ria ls is so e n o r­ Flow m o u s th a t ‘ro c k s lid e s c an b e d ra m a tic fo rm s o f D iag o n al d o w n slo p e m o v e m en t o f ro ck frag­ slk fin g m a ss-w a stin g i f la rg e m a sse s o f u n w eath e red m e n ts and so ils alo n g slid in g p la n e w ith enough roek slid e d o w n h ill alo n g a slo p in g jo in t o r a b ed d in g

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WEATHERING AND MASS M OVEM ENT

M u d flo w d iffers fro m ea rth flo w in th a t fo rm er m ay be n o ticed by th e o b se rv e r w h ile th e latter can n o t be n o ticed b eca u se e a rth flo w is .n o t very com m on. T h e w ater c o n te n t is m o re in m u d flow than in d eb ris flo w an d ea rth flo w . M ud flo w is m o st co m m o n alo n g v alle y sid e s o f a llu v ia l riv e rs and the d eb ris (m u d ) so p ro d u c e d is tra n s p o rte d by the rivers. T h e n ece ssary c o n d itio n s w h ich p ro m o te m ud flow in clu d e (1 ) steep an d v ertical slo p e, (2 ) p resen ce o f u n co n so lited m a te ria ls on th e u p p e r su rface so th a t th ese, w h en m ix e d w ith w ater, b e ­ com e viscous flu id and slip p e ry , (3 ) in te rm itte n t supply o f su fficie n t w ater as lu b ric a n t, a n d (4 ) a b ­ sence o f v eg etatio n . B ased on th e se fa c to rs E llio t B lack w eld er (19 2 8 ) c o n sid ere d arid re g io n s as m o s t favourable fo r m u d flow . C .F .S . S h a rp e (1 9 3 8 ) h as d ivided m ud flow into th ree c a te g o rie s on th e b a sis o f spatial ch ara cteristics e.g. (1) m u d flo w o f a rid regions, (2) A lp in e m ud flo w , an d (3 ) v o lc a n ic m u d flow .

w ater is called flow (w hich is fu rth er d iv id ed into so liflu ctio n , d eb ris flo w , m ud flo w , ea rth flo w , rock a v a la n ch e etc.). F low in v o lv es d o w n slo p e rapid m o v e m en t o f ro ck d eb ris or soils satu rated w ith w ater like visco u s fluid. ‘D ry flow s in san d or silt are kn o w n , b u t m o st flow s are satu rated w ith w ater. R ates o f m o v e m e n t are g reater than for creep but ran g e from im p ercep tib ly slow to trag ically rapid m u d flo w s an d av alan ch es. F low s typically m ove as lo b e s o r to u n g e s ’ (A .L . B loom , 1978). D e b r is flo w in v o lv es d o w n slo p e m o v em en t o f e n o rm o u s a m o u n t o f v isco u s soils and boulders either sep arately o r m ix ed together, and occurs m ostly along riv e r v alley sid es. T h e d iffere n ce betw een d eb ris flo w , ea r th flo w an d m u d flo w is related to size o f p a rtic le s an d a m o u n t o f w ater. T h e size o f p article d e c re a se s fro m d eb ris flow to m udflow . ‘T h e th ree te rm s fo rm a series o f pro g ressiv ely h ig h e r w a te r c o n te n t (i.e. w ater co n ten t increases fro m d e b ris flo w th ro u g h earth flow to m ud flow ) but are o ften u se d in terch an g ea b ly . D ebris flow s have 2 0 -8 0 p e r c e n t p a rtic le s c o a rse r than sand sizes, w h ereas ea rth flo w s an d m u d flow s are 80 per cent o r m o re m u d an d sand. M u d flow is the m o st liquid “ en d m e m b e r” o f th e s e rie s ’ (A .L . B loom , 1978). D e b ris flo w o c c u rs m o stly d ue to av ailab ility o f w ater, p re s e n c e o f lo o sely d ep o sited so ils and fine ro c k m a te ria ls , la c k o f v eg etatio n co v er, clay m in er­ als in th e so ils , u n sta b le slo p e, u n d ercu ttin g o f slope (v a lle y sid e s ) by stream s, earth trem o rs etc. ‘D ebris flo w s ra n g e in size fro m a few m eters to o v e r 1000 m e te rs in w id th a n d m ay be ten s o f m eters th ick in p la c e s; m o re c o m m o n ly th ey are 1 to 5 m th ic k ’ (M .J. S elb y , 1982). D e b ris flo w is m o st c o m m o n on gully h e a d s in th e riv e rin e tra c ts o f m a jo r a llu v ial riv ers.

Creep V ery slow and im perceptible d o w n slo p e m o v e ­ m en t o f m aterials (co llu v iu m ) is c a lle d creep . O n th e basis o f m aterials in v o lv ed in su ch m o v e m e n t c re e p is div id ed into (1) so il c re ep (fin e w e a th e re d ro c k debris as w ell as soil) and (2) ro ck creep (u n w eath ered jo in t b locks). It m ay be p o in te d o u t th a t th e ra te o f m o v em en t o f m a teria ls (c o llu v ia ) u n d e r c re e p is so slow (a few m illim e te rs p e r y e a r) th a t it b e­ co m es p ractically d iffic u lt fo r th e o b se rv e rs to n o ­ tice it. S o il creep is also c a lle d as s o liflu c tio n w h ic h o ccu rs in a v ariety o f c lim a tic c o n d itio n s ra n g in g from tro p ical h u m id to p e rig la c ia l c lim a te s . T h e p ro cess o f d e b ris m o v e m e n t in p e rig la c ia l re g io n s

E a r th flo w is p ro m o te d by e x c e ssiv e w ater re c e iv e d m o s tly th ro u g h ra in fa ll so th a t th e m a teria ls are o v e rsa tu ra te d . E a rth flo w is m o re co m m o n on p la n a r h ills id e s o r v a lle y s id e s h a v in g a llu v iu m , rich

has been v ario u sly d e fin e d an d a n u m b e r o f te rm s h av e b een su g g e ste d . F irs t J.G . A n d e rs o n (1 9 0 6 ) p ro p o se d th e te rm solifluction (s o lu m -s o il, flu e re

in clay m in e ra ls.

flo w ) fo r slo w m o v e m e n t o f d e b ris , s o a k e d w ith

D e b ris flo w o f v o lc a n ic m a te ria ls satu rated w ith w a te r on v o lc a n ic c o n e s is c a lle d la h a r . H eav y d o w n p o u r m ix in g w ith fallin g v o lc a n ic d u sts c au ses en o rm o u s m u d flo w as la h a r on the ste e p slo p es o f v o lcan ic c o n e s w h ic h in flic ts g re a t d a m a g e to h u ­ m an h e a lth an d w ealth . F o r e x a m p le g re a t la h a r c re a te d on th e ste e p slo p e s o f K e lu t V o lc a n o in Ja p a n in 1919 k ille d 5 5 0 0 p erso n s.

w ater, fo rm h ig h e r to lo w e r slp e s. S o liflu c tio n te rm w as re p la c e d by congelifluction o f J. D a y lik (1 9 5 1 ) to in c o rp o ra te o n ly s o il-flo w in th e p e rig la c ia l c li­ m a te h a v in g p e rm a fro s t b e lo w a n a c tiv e la y e r. K . B ry an (1 9 4 6 ) u se d th e te rm cryoturbation w h ic h in c lu d e d all ty p e s o f m a ss m o v e m e n t o f re g o lith s u n d er p e rig la c ia l e n v iro n m e n t. R e c e n tly , gelifhiction

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is u se d in p la c e o f c o n g e liflu c tio n .

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14.6 TOPOGRAPHIC EXPRESSIONS OF IIA88 WASTING AND MASS MOVEMENT D iffe re n t ty p e s o f m a s s w a s tin g a n d mass

Rock creep in v o lv e s d o w n h ill m o v e m e n t o f ro c k d e b ris h a v in g re la tiv e ly g re a t d e p th (u p to 3 0 0 m ) bu t the m o v e m e n t is v ery slo w a n d ra n g e s b e ­ tw e e n o n e m e te r to ten m e te rs p e r y ear. ‘It is d is tin ­ g u ish e d from so il c re e p b y its g re a t d e p th a n d is o la ­ tion from d aily a n d se a so n a l c lim a tic c o n d itio n s , an d fro m lan d s lid in g by th e la c k o f a sin g le c le a rly d efin e d fa ilu re p la n e an d slo w ra te o f d e fo rm a tio n ’ (M .J. S elb y , 1982). T h e fo llo w in g c o n d itio n s p ro ­ m o te ro c k c re e p — d e fo rm a tio n o f ro ck s th ro u g h b e n d in g , fo ld in g , b u lg in g , fra c tu rin g , sp read in g ; d isto rtio n and b u ck lin g o f in clin ed ro ck b ed s o f v ary ­ ing resistan ce, m ech an ical d isin teg ratio n o f rocks etc.

m o v e m e n t c re a te d is tin c tiv e m o rp h o lo g ic a l fe a tu re s on h ills lo p e a n d r iv e r v a lle y s id e s a n d c o a s ta l la n d s. It m a y be p o in te d o u t th a t on o n e h a n d th e re is w ide ra n g e o f v a ria tio n in m a ss m o v e m e n t b e c a u se o f v a ry in g c o n tro llin g fa c to rs a n d c o n d itio n s , th e re is a lm o s t u n ifo rm ity in th e r e s u lta n t m o rp h o lo g ic a l fe a tu re s. T h e to p o g ra p h ic fe a tu re s p r o d u c e d b y m a ss m o v e m e n t say la n d slid e s in c lu d e sc a rs , rip p le m a rk s,

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te rra ces, m e a n d e r w id e n in g etc.

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C la s s if ic a tio n o f s l o p e s ; s lo p e e l e m e n t s ; a p p r o a c h e s to th e s tu d y o f s lo p e d e v e lo p m e n t-s lo p e ev o lu tio n ap p ro a c h an d p ro c e s s -fo rm a p p ro a c h (m o n o ro c e s s c o n c e p t a n d p o ly -p ro c e s s c o n c e p t) ; s lo p e d e c lin e th e o ry o f >avis ; s lo p e r e p la c e m e n t th e o ry o f P e n c k ; A . W o o d 's m o d e l o f s lo p e e v o l u t i o n ; h ills lo p e c y c le th e o ry o f L .C . K i n g ; c o n c e p t o f R . A .S a v ig e a r ; F is h e r - L e h m a n n m o d e l o f slo p e e v o lu tio n ; p r o c e s s - r e s p o n s e m o d e l o f A . Y o u n g ; s lo p e fa ilu re ; h ills lo p e p ro c e s s e s a n d e ro s io n .

E

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CHAPTER 15

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15 HILLSLOPE

slope profile o f e ith er h illslo p e o r v alley sid e slo p e. T he ch a ra cteristic a n g le or c h a r a c te r is tic slo p e ‘is that w hich is m o st freq u en tly id e n tifie d u n d e r p a r ­ ticu lar co n d itio n s o f ro ck ty p e an d c lim a te ’ (R .J. C horley et. al, 1985) and has m a x im u m fre q u e n c ie s o f slope angles o f h illslo p e.

S lo p e , d efin e d as an g u lar in clin atio n s o f te r­ rain b e tw e e n hill to p s (crests) and valley bottom s, re su ltin g fro m the co m b in atio n s o f m any causative facto rs like g eo lo g ical stru ctu re, clim ate, vegetation c o v e r, d ra in a g e te x tu re and freq u en cy , d issection in d e x , re la tiv e reliefs (and o f co u rse d en u dational p ro c e s s e s , in c lu d in g w eath erin g , m ass w asting and m a ss m o v e m e n ts o f ro ck w astes, ero sio n and tran s­ p o rta tio n o f e ro d e d m a teria ls d o w n slo p e) etc. are sig n ific a n t g e o m o rp h ic attrib u tes in the study o f la n d fo rm s o f a (flu v ia lly o rig in ated ) d rain ag e basin (S a v in d ra S in g h an d R. S riv asta v a, 1975). S lope is, th u s, u p w a rd o r d o w n w a rd in c lin atio n o f su rface b e tw e e n h ills a n d v alley s an d form m o st sig n ifican t a s p e c t o f la n d s c a p e a sse m b la g e s. E x ce p t p lain s and te rra c e s s lo p e s are a lw a y s p e rc e p tib le and are m ore s ig n ific a n t in m o u n ta in o u s reg io n s. In fact, m o rp h o ­ lo g ical c h a ra c te ris tic s o f a g iv en reg io n are d e te r­ m in ed by s lo p e s o f th a t reg io n b eca u se p h y sical la n d sca p es a re the re s u lt o f c o m b in a tio n s o f slo p es. N ot o n ly th is, slo p e s in tro d u c e v a ria tio n s an d c o m ­ p lex ity in th e la n d sc a p e s o f a reg io n . T h is is w hy the

T he study o f slope in g e o m o rp h o lo g y is g iv e n m ore im p o rtan ce b eca u se , ‘n o t o n ly slo p es d o c o m ­ p rise the g reater p art o f th e la n d sc a p e , b u t as an integral p art o f the d ra in a g e sy stem th ey p ro v id e w ater and sed im en t to stream s. T h e re fo re , h ills lo p e s are an im p o rtan t co m p o n e n t o f th e c o m p le x la n d ­ scape th a t fo rm s a d ra in a g e b a s in ’ (R .J. C h o rle y et. al 1985). T h e slo p e stu d y in v o lv e s c la ss ific a tio n , d ev elo p m en t and ev o lu tio n o f h illslo p e s. B e sid e s, slo p e p ro file stu d y b ased on field m e a s u re m e n t o f slope an g les and in s tru m e n ta tio n o f p ro c e s s e s actin g on h illslo p e h as b e c o m e m o re im p o rta n t fo r q u a n ti­ tativ e a n a ly sis o f h illslo p e s an d c o n s tru c tio n o f th e o ­ ries and m o d e ls o f th e ir d e v e lo p m e n t and e v o lu tio n . H istorically, the research es on slope stu d y and analysis m ay be g ro u p e d in tw o p h a se s. Initial phase w as d o m in a te d by in te rp re ta tio n o f d iffe re n t a s p e c ts o f h illslo p e an d v alle y sid e slo p e d e v e lo p m e n t o n th e b asis o f fie ld o b s e rv a tio n . D a v is ia n (slope decline)

study o f d iffe re n t a sp e c ts o f slo p e s h as a lw a y s been a focal th e m e in g e o m o rp h o lo g y . T w o te rm s, re la te d to slo p es i.e. an g le o f in c lin atio n a n d in c lin e d su rfa c e , n eed c la rific a tio n .

an d P e n c k ia n (parallel retreat a n d slope rep lace­ m ent) m o d e ls o f s lo p e d e v e lo p m e n t b e lo n g to th is p h a se o f q u a lita tiv e stu d y o f s lo p e s . M odern phase

v alley sid e slo p e w h e re a s in c lin e d s u rfa c e m e a n s

is d o m in a te d by q u a n tita tiv e a n a ly s is o f s lo p e s b a s e d https://telegram.me/UPSC_CivilServiceBooks

A n g le o f in c lin a tio n o f th e su rfa c e o f a slo p e m e an s slo p e a n g le o r sim p ly slo p e w h e th e r h ills lo p e o r

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m ents o f slope p rofile are called slo p e elem ents or slop e segm en ts. It is not alw ays necessary that all the slope profiles w ill com prise all the four elem ents but an ideal h illslope p rofile is th at w hich consists of sum m ital co nvexity, free-face, rectilinearity and ba­ sal concavity. T h e ex isten ce o f free face (cliff) ele­ m ent depends on the p resen ce o f resistant strata (in the case o f hillslo p e) or u n d ercu ttin g (cliffing) at the slope base (in the case o f co astal slope) (fig. 15.1).

on data derived from topographical m aps and aerial photographs, m easu rem en t o f slope angles in the field, and instrum entation o f p ro cesses (w eathering, m ass w asting and m o vem ent, and ero sio n ) acting on the hillslopes. The w orks o f R. A. Savigear, A. Y oung, W aters etc. belong to this phase. B esides, sig nifican t contributions in slo p e study h av e been m ad e by J.L. R ic h (1 9 1 6 ), C .K . W en tw o rth (1930), A .C . L aw son, E. R aize an d J. H enry (1 9 3 7 ), R .E. H orton (1945), H. B aulig (1935), W .C . C a le f (1950), W .C . C a le f and R. N ew C o m b (1953 ), A. W ood (1942), L .C . K ing (1953), A .N . S trah ler (1950), C .A . C o tton, O .M . M iller and C .H . S u m m arso o (1960), S.A . S chum m (1 9 5 6 ,1 9 6 7 ), S. A. S chum m an d M .P . M osley (1973), M .J. S elby (1 9 8 2 ), R .J. Sm all and M .J. C lark (1982), Y o u n g (1 9 6 1 , 1963, 1972), S av in d ra S in g h and R. S riv a sta v a (1 9 7 5 , 1977), S av in d ra S ingh (1979). S a v in d ra S in g h an d R .S. P an d ey (1982, 1987), S av in d ra S in g h and S.P. A g n ih o tri (19 8 2 ) etc.

IDEAL HILLSLOPE PRO FILE 1

Summital Convexity 2

Free Face \ s l e c t iii near

3

15.1 CLASSIFICATION OF SLO PE S T h ere is m a rk e d v ariatio n in op in io n s reg ard ­ in g the c la ssific a tio n o f h illslo p es b ecau se o f co n fu ­ sio n s re g a rd in g the elem en ts (seg m en ts) and form o f slo p es. C o n v ex , free face, re c tilin e a r and co n cav e seg m en ts are in fact elem en ts o f h illslo p e p ro file and the fo rm s o f slo p e b u t a few g eo m o rp h o lo g ists have re c o g n iz e d th em as ty p es o f h illslo p es. It m ay be p o in te d o u t th a t no h illslo p e m ay be only e ith er re c tilin e a r (stra ig h t), o r c o n v e x or co n c a v e ra th e r it c o m p rise s m o re th an o n e e le m e n t (seg m en t), th o u g h o n e e le m e n t m ay be m o st e x te n siv e an d d o m in an t. G e n e ra lly , so m e g e o m o rp h o lo g ists classify slo p es fo r s im p lific a tio n in to c lif f slo p e (free face o r scarp s lo p e ), re c tilin e a r o r s tra ig h t slo p e, c o n v e x slo p e and c o n c a v e slo p e , an d if m o re th an o n e fo rm s are p re s e n t, th e y c all th e m c o m p o site slo p e. B u t this c la s s ific a tio n is n o t ju s tifia b le b e c a u se th e se are n ot s lo p e ty p e s (slo p e fo rm s) ra th e r th e se are slo p e e le m e n ts (s lo p e se g m e n ts ). S o let us first e x a m in e th e c h a ra c te ris tic s o f slo p e e le m e n ts.

k

----------- ^ " x B a s a l ^ ^ Q oncavity

Fig. 15.1 : Elements (Segments) o f Hillslope. (1) S u m m ita l C o n v e x ity — T h e c o n v e x seg­ m e n t is fo u n d a t th e h ill c re s t (h ill to p ) an d this e lem en t is ca lle d su m m ita l c o n v e x ity (o r sim ply co n v ex e lem en t). C o n v e x slo p e , if c o n v e x elem ent d o m in a te s th e h ills lo p e p ro file , is a lso c a lle d as w a x in g slo p e b e c a u se it g ro w s in h e ig h t u p w a rd and in d im e n sio n d o w n w a rd b u t th is te rm in o lo g y o f W. P en ck is n o t ju s tif ia b le b e c a u se n o t all c o n v e x slopes n e c e ssa rily g ro w in h e ig h t a n d d im e n s io n (size). M o st o f th e s u m m ita l c o n v e x ity d e v e lo p s b ecause of d e n u d a tio n a l p ro c e s s e s e .g . ra in w a s h o r so il creep, th is is w h y c o n v e x slo p e is a lso c a lle d u p p e r w ash slo p e . (2 ) F ree F a c e — F re e fa c e e le m e n t o f hillslope re p re s e n ts w a ll-lik e p re c ip ito u s s lo p e o f b are rocks an d is d e v o id o f an y d e b ris . T h e s lo p e is so steep and p re c ip ito u s th a t n o w e a th e re d m a te ria ls c an re st on it. T h e e le m e n t o r s e g m e n t is c a lle d s lo p e o f d eriv a ­ tio n b e c a u se th e re is in s ta n ta n e o u s d o w n s l o p e trans­ p o rt o f m a te ria ls . T h e fre e fa c e e le m e n t is subjected to b a c k w a s tin g w h ic h f a c ilita te s p a ra lle l retreat of th is e le m e n t.

Slope Elements I f th e lo n g itu d in a l p ro file o f a h illslo p e o r coastal slo p e is c o n s id e re d , it b e c o m e s a p p a re n t th at th e re is n o u n ifo rm ity in th e slo p e p ro file in rela tio n to s lo p e a n g le s fro m h ill to p s to v alley flo o rs o r from c l i f f c r e s t to s e a sh o re . T h e e n tire slo p e p ro file is p u n c tu a te d b y th e p re s e n c e o f c o n v e x ity , c o n c a v ity , r e c tilin e a rity a n d fre e face . T h e s e d is tin c tiv e seg https://telegram.me/UPSC_CivilServiceBooks

( 3 ) R ectilinear E lem ent — T h e straight o r lin e a r s e g m e n t o f h ills lo p e p r o f ile b e tw e e n uppeT

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HILLSLOPE

free face an d lo w er co n cav e elem en t is called recti­ linear elem en t. It is also term ed as co n sta n t or u n iform o r r e g u la r slo p e b ecau se o f uniform nature o f slo p e an g le (i.e. g rad u al in crease upslope

slope. T his is why rectilin ear elem en t is called d e ­ bris con trolled slop e. (4) C o n ca v e E lem en t— T h e basal seg m en t o f an ideal hillslo p e p ro file is alw ay s ch ara cterized by concave elem ent. T he slope an g le d ecreases as the segm ent o f basal co n cav ity in creases (in length). T his is w hy this seg m en t is called w a n in g slo p e. T his elem en t is also called v a lley flo o r b a sem en t slop e or lo w er w a sh slo p e. B asal c o n ca v ity is g e n ­ erally originated because o f activ e d en u d atio n m ainly by rainw ash, rill and gu lly eo rsio n . T h is seg m en t m ay be covered w ith d eb ris o r m ay be o f b are ro ck s.

Classification on the Basis of Slope Elements It m ay be p o in ted o u t th a t th e o re tic a lly it is conceived that an ideal h illslo p e p ro file c o n sists o f all the four elem en ts viz. su m m ital co n v e x ity , free face, rectilin ear and basal c o n ca v ity b u t in p ra c tic e it is not alw ays possible. T he escarp m en ts o f B h a n d e r plateau (M .P., fig. 3.8) p resen t a ty p ical e x a m p le o f ideal h illslope p ro file c o n sistin g o f all th e fo u r slo p e elem ents b ut sum m ital co n v ex ity is n o t w ell p ro ­ nounced. S lightly u p slo p e d ip p in g b ed s o f m a ss iv e V indhyan san d sto n es o f 152 m th ick n ess re stin g as cap ro ck on u n d erly in g sh ales and m u d sto n e s h a v e given birth to steep and p re c ip ito u s scarp s as free face elem en t. T h e th ic k d e p o sits o f sh a le s an d m u d sto n es b elow san d sto n es h av e b een re sp o n sib le fo r the d ev elo p m en t o f re c tilin e a r an d b asal c o n c a v e elem en ts. It m ay be m e n tio n ed th a t sin ce d e v e lo p m e n t o f ideal h illslo p e p ro file w ith fo u r e le m e n ts d ep e n d s on local co n d itio n s (e.g. n atu re an d d isp o sitio n o f ro ck s, stru ctu re o f ro ck s, n a tu re o f d e n u d a tio n a l p ro cesses, c lim a tic c o n d itio n s, v e g e ta tio n e tc .) and o r g ra d u a l d e c re a s e in s lo p e a n g le d o w n slo p e ). B e ­

h en ce so m e tim es o n e o r tw o e le m e n ts d o n o t d e ­

c a u s e o f p r e s e n c e o f ro c k d e b ris th is e le m e n t is

velop. F o r e x a m p le , fre e fa c e d o e s n o t d e v e lo p in th e

c a lle d d e b r is s lo p e . T h e a n g le o f re c tilin e a r se g ­

re g io n s o f lo w o r su b d u e d re lie fs. O n th e o th e r h an d ,

m e n t is c o n tro lle d a n d d e te rm in e d b y a n g le o f rep o se

if resistan t and w eak ro ck s are a ltern ated in a h illslo p e,

o f d e b ris a n d it is a ls o c a lle d as r e p o s e s lo p e . It m ay

th e re is re p e titio n o f fre e fa c e (o n re s is ta n t ro c k s)

b e p o in te d o u t th a t th is te rm in o lo g y is c o n fu s in g a n d g iv e s th e im p re s s io n th a t r e c tilin e a r e le m e n t is a l­

an d re c tilin e a r e le m e n ts (o n w e a k ro c k s). T h e s e ­ q u e n c e o f th e se fo u r e le m e n ts c h a n g e s w ith the'

w a y s th e re s u lt o f a c c u m u la tio n o f d e b ris b u t th is is

p a s s a g e o f tim e d u rin g c y c le o f e ro s io n . F o r e x a m ­

n o t a lw a y s tru e b e c a u s e s o m e tim e s r e c tilin e a r s e g ­ m e n t is o rig in a te d d u e to d e n u d a tio n a n d e x p o s u re o f b are ro c k su rfa c e . It m a y b e m e n tio n e d th a t th e re

p le , su m m ita l c o n v e x ity is d o m in a n t e le m e n t in

m a y b e a th in la y e r o f d e b ris b u t th is is n o t s ta tio n a ry

o f la n d sc a p e . W a n in g s lo p e o r c o n c a v e e le m e n t

ra th e r it is m o b ile i.e. it is in c o n s ta n t m o tio n d o w n ­

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y o u th fu l sta g e w h ile re c tilin e a rity b e c o m e s m o re p ro n o u n c e d in m a tu re sta g e o f c y c lic d e v e lo p m e n t

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Fig. 15.3 : Different types o f composite slopes (after R.J. Small, 1970). m ay label a slope on the b asis o f a sin g le elem en t su ch as co n v ex slo p e, rec tilin e a r (straig h t slope), free face slope o r co n ca v e slo p e b u t in reality this is rarely p o ssib le. A ny slo p e p ro file co n sists o f at least tw o elem en ts and in ste ad o f m o n o -elem e n t slope th e re is c o m p o s ite o r c o m p o u n d s lo p e w h ich m ay be o f d iffe re n t ty p e s d e p e n d in g on v ary in g c o m b i­ n atio n s o f slo p e elem en ts. (i) C o n v ex o -co n c av e slope

(iii) F ree face -re c tilin e a r-c o n c a v e slope (iv) C o n v e x -re c tilin e a r-fre e face-rectilinearco n ca v e slo p e (v) C o n v ex -rec tilin ear-free face slope (coastal area, basal free face is d ev elo p ed due to basal cliffin g by se a w aves), and so on

Genetic Classification G e n e tic c la ss ific a tio n o f h illslo p e s involves th e m o d e o f th e ir o rig in a n d d e v e lo p m e n t. Thus, on

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(ii) C o n v ex o -rec tilin ear-co n c av e slope

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slope angles w hich are deriv ed eith er from to p o ­ graphical m aps and aerial p h o to g rap h s or on the basis o f m easurem ent o f slope an g les w ith suitable instrum ents in the field. A. Y oung classified slopes on the basis o f slope angles o f slope p ro files into seven m ajor categ o ries as follow s.

the basis o f m ode o f gen esis hillslopes may be classified into three types. (1) T ecto n ic slo p e is generally fo rm ed due to tensional and co m p ressio n al forces resu ltin g into the form ation o f fau lt scarps th ro u g h fau ltin g in crustal rocks, and due to tiltin g o f ro ck beds. Scarp slope is ideal ex am p le o f tecto n ic slope. It m ay be m entioned th a t o rig in al scarp slopes are m o d ified by denudational p ro cesses and hence they becom e com ­ pound slopes.

1. Level to gentle slope

(2) E ro sio n a l slo p es are generally form ed due to ero sio n by riv ers, glaciers and sea w aves. E rosional slo p es d ev elo p ed by riv ers undergo se­ quential tran sfo rm a tio n and changes in different stages o f cy cle o f ero sio n i.e. convex slope in ju v e ­ nile (y o u th ) stage, re c tilin e a r slope in m ature stage and co n cav e slope in old stage. M ost o f the erosional slopes by the stream s are valley side slopes. Sea w aves form steep c liff slope through basal u n dercut­ ting o f coastal rocks. F ree face and pedim ent slopes are fo rm ed by fluvial erosion in arid and sem i-arid regions.

(a)

level slope (0°— 0.5°)

(b)

alm ost level slope (0.5°— 1.0°)

(c)

very gentle slope (1.0°— 2.0^)

2. G entle slope (2.0°— 5.0°) 3. M oderate slope (5.0°— 10.0°) 4. M oderately steep slope (10.0°— 18.0°) 5. Steep slope (18.0°— 30.0°) 6. Very steep slope (30.0°— 45.0°) 7. Precipitous to vertical slope (45.0°— 90.0°) (a) precipitous slope (45.0°— 70.0°) (b)

w all-like slope (70.0°— 90.0°)

15.2 APPROACHES TO THE STUDY OF SLO PE (3) S lop e o f accum ulation - The slopes formed DEVELOPMENT due to dep o sitio n o f eroded sedim ents by different In the initial stage the study o f slo pe d e v e lo p ­ d en u d atio n al p rocesses are called aggradational m ent was based on qualitativ e in fo rm atio n rece iv ed slop es or slo p es o f accum ulation. For exam ple, from field observations b u t the p ro b lem s o f slope slopes o f allu v ial fans and cones form ed by rivers, continued to increase. T he m ajo r p ro b lem s o f slope sand d unes by w ind, m orainic ridges deposited by developm ent before the g eo m o rp h o lo g ists in c lu d e glaciers etc. are aggradational slopes. V olcanic the follow ing aspects, 1. v arying fo rm s o f slo p es, 2. cones also com e u n d er this category. m ost slopes are co m pound slopes as they c o n sist o f On the basis o f stage o f form ation slopes are m ore than one slope elem en ts, 3. slo p e is a th reeclassified into (i) prim ary slopes and (ii) secondary dim ensional feature, 4. d en u d atio n b alan ce o f slope, slopes. P rim a ry slop es are form ed due to erosion by 5. progressive changes in the form an d g rad ie n t o f stream s, glaciers, sea w aves etc. V shaped valleys slopes w ith tim e, 6. relatio n sh ip s b etw een the p ro c ­ w ith convex plan o f gorges and canyons, c liff slopes esses o f m ass m o v em en ts (slid es, creep, flow etc.) carved out by sea w aves,, steep slopes o f U shaped and slopes, 7. p arallel retreat o f slo p e and g rad in g o f glacial valleys etc. are ex am p les o f prim ary or ero ­ slope profile, 8. influ en ce o f g eo lo g ical stru ctu re sional slopes. T ecto n ic slopes (e.g. fault scarp slope) and lithology (rock ty p es) on slo p e fo rm s, 9. re la ­ also fall in this category. S eco n d a ry slo p es rep re­ tionships b etw een clim ates and slo p e fo rm s etc. T h e sent m inor slopes dev elo p ed at the base o f prim ary problem s o f slope ev o lu tio n an d d ev elo p m en t m ay slopes. T hese slopes are form ed due to surface be approached in th ree w ays viz. th eo retica l a p ­ erosion and w eathering. S lopes o f talus con es are p roach , ex p erim en ta l a p p ro a ch (lab o rato ry ex ­ form ed due to accum ulation o f deb ris/ scree com ing p erim ents) and em p irica l a p p ro a ch (i.e. field stu d ­ dow n from the hillslope as a result o f m as m ovem ent ies). T hese three ap p ro ach es m ay be fused in tw o o f rock w astes. categ o ries viz. (1) slo p e-ev o lu tio n a p p ro a ch and (2) p ro cess-fo rm a p p ro a ch . Quantitative Classification

(1) S lo p e ev o lu tio n a p p ro a ch in volves th study o f historical developm ent o f hillslopes. D avisian

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Q uantitative classification involves classifi­ cation o f slopes into certain types on the basis o f

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m odel o f slope decline co m es u n d er th is ap proach. T here are certain pro b lem s w h ich are faced by the g eom oiphologists w hile attem p tin g to d escrib e h is­ torical evolution o f slo p es e.g. 1. It b eco m es n eces­ sary to find o u t origin al and initial form o f presen t day hillslope but this b eco m es very d ifficu lt because o f lack o f req u isite e v id en ces and h en ce th e study is based on sp ecu latio n s, d ed u ctio n s and th oughtful reasoning. M o st o f th e g eo m o rp h o lo g ists generally assum e th at the initial fo rm o f slo p e m ig h t have been vertical c liff or scarp , w hich m ig h t have undergone su bsequent ch an g es and tran sfo rm atio n due to w eath­ ering, m ass m o v e m e n t and ero sio n and resultant p ro g ressiv e d eclin e in slope g rad ien t to attain the p re se n t form . 2. T h e d atin g o f slope d ev elo p m en t p oses a serio u s p ro b lem before the in v estigators. In o rd er to so lv e this p ro b lem attem p ts are m ade to study the m e ch an ism s and p ro cesses o f ch an g es in slo p e form and slope declin e. S lopes are p laced in tim e seq u en ce on the basis o f study o f slope profiles in the field and th e ir relativ e d ating. T h is m ethod m ay be ap p lic a b le to only th o se areas w here slope d eclin e has not been o b stru cted . T h is m eth o d w ould n ot be ap p lic a b le to v alley sid e slo p es o f such rivers w h ich have fo rm ed in cised m ean d ers d ue to reju v e­ n atio n b eca u se the to p o g rap h ic d isco rd an c e (the se g m e n ts o f valley sid es bein g o ld er than the low er s e g m e n ts) d istu rb s the seq u e n tia l pattern o f slope e v o lu tio n in tim e seq u en ce. In sp ite o f th ese p ro b ­ le m s W .M . D av is p o stu la te d his co n ce p t o f slope d e c lin e (slo p e e v o lu tio n ) w h erein slo p e u n d erg o es se q u e n tia l b u t p ro g re ssiv e ch an g e s (d eclin e in slope g ra d ie n t) fro m very steep slo p e d u rin g y o u th thro u g h

esses activ e on slo p e profiles o r betw een slope types and denudation. T he intensity o f d enudational proc­ esses is influenced by structure, rock types, clim atic con d itio n s, vegetation cover, re lie f etc. The relative variations in the influences o f these con trolling vari­ ables introduce v ariatio n s in slo p e form s and slope types. F o r exam ple, lim esto n es an d clay in hum id clim ate will d ifferen tly resp o n d to flu v ial processes and hence resu ltan t slope w o u ld g reatly vary e.g. soil creep w ould be m o st activ e on lim esto n e s resu ltin g in the form ation o f co n v ex fo rm o f slo p e w hereas clay rocks w ould be su b jected to in ten se rain w ash resu ltin g in the d ev elo p m en t o f co n c a v e slo p e form . T his ap p ro ach also su ffers fro m so m e d iffi­ cu lties and pro b lem s. 1. S lo p e fo rm in g p ro cesses operate so slu g g ish ly th at it b eco m es very d iffic u lt to observe, m easu re and reco rd th e ir ra te s o f actio n 2. It is also d ifficu lt to iso late su ch p ro c e sse s w h ich affect slppe d ev elo p m en t b eca u se not all p ro cesses acting on h illslo p e p ro file are c o n ce rn ed w ith slo p e developm ent. F o r ex am p le, W . P e n c k re c o g n iz e d soil creep, rain w ash etc. as tra n sp o rtin g a g en ts w hich tran sp o rt the w eath ered m a te ria ls d o w n slo p e . In fact, these p ro cesses ero d e and tra n sp o rt on ly w e a th ­ ered reg o lith s and n ot the so lid rock su rface. 3. M o st o f the p resen t day slo p es do n ot ap p ea r to be the resu lt o f p resen t day g e o m o rp h ic p ro c e sse s, ra th e r they rep resen t re lic t featu res o f fo rm erly d e v e lo p e d slopes. T he ad v o ca tes o f p ro c e ss-fo rm a p p ro a c h argue th at the re la tio n sh ip b etw ee n p re s e n t d ay p ro c ­ esses and slo p es is w ith o u t d o u b t e s ta b lis h e d at le a st in the areas o f so ft ro ck s w h ich a re s u b je c te d to c o n tin u o u s d en u d atio n . 4. I f th e c o n c e p t o f a s s o c ia ­ tion b etw een d e n u d a tio n a l p ro c e s s e s a n d s lo p e form is acc ep ted then it sh o u ld a lso b e a c c e p te d th a t there m u st be clo se re la tio n s h ip b e tw e e n c lim a tic c o n d i­

re c tilin e a r (u n ifo rm o r stra ig h t slo p e) slo p e o f m o d ­ e ra te a n g le d u rin g m a tu rity to co n c a v e slo p e o f very lo w a n g le d u rin g old age. O n th e o th e r h an d , som e g e o m o rp h o lo g is ts h av e o p in e d th a t it is n o t alw ay s n e c e ssa ry th a t all th e slo p e s u n d e rg o p ro g re ssiv e d e c lin e in s lo p e g ra d ie n t th ro u g h tim e ra th e r som e s lo p e s m a in ta in th e ir a n g le s th ro u g h p a ra lle l retreat. A c c o rd in g to a d v o c a te s o f d y n a m ic e q u ilib r iu m th e o ry tim e p la y s no sig n ific a n t ro le in slo p e d e v e l­ o p m e n t b e c a u se th is p ro c e ss is tim e -in d e p e n d e n t a n d slo p e p ro c e s s e s p lay m o s t im p o rta n t ro le in th e ir d e v e lo p m e n t.

tio n s and slo p e fo rm s (i.e. slo p e fo rm s v a ry fro m one clim a tic ty p e to o th e r) b e c a u s e c lim a te d e te rm in e s th e n atu re o f d e n u d a tio n a l p ro c e s s e s . F o r e x a m p le , fin e m a te ria ls (d e b ris ) are p ro d u c e d d u e to m ore ac tiv e c h e m ic a l w e a th e rin g in h u m id c lim a te b e­

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c a u s e o f a b u n d a n c e o f ra in w a te r. O n th e o th e r hand, c o a rs e m a te ria ls (d e b ris ) a re p ro d u c d d u e to m o re in te n se m e c h a n ic a l w e a th e rin g in a rid c lim a te . B ut th e c ritic s o f th e c o n c e p t o f c lim a tic g e o m o rp h o lo g y (2 ) P r o c e s s-fo r m a p p r o a c h is based on the h av e re fu te d th is c la im an d m a in ta in th a t it is not co n cep t that there is direct relationship betw een n e c e ssa ry th a t d is tin c tiv e s lo p e s a re p ro d u c e d in slo p e typ es and slo p e form s, and geom orp h ic procd if f e r e n t c lim a te s . F o r e x a m p le , c lim a tic

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geomorphologists take pediments o f concave sur* face as representativ es o f arid and scmi-arid cli­

cause o f substantial increase in surface ru n o ff (com ­ ing from upslope) and debris w ith the resu lt the basal segm ent o f hillslope is subjected to m axim um ero­ sion w hich ultim ately form s co n cav e slope. T hus, the convexo-concave hill slope p rofile is the resu lt o f least erosion at the h ill-crest and m axim um erosion at the basal segm ent by surface runoff. T h is concept o f Fcnnem an suffers from the w eakness th at it does not accom m odate the process o f soil creep w hich is m ost dom inant in hum id regions because a sizeable portion o f rainw ater infiltrates in the p o ro u s and perm eable rocks and thus m otivates soil creep on one hand, and infiltration o f rain w ater lessens the intensity o f erosional w ork o f surface r u n o ff becau se of reduced (due to m ore infiltratio n ) ru n o ff and resultant overland flow on the o th er hand.

mates but such p ed im en ts are now also found in subhumid tropical as w ell as h u m id tem p erate clim ates In fact, except perig lacial clim ate d ifferen t co m b i­ nations o f all the fo u r elem en ts (convex, free face, rectilinear and co n ca v e) are found in alm ost all the climates. T his is w hy L .C . K ing p ropounded the concept o f c lim a tic u n ifo r m ita r ia n is m . It m ay be c o n c lu d e d th at slope form s arc co n t­ rolled not by a u n iq u e facto r rath e r these arc co n tro l­ led by several facto rs su ch as g eo logical structure, rock types, v eg etatio n c o v e r, d ifferen t types o f w eath­ ering, dip an g le, m ass w astin g and m ass m ovem ent of rockw astes (creep , slid e s, flow etc.), earth m o v e­ ments, reliefs etc. It m ay be pointed out that in certain e n v iro n m e n ta l c o n d itio n s one o f these fac­ tors m ay be m o st d o m in a n t but that cannot be the only factor o f slo p e d ev elo p m en t.

G. K. G ilbert (1909), on the o th er hand, at­ tached m ore im portance to soil creep in the d ev elo p ­ m ent o f convex slope o f such a h illslo p e p ro file w hich has a cover o f soils. T he w ater- so aked so ils It is now g en e ra lly b eliev ed that geom orphic m ove dow nslope. A s we m ove aw ay fro m h ill-to p processes ( o f w e a th e rin g , m assm o v em en t, erosion dow nslope the am ount o f soil to be m o ved d o w n ­ etc.), w ith o u t d o u b t, c o n tro l the form s o f slopes and w ard increases. Thus, there is m in im u m soil c o v er at their d e v e lo p m e n t b u t q u estio n arises as to w hether the hill crest w hile it is m axim um at the lo w er slope form is a fu n ctio n o f m o n o -p ro cess or p oly­ segm ent o f the slope b ecau se o f soil co m ing fro m processes. T h u s, th e re are tw o ap p ro ach es to the upslope. It is evident from fig. 15.4 th a t the a m o u n t study o f p ro c e ss-fo rm ap p ro ac h o f slope studies viz. o f soil to pass from point A d o w n w ard is m in im u m (i) m o n o -p r o c e s s c o n c e p ts , and (ii) p o ly -p r o c e s s w hile it is m axim um at p o in t C in a h illslo p e h av in g co n c e p t. a m obile soil layer. A cco rd in g to G .K . G ilb e rt the It m a y be m e n tio n e d th at sig n ifican t slope total rem oval o f soils from the h illslo p e m ay b e form ing p ro c e s s e s are s o il c r e e p (so liflu ctio n , earth po ssib le only w hen th e slo p e b e c o m e s s te e p e r creep, d e b ris c re e p ), s lid e s (earth slides, d eb ris slides dow nslope so that the slope load (so ils) m ay be etc.), f lo w s (e a rth flo w s, d eb ris flow s, m ud flow s), totally rem oved thro u g h the p ro cess o f soil creep . In slope ero sio n by o v erla n d flow , rain splash, rainw ash, such condition the h illslo p e assu m es co n v ex form so rill and g u lly e ro sio n etc. as to allow effectiv e m o v e m en t an d e v a c u a tio n o f (i) M o n o - p r o c e s s c o n c e p t is based on the debris from the slope. It m ay be p o in ted o u t th a t in basic te n et th a t a p a rtic u la r d en u d atio n al process or such situ atio n (in cre ased tra n sp o rt o f soil la y er process o f m a ss m o v e m e n t p ro d u ces a d istin ctiv e d o w n slo p e) there w o u ld be a c c u m u la tio n o f h u g e slope form . F o r e x a m p le , co n v ex ity and co n cavity am ount o f d eb ris at the slo p e b ase and h en ce the are c o n sid ere d to be the o u tc o m e o f soil creep and basal slope w ill b eco m e c o n ca v e ra th e r th an co n v ex . rainw ash resp ectiv ely . A cco rd in g to N .M . F ennm an G ilb ert's a ssu m p tio n , th a t soil la y er on slo p e p ro files (1908) co n v e x o -c o n c a v e h illslo p es are the resu lt o f is alw ay s m o b ile, is n ot a lw ay s true b eca u se it is no t d ifferential actio n s o f su rface ru n o ff and resu ltan t alw ay s n ecessary th at soil la y e r on h illslo p e , e v en in o verland flow i.e. the crestal areas o f h illslo p e are hum id reg io n s, is m o b ile. le ast ero d ed by su rface ru n o ff becau se o f lim ited am ount o f w ater (due to lim ited space) and ero sio n A cco rd in g to A .C . L a w so n (1 9 3 2 ) su m m ita l tools and hence rain w ash only sm o o th en s the hill convexity o f h illslope, like N .M . F en n em an , is fo rm ed crest so as to give it co n v ex plan but the in ten sity o f by rain w ash e ro sio n b u t L a w so n 's c o n c e p t d iffe rs erosional w ork o f ru n o ff in creases d o w n slo p e b e ­ https://telegram.me/UPSC_CivilServiceBooks

sig n ifican tly fro m F e n n e m a n 's c o n c e p t a s th e fo rm e r

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Fig. 15.4 : Relationship between soil creep and development o f convex slope (After G.K. Gilbert, 1909, in R.J. Sm all, 1970). the zone o f erosion. The radius o f the curvature o f longitudinal profile o f h illslo p e in creases b eca u se o f continuous d ep osition o f m aterials at the slo p e b ase com ing dow n from upslope. T h is resu lts in the

postulated m ost effe c tiv e rainw ash at and near h ill­ tops due to under-loaded surface runoff w h ile the latter (Fennem an) postulated least effectiv e rainwash at the h ill-top b ecau se o f less volu m e and velo city o f surface runoff. A ccord in g to L aw son erosion by surface ru n o ff at and near h ill-top s b eco m es m ost a ctive b ecau se surface ru n off carries le ss debris and h en ce there is m ore available energy o f surface ru n off for erosion w hich very e ffe c tiv e ly rounds the crestal part o f the hill g iv in g it co n v ex slo p e at the top w h ile the lo w er seg m en t o f the h illslo p e b e­ co m es co n ca v e due to d ep osition o f sed im en ts co m ­ in g d ow n from u p slop e segm en ts. A s per postulation o f L aw son there is tw o d istin ct zo n es in h illslo p e p rofile viz. 1. z o n e o f e r o s io n , and 2. z o n e o f a g g r a d a tio n (fig . 15.5). T he upper seg m en t is sub­ jected to m axim um erosion by rainw ash w herein lu n e shaped m aterials are rem oved from the crestal part and are transported d o w n slo p e and are d ep o s­ ited at the fo o t-h ill o f the slo p e. T he con tin u ou s rem oval o f m aterial through rainw ash results in the d ev elo p m en t o f sum m ital c o n v ex ity representing

form ation o f basal co n ca v ity . T h is lo w er zo n e is in fact erosion less zon e b eca u se o f the fa ct that, ac­ cording to L aw son , erosion al en ergy o f surface runoff decreases d o w n slo p e (in sp te o f d o w n slo p e increase in the v o lu m e o f ru n off) b eca u se the am ount o f debris in creases d o w n slo p e and h en ce surface runoff b eco m es o v erlo a d ed and slu g g ish . R .J. Sm all has rem arked, ‘A corollary o f L a w so n 's theory is, therefore, that w ith tim e r e lie f is d im in ish ed , d ivid es are w asted, and s lo p e a n g les stea d ily d e c lin e , m uch as en v isa g ed by D a v is ’ (R .J. S m a ll, 1 9 7 0 ). Several q u estio n s are raised again st the c o n c e p t o f L aw son . T h e m ost e ffe c tiv e ero sio n b y su rfa ce r u n o ff at the h ill-to p s in sp ite o f lo w v o lu m e o f w ater d u e to lim ited sp a ce, and lea st ero sio n or n o ero sio n at the slo p e b ase in sp ite o f in crea sed v o lu m e o f surface

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ru n off w ith h igh flo w v e lo c ity , as cla im ed by L aw son,

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275 HILLSLOPE

is every likelihood that m ore than one denudational processes act on slope p rofiles. H. B aulig postulated the poly-process co n cep t o f slo p e developm ent in Lawson, is not o f com m on occurrence, instead, 1950 and refuted th e co n cep t o f ‘one process-one active erosion (m ainly by rill and gully erosion) at slope f o r m ' (m o n o -p ro cess co n cep t). A cco rd in g to the slope base is responsible for the developm ent o f B aulig different d en u d atio n al p ro cesses o p erate in­ basal concavity because the erosive energy o f sur­ dependently on slo p e p ro file (they do not operate face ru n o ff increases dow nslope due to substantial together). It m ay be p o ssib le th a t a particular pw**** increase in the volum e and velocity o f runoff. It may m ay be m ost active on one seg m en t o f the h illslo p e be opined that concav ities m ay be depositional or w hile other process m ay be m o re activ e on th e o th e r erosional depending on local conditions. segm ent o f slope. Soil cree p (so liflu c tio n ) and (ii) P o ly -p ro c e ss c o n c e p t o f the study o f rainw ash have been reco g n ized by B au lig as m ost slope developm ent is n earer to reality because there cannot be easily digested. T he developm ent o f basal concavity by deposition o f debris, as claim ed by

Fig. 15.5 : Development o f convexo-concave slope by rainwash erosion at the hill-top and deposition at the slope base (after A. C. Lawson, 1932, in R.J. Small, 1970). and hence the process o f soil creep b eco m es slu g g ish (with tim e) but rainwash still con tin u es. T h is results in extension o f length o f co n ca v e unit u p slop e at the cost o f sum m ital co n v ex ity and ultim ately co n ca v e form spreads over entire h illslo p e. It appears from Baulig's con cep t that slo p e p ro cesses operate in opposite directions but this is not the truth b ecau se m ost o f denudational p ro cesses operate togeth er on h illslop e.

active slope form ing processes. Soil creep becom es more active on upper segm ent o f con vexo-con cave hillslope in humid tem perate regions with the result summital con vexity is d eveloped (due to soil creep). On the other hand, m ore rills and gu llies are d ev el­ oped due to large volum e o f water on the low er segm ent o f hillslop e and hence basal segm ent is subjected to intense fluvial erosion by rills and gullies w hile soil creep is significantly retarded, with the result basal concavity is form ed (due to fluvial erosion, sim ply called as rainwash). The relative significance o f soil creep (active on upper segm ent) and rainwash (active on low er segm ent) changes w ith tim e. There is marked low erin g o f relief (relief reduction) due to continued denudation https://telegram.me/UPSC_CivilServiceBooks

C o n tr a ry to th e a b o v e c o n c e p t s o m e g eo m o ip h o lo g ists have o p in ed that so il creep and rainw ash are n ot slo p e form ing p ro cesses rather they are agents o f transportation o f d eb ris p rod u ced b y w eathering o f slo p e m aterials.

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15.3 MODELS OF SLOPE EVOLUTION The origin (evolution) and developm ent o f hiHslopes and valley-sides slopes m ay be approached

in three w ays viz. (i) theoretically (theoretical m od­ els), (ii) experim entally (experim ental design mod­ els) and (iii) em pirically. A ny m odel or theory o f slope developm ent m ust have solutions o f a few im portant questions related to slope developm ent e.g. (i) w hether slopes undergo parallel retreat and m aintain their slope angles or (ii) there is progres­ sive decrease in slope angles w ith tim e, (iii) w heth­ er hillslopes are subjected to dow nw asting or backw asting etc. B ased on these tw o basic issues o f slope developm ent (i.e. parallel retreat and constant slope angle - dynam ic equilibrium , and progressive slope decline with tim e) there are tw o distinct schools o f thoughts w hich are based on con trasting view s o f W. Penck and W .M . D avis (fig. 15.6). S ignificant m odels (theories) o f slope developm ent include slope decline m odel o f W .M . D avis, slope rep lacem en t m odel o f W . Penck, slope evo lu tio n m odel o f A. W ood, hillslope cycle m odel o f L .C . K ing, co n ce p t o f slope developm ent by R .A . S av ig ear, co n ce p t o f

Y /m /m

Fig. 15.6 : Slope development according to W.M. Davis (A) and W. Penck (B).

Fig. 15.7:

Three hypotheses o f slope evolution : A. slope decline (progressive decline in slope angles as indicated by sequential profiles from 1 to 5), B. slope repl cment, C. parallel retreat in hillslope with scarps or free face, D. parallel retreat without free fa ce (after Young, 1972).

placem ent theory, and (3 ) parallel retreat theory

A .N . Strahler, p rocess-response m odel o f Fisher and Lehmann, Process-response model o f A. Young etc. F o llo w in g A . Y oun g (1 9 7 2 ) all the theories o f slop e d evelop m en t m ay be grouped in 3 broad cat­ eg o ries v iz. (i) slope decline theory, (2 ) slope re­

(fig. 15.7).

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'1. ‘Slope decline - the steepest part o f the slope progressively decreases in angle, accom panied by the developm ent o f a convexity and con cavity’.

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h il l s l o p e

2. ‘S lop e rep la cem en t - the m axim um angle decreases th ro u g h rep lacem en t from below by g en­ tler slopes, cau sin g the g reater part o f the profile to become occu p ied by the concavity. T he concavity may be eith er sm oo th ly curved or seg m en ted ’. 3. ‘P a ra llel retr ea t - the m axim um angle remains constant, the ab so lu te lengths o f all parts o f the slope ex ce p t the co n ca v ity rem ain constant, and the concavity in c re ases in len g th ; the angle at any (vertically co n stan t) p o in t on the concavity decreases. This type o f ev o lu tio n m ay be su b -d iv id ed into parallel retre at w ith a free face and w ith o u t a free face’. A. Y oung, 1972

(1) Slope decline theory of W.M. Davis D avis' th e o ry o f slo p e d eclin e has its roots in his essays on ‘the c o n v e x p ro file o f b ad lan d d iv id es’ (1892), ‘the g ra d in g o f m o u n ta in s lo p e s’ (1898), ‘the geographical c y c le ’ (1 8 9 9 ), ‘p ied m o n t bench lands and p rim a ru m p fe ’ (1 9 3 2 ) etc. L ik e the cyclic developm ent o f la n d sc a p e s D av is' h illslo p e and valleyside slope a lso u n d e rg o the p ro cess o f cyclic developm ent w h e re in th e re is p ro g ressiv e declin e in slope angle an d s e q u e n tia l c h a n g e in slo p e form from you th (c o n v e x fo rm ) th ro u g h m atu re (rectilin ­ ear or u n ifo rm slo p e fo rm ) to o ld (co n cav e form ) stages. S te e p c o n v e x slo p e ev o lv e s d u rin g youth stage o f cy cle o f ero sio n b ecau se o f active dow ncutting resulting in to v a lle y d e e p e n in g and w eath erin g p ro cesses. T h e re is v e ry lim ite d slo p e retre at and p ractically th e re is n o d e c lin e in slo p e an g le rath e r it

Fig. 15.8 : Sequential stages o f slope evolution accord­ ing to W.M. Davis. D avisian m o d el o f slo p e e v o lu tio n in c lu d es three aspects viz. ro u n d ed c o n v ex ity o f h ill to p s and interfluves, g ra d ed w a ste sh eets on slope p ro ­ files and g rad ed va lley sid es, w h ich D av is trie d to explain in term s o f th eir sig n ifican ce an d o rig in . A ccording to D avis su m m ital ro u n d e d c o n v ex ity results from the action o f so il cre ep in h u m id clim ate. ‘H e d escrib e d q u alita tiv e ly th e m e c h a n ism o f m o v em en t in creep , by a lte rn a te d ia la tio n an d co n tractio n o f th e soil u n d e r th e in flu e n c e o f g rav ity , and co rrectly in te rp re te d o u tc ro p c u rv a tu re ’ (A . Y o ung, 1972). S oil c ree p is m o tiv a te d b e c a u se o f rain w ash the in ten sity o f w h ich in c re a se s d o w n slo p e. ‘R easo n in g on p rio ri g ro u n d s th a t su rfa c e w ash in creases in v o lu m e d o w n slo p e , h e s u p p o se d th a t, n ear d rain a g e d iv id e s, th e ra tio o f c re e p to w ash is large. C reep (soil c re e p ) p ro d u c e s ‘ro u n d e d c o n ­ to u rs ’, and is re s p o n s ib le fo r c o n v e x p ro file o f d i­ v id e s ’ (A . Y o u n g , 1972).

is in c re a se d (fig s. 15 .8A an d 15.9). L ateral ero sio n d o m in ates o v e r v e rtic a l ero sio n an d d iv id e su m m its are ero d e d (d o w n w a s tin g o f w a te r d iv id e s) d u rin g m ature sta g e . T h u s, d o w n w a s tin g o f w a te r d iv id es results in d e c re a s e in slo p e a n g le (an d h e n ce slo p e d ecline) an d slo p e p ro file o f sm o o th cu rv e is fo rm ed . Slope b e c o m e s g ra d e d (fig s. 15.8B an d 15.9B ) b e ­ cause at e a c h p o in t on th e s lo p e p ro file th e g ra d ie n t is such th a t w e a th e re d d e b ris m a y be tra n sp o rte d

T h e w e a th e re d m a te ria ls e x is tin g o n slo p e p ro file s (b o th h ills lo p e an d v a lle y s id e s lo p e s ) h a s b een te rm e d as w a s te s h e e t w h ic h c o n s ta n tly m o v e s d o w n slo p e by th e a g e n ts o f tra n s p o rta tio n u n d e r th e in flu e n c e o f g ra v ity fo rc e . T h e n a tu re , ra te a n d m a g n itu d e o f d o w n s lo p e tra n s fe r o f d e b ris d e p e n d s

(rem oved) d o w n s lo p e . B e c a u s e o f c o n tin u e d lateral erosion a n d d o w n w a s tin g o f w a te r d iv id e s (and hence m a rk e d lo w e rin g o f re lie f) th e re is m a rk e d slope d e c lin e in o ld s ta g e so th a t th e g e n e ra l slo p e form b e c o m e s c o n c a v e a n d n o w h e re s lo p e an g le

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becom es m o re th a n 5 d e g re e .

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OEOMORPHOLOOV

Fig. 15.9 : Sequential stages o f evolution o f valley side slope according to Davis (after, R.J. Rice, 1977). on the n atu re and am o u n t o f d eb ris and tran sp o rtin g cap acity o f d en u d atio n al p ro cesses. W hen the tran s­ p o rtin g cap a city o f the tra n sp o rtin g ag en ts (actual a v ailab le en erg y o f tra n sp o rtin g ag en ts) eq u als the req u ire d e n erg y (fo rce) to tra n sp o rt the m a teria ls i.e. w hen the a v a ila b le en erg y fo r tran sp o rta tio n o f d e ­ b ris an d the w o rk to be d o n e (d eb ris to be tran sp o rte d d o w n slo p e ) are e q u a lly b alan ced , the lay er o f w aste sh e e t o f d e b ris on slo p e p ro file is called ‘g r a d e d w a s te s h e e t ’. In y o u th fu l stag e u p p er slo p es are so s te e p th a t a v a ila b le en e rg y (tra n sp o rtin g cap acity ) fa r e x c e e d s th e w o rk to be d o n e (d eb ris to be m o v ed d o w n slo p e ) and h e n c e d e b ris is q u ic k ly rem o v e d a n d tra n s p o rte d d o w n slo p e b e c a u se o f th e fact th at

co v ered w ith slu g g ish ly m o v in g ro c k w aste. D av is has d efin ed g rad ed w aste sh e e t as fo llo w s, ‘a g rad ed w aste s h e e t........ is o ne in w h ich th e ab ility o f the tran sp o rtin g fo rces to do w o rk (to tra n s p o rt) is eq u al to the w ork th a t th ey h av e to d o .........fro m an y p o in t on such su rface a g rad ed slo p e le a d s th e w aste dow n to the stream s. A t any p o in t th e a g e n c ie s o f rem o v a l are ju s t able to c o p e w ith th e w a ste th a t is th ere w eath e red p lu s th a t w h ich c o m e s d o w n fro m fu rth e r u p h ill’ (W .M . D av is, 1899). T h u s, th e re is b alan ce b etw een su p p ly ,o f d eb ris an d re m o v a l o f d e b ris at all p o in ts o f g rad ed slo p e p ro file . H e h as fu rth e r o b s e rv e d th a t in the in itial stag e g rad ed slo p e p ro file h as s te e p e r g ra d ie n t and thin v en ee r o f c o a rse d e b ris (w a ste ) b u t as th e stag e o f c y c le 'o f e ro sio n a d v a n c e s th e g ra d ie n t o f g ra d e d slo p e p ro file d e c lin e s , d e b ris b e c o m e s fin e r and d e b ris th ic k n e s s in c re a se d . S im ila rly , th e g rad ie n t o f

m a te ria ls s u p p lie d th ro u g h w e a th e rin g and ero sio n o f u p s lo p e s e g m e n t a re fa r le ss th an the tra n sp o rtin g c a p a c ity o f th e d e n u d a tio n a l p ro c e s s e s d u e to steep g rad ie n t' o f slo p e . It m a y be p o in te d o u t th a t c o n d i­ tio n o f g ra d e d w a ste sh e e t b e g in s fro m th e b ase o f th e s lo p e a n d g ra d u a lly p ro c e e d s u p slo p e . T h e e n tire s lo p e p ro file is g ra d e d (i.e. tra n s p o rtin g c a p a c ity e q u a ls th e to ta l a m o u n t o f d e b ris to be tra n s p o rte d d o w n s lo p e th ro u g h o u t th e p ro file le n g th ) by th e a tta in m e n t o f o ld s ta g e o f c y c le o f e ro s io n a n d the w h o le s lo p e p ro file fro m h ill to p to th e b a se is https://telegram.me/UPSC_CivilServiceBooks

v alley sid e slo p e a lso d e c re a se s as th e stag e o f c y cle o f e ro s io n a d v a n c e s w ith tim e (fig . 15.9). T h e v a lle y s id e slo p e is d o m in a te d by s u m m ita l c o n v e x ­ ity (in th e u p p e r p art o f th e v a lle y sid e s ) a n d b asal c o n c a v ity . W ith th e a d v a n c e m e n t o f c y c lic tim e (s ta g e s ) th e ra d iu s o f th e c u rv a tu re o f v a lle y sid e

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HILLSLOPE

tion . A cco rd in g to P en ck th e m o d e o f o p eratio n o f exogenetic p ro cesses d ep en d s o n th e c h ara cteristics o f reg o lith s on hillslo p e, m e ch an ism o f w eath e rin g and su rface pro cesses. H e attach e d m o re im p o rtan ce to the p ro cess o f m ass m o v e m en ts in c o m p a riso n to other su rface p ro cesses. H e h as an aly se d th e m e ch a­ nism o f p ro cesses w ith so u n d re a so n in g an d h as attem pted to estab lish rela tio n sh ip b etw een m e ch a­ nism o f p ro cesses and ch a ra c te ristic s o f fo rm s. A c ­ cording to P en ck th e m e ch an ism o f su rfa c e p ro c ­ esses is affected and c o n tro lled by th e p ro p e rtie s o f form s and in turn th e la tter a lso u n d e rg o c h a n g e s an d transform ation. P en ck has d e sc rib e d se v e n p ro p e r­ ties o f form s and 3 p ro p erties o f p ro cesses.

slope profile increases because o f gradual increase in the length o f convex and concave segm ents. T his again denotes flattening o f slope p rofile and resu lt­ ant decline in slope angle (fig. 15.9). L ike cycle o f ero sio n D av is’ m odel o f slope evolution also depen d s on crustal stability for longer duration afte r the in itiatio n o f cycle. A. Y oung has rem arked that, ‘the D avisian m odel has, how ever, great flexibility co m b in ed w ith in tem al.logical co n ­ sistency, w hich ren d ers it un assailab le by critical attacks w ith its ow n w eapons, those o f abstract q u alitativ e re a s o n in g ... D av is'th e o ry is a clear state­ m ent o f a type o f slo p e ev o lu tio n , and is cap ab le o f being tran slated into q u an titativ e term s. C o n so n an t w ith its rela tio n to the cy cle o f ero sio n , the theory im plies th at slo p e form is tim e-d ep en d en t; possibly this is the featu re th a t o ffers an o p p o rtu n ity o f in d irect te stin g by field o b se rv a tio n ’ (A. Y oung, 1972)

Properties of Forms 1. D eg ree o f r e d u c tio n o f r e g o lith m e a n s degree o f b reak in g o f reg o lith s in to fin e m a te ria ls . It m ay be m en tio n ed th at fine te x tu re d soil h as h ig h e r degree o f red u ctio n than co a rse te x tu re d sto n y so ils. The degree o f red u ctio n (c o m m in u tio n ) d e p e n d s on the rate o f w eath erin g an d d e n u d a tio n . It m a y b e m entioned th at P en ck u sed d e n u d a tio n as th e p ro c ­ ess o f rem oval (tra n sp o rta tio n ) o f slo p e d e b ris an d not as erosional p ro cess.

(2) Slope Replacement theory of W. Penck It m ay be m entioned, at the outset, that W alther P en ck is m o st m isu n d e rsto o d g eo m o rp h o lo g ist o f the w orld b eca u se o f his (i) in co m p lete w ork due to h is u n tim ely d eath , (ii) his o b scu re com p o sitio n in d iffic u lt G e rm a n lan g u ag e, (iii) ill d efin ed te rm in o l­ o gy, (iv ) m isle a d in g rev ie w by W .M . D avis, (v) fau lty tra n sla tio n in E n g lish language, (vi) som e c o n tra d ic to ry id e as etc. P rev io u sly P en ck 's co n cep t o f slo p e d e v e lo p m e n t w as w id ely u n d ersto o d as ‘p a r a lle l r e tr e a t o f s lo p e ’ but his real co n cep t on slo p e e v o lu tio n c a m e to lig h t, as slo p e rep la cem en t m o d e l, a fte r E n g lis h tran sla tio n o f P en ck 's w ork i.e. ‘D ie M o rp h o lo g isc h e A n a ly s e ’ by C zech and B osw ell and ag ain by M . S im o n s in 1962.

2. M o b ility o f r e g o lith is th a t p ro p e rty o f reg o lith w hich d eterm in e s d o w n slo p e m o v e m e n t o f slope m aterials by d e n u d a tio n a l p ro c e s s e s ( o f m a ss m o v em en t). H ig h er the m o b ility o f re g o lith , m o re th e d o w n s lo p e tr a n s f e r o f s lo p e m a te r ia ls b y d en u d atio n al p ro cesses an d v ic e v ersa. M o b ility d ep en d s on ro ck p ro p e rtie s an d d e g re e o f re d u c tio n . *3. T h ic k n e s s o f r e g o lith m e a n s th ic k n e s s o f m o b ile o r static c o v e r o f lo o se m a te ria ls o n h ills lo p e and it is d irec tly re la te d to d e g re e o f re d u c tio n ( th e h ig h e r th e d e g re e o f re d u c tio n o f r e g o lith th e th ic k e r th e reg o lith an d v ise v e rsa ) a n d is in v e rs e ly re la te d to rates o f d e n u d a tio n a l p ro c e s s e s (th e h ig h e r th e rate o f d e n u d a tio n a l p ro c e s s i.e. th e h ig h e r th e ra te o f rem o v a l o f slo p e m a te ria ls d o w n s lo p e , th e th in n e r th e c o v e r o f re g o lith a n d v ic e v e rs a ).

T h e m a in g o a l o f P e n c k 's m o rp h o lo g ic a l sy s­ tem w as to fin d o u t th e m o d e o f d e v e lo p m e n t and causes o f cru sta l m o v e m e n t on the b asis o f ex o g cn etic p ro cesses an d m o rp h o lo g ic a l c h a ra c te ristic s. In o th er w ords, P en ck so u g h t th e in te rp re ta tio n o f d ia stro p h ic h istory o f re g io n s on th e b a sis o f th e in te rp re ta tio n o f p resen t-d a y la n d fo rm c h a ra c te ristic s . T h e r e fe r e n c e sy stem o f P e n c k 's m o d e l is th a t th e c h a ra c te ris tic s o f la n d fo rm s o f a g iv e n re g io n are re la te d to the te c ­ to nic a ctiv ity o f th a t reg io n .

4. E x p o s u r e o f s lo p e s u r fa c e m e a n s d e g re e to w h ich th e ro c k s u n d e r re g o lith a re u n c o v e re d a n d a re m a d e a v a ila b le fo r re d u c tio n b y w e a th e rin g . E x p o ­ su re o f ro c k s u n d e r re g o lith is in v e rs e ly r e la te d to the th ic k n e s s o f re g o lith .

5. Rock property includes all those aspects of lithology and geological structure which influence the degree of reduction and mobility o f regolith. https://telegram.me/UPSC_CivilServiceBooks

P e n c k 's ‘m o rp h o lo g ic a l s y s te m ’ h a s tw o im ­ p o rta n t p a rts viz. ( 1) m a n n e r (m o d e ) o f a c tio n o f p r o c e sse s, a n d (ii) d e d u c tiv e m o d e l o f s lo p e e v o lu ­

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esses). It dep en d s on the rate o f d enudation o r rate o f rem oval o f m aterials from slope surface (direct relationship). It m ay be clarified that renew al o f exposure is not a process in itse lf b ut is the result o f rem oval o f regoliths becau se bare rock surface, after the regolith m aterials are rem o v ed and are trans­ ported d o w nslope, is ex p o sed to w eathering p ro c­ esses.

6, C lim ate - O nly those aspects o f clim ate are considered w hich influence the rate o f reduction and denudation (rem oval o f m aterials). 7. Slope a n g le becom es independent variable in the initial stage o f slope evolution but later on it becom es partly dependent on the rate o f denudation.

Properties of Processes 1. R ed u c tio n m eans m ode o f breaking o f regolith into fine particles. The m echanism o f co m ­ m inution o f regolith is con tro lled by w eathering p rocesses w hile the rate o f breaking o f regolith depends on rock characteristics, clim ate and ex p o ­ sure o f slope surface o f solid rocks.

T hough W . P en ck has d escrib e d in terrela­ tionships betw een the p ro p erties o f fo rm s an d p ro c­ esses but A. Y oung has fu rth er a tte m p te d to ratio n ­ alize these in terrelatio n sh p s afte r m a k in g certain m odifications and has tran sfo rm e d P en ck 's m o d e l o f slope evolution into p r o c e ss-r e sp o n se m o d el o f slop e evolu tion (fig. 15.10).

2. D e n u d a tio n is related to rem oval (trans­ p o rt) o f regolith m aterials dow nslope. T he rate o f d enudation o f m aterials (rem oval o f m aterials) is dependent on the rate o f w eathering, m obility o f regolith. clim ate and slope angle.

B a sic P rem ises of P e n ck 's Model

(1) T he form o f h illslo p e is d e p e n d e n t on th relativ e rates o f v ertical ero sio n by s tre a m s a t th e slope base and d en u d atio n (rem o v a l o r tra n s p o rta ­ tion o f slope d eb ris d o w n slo p e o r fro m th e slo p e base).

3. R en ew al o f ex p o su re m eans uncovering o f solid ro ck surface, u n d er regolith, and its ex p o ­ sure to the processes o f reduction (w eath erin g p ro c­

^ R ED U C TIO N

MOB ILI TY^

A I _ I i ( r

CffT x Ep o su R E F

d en u d atio T T )—

c |

A

I

-f-

S L O P E ANGLE

| Independent variable

) Dependent variable Mobility property of process Denudation property ot process * Relation not direct

(

j) \o

---- > Short term effect

+ Direct relation

---- > Medium term effect

- Inverse relation

c= £ > Lo n g term effect

4 Differential relation O Non-parametric relation

Fig. 1 5 .1 0 . D iagram atic representation o f the system o f interaction betw een p ro c esse s a n d fo r m p ro p erties a s described https://telegram.me/UPSC_CivilServiceBooks

b y W. P en ck m the fo r m o fp ro c e ss-re sp o n se m o d el b y A. Young (1972).

*

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281 fdULSLOPE

lesser duration. S uch effects are called sh o rt-term effects. T he factors affecting reg o lith s are o f d iu m -term effects (1 ,0 0 0 to 10,000 y ears) w h ile th e factors affecting slopes are o f lo n g -term effects (10,000 to 100,000 years). Y oung has fu rth er su g ­ gested that for the testin g o f P en ck ian m odel on quantitative base ro ck p ro p erties sh o u ld be d iv id ed into tw o independent variab les o f ro ck p ro p erty viz. (i) those w hich in fluence d eg ree o f red u ctio n and (ii) those w hich d eterm ine m ob ility o f reg o lith . S im i­ larly, clim atic variables should be g ro u p ed in to tw o independent variables viz. (i) th o se w h ich in flu en ce degree o f reduction and (ii) th o se w h ich in flu en ce rate o f denudation (rem oval o f w eathered m aterials).

(2) The form o f hillslope is not directly con­ trolled by slope processes because these act as agents of removal (transportation) o f w eathered slope de­ bris dow nslope. The m ajor role o f denudational processes (transportational processes) is to expose bare rock slope surface (by rem oving slope debris) for w eathering processes. (3) The retreat o f slope unit backw ard de­ pends on the gradient o f hillslope. S teeper gradient facilitates m ore active retreat than gentle gradient. (4) T he slope retreat occurs in parallel m anner which results in the replacem ent o f low er segm ent of slope profile by new unit o f gentle gradient. (5) ‘F lattenin g o f slopes alw ays takes place from below u p w ard ’.

Explanation of Penck's Model

(6) T he form o f valley side slopes o f stream s depends on 3 rates o f stream erosion i.e. 1. acceler­ ating rate o f erosion, 2. d ecelerating rate o f erosion, and 3. co n stan t rate o f erosion, w hich produce co n ­ vex, concave and rectilinear slope form s respec­ tively.

(Deductive Model of Slope Evolution)

For the ex p lan atio n o f ev o lu tio n o f hill slo p e Penck selected a steep rock c liff o f h o m o g e n e o u s com position. The up p er surface o f the slo p e u n it is surm ounted by level su rface (fig. 15.11, A '— F '). There is a river at the foot o f the slo p e w h ich is neither eroding nor d ep o sitin g b u t is c a p a b le o f rem oving all the m aterials c o m in g at the fo o t-h ill from upslope seg m en ts (fig. 15.11(A )) ‘In a u n it tim e a superficial layer o f rock, o f a d e fin ite th ic k ­ ness the sam e every w here, is lo o sen ed an d rem o v e d . The method o f rem oval is that loosened particles o f rock crum ble aw ay and fall dow n. F o r this to happen the gradient must be too great to allow the little pieces of rock, just loosened by w eath e rin g b u t n o t fu rth e r co m m in u ted and red u ced , to rem a in at rest. T h is gradient is av ailab le for each u n it o f ro c k face e x c e p t the lo w est’ (P en ck , 1924). T h u s, th e slo p e face except the lo w est seg m en t (A-B, fig . 15.11 (A) und erg o es p arallel re tre a t d u e to u n ifo rm ra te o f w eath erin g and in sta n ta n e o u s re m o v a l o f w e a th ­ ered m a teria ls fro m slo p e se g m e n ts. T h e lo w e s t seg m en t d o es n o t e x p e rie n c e p a ra lle l re tre a t b e c a u se its slo p e an g le is n o t su c h th a t it m a y atta in re q u ire d m o b ility w h ich m ay h e lp in th e re m o v a l o f w e a th ­ ered m a te ria ls. T h u s, th e lo w e s t s e g m e n t o f in itia l c lif f slo p e fa c e (A - A', fig . 15.11(A) is re p la c e d by a n ew (y o u n g ) u n it o f g e n tle slo p e a n g le (A-B) a n d th e c lif f slo p e p ro file n o w c o n s is ts o f tw o slo p e u n its viz. A-B an d B- B' (fig . 15.11 (A a n d B).

(7) T here is uniform rate of w eathering o f rocks o f slopes. (8) A s Ihe required m obility is attained, the rem oval o f w eathered m aterials begins and the rate o f rem oval (d en u d atio n ) m atches with the rate o f w eathering. (9) T h ere are an g u lar breaks o f slope (breaks o f g ra d ie n t) h av in g b ase-lcv els and the developm ent o f g rad ien t o f slo p e seg m en t is independent o f low er basal lev els o f break s o f g radient. A. Y oung (1972) has rem ark ed , ‘Pcnck d o es not say, as som e c o m ­ m en tato rs h av e assu m ed he does, that a sm oothlya cceleratin g rate o f ero sio n w ill produce an angular b reak o f s lo p e .’ A. Y o u n g (1 9 7 2 ) afte r m ak in g certain m o d i­ fications in the re la tio n sh ip s, in terrela tio n sh ip s and interactions in the p ro p e rtie s o f fo rm s and p ro cesses, as described by P enck , has p resen ted P en ck ian m odel in the form o f 4p ro cess-resp o n se m o d e l’ (fig. 15.10). F irstly, he d iffere n tiated b etw een d e p e n d e n t and in d ep en d en t v ariab les. S eco n d ly , he id e n tified th ree closely rela ted pairs o f p ro p erties e.g. (i) d eg ree o f red u ctio n and m o b ility , (ii) th ick n ess o f re g o lith an d exposure, and (iii) d en u d atio n al p ro cesses and re ­ new al o f ex p o su re. T he effects on the ra te o f p ro c ­ esses becom e o p eratio n al fo r the w h o le y e a r o r fo r

The same process is repeated during second

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time interval all along the slope except A- B' scg

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(profile B in fig. 15.11 (B ). T h e sam e process is rep eated durin g th ird tim e interval (uniform rate o f w eath erin g and p arallel retreat) and th e initial slope profile (A -A ‘) reach es th e p o sitio n o f D -D ' and gentle slope seg m en t ex ten d s from A to D due to p arallel retreat. D u e to rep etitio n o f the sam e m echa­ nism durin g su cceed in g stages o f tim e intervals the po sitio n s o f slope p ro files shift to E - E \ F - F ... and so on and lo w est g entle slope se g m e n t extends grad u ally from A to A -B , A -C , A -D , A -E , A -F p o s itio n s ........ and so on. T h u s, sm all seg m en ts o f new basal slopes i.e. A -B , B -C , C -D , E -F co m b in e to g eth er and form a co n tin u o u s slo p e o f u n ifo rm grad ien t (A -F). ‘T his is the b asal slope ... the fo llo w ­ ing statem en t m ay th erefo re be m ad e; a steep ro ck face left to itself, m oves b ack u p slo p e, m a in tain in g its original grad ien t; and a b asal slo p e o f le sse r g radient develops at its ex p en se' (P en ck ). It m ay b e pointed out th at step p ed slo p e p ro file d u e to p a ra lle l retreat o f ro ck face o f th e slo p e as sh o w n in fig . 15.11 (A ) is not p o ssib le in reality . ‘It re su lts m e re ly fro m the arbitrary d iv isio n o f slo p e d e v e lo p m e n t in to ‘given in terv als o f tim e ’. In th e field th e slo p e w o u ld be rectilin ear one, and w o u ld b e o c c u p ie d by u n i­ form ly th ick la y e r o f w e a th e re d d e tritu s m o v in g dow n from the b ase o f th e o rig in a l slo p e u n it to the riv e r’ (R .J. S m all, 1970). In fact, the parallel retreat o f slo p e segm en ts except the lo w est segm ent, ultim ately results in the developm ent o f con cave slo p e profile (sh ow n by dotted line in fig. 15.11 (B )). If the parallel retreat is occurring on both the sid es o f an interfluve, then after the rem oval o f free face or steep segm en ts on both the sid e s the in te r flu v e is s u b je c te d to dow nw asting and thus there is su c c e ssiv e low ering o f altitude (a to b, b to c, c to d, fig . 1 5 .1 1(C )) and consequent slop e flattening results in slo p e d eclin e. Penckian m odel o f slop e d evelop m en t is nearer to reality in the field . If w e look at Bhander plateau

Fig. 15.11: Parallel retreat and slope replacement model o f Penck (M odified after W. Penck, also in R.J. Small, 1970).

(M .P., fig. 3.8►) it clearly appears that the upper free face segm ent o f m a ssiv e sandstones o f Bhander

m en tan d thus the n ew slo p e face (B - B ') undergoes parallel retreat and reaches the p osition o f C -C (in fig. 15.11 (A ) and B in 15.11 (B ). The B -C segm en t

escarpm ents is under the p rocess o f parallel retreat and the low er seg m en t o f rectilinear slo p e ( o f shales) is exten d in g u p slope. T he parallel retreat is com p lete

does not exp erien ce parallel retreat due to lack o f required degree o f m ob ility and thus non -rem oval o f

over Sharda Pole hill (fig. 3.8) and hence dow nw asting has resulted into the d ev elo p m en t o f co n v ex o -co n https://telegram.me/UPSC_CivilServiceBooks

m aterials, w ith the result the lo w e st seg m en t (A -B ) is again replaced by n ew unit o f further g en tle slo p e

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inclination extends upslope. A new slope unit o f still gentle gradient develops at the base o f initial basal slope. Later on, this slope unit is also replaced by new slope unit o f further gentle gradient. T his m echa­ nism repeats itself and thus a new slope unit o f gentle slope is produced at the base o f slope segm ent. T hus, ‘flattening o f slope alw ays occurs from slope base and proceeds u p slo p e’.

A

A. Y oung (1972) has pointed o ut tw o errors in this second m odel o f Penck e.g. (i) the ap plication o f the m echanism o f rockfall and instan taneous re ­ moval o f debris from c liff slope to the reg o lith covered slope is unrealistic, and (ii) P en ck's assu m p ­ tion that all parts o f basal slopes o r its la ter re p la c e ­ ment slopes are equally ex p o sed to w eath erin g is erroneous. It m ay be m en tio n ed th at p erio d o f e x p o ­ sure is m axim um at the lo w est seg m en t o f slo p e profile and decreases upslope and b eco m es zero a t the intersection p o int o f upper slo p e segm ent.

e

c

Penck has also pro p o sed a m o d el o f d e v e lo p ­ m ent o f slope form s on the b asis o f rate o f stream erosion. In the case o f acceleratin g rate o f stream erosion, the valleyside slope beco m es co n v ex w h ile decelerating rate o f erosion cau ses the d e v e lo p m e n t of concave valley side slope. O n the o th e r h an d , constant rate o f stream ero sio n resu lts in the d e v e l­ opm ent o f rectilin ear slo p e (fig. 15.12).

Fig. 15.12 : P enck’s model o f slope development. A. Con­ cave slope form due to decelerating rate of stream erosion, B. Constant or rectilinear slope due to constant rate o f erosion, C. convex slope due to accelerating rate o f ero­ sion. A - E = successive river positions.

Evaluation

It m ay be em p h asised th a t ‘the in itial n e g lect, subsequent m isinterpretations (by W .M . D av is, faulty translation in E n g lish ) and b e lated re c o g n itio n o f Penck's w o rk ’ (A . Y o u n g , 1970) has g iv e n b irth to num erous co n tro v e rsies re g a rd in g P e n c k 's d e d u c ­ tive m odel o f slo p e ev o lu tio n . A c c o rd in g to Y o u n g , P enck no w h ere m e n tio n e d a n g u la r b re a k o f slo p e in his orig in al w ritin g s in the c a se o f a c c e le ra tin g ra te o f stream ero sio n . It m ay be th a t in c a s e o f a b ru p t increase in the rate o f e ro sio n c o n v e x b re a k o f g rad ien t is p ro d u ced , ‘th e ste e p e r slo p e b e lo w it w ill w ork p ro g ressiv ely u p slo p e , re p la c in g the g e n tle r unit a b o v e ’ (A . Y o u n g , 1972).

P en ck has a lso ap p lied his m odel to explain the d e v e lo p m e n t o f d e b ris co v ered slope. R ed u ctio n begins at the basal slo p e and co n tin u es till the entire basal slo p e a tta in s such m o b ility that the debris resting on the slope b eco m es m obile. T he req u ired m obility (fo r the rem o v a l o f d eb ris) on basal slope is m uch h ig h er than on the c lif f slo p e becau se the g radien t o f the fo rm er is very low in co m p ariso n to the latter. W ith the atta in m e n t o f this situ atio n (re ­ q uired m o b ility at the basal slo p e) the ro ck d eb ris b egins to m o v e d o w n w a rd on all seg m en ts o f slope profile. T h e d eb ris d o es not m ove from the lo w est

It is not c le a r fro m th e c lo se p e ru s a l o f th e in te rp re ta tio n s o f P e n c k 's m o d e l by so m e s c ie n tis ts as to w h e th e r slo p e d e v e lo p m e n t ta k e s p lace in d iscrete stag es, ‘o r w h e th e r th is w as ju s t a d e v ic e to be ad o p ted fo r e x p la n a to ry p u rp o s e s , th e s ta g e s

gradient. T h u s, the basal slo p e h av in g its o rig in al

b ein g re g a rd e d as in fin ite ly s m a ll’ (Y o u n g ). It is a lso

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slope seg m en t b eca u se o f lack o f su ffic ie n t slo p e

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n o t c le a r w h e th e r slo p e p ro file , d u rin g its e v o lu tio n ­ a ry s ta g e s , c o n s is ts o f se v e ra l in te rs e c tin g re c tilin e a r s e g m e n ts (fig . 1 5 .1 1 (B )) o r it b e c o m e s co n c a v e p ro file . ‘T e x tu a l p a s s a g e s can b e fo u n d in su p p o rt o f e ith e r in te rp re ta tio n o f h is m e a n in g ; on b a la n c e the la tte r a lte rn a tiv e se e m s in d ic a te d , an d is su p p o rte d by th e d ia g ra m o f slo p e e v o lu tio n in h is la ter p a p e r on th e B la c k F o re s t (A . Y o u n g , 1972). H. M o rten sen (1 9 6 9 ) h as e x p re s s e d d o u b t a b o u t the p a ra lle l retreat o f c lif f ro c k fa c e by u n ifo rm rate o f w ea th e rin g . It m a y be m e n tio n e d th a t P e n c k 's m o d e l b eco m es v alid w h e re th e re is c o n tin u o u s an d in sta n ta n e o u s re ­ m o v a l o f re g o lith s fro m slo p e b u t th e m o d el b e ­ c o m e s in v a lid w h e re re m o v a l o f re g o lith o ccu rs in s ta g e s (M o rte n s e n ). A . Y o u n g (1 9 7 2 ) h as e v alu ate d P e n c k 's m o d e l a n d h as re m a rk e d , ‘G iv en th ese q u a li­ fic a tio n s , P e n c k ’s m o d e l p re se n ts a h y p o th e sis o f slo p e ev o lu tio n in w h ich g entle slope succeeds steeper, b u t in a m a n n e r v ery d iffe re n t fro m th e D av isian s c h e m e ........... In P e n c k 's sy stem p arallel re tre a t o nly ta k e s p la c e on an in itia l re c tilin e a r slo p e; su ch a slo p e is tra n s fo rm e d in to a c o n c a v e fo rm rela tiv e ly e a rly in its e v o lu tio n a ry h isto ry , and d e v e lo p m e n t th e n p ro c e e d s by su c c e s s iv e re p la c e m e n t fro m b e ­ lo w . I f th is re p la c e m e n t is n o t in d isc re te sta g e s b ut c o n tin u o u s , th en no p a rt o f the c o n c a v e slo p e re ­ tre a ts p a ra lle l to i t s e l f (A . Y o u n g , 1972).

o f free face d e c re a se s). T h u s, the lo w e r segment o f free face b u ried u n d e r d e b ris c o v e r b eco m es con ­ sta n t slo p e . A c o n v ex ro ck slo p e is fo rm ed under d e b ris co v e re d c o n s ta n t slo p e. T h is sim p le co n d itio n m ay get d is tu rb e d w h en a stre a m b e c o m e s activ e on d e b ris c o n s ta n t slo p e. D u e to stream actio n w eath­ ered m a te ria ls are re m o v e d from th e lo w e r segm ent o f slope. A cco rd in g to W o o d ’s assu m p tio n the stream s are c o m p e te n t e n o u g h to re m o v e all the m aterials co m in g from u p slo p e s e g m e n t o f free face d u e to w eath e rin g . In o th e r w o rd s, th e re is b a la n c e b e­ tw een th e a v a ila b le e n e rg y (tra n s p o rtin g c a p a c ity o f stre a m s) an d th e w o rk to be d o n e (w e a th e re d d e b ris to be rem o v e d ). T h is situ a tio n ( o f e q u ilib riu m ) leads to the d e v e lo p m e n t o f c o n s ta n t d e b ris slo p e i.e. the len g th o f d e b ris slo p e re m a in s c o n sta n t. T h e re g u la r re tre a t o f free face th ro u g h b a c k w a s tin g re s u lta n ts in re g u la r e x te n sio n o f c o n s ta n t slo p e b u t no d e b ris can rest on this e x te n d e d c o n s ta n t slo p e , w h ic h is c a lle d d e n u d a tio n a l c o n s ta n t s lo p e (fig . 1 5 .1 3 B ). T h u s, c o n sta n t slo p e c o n s is ts o f tw o s e g m e n ts e .g . (i) lo w e r a g g r a d a tio n a l s e g m e n t (fig . 1 5 .1 3 B ) a n d (ii) d e n u d a tio n a l s e g m e n t (fig . 1 5 .1 3 B ) w h ic h is a lso ca lle d as s lo p e o f d e r iv a tio n o r tr a n s p o r ta tio n s lo p e an d is an e x a m p le o f r e c tilin e a r s lo p e . A s th e m ech an ism o f re tre a t o f free face th ro u g h b a c k w a stin g c a u se d by w e a th e rin g c o n tin u e s , th e r e c tilin e a r se g ­

(3) A. Wood's Model of slope evolution A . W o o d p re se n te d h is c o n c e p t o f slo p e d e ­ v e lo p m e n t in 1942 th ro u g h h is re se a rc h p a p e r e n ti­ tled , ‘th e d e v e lo p m e n t o f h ills id e s lo p e s ’ p u b lish e d in th e P ro c e e d in g s o f G e o lo g ic a l A sso c ia tio n o f L o n d o n . W o o d 's c o n c e p t o f slo p e d e v e lo p m e n t in ­ c lu d e s (a) p a ra lle l re tre a t o f slo p e an d (b) a d ju stm e n t b e tw e e n ra te o f w e a th e rin g an d rate o f tra n s p o rta ­ tio n o f w e a th e re d m a te ria ls (d o w n slo p e re m o v a l o f m a te ria ls ). H e s e le c te d a c lif f slo p e w ith free face e le m e n t fo r th e p u rp o s e o f e lu c id a tio n o f h is c o n ­ c e p t. T h e c l if f s lo p e m ig h t h a v e b een fo rm e d e ith e r te c to n ic a lly (fa u ltin g ) o r e ro s io n a lly . T h e free face is s u b je c te d to w e a th e rin g w h ic h c a u s e s its re tre a t th ro u g h b a c k w a s tin g . T h e w e a th e re d d e b ris is tra n s ­ p o rte d d o w n s lo p e a n d a c c u m u la te s a t th e slo p e fo o t

m e n t e x te n d s u p slo p e a n d u ltim a te ly re a c h e s th e d iv id e su m m it w h en th e free fa c e e le m e n t c o m ­

(b a s e ). T h e a c c u m u la tio n o f d e b ris e x te n d s u p s lo p e w ith the re s u lt th e lo w e r s e g m e n t o f free face s lo p e is c o n tin u o u s ly c o v e re d w ith d e b ris a n d th u s is b u rie d u n d e r d e b r is c o v e r (fig . 1 5 .1 3 A ). C o n s e ­ q u e n tly , the le n g th o f fre e fa c e d e c re a s e s (i.e. h e ig h t

p e n se o f re c tilin e a r u n it. U ltim a te ly , th e re c tilin e a r

p le te ly d isa p p e a rs o r say w h en th e p ro c e s s o f re tre a t o f free face is fin a lly c o m p le te d . T h e d is a p p e a ra n c e o f free fa c e re s u lts in the ro u n d in g o f d iv id e (in te rflu v e ) s u m m it an d th e d e ­ v e lo p m e n t o f s u m m ita l c o n v e x ity . W ith th e m a rc h o f tim e lo w e r s e g m e n t o f c o n s ta n t slo p e b e c o m e s c o n c a v e (fig . 15.13 C ). T h u s , th e e n tire slo p e p ro file c o n s is ts o f th re e e le m e n ts i.e. s u m m ita l c o n v e x ity , r e c tilin e a r ity a n d b a s a l c o n c a v ity a n d an id e al c o n v e x o -r e c tilin e a r -c o n c a v e s lo p e is fo rm e d . T he e v o lu tio n o f slo p e still c o n tin u e s a n d th e su m m ital c o n v e x an d b a sa l c o n c a v e u n its e x te n d at th e e x ­ u n it d isa p p e a rs and c o n v e x o -c o n c a v e slo p e is form ed. W ith th e m a rc h o f tim e th is slo p e is s u b je c te d to g ra d u a l w a s tin g a n d d e c lin e in s lo p e a n g le (fig-

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15.13 C ).

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Fig. 15.13 : Slope development according to A. Wood (in R.J. Small, 1970). Evaluation T h e m odel o f slo p e ev o lu tio n o f A. W ood is, in fact, g e o m e tric a l in ap p ea ran ce and indeed is very in terestin g b u t it is u n ab le to so lv e som e p ro b lem s

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and co n fu sio n s (reg ard in g the slo p e d ev e lo p m e n t) in reality. It is not likely th at c o n v ex ity and c o n ca v ity w ould ap p ea r in the last p h ase o f slo p e d e v e lo p m e n t. W o o d fails to p resen t c o n v in c in g a rg u m e n t fo r th is

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situation. The transform ation o f co n stan t debris slope into concave form m ay be accep ted , at least co n cep ­ tually, but it is very d ifficu lt to d ig est the arbitrary assum ption o f tran sfo rm atio n o f convex rock slope buried u n d er co n stan t d eb ris slope into concave slope form . If the co n stan t d eb ris slope is once transform ed into co n ca v e form due to stream ero ­ sion, then this co n cav e form is sup erim p o sed on the convex rock slope w h ich is ero d ed dow n due to rem oval o f o v erly in g debris.

A lthough this is severe in d ictm en t o f any study of the natural env iro n m en t, it co u ld be m aintained that he w as co n cern ed to p resen t a broad view, based upon reco n aissan ce ob serv atio n s on a w orld scale previously unp aralleled in geom orphology, and that this is no bar to su b seq u en t d etailed investigations of p articu lar aspects' (A. Y oung, 1972). A fter a careful ob serv atio n o f S outh A frican landscape he o b serv ed that h um id tem p erate clim ate and areas could not be ‘n o r m a l’ for landscape de­ velopm ent, as claim ed by W .M . D av is, because m ost o f the top o g rap h ic featu res had been m odified by P leistocene glaciatio n ; and m o st o f the p resen t day landform s are relict featu res d ev elo p ed in the past periglacial clim ate. T hus, acco rd in g to K ing, subtropical sem i-hum id reg io n s are ‘m o st n o r m a l’ for landscape ev o lu tio n . H e also reje cted th e basic tenet o f clim a tic g eo m o rp h o lo g y th a t ‘th e v a ria ­ tions in slope fo rm s and slo p e elem en ts d e p e n d on clim atic types i.e. slope fo rm s vary from o n e cli­ m atic type to the o th e r’ and m a in ta in e d th a t, ‘o u r th e sis w ill be th a t th e b a s ic p h y s ic a l c o n tro ls o f la n d sc a p e re m a in th e sa m e in all c lim a tic en v iro n m en ts sh o rt o f frig id o r e x tre m ely a rid ’ (K ing, 1957).

(4) H illslope c y c le theory of L .C . King

T h e K ing's co n ce p t o f.h illslo p e evo lu tio n is, in fact, re la te d to his b ro ad er schem e o f landscape ev o lu tio n , w hich in clu d es the concepts o f ‘river c y c le ’ (1 9 5 1 ), ‘h illslo p e c y c le ’ (1953) and ‘la n d ­ sca p e c y c le ’ (1962). It m ay be m en tio n ed that K ing's m o d e l is b ased on the co n cep ts o f A. W ood (1942), T .J. F a ir (1 9 4 8 , 1949), W . P en ck (1924) and R.E. H o rto n (1 9 4 5 ). In fact, K ing d id not survey slopes in th e field and did not m easu re slope m aking and co n tro llin g p ro cesses but based his m odel on field o b serv atio n s alone. ‘K ing's o p inions lack a basis o f d etaile d field o b serv atio n s; no m easu rem en t o f e i­ th e r p ro cess or form ap p ears in his pub lish ed w ork.

Fig. 15.14 : Components o f a standard hillslope (L.C. King. 1962). A ccord in g to K ing a standard (id ea l) hill

co n v ex ity , free fa ce, rectilinearity and basal concavity (fig- 15.1 4 ). S uch ideal h illslo p e is the na https://telegram.me/UPSC_CivilServiceBooks

s lo p e c o n s is ts o f all the four elem en ts e.g . sum m ital

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products o f norm al processes o f slope evolution in v o lv in g flu v ia l p ro c e ss (flo w in g w ater) or m assm ovem ent or both. He further m aintains that full developm ent o f slope elem ents depends on local conditions i.e. resistant and strong bedrocks and bold and sufficient reliefs. If either o f these condi­ tions are absent, free face (scarp) and debris slope do not develop and thus a w aning convexo-concave slope is form ed. The evolution o f hillslope is, in fact, related to the concept o f pediplanation cycle of King. A ccord­ ing to King pediplain form ation is the result o f twin processes i.e. scarp retreat and pedim entation. In the initial stage o f hillslope developm ent, the scarp face experiences parallel retreat due to backwasting caused by w eathering of exposed rocks. This parallel retreat of scarp contrails the evolution of entire hillslope. A ccording to King the debris slope just below the scarp or free face does not extend upslope and hence it is neither capable o f obscuring nor destroying the free face element. It means that there is a balance between the supply of debris from upslope and rem oval o f debris dow nslope on this section i.e. debris slope. Sim ilarly, there exists balance between debris supply and debris removal at hill crest and hence it rem ains constant. The gradual parallel retreat o f free face and rectilinear elem ents (debris slope) results in the form ation o f pediment o f concave plan at the base o f the hillslope. As the parallel retreat of free face and debris slope continues, their lengths decrease and pedim ents extend upslope at the cost of rectilinear and free face elem ents. Ultimately, upper slope elem ents m ainly free face disappear and pedi­ m ents extend upto hill crest. Thus, an extensive erosion surface o f concave slope is formed. This surface is called pediplain, which is, in fact, the product o f coalescence o f several pedim ents (fig. 15.15). Thus, the w hole process o f developm ent of pediplain and the entire process o f slope evolution during this period is called ‘pediplanation cycle and ‘hillslope evolution cy cle’.

Fig. 15.15 : Pediplanation cycle and hillslope evolution according to L.C. King.

face) is the active elem ent o f slope retreat. W herever scarp is absent, the present slope form is not the result of present geom orphic processes. B ased on this as­ sumption King has maintained that extensive convexoconcave slopes o f the tem perate regions o f the north­ ern hem isphere are, in fact, relict form s and are not the result o f present day processes rather they are the result o f past periglacial processes. T hese have not been formed due to scarp retreat rather these have developed because o f w eathering and erosion o f weak rocks. Evaluation

King’s model o f slope evolution is, in fact, not original concept o f his ow n rather it is am algam ation of views o f earlier w orkers like A. W ood, T.J. Fair, R.E. H orton etc. and is a story-like deductive and im practicable b ut interesting description. H is co n ­ cept is based on extensive reconnaissance o b serv a­ tions alone. N o field survey and instrum entation of slope m aking processes have been undertaken. King has used assertions rather than ev id ences in support o f his m odel. ‘E vidence from the w ork of o thers is selectively m ustered in support o f preconceived views. An exam ple is the arg u m en t p u t fo rw ard involving the hydrological distinction b etw een laminar and

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K ing has further m aintained that if the scarp is absent due to lack o f resistant rocks and bold relief then the process o f slope evolution would be entirely different from the process o f parallel retreat and pediplanation. There will be regular decrease in m axi­ m um slope angle and extensive convexo-concave slope is produced. A ccording to K ing the hillslope w ith scarp is considered norm al because scarp (free

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turbulent flow o f water; it is asserted that turbulent flow on the upper slope elem ent gives place to lam inar flow on pedim ent. This has never been experim en­ tally verified. W here there exists clear field evidence contradicting a view, the circum stances are dism issed as not ‘norm al’ (A. Y oung, 1972)’. A. Y oung (1972) has further rem arked, ‘his w riting lacks objectivity. The value o f King's w ork is in setting up hypotheses for testing; they are not p ro v en ’.

slope elem ents (crestal convex, free face, rectilinear and basal concave elem ents) whrein scarp or free face elem ent is subjected to parallel retreat. A few of the detached hills having sandstone capping are the exam ples o f m esas characterized by flat top but steep scarps on all sides. T hese scarps are experienc­ ing parallel retreat due to backw asting. (fig. 3.8).

If K ing's view s are considered in Indian con­ text, som e o f his ideas becom e relev an t and are validated. The author after having close observa­ tions and selective m easu rem en t o f slope angles in certain parts o f foreland o f Indian peninsula viz. C h o ta n ag p u r h ig h lan d s (B ih ar), R ohtas plateau (B ihar), K aim ur ranges, R ew a plateau, B hander pla­ teau (M .P .) etc. has n o ticed precipitous scarps (free face elem en t) w hich are undergoing the process o f parallel retreat due to backw asting caused by physico­ c h e m ic a l w e a th e rin g o f w ell jo in te d m assiv e sandstones (of scarps) resting over shales and siltstones form ing d ebris or rectilin ear slope. B hander plateau having im posing escarpm ents on its eastern, south­ eastern and southern sides consists o f all the four

U n lik e o th e r g e o m o rp h o lo g is ts B ritis h geom orphologist R.A. S avigear (1952) postulated his concept o f slope d evelopm ent on the basis of field evidences rather than on deductive reasoning. H is concept, thus, is nearer to reality and ground truth. He closely observed and studied the ch aracter­ istics of a few slope profiles at the head o f the C arm arthen Bay in South W ales and postulated that both parallel retreat and slope d ecline played im por­ tant role in slope developm ent. Som e tim es these processes work together in a single region o f u n i­ form environm ental conditions. H e id entified one such slope profile in the eastern part o f the C arm arthen Bay w hich consisted o f basal free face (scarp slope), m iddle rectilinear elem ent w ith slope angle o f 32°

(5) Concept of R.A. Savigear

Limited Summital -------------^ ^ C o n v e x ity ® Major rectilinear Slope Section

V_/

Broader Summital # _______Convexity

N. Limited rectilinear Slope Section 28° )

\ \

Recently active cliff

Basal ^ ^ -«^ concavity

• Marsh

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Fig. 15.16: Slope development - cliff slope according to R.A. Savigear, 1952).

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h il l s l o h e

field d ata o b tain ed th ro u g h m e a su re m e n t o f slo p e angle in carefully selected areas o f u n ifo rm co n d i­ tions in term s o f g eological stru ctu re, lith o lo g ical ch aracteristics, clim atic co n d itio n s, v eg etatio n , soil, relief etc. He first m easu red m a x im u m slo p e a n g les in a region o f uniform en v iro n m en tal co n d itio n s and then calcu lated m ean m a x im u m slo p e a n g le a fte r averaging m axim um an g les. O n th e b asis o f clo se perusal o f m axim um slope an g les and m ean m ax i­ m um slope angles he p o stu la ted th at th ere w as very little deviation o f in d ividual m ax im u m slo p e an g les from m ean m axim um slo p e an g les in a given lo c a l­ ity. A ccording to S trah ler the h illslo p es h av e d e v e l­ oped uniform m axim um slo p e an g le so th a t slo p e m aterials m ay be effectiv ely tran sp o rte d d o w n slo p e. Such slope, from w hich debris is rem o v ed d o w n slo p e easily, has been term ed by S trah ler as eq u ilib riu m slo p e w hich is co n tro lled not by a sin g le fa c to r b u t by all those factors w hich are in v o lv ed in th e d e v e l­ opm ent o f slopes. S lig h t ch an g e in any one o f the factors m ay distu rb the eq u ilib riu m slo p e a n d m a y w arran t re a d ju stm e n t o f e q u ilib riu m c o n d itio n . S trahler drew th ree in feren ces fro m his em p irica l study o f slope d ev elo p m en t—

and u p p er lim ited su m m ital co nvex elem ent. T he angle o f re c tilin e a r slo p e is such that the w eathered and eroded m aterials are rem oved from the rectilin ­ ear slope d u e to g rav ity and are tran sp o rted dow n the free face o r scarp slope. T he m aterials co m in g at the base o f free face are in stan tan eo u sly rem o v ed by sea w aves and thus basal free face slope is m aintained but it ex p erien ces p arallel retreat (fig. 15.16A). On the other hand, th ere are co n v ex -rectilin ea r-co n cave slo p es in the w estern p art o f the C arm arthen Bay w here basal c o n ca v e slope has dev elo p ed b e­ cause o f a c c u m u la tio n o f d eb ris at the slope base (fig. 15.16B ) as the m a teria ls co m in g from upslope cannot be rem o v e d by sea w av es b ecau se the sea has receded an d th u s sea w av es do n ot reach the slope base. A c co rd in g to S av ig e ar rectilin ear slope at c o n sta n t an g le o f 32° can be m ain tain ed only when th ere is effectiv e rem o v al o f m aterials from its base but if th ere is no rem o v al o f m aterials from slope base, the d e b ris com in g from upslope accum ulates at the b ase o f re c tilin e a r slope and thus the low er se g m e n t o f slo p e is p ro tec ted from w eathering and e ro sio n al p ro c e sse s w h ereas the upper part o f the slope p ro file co n tin u e s to retreat due to w eathering. T h is m e c h a n ism effects d eclin e in slope angle. He has also in fe rre d th a t the d ev elo p m en t o f sum m ital c o n v ex ity ta k e s p la ce at the later stage o f slope d e v e lo p m e n t. ‘It d o es seem p o ssib le, to ju d g e from the e v id e n c e in C arm arth en B ay, that both parallel retre at an d slo p e d e clin e are ten ab le theories, and th a t tw o m e c h a n ism s can ex ist to g eth er in one small area in re s p o n s e to d iffe re n t co n d itio n s o f slope-foot ero sio n o r d e p o s itio n ’ (R .J. S m all, 1970).

(i) In any one lo cality th ere is u n ifo rm m a x m um slope angle on all retreatin g slo p es, (ii) it is no t necessary th at all such slo p es are o f the sam e ag e in term s o f ev olution, and (iii) it is m o re likely th a t slopes having un ifo rm m ax im u m slo p e a n g le s u n ­ dergo the p ro cess o f p arallel retre at ra th e r than slo p e decline. A fter a d etailed stu d y o f slo p e p ro file s in fluvially originated drainage basins S trahler attem pted to establish co rrelatio n b etw een v alley side o r g ro u n d slope and ch an n el g rad ien t o r slo p e. A c c o rd in g to him valley side slo p es are p o sitiv e ly c o rre la te d w ith channel slope i.e. w h erev er v alley side slo p e is steep, the ch an n el slo p e is also sle e p so th a t w e a th ­ ered and eroded m aterials co m in g fro m the v alley sid e slopes m ay be e ffectiv ely tra n s p o rte d by the ch an n e l d o w n stream . C o n v e rse ly , g en tle v alley sid e slopes gen erate relativ ely le sse r d e b ris an d h e n ce th e chan­ nel g rad ien t a lso b eco m es g en tle. F o llo w in g S tra h le r su ch c o n d itio n (p o sitiv e re la tio n s h ip b e tw e e n v alley side slope and c h a n n e l slo p e ) m a y be possible on ly w hen the slo p e m a k in g an d c o n tro llin g factors re­ m ain co n stan t. C h an g e in any of the factors may

It m a y be m e n tio n e d th a t d iffe re n c e in lith o lo g ical c o n d itio n s m ay also lead to parallel retreat as w ell as slo p e d eclin e in clo se p roxim ity. F or e x a m p le , the B h a n d e r scarp s and d etach ed hills (m esas) h av in g sa n d sto n e cap p in g resti ng ov er shales (fig. 3.8) (S a tn a d istric t, M .P .) are ex p erien cin g p arallel re tre a t w h ile th e S h ard a P ole hill located very clo se to B h a n d e r scarp s is e x p erien cin g slope decline as the sa n d sto n e c a p p in g has been finally rem oved due to p arallel re tre a t and thus the hill is s u b jected to d o w n w a stin g w h ich h as p ro d u ced co n v ex o -co n cav e slope.

(6) Concept of A.N. Strahler T h e co n cep t o f A .N . S trah ler, an A m erican g eo m o rp h o lo g ist, on slo p e d ev elo p m en t is b ased on

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d istu rb such c o n d itio n and relationship. For exam-

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GEOMORPHOLOGY

pie, tw o valley sid e slo p e s h av e u n ifo rm an g le but o ne is co v ered w ith v e g e ta tio n w h ile the o th e r one

the ch an n el d o w n stream easily. O n the o th e r hand, if the channel is aw ay from the base o f valley sides, the m aterials co m in g from up slo p e are d ep o sited at the base and hence the valley side slope becom es gentle.

is devoid o f v eg etatio n . T h e v e g e ta tio n -c o v e re d valley side w o u ld g e n e ra te less d e b ris b eca u se o f low d eg ree o f d e n u d a tio n and h en ce ch an n e l g rad i­ en t w ould b e g en tle in c o m p a riso n to v eg etatio n -fre e valley sides an d a sso c ia te d ch an n e l g rad ie n t b ecau se in this case v alley sid es are su b jected to m o re d e n u ­ datio n , g en era te m o re d eb ris and h en ce channel g rad ie n t w o u ld be rela tiv e ly steep e r so th a t all the loads m ay be tra n sp o rte d d o w n stream .

It m ay be m e n tio n ed th a t th e view s o f S trahler very m u ch fall in lin e w ith th o se o f R .A . S avigear in that p arallel retre at and m a in ten a n ce o f steep slope angle is possible w here m aterials co m in g from upslope are rem o v ed from the slo p e b ase an d slope d eclin e b eco m es o p erativ e w h ere m a te ria ls a c c u m u la te at the slope base.

A cco rd in g to S trah ler, if the ch an n el g rad ien t is steep e n ed e ith e r d u e to reju v e n atio n or due to ch an g e in d is c h a rg e -lo a d ratio co n seq u en t upon cli­ m a tic ch a n g e , the steep e n in g o f valley sid e slope b eco m es m o re co m m o n than p arallel retreat. He fu rth e r m a in ta in s th a t w h e re v e r the la n d sca p e ev o lu ­ tion is c o n tro lle d by b a se-le v el, w h erein slo p e g rad i­ en t d e c re a se s w ith tim e, then the p o ssib ility o f slope d e clin e c a n n o t be ru led out. T h e valley side slope b eco m es s te e p e r o nly in th o se c o n d itio n s w here riv e r ch an n e l to u c h es th e slo p e b ase so th at all the d eb ris co m in g fro m u p slo p e m ay be tran sp o rte d by

(7) Fisher-Lehmann Model of slope evolution

O. F ish er (1 8 6 6 ) an d O. L e h m a n n (1933 1934) p o stu la ted p ro cess r e sp o n se m o d el o f slope ev o lu tio n . It m ay b e p o in ted o u t th a t p ro c e ss-re sp o n se m o d el is rela ted to th e m e c h a n ism o f slope fo rm in g p ro cesses and th e ir re sp o n se i.e. c h a n g e s in slope form d u rin g a giv en tim e sp an . F irst, F ish e r ap p lied this m o d el fo r the e x p la n a tio n o f c liff re c e s ­ sion in 1866. T h e in itial fo rm o f th e c liff slo p e is w all-lik e steep slo p e w ith p re c ip ito u s scarp (free face elem en t) and th ere is a c c u m u la tio n o f d e b ris at the fo o t o f the cliff. A .C . L a w so n also d e d u c e d the

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Fig. 15.17: Slope development according to O. Lehmann. Parallel retreat o ffre e face-1, II, III successive recessions o f fre e face, p - C liff angle, a - scree angle, FABCH- rock core, Source : A. Young, 1972.

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process o f c liff rece ssio n in 1915 w h ile O. L eh m an n added a few m o re v aria b les for the ex p lan atio n o f developm ent o f c liff slo p e in 1933 and 1934 e.g. angle o f cliff, scree an g le, ratio o f ro ck -scree v o l­ ume. T hey d ed u ce d the c o n c e p t o f p arallel retre at o f scarp (free face ) sectio n o f the c lif f slo p e. T h e model of cliff recession is g en era lly k now n as FisherLehmann model (A . Y o u n g , 1972).

co n tin u o u s p arallel retre at d u e to b a c k w a s tin g cau sed by w eath e rin g o f e x p o se d ro c k s o f fre e fa c e elem en t. A ro ck c o r e is fo rm ed d u e to c lif f re c e s s io n a n d acc u m u la tio n o f sc re e at th e fo o t o f c liff. T h is r o c k co re is b u ried u n d e r sc re e c o v e r. It is a s s u m e d th a t there is no c h a n g e in th e fo rm o f r o c k c o r e . T h e h eig h t o f free face (h ) g ra d u a lly d e c re a s e s b e c a u se o f g rad u al a c c u m u la tio n o f scree.

A cco rd in g to th is m o d el it is assu m ed th at the initial form o f slo p e is re c tilin e a r-fre e face w h erein initial h e ig h t is d e n o te d by h and slo p e an g le by (3 (fig. 15.17). It is fu rth e r assu m ed th at the su rface at the top o f c lif f and at its b o tto m is flat and plain (as show n in fig. 15.17). T h e en tire sectio n o f free face (scarp) is e x p o s e d to w e a th e rin g p ro cesses. B ecau se o f im p act o f w e a th e rin g p ro c e sse s a w eath e red lay er o f u n ifo rm th ic k n e s s is d e ta c h e d and falls d o w n in a unit tim e. T h is d is lo d g e d w eath e red la y er o f scree a c c u m u la te s at th e fo o t o f the slo p e. T h u s, the slope an g le o f n ew ly fo rm e d re c tilin e a r slo p e d ue to acc u ­ m u la tio n o f scree is less than the angle o f original (in itia l) free face ((3). C liff u n d erg o es the p ro cess o f

T h is m o d el e n v isa g e s th a t if th e c lif f fa c e is su b jected to g rad u al p a ra lle l re tre a t a n d th e re is acc u m u la tio n o f scree at its (c liff) b a se th e n th e fo rm o f ro ck co re b u ried u n d e r sc re e w o u ld b e c o n v e x . T h e lo w est p o in t o f c o n v e x ro c k c o re b e c o m e s tan g en t to initial c lif f a n g le ((3). W h e n th e p ro c e s s o f p arallel retre at is c o m p le te a n d th e e n tire fre e fa c e (scarp ) is rem o v e d d u e to p a ra lle l re tre a t a n d sc re e accu m u latio n , th e h ig n e st p o in t o f c o n v e x ro c k c o re (H ) b eco m es ta n g e n t to sc re e a n g le ( a ) . T h e c h a n g e s in initial c liff an g le ((3) h av e in s ig n ific a n t c o n tro l on the shape o f sc re e -c o v e re d ro c k c o re as it re m a in s co nvex in plan. I f the c lif f a n g le ((3) is le ss th a n 90°, the lo w er se g m e n t o f ro ck c o re is re la tiv e ly o f g e n tle

F G

A

B I D = D

A FG I

A

CJE = D

H

H

A F H J

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Fig. 15.18: A m odel o f c liff recession and slope development according to O. Fisher. Source : R.J. Rice, 1977.

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GEOM ORPHOLOGY

m ter).

slope. On the other hand, ch an g e in scree an g le ( a ) has greater im pact on the form o f rock core. A s the angle o f scree slope increases, the steep n ess o f rock core also increases and vice-versa. If the entire m ass o f scree is rem o v ed du rin g parallel retreat o f cliff, the resu ltan t relict ro ck su r­ face p roduced by parallel retreat gives birth to re c ti­ linear slope w hich is called d en u d a tio n a l slo p e. It m ay be m en tio n ed th at the steep n ess o f ro ck core depends on the steep n ess o f initial c liff an g les i.e. if the initial c liff slope an g le is very steep, the co n v ex ­ ity o f rock co re w o u ld also be very steep as show n in fig. 15.18,

M o d e l 2.

Fig. 15.18 portray s an exam p le o f very steep c liff hav in g initial angle o f 90°. B ecau se o f w eath e r­ ing o f ex p o sed rocks o f c liff face a lay er o f w eath ­ ered ro ck o f uniform th ick n ess (A F G I in fig. 15.18) is rem o v ed an d is d ep o sited at the c liff fo o t (A B ID ). It m ay be m en tio n ed th at the v o lu m e o f w eath ered ro ck equals the v olum e o f scree d ep o sited at the c liff b ase as fo llo w s—

M o d el 3a.

(ii)

S lo p e retre at su b ject to control by rem o v al (o f accu m u lated scree).

(iii)

R em o v al o f reg o lith (scree) e n ­ tirely by su rface tran sport.

(iv)

R ate o f su rface tran sp o rt (is) p ro ­ p o rtio n al to sin an d d istan ce from c re st (o f cliff), (fig. 15.19A 1).

(i)

U n im p e d ed rem o v a l (o f reg o lith ), b u t no basal e ro sio n (o f a c c u m u ­ lated reg o lith by flo w in g w ater).

(ii)

S lo p e retre at su b je c t to co n tro l by rem o v al (o f acc u m u la te d reg o lith ).

(iii)

R em o v al o f re g o lith e n tire ly by su rface tran sp o rt.

(iv)

R ate o f su rface tra n s p o rt (is) p ro ­ p o rtio n al to sin » 1 • .£ %

s B

3 o

6. Stream F requ en cy S tream freq u en cy or d rain a g e freq u en cy is the m easu re o f n u m b er o f stream s p er u n it a re a (m ay be sq u are m eter, sq u are k ilo m eter an d so on). F o r th e co m p u tatio n o f stream freq u en cy (S F ), th e b asin is c o n v e n ie n tly d iv id e d in to g rid s q u a re s (m o re co m m o n ally one sq u are m ile /k ilo m e te r) d e p e n d in g on m ap scale and areal co v era g e o f th e b asin an d th e n u m b er o f stream s in each g rid is c o u n te d , ta b u la te d and q u an tified . T h e d a ta o f stream fre q u e n c y are

/

2000-

/

classified into certain c a teg o ries d e p e n d in g u p o n the n atu re o f data. T h e sp atial p a tte rn o f stream fre ­ q u en cy is stu d ied th ro u g h iso p le th (fig . 19.8A ) o r ch o ro p le th m ap s. T h e g en era l c a te g o rie s o f stream freq u en cy are (i) very p o o r (S F vp), (ii) p o o r (S F p), (iii) m o d e ra te (S F M), (iv ) h ig h (S F „) an d (v) very

i /

1 0 0 0: 800 600-

/ / /

1*00200-

/

/

/

r = 0 • 9 991 r 2= 99 • 8 %

h ig h (S F vh). I1 1

i*nVInu ------- j— i— r i i m j i i m to 20 UO 60 80100 200 *00 600 Cutty b a s i n a r e a ( K m ^ )

7. Drainage Density D ra in a g e d en sity refers to to tal stre a m le n g th s p e r u n it area. R .E . H o rto n (1 9 4 5 ) d e fin e d d ra in a g e d en sity as a ra tio o f to ta l le n g th o f all stre a m se g ­

Fig. 1 9 .7 : Law o f allometric growth - regression line o f positive p o w er fu n ctio n model. Source : Savindra Singh and A. Dubey, 1996.

m e n ts in a g iv e n d ra in a g e b a sin to th e to ta l a re a o f

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th a t b a sin an d th u s it c a n b e d e riv e d as fo llo w s—

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STREAM F REQUENCY

D R A IN A G E DENSITY

SANKH

A

B

Stream rcqucn cy

0 -2 2

-

DRAINAGE TEXTURE

6

Od

VH

> 8 Ora i nage Tex t a r e i n m il e

Fig. 1 9 .8 : Spatial patterns o f stream frequency (A ), drainage density (B) and drainage texture (C) o f Sankh basin, Randu plateau, Bihar. Source : Savindra Singh, 1978a.

squares of one square mile or one square kilometer each and to measure the total stream lengths in each grid square and to group the derived data into drain­ age density categories viz. (i) very low (Dd^), (ii) low (DdL), (iii) moderate (DdM), (iv) high (D d^ and (v) very high (DdVH) drainage density categories and to prepare isopleths (fig. 19.8B) for the study of spatial distributional pattern.

D d = -^ Ak

where LK = total lengths of all stream seg­ ments of a basin Afc = total area of the basin Horton's method (as given above) yields only a single value (of drainage density) for the entire basin and hence it cannot be applied for the study of spatial variations o f drainage density within a given basin. The simplest way to calculate drainage den­ sity on a regional scale is to divide the basin into grid https://telegram.me/UPSC_CivilServiceBooks

It may be pointed out that the m easurem ent of stream lengths in grid square (with the help of opisometer or threads) is very tedious, difficult and time consuming procedure and hence

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373

MORPHOMETRY o f d r a i n a g e b a s i n s

h *ioh t curve C—percentage hypsometric curve and D—model o f percentFig. 19.9 : A -H ypsom etric curve, B - a r e a - h e i ig >^ dev age hypsometric curve fo r the determin B = equilibrium stage and H = penultimate (o stag ,, , a tn v iz ‘th e p ro b a b ilistic lin e in tersectio n m e th o d o t m easu rem en t o f stream le n g th s p e r u n it are a i C \ V .C a r ls t o n W ^ B .L a n g b e in ( I960), T .W ilg a l's calcu late d ra in a g e d e n sity h as d iv e rte d th e a tten tio n m eth 0(j (] 966) b ased on th e m e an d ista n ces fro m th e o f interested w o rk ers in th is fie ld to e x p lo re a te n ?j* n e a re st w ater co u rse, ‘th e p h o to m e c h a n ic a l m e th o d tiv e m eth o d s o f ra p id e s tim a tio n o f d ra in a g e en si y d

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a n

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GEOM O RPH O L

374

o f V in c e G a rd in e r (1 9 7 4 ), J J . D o n a h u e 's m e th o d (1 9 7 2 ) u sin g th e n u m b e r o f p o in ts on a re g u la r g rid th a t to u c h a stre a m as an in d ex o f stre a m le n g th , d ig itiz a tio n o f d ra in a g e lin es, S a v in d ra S in g h 's d ra in ­ ag e te x tu re m e th o d (1 9 7 6 , 1978, 1981 etc.).

t=

V2

whe nt i , t 2 = number of intersections between th e s tre a m n e tw o rk and grid > sq u are d ia g o n a ls

T h e s p a tia l v a ria tio n in d ra in a g e d e n sity h as b een re la te d to p re c ip ita tio n e ffe c tiv e n e ss (M .A . an d p =

M elto n , 1957), v eg e ta tio n in dex (R .J. C h o rley , 1957), p e rm e a b ility o f te rra in (C .W . C a rlsto n , 1963), c li­

T h e d a ta o f d ra in a g e te x tu re, so d e riv e d (on th e b asis o f ab o v e e q u a tio n ) are c la ss ifie d into five D t c a te g o rie s e.g. (i) v ery c o a rse d ra in a g e texture (ab o v e 0.8, D tvc), (ii) c o a rse d ra in a g e te x tu re (0 .8 — 0.6, D tc), (iii) m o d e ra te d ra in a g e te x tu re (0.6— 0.4, D tm), (iv) fin e d rain a g e te x tu re (0 .4 — 0.2, D tp), and (v) very fine d ra in a g e te x tu re (0 .2 — 0.0 0 1 , D ^ ) , valu es in d icate stream sp acin g in m ile o r k ilo m e te r in g rid sq u ares. Iso p leth s (fig. 19.8C ) are p rep ared on th e b asis o f co m p u ted d ata o f d ra in a g e te x tu re o f a given basin fo r the study o f its (d rain a g e te x tu re) spatial pattern s.

1962 ; M .A . M e lto n , 1957), stru c tu re , p a rtic u la rly ro c k ty p e , re la tiv e ea se in in filtra tio n o f p re c ip ita ­ tio n in to g ro u n d s u rfa c e an d v eg etal c o v e r (S av in d ra S in g h a n d R e n u S riv a sta v a , 1974) etc. 8. D ra in a g e T extu re ‘A n im p o rta n t g eo m o rp h ic c o n c e p t is d ra in ­ a g e te x tu re by w h ich w e m ean th e re la tiv e sp acin g o f d ra in a g e lin e s ’ (G .H . S m ith , 1950). H o rto n (1945) d e fin e d d ra in a g e te x tu re on th e b asis o f stream fre q u e n c y (n u m b e r o f stream s p er u n it area). In fact, th e te rm ‘d ra in a g e te x tu re ’ h as b een u sed lo o sely a n d no s u c c e s s fu l a tte m p t has b een m ad e to search o u t a q u a n tita tiv e p a ra m e te r fo r its calcu latio n . A c ­ c o rd in g to S a v in d ra S in g h (1 9 7 6 , 1978) the term d ra in a g e te x tu re m u st b e u sed to in d icate relativ e s p a c in g s o f th e stre a m s in a u n it area alo n g a lin ear d ire c tio n . T h u s, h e atte m p te d to fin d o u t a new p a ra m e te r in te rm s o f d rain a g e tex tu re (S av in d ra S in g h , 1976, 1978) to rep la ce the d eriv a tio n o f d ra in a g e d e n sity . A c c o rd in g to him , ‘d rain a g e te x ­ tu re re fe rs to re la tiv e sp acin g o f stream s p er unit le n g th in g rid sq u a re s (o n e m ile X o n e m ile o r one k ilo m e te r X o n e k ilo m e te r). T h e d eriv a tio n o f d ra in ­ a g e te x tu re is q u ic k an d e a sie r m e th o d , as it in v o lv es o n ly th e c o u n tin g o f stream c ro ssin g s alo n g the four e d g e s o f e a c h g rid an d its tw o d ia g o n als ra th e r than m e a su rin g th e le n g th s as is d o n e in d rain a g e den sity . T h e fo llo w in g e q u a tio n is su g g ested to calcu la te

S av in d ra S ingh, V. G a rd in e r and S .S . O jh a. (1986) using a ‘p rin cip al co m p o n en t a n a ly s is ’ te c h ­ n iq u e for the an aly sis o f ‘spatial v ariation o f d ra in ­ age density in P alam au u p la n d ’ cam e to the c o n c lu ­ sion that d rain ag e den sity and d rain ag e te x tu re have alm o st identical lo ad in g s on the stream co m p o n en t and ap p ear to y ield v irtu ally id en tical in fo rm atio n co n cern in g this a sp ect o f b asin form . S a v in d ra Singh (19 8 1 ) o b tain ed very h ig h c o rre la tio n co efficien t (ab o v e 0.9) b etw een d rain a g e d en sity and drainage tex tu re d u rin g th e c o m p arativ e study o f these tw o variab les in P alam au u p lan d (29 d ra in a g e basins), R an ch i p lateau (23 b asin s), S.E . C h o ta n a g p u r region (30 b asin s), B h an d er u p lan d in M .P . (15 b asin s) and N orth A rav alli reg io n (31 b asin s) an d concluded that, d eriv atio n and use o f d rain a g e d en sity m ay be rep laced by d rain a g e te x tu re as th e latter is m uch effectiv ely related to o th e r m o rp h o m etric variables as d rain a g e d en sity an d its (o f d rain ag e texture) d eriv a tio n is q u ic k e r and e a sie r than drainage den- | sity. So, the five c ateg o ries o f d rain ag e texture (Dt) 1.e. very co arse, co arse, m o d erate, fine and very fine ..j should be taken as rep resen tativ es o f extrem ely lo w> low , m o d erate, hig h and very high D d c a t e g o r i e s J

d ra in a g e te x tu re : 1

...S a v in d ra S ingh, 1 9 8 1

(t + p ) / 2

w h e re D t = d rain a g e tex tu re

resp ectiv ely . https://telegram.me/UPSC_CivilServiceBooks

A S = av era g e sp acin g b etw een tw o stream s

P1 + P 2 + P 3 + P 4

w h ere P] to P 4 = n u m b e r o f in tersectio n s between th e stre a m n e tw o rk an d grid sq u are ed g es.

m a tic c h a ra c te r (w h e re so il, re lie f, ro c k ty p e o r vegetal c o v e r a ffe c t p recip ita tio n , C .A . C o tton, 1964), ra in fa ll in te n s ity (R .J. C h o rle y an d M .A . M o rg an ,

Dt = AS =

(t i + t 2 ) / 2

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jylORPHOMETRY OF D RAIN AGE BASINS

375

19.7.3 RELIEF ASPECTS OF THE BASIN

plotted on the o rd in ate and relativ e area (a/A , w h ere a d enotes area betw een tw o su ccessiv e co n to u rs an d A indicates total basinal area) p lo tted on the a b ­ scissa in term s o f p ercen tag e (cu rm u lativ c) (fig. 19.9C and D) has pro v ed fruitful in p ro v id in g a b asis for reco g n izin g stag es o f the cy cle o f ero sio n in a d rain ag e basin (S av id n ra S ingh and R. S riv asta v a, 1975). It is im p o rtan t to n o te th a t th e ty p e o f hypsom etric an aly sis o f the basin ad v o cated by A .N . S trah ler is not prim arily d esig n ed to show ero sio n levels but is m ore for co m p arativ e p u rp o se s b etw ee n one basin and an o th er to estab lish stag es o f d e n u d a ­ tion that can be ap p lied to any basin (C .A .M . K in g , 1966). H y p so m etric and ero sio n in teg rals (H I an d E l) m ay be calcu lated eith er by p la n im e te r o r m a th ­ em atically. T he h y p so m etric in teg ral is th e ra tio o f volum e o r p ercen tag e o f total v o lu m e o f th e b a sin area below the curve.(fig. 19.9 C ) and th u s it re v e a ls the volum e o f the area u n co n su m ed by the d y n a m ic w heels o f erosion (30% in fig. 19.9 C ) w h ereas th e erosion integral (E l) is a p ro p o rtio n a te are a a b o v e the cu rv e (fig. 19.9C, E l= 70% ) and th u s it in d ic a te s the volum e o f area w hich has been ero d e d by e ro ­ sional processes. T h e h y p so m etric in teg ral h as b e e n accepted as an im p o rtan t m o rp h o m etric in d ic a to r o f the stage o f basin d ev elo p m en t. A .N . S tra h le r (1 9 5 2 ) related the h y p so m etric in teg ral o f a b o v e 0 .6 (o r 60% ), 0 .6 — 0.35 (o r 60 % — 35% ) and b elo w 0 .3 5 (o r 35% ) to the y o u th fu l (fig. 19.9D — E ), e q u ilib riu m (fig. 19.9D — B) and m o n a d n o ck (o ld ) sta g e s (fig . 19.9D — H ) o f b asin d ev elo p m e n t re sp e c tiv e ly . B u t m o re co m m o n ally y o u th fu l, m a tu re an d o ld sta g e s are rep resen ted by h y p so m etric in te g ra ls o f m o re than 60% , 6 0 % — 30% and less th a n 30% re s p e c ­ tively. It m ay be p o in ted o u t th at ‘th e low v alu e (o f h y p so m etric in teg ral, b elo w 3 0 % ) is o n ly m a in ­ tain ed as long as a few m o n a d n o c k s g iv e re la tiv e ly g reat d iffere n ce in h eig h t b e tw e e n th e h ig h e st an d the lo w est p o in ts, w h en th e m o n a d n o c k s are c o n ­

The re lie f asp ects o f the d rain ag e b asin s arc related to the study o f th ree d im en sio n al featu res o f the basins inv o lv in g area, vo lu m e and altitu d e o f vertical d im en sio n o f lan d fo rm s w h erein d iffere n t m orphom etric m eth o d s are used to an aly se terrain characteristics, w hich are the resu lt o f b asin p ro c­ esses. T hus, this asp ect in c lu d es the an aly sis o f the relationships betw een area and altitude (hyposom etric analysis), altitu d e and slo p e an g le (clin o g rap h ic analysis), av e ra g e g ro u n d slo p e, relativ e reliefs, relief ratio, d isse c tio n in d ex , p ro files o f terrain s and the rivers etc.

1. Hypsometric Analysis H y p so m e try in v o lv es the m e asu re m en t and analysis o f re la tio n s h ip s b etw ee n altitu d e and basin area to u n d e rsta n d the d e g re e o f d issectio n and stage o f cycle o f e ro sio n . A re a -h e ig h t cu rv es, h y p so m etric curves an d p e rc e n ta g e h y p so m e tric cu rv es are g en ­ erally u sed to sh o w th e re la tio n sh ip s b etw een alti­ tudes an d a re a o f th e b asin . T h e b asic d ata req u ired for the stu d y o f a re a -h e ig h t re la tio n sh ip s are areas betw een s u c c e s s iv e c o n to u rs and th eir resp ectiv e heights. T h e a re a m ay be m e asu re d w ith th e help o f p la n im e te r o r m a y b e e stim a te d by in tercep t m ethod. T he h e ig h t is o b ta in e d fro m th e c o n to u r m ap. A r e a - h e i g h t c u r v e s in d ic a te actu al areas b e­ tw een tw o s u c c e s s iv e c o n to u rs and h en ce h o rizo n tal axis re p re s e n ts a re a in te rm s o f p e rc e n ta g e o f total area an d th e v e rtic a l ax is sh o w s h e ig h t (fig. 19.9B). T h o u g h a re a -h e ig h t c u rv e d e n o te s re la tio n sh ip b e­ tw een a ltitu d in a l z o n e s an d c o rre sp o n d in g areas but this d o e s n o t re p re s e n t actu al p ro file o f terrain and h en ce b re a k s in slo p e a re c o m p le te ly c o n ce aled . H y p s o m e tr i c c u r v e is g e n e ra lly u sed to show the p ro p o rtio n o f a re a o f th e su rfa c e at v ario u s e le v a tio n s a b o v e o r b e lo w a d a to m (F .J. M o rk h o u se an d H .R . W ilk in s o n , 1967) an d th u s th e v alu es o f a re a a re p lo tte d as ra tio s o f th e to tal area o f the basin a g a in st th e c o rre s p o n d in g h e ig h ts o f th e c o n to u rs an d h e n c e th e are a is re p re s e n te d by c u m u la tiv e p e rc e n ta g e (fig . 19.9 A ). H y p so m e tric c u rv e also d o es n o t rev ea l th e actu al p ro file o f te rra in .

su m ed , the in teg ral re tu rn s to a b o u t 0 .4 to 0 .6 ’ ( 4 0 ^ to 6 0 % ) (C .A .M . K in g , 1966). It m ay b e m e n tio n e d th a t h y p s o m e t r i c i n t e g r a l is v e ry d e l i c a t e m o rp h o m e tric v aria b le an d h e n c e it s h o u ld b e u se d w ith u tm o st c a re and field c h e c k s, o th e rw is e it m a y re n d e r m isle a d in g resu lts.

P e r c e n ta g e h y p so m e tr ic c u r v e (su g g ested b y A .N . S tra h le r, 1952) in v o lv in g tw o ratio s o f re la tiv e h e ig h t (h /H , w h ere h d e n o te s h eig h t b etw een tw o s u c c e s s iv e c o n to u rs a n d H in d icate s to tal h eig h t)

2. Clinographic Analysis

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Cl i n o g ra p h ic c u rv e s re p re s e n t a v e ra g e s lo p e s b etw ee n s u c c e s s iv e c o n to u rs an d th u s p re s e n t p a n o -

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376

C l (in fe e t) s lo p e a n g le = ta n * = A d (jn ^

ram ie view o f th e te rra in . U n lik e ar e a -h e ig h t an d h y p so m etric c u rv es, c lin o g ra p h ic c u rv e s re v e a l th e b reak s in slo p e a n d su d d en c h a n g e s in th e r e lie f o f th e area and th e y a lso p re s e n t th e g e n e ra l tre n d o f th e su rfaces. T h e c o n stru c tio n o f c lin o g ra p h ic c u rv e s re q u ire d a ta o f slo p e an g le s b e tw e e n s u c c e s s iv e co n to u rs, c o n to u r le n g th s, h e ig h ts an d a re a s b e ­ tw e e n su c c e ssiv e c o n to u rs, fo r w h ic h th e fo llo w in g te ch n iq u es h a v e b een su g g e s te d —

w h e re A d =

w hen A d =

a v e ra g e d is ta n c e b e tw e e n tw o sue c e s s iv e c o n to u rs a n d C l is contour in te rv a l. "•fr rl - ru w h ere rl is the radius o f lower co n to u r and ru is the radius of upper con tou r.

(i) Flnsterwalder's (1980) clinographic curve : A v erag e slo p e an g le b etw ee n tw o su ccessiv e co n to u rs = tan (j) =

Cl x L w h e re a is th e a r e a a b o v e th e c o n to u r. The

w h ere C l = c o n to u r in terv al L = total le n g th o f c o n to u r

slo p e a n g le s so d e r iv e d a re p lo tte d fro m above d o w n w a rd (fig . 1 9 .1 0 C ). W h e n s lo p e a n g le s are v ery lo w , th e n th e s e a re e x a g g e r a te d b y m u ltip ly in g

A = to tal are a b etw ee n co n to u rs F in s te rw a ld e r s u b stitu te d h is c lin o g ra p h ic c u rv e

by a c o n s ta n t.

(1980), w h erein len g th s o f co n to u rs are p lo tted ag ain st

(iii) M ean Slope C urve o f A .N . S trah ler (1952)

th e ir re s p e c tiv e h e ig h ts (fig. 19 .1 0 A ) by a n o th e r

A v e ra g e s lo p e a n g le b e tw e e n tw o successive

c u rv e k n o w n as h y p s o c lin o g ra p h ic c u rv e w h ic h in ­

c o n to u rs is

c o rp o ra te s h y p so m e tric d a ta and c o n to u r le n g th s alik e. C u m u la tiv e a re a is p lo tte d a lo n g th e h o riz o n ­

= tan (j) =

tal ax is an d c u m u la tiv e c o n to u r le n g th s a re p lo tte d

C l (in A w (in

f e e t) f e e t)

alo n g th e v ertical ax is (fig. 1 9 .10B ). A v e ra g e slo p e w h e re A w =

is c a lc u la te d ac c o rd in g to fo llo w in g e q u a tio n — S lope an g le = tan 7- i w o i 6 o *i c o 5- i o k - -r«•o 3- ii JZ 2“ i c 1- i i w mJ n 1

Cm ) 2 5 0 -t

Mean slope

2

1---------1--------- r 3 k

contour Length C 0 0 0 in') 2

I*

6

8

10

12

I 1*

16

Area between successive c o n t o u r s ( 0 0 0 0 0 m )

D

(m ) 250150-

50 0 1*0 80 120 Mean inter contour width ( m )

strahler’s Method

(m )

250150-

50i

5

i

i

i

i

10

i* I

i

i

i

'15

Slop e

>‘

202 5 30

angle ( d e g r e e )

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Fig. 19.10: Clinoglraphic curves o f {A) Finsterwalder, (B) Finsterwalder, (C) Hanson-Lowe, (D) D e Smet, a n d (E) M oseley. .

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SPOT HEIGHTS +SUMMITS

Ol

C la ss

1500H

%) I

--------

----

in te rv a l ( f e e t ) 50

100

-------- 150

---* 20

30