Patterns of Burning over Archaeological Sites and Landscapes: Prospection and analysis 9781407307879, 9781407322032

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BAR 531 2011 MARSHALL PATTERNS OF BURNING OVER ARCHAEOLOGICAL SITES AND LANDSCAPES

B A R

Patterns of Burning over Patterns of Burning Archaeological Sitesover and Archaeological Sites and Landscapes Patterns of Burning over Landscapes Prospection and analysis Archaeological Sites and Prospection and analysis

Landscapes

Prospection and analysis

Alistair Marshall Alistair Marshall Alistair Marshall

BAR British Series 531 2011 BAR British Series 531 2011 BAR British Series 531 2011

Patterns of Burning over Archaeological Sites and Landscapes Prospection and analysis

Alistair Marshall

BAR British Series 531 2011

ISBN 9781407307879 paperback ISBN 9781407322032 e-format DOI https://doi.org/10.30861/9781407307879 A catalogue record for this book is available from the British Library

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PUBLISHING

CONTENTS Page 1

Preface

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Paper 1 Improved determination of magnetic susceptibility (MS) by ground- insertable probe: patterns over Iron Age settlement enclosures in the Cotswolds (Gloucestershire, UK).

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Paper 2 Patterns of magnetic susceptibility (MS) within and around Iron Age hillforts, and their interpretation: case studies from the Cotswolds (Gloucestershire, UK).

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Paper 3 Use of magnetic susceptibility (MS) to determine patterns of activity over settlement areas: case studies on larger Iron Age and Roman settlements in the Cotswolds (Gloucestershire, UK).

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Paper 4 Magnetic susceptibility (MS) survey at early Bronze Age round barrows in the Cotswolds (Gloucestershire, UK): evidence for structure and association with settlement or activity areas.

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Paper 5 Variation of magnetic susceptibility (MS) over the landscape, and its use for assessment of the scale and distribution of early land clearance: a case study from the Cotswolds (Gloucestershire, UK).

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ITEMS COMMON TO ALL PAPERS.

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PREFACE This programme of geophysical analysis arose from specific problems encountered during survey and excavation of mid-later Iron Age settlement enclosures by the author in the Gloucestershire Cotswolds (UK). At these sites there was a need to supplement detailed magnetic mapping from gradiometry with higher quality data on magnetic susceptibility (MS), in order to establish a more viable basis for assessing patterns of burnt material over and around clearly defined archaeological structures. Standard prospection over most archaeological sites is based firmly on gradiometric magnetic mapping and resistivity (earth resistance) survey, with MS survey usually occupying a more marginal role as a technique of less certain application and interpretation, given its lower quality, quantity, and spatial resolution of data. The principal method currently available for logging MS uses a field coil such as a Bartington D-head pressed against the ground surface, often in uncertain and irregular contact, and anyway at some distance from less-contaminated buried archaeological surfaces. However, this method does allow rapid survey, with MS data available immediately, and if nothing more than general distributions are required, a certain level of noise within the data is deemed acceptable. The second method, far slower and more labour-intensive, but more accurate and in closer contact with buried features, involves removal of sediment samples from known depths and, after preparation, assaying them using a laboratory MS coil, with consequent delay in any availability of data. In order to remedy these problems a ground-insertable probe was developed, incorporating the advantages of both methods: speed of survey, independence from variable and irregular surface conditions, measurement of MS whilst totally immersed in buried sediment and hence in greater proximity to buried archaeological surfaces. Successful pilot application of the technique to the enclosures and their environs justified extension of the programme to include use of the probe at other types of site in the same region, carefully maintaining a common geology and soil cover to allow a clear basis for direct comparison. A range of Neolithic to Bronze Age funerary areas, Iron Age hillforts, larger Roman settlements, and a large-scale landscape survey were therefore added. The entire programme is the largest and most consistent study using higher-grade MS data currently known to the author. It is published here in the hope that MS survey using the probe, equipment now commercially available (Bartington Instruments, Oxford OX8 7GE, UK), will become more standard procedure. The interpretation of these MS surveys has been developed as far as possible. In all of the projects distributions of MS are certainly relevant to immediate questions of function at and around sites, but in two studies there are wider implications. Amongst hillforts additional discussion is developed around the theme of on-site metal-working within the context of the broader iron trade (paper 2). Also, the larger landscape study has interesting relevance to patterns of early land clearance, erosion of sediment, and consequent environmental change (paper 5). Paper 1: small Iron Age enclosures The first project introduces the MS probe within a detailed analysis, supported by data from a range of related sites. These smaller, lightly-defended ditched enclosures, often complexes with appended subenclosures, usually clearly definable by gradiometry, are a common type in the Cotswold and English Midland areas, and present excellent subjects for functional analysis using MS and other geochemical data. Few such detailed studies exist. Paper 2: Iron Age hillforts The Cotswold area contains a series of hillforts, ranging from smaller and more modestly defended, to larger and often highly defensive sites. This diversity, and the relative absence of data on internal, and especially on any external features, present clear grounds for investigation. MS surveys over the interiors of Iron Age hillforts and adjacent extra-mural areas indicate patterns of activity highly relevant to their functional interpretation, and allow comparison with data from broadly contemporary ditched enclosures. Paper 3: larger Roman settlement Survey of extensive and complex areas of Iron Age and Roman settlement provides data relevant to their layout and operation. One Roman 'small town' site and one complex of more agrarian Roman settlement, both with known mid-later Iron Age antecedents, were selected, both on a similar gravel substrate.

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Paper 4: early Bronze Age round barrows The region includes many funerary areas of Neolithic to Bronze Age date, containing mainly round, but often long barrow sites. The majority of these monuments and their surrounding areas have been heavily damaged by ploughing, and consequently many structures relevant to general interpretation are unlikely to survive sufficiently to be detectable by magnetic gradiometry or excavation. However, MS survey can be applied with some confidence to map distributions of burnt sediment over and around them, and to retrieve at least some information, since MS features are more resilient to destruction and displacement by ploughing. Detailed survey of barrow monuments and their surrounding areas provides data on the properties of known sites as MS anomalies, indicates the potential for detection of terminal sites, truncated and without apparent above-ground features, and allows association between round barrows and areas of nearby settlement or activity to be assessed. Paper 5: Cotswold area Extensive survey over a 40 km2 sample of the Cotswold upland and dip-slope reveals patterns of MS relevant to discussion of early land clearance and use, to development and organisation of settlement in the area, also to environmental events in this catchment area of the upper Thames valley and their effects on its hydrology.

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MS over Iron Age enclosures

PAPER 1 Improved determination of magnetic susceptibility (MS) by ground-insertable probe: patterns over Iron Age settlement enclosures in the Cotswolds (Gloucestershire, UK). ABSTRACT A prototype ground-insertable probe (Bartington Instruments, UK) was used to determine the distribution of magnetic susceptibility (MS) in basal topsoil over surviving archaeological surfaces at small enclosed settlement sites of Iron Age to Roman date, located in the Cotswolds (Gloucestershire, UK). This project provides a series of extensive MS surveys, rapidly obtained at high-resolution, from geologically similar areas, which are relevant to comparative, period-specific study of these small settlement sites in terms of their function and organisation, acting as an important supplement to detailed ground-plans obtained by magnetic gradiometry. In terms of methodology for prospection, this work also enabled the effectiveness of the probe to be assessed in large-scale use, and to be compared with existing methods of MS survey. High quality MS data-sets, obtained using the probe, during direct contact with buried archaeological surfaces, were compared with the more usual type of measurement made under conditions of variable contact at the ground surface using the D-head (Bartington Instruments, UK). Comparison was also made with survey based on laboratory-based analysis of removed sediment samples. Use of such distributions of MS as a method of initial prospection over archaeologically-unknown areas, unsupported by further data, is also considered. This project forms the initial analysis in a programme of related studies, in which the MS probe is used to examine a range of sites of different types and periods. Keywords: prospection, magnetic susceptibility, Iron Age, settlement.

INTRODUCTION

industrial processes.

The problem: achieving higher quality, rapid MS survey The pattern of magnetic susceptibility (MS) over the surviving upper surfaces of archaeological sites provides information on the distribution of sediment the iron minerals of which have been magnetically enhanced by burning or by microbial decay of organic debris (brief review: Marshall 2001). Such enhanced sediment either remains in its original position, retaining some integrity and association with the parent feature ('in situ enhancement'), or becomes displaced elsewhere, by dumping or operation of sedimentary processes ('displaced enhancement'), in both cases providing important additional detail on structure and activity over the area.

Data on MS is at its most useful when uniformly gridded and viewed against site-plans obtained either from magnetic gradiometry, resistivity (earth resistance) survey, or by excavation ('supplementary MS survey'). Alternatively, MS data, not necessarily gridded, unsupported by such structural data, can be used for general assessment of an area, as for instance in determining the possible presence, layout, and limits of a settlement site or activity area ('primary MS survey'). However, before more detailed use of MS as an independent method for prospection, any inherent problems need to be assessed by analysis of specific case-studies, covering a comprehensive range of sites. A series of such 'supplementary' and 'primary' MS surveys are discussed in this paper.

Such information is useful in discussing the location at a site of the various activities which generated magnetically-enhanced sediment, for instance domestic hearths, metalworking areas, pottery kilns, agrarian bonfires, manuring scatters, and middens where refuse was dumped. The intensity of enhancement also gives some indication as to the nature of burning at a site, whether low-grade and domestic in scale, or indicative of higher temperature

Two alternative methods for MS survey on archaeological sites are given in Cole et al. 1995, and endorsed in Thomas 2005, where the second method is used over the interior of Conderton Camp hillfort. Removal of samples for laboratory analysis is cited as a slower option, with quicker measurement preferred using the Bartington MS2 meter and field-coil type D to log data at 5m sampling intervals from the surface. The latter method is considered

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MS over Iron Age enclosures data, encountered during investigation of Iron Age enclosures by the author (AJM), as at The Bowsings (Marshall 2001, 2007), prompted the need to introduce new equipment, for routine, large-scale, standardised MS-logging at depth. An approach was made to Bartington Instruments (Geoff Bartington), who made a prototype probe available for application in the field (FIGS 1, 2). The probe has been assessed by AJM during an extended programme of field-research at a range of sites, the initial project being presented in this introductory paper on iron age ditched enclosures.

adequate for outlining variation in MS and defining localised concentrations, as at Nettlebank Copse banjo enclosure, Hants (Payne 1996; 2000). In general, problems from uneven ground contact and distance from buried target are all too rarely stressed, and too few comparative studies presented. This paper presents the results of MS survey using a third method, a probe, which combines the speed of the MS2/D-head system with some of the accuracy obtained from laboratory analysis of removed sediment samples. Comparison of repeat surveys also details discrepancies between data gathered from the surface and at depth.

Detailed and extensive analysis of MS over sites, either before excavation, or over exposed structures, is all too seldom carried out, despite its potential for interpretation of the area under examination. In specific relation to this paper, comprehensive, detailed MS survey over later prehistoric enclosure sites, and also Iron Age hillforts, is generally lacking. For instance, in the case of the extensive and varied group of Iron Age settlement sites on upper Thames gravel, useful comparative studies of MS over complex habitation, agricultural, and other working areas could certainly have been undertaken (examples: Lambrick and Robinson 1979; Allen 1990; Allen et al. 1993; Allen and Robinson 1993; Hardy and Cropper 1999; Lambrick and Allen 2004; Jennings et al. 2004). However, the general utility of MS survey as a supplement to magnetic and resistivity mapping is well proven, with improved technology and a growing body of positive results increasing its use on a wide range of sites (recent work including: Barton and Fenwick 2005; Slater et al. 1996; Batt and Dockrill 1998; Aidona et al. 2001; Kattenberg and Aalberg 2004).

Another non-invasive technique, in less common use than the Bartington MS2 meter/D-head combination, involves use of 'Slingram'-type instruments, such as the EM88, SH3, CS60, and CS150, which provide dual MS and soil conductivity measurement from the surface, with certain calibrating and comparative studies showing applicability to survey of shallower archaeological contexts (Benech and Marmet 1999; Thiessen et al. 2009). The problem in providing extensive, high-quality data, at high resolution, suitable for producing adequate MS surveys, has been the lack of suitable equipment for survey over unexcavated sites. MS data need to be logged directly, at depth within the soil-cover, hence away from surface contamination, whilst maintaining direct, reproducible contact between instrument and sediment, and always in close proximity to the archaeological site. It also seems better practice to determine MS from deposits in situ, where removed sediment has not been subjected to any sub-sampling or sorting during preparation for laboratory assay, and hence remains more truly representative of the buried site.

An example of current survey practice: MS data from Conderton Camp (Worcs.) Published data from this site illustrate common methods of MS survey in current use and provide useful information on the suitability of the D-head for use on grassland surfaces similar to those encountered for several sites in this study (The Park-Bowsings, Middle Ground, and the Paddocks). Brief details of other recent MS surveys which do not form part of this programme, are given in TABLE 1.

Survey involving removal and preparation of sediment samples for analysis by laboratory coil is time-consuming, damages the ground surface, and since the results take some time to process and be made available, interactive use at the site itself is not possible. Problems relating to speed of survey can certainly be overcome by use of a D-head and MS2 meter (Bartington Instruments), but with the major disadvantage that reading from the surface encounters the problems of variable contact with sediment, through ground-cover or uneven surface conditions, and also involves remoteness from the source to be assessed. This is a usage for which the equipment was not originally intended, with deployment on flat, prepared surfaces being specified (Bartington Instruments: manual for the D-head). Since only part of the 'recording field' of the D-head actually penetrates the ground (FIG 3) the reading must be calibrated to provide some estimate of the actual underlying MS, another source of uncertainty, one absent in the case of the probe.

This small hillfort lies on the flank of an outlier to the Cotswolds, in a similar topographic and geological location to many of the ditched enclosures of the type examined in this analysis, with similar close-cropped grass and thin pastureland soil covering limestone bedrock (Thomas 2005). It is comparable in middle Iron Age date and in plan to the double enclosure seen at The Park, both sites divided between a habitation zone, containing huts surrounded by pits, and a relatively open possible working area, with evidence for adjacent fields. However, at 1.95 hectares Conderton Camp is some 4 times larger than The Park, and more defensive in position and perimeter.

Such problems relating to inherent quality of MS

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MS over Iron Age enclosures survives below the near-ubiquitous plough-zone, are brown, relatively stoneless clays, of fairly uniform, low MS (FIG 5).

MS survey using the Bartington D-head at 5m intervals over the interior of this hillfort indicates minor areas of enhancement marginal to the main group of huts in the W half, with some increase visible over a large pit group occupying much of the E interior (Thomas 2005, figs. 4,5; PAPER 2, fig. 12). Values for v (volume) MS show a peak at 100-150 (SIx10-5), but are of little use in terms of absolute comparison against other sites since only measured by variable contact at the ground-surface. The reduced quality of data and restricted coverage of the surrounding area do not do justice to the potential contribution MS survey could make to interpretation of the site.

Such thin, iron-rich, magnetically-responsive soils which directly overlie archaeological sites make the area ideal for magnetic gradiometry, and for logging MS by probe, which can easily be inserted into the base of topsoil, and placed in close contact with surviving upper archaeological surfaces. Large expanses of the area are arable, consequently with rough, variable surfaces, and little close-cropped pasture exists, making realistic use of the D-head difficult. -the sites The enclosures in this analysis were small Iron Age farmstead and stronghold enclosures, typically about 0.5 hectares in extent, were involved in subsistence agriculture, with craft-work low-grade and utilitarian, appear relatively self-sufficient, and could have directly supported a population of little more than an extended family. They are typically located on small spurs or valley slopes adjacent to a stream, and are common in the region, singly or as small complexes (RCHME 1976; Marshall 2007).

Only the broadest conclusions on relative internal distributions of MS could be drawn: that direct settlement was suggested by the higher MS in the upper camp, whilst the reduced values in the lower camp indicated other functions, or perhaps that this area was abandoned early. A possible E-W division of the upper camp into discrete areas with specific functions was also noted, with an axial zone of lower MS separating the two lateral areas of higher MS. Subsequent sampling by the author (AJM) at the site, using the probe, indicates that values at buried surfaces (mean vMS 248) are higher than those cited from the surface by a factor of about 2, and that the contribution of pit groups to MS has been underestimated, since they do show up as distinct positive MS anomalies. The original survey with the D-head was also confined to the interior of the hillfort, and hence data are lacking on any external anomalies, suggestive of external burning and dumping, of the type found by the author (AJM) at other enclosure and hillfort sites (PAPER 2). Magnetic gradiometry was also confined to the interior and so broader structural data is also absent.

Six sites were chosen for this analysis, all in the Guiting Power-Temple Guiting sector of upper Windrush valley. Four of these enclosures (The Park, Middle Ground, The Paddocks, and Lot's Barn) appear to be essentially single phase, and mid to later Iron Age in date (c.400BC to early decades AD). The remaining two (The Bowsings, Wharton's Furlong) show clear evidence from scatters of finds and excavated structures for continuation into the Roman period. Evidence for occupation earlier than the Iron Age appears absent, with no structures found, and scatters of flint-work no higher than the general background level. The sites are therefore relatively uncomplicated by multi-period occupation, and provide excellent subjects for functional analysis involving distributions of MS.

The sites and their environment -the survey area and its geology The Iron Age enclosures included in this analysis are located on the Cotswolds, a range of limestone hills running across midland England, its steep main scarp and adjacent upland flanked to the NW by the river valleys of the Severn and Avon, with its dip-slope descending gently SE towards the upper Thames (OS 1981, 1985, 2002). These enclosures lie on the upper dip-slope of the Cotswolds, in and around the head-water stream valleys of the River Windrush, a tributary of the upper Thames (FIG 6). The Cotswolds are formed from Jurassic limestone, and their shallow soils, which directly overlie bedrock, are mainly rendzinas, typically 20-40cm thick, naturally calcium-poor, and variably altered by agriculture in terms of texture, chemical content, and MS (BGS 1998; IGS 1975, 1977, 1979; SSEW 1983). Parent subsoils, seen where deeper stratigraphy

This sample of enclosures appears to represent expansion of settlement and agriculture from more heavily exploited, traditional areas of the upland limestone (PAPER 5), into relatively under-utilised valleys, to form farmstead and stronghold sites for small kin-groups, some of which continued in Romanised form. Given basic utilitarian simplicity, and continuation of native tradition over a life-span of a only few centuries, the layout of such sites, and the pattern of MS over them, are likely to remain relatively uncomplicated by major structural change. Patterns of MS over a site are, of course, cumulative, but these enclosures provide a case-study in which the palimpsest is likely to be simpler, allowing fairly direct correlation between MS and component features of the site.

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MS over Iron Age enclosures reflecting a reading near unenhanced bedrock, or one with poor ground contact using the D-head, the latter possibly the result of proximity to intrusive ferrous debris. Plotting MS data as multiples of unenhanced background levels, in addition to scaling as absolute values, is very rare indeed. In the case of work with the D-head, it is usually unstated whether quoted MS data are direct instrument readings, although this is the norm, or have been calibrated to estimate an actual value for surface sediment (see: Bartington Manuals for MS2 meter and D-head).

EXAMPLES OF EXISTING MS SURVEYS The standard method for archaeological prospection in general use, as for instance providing commercial site evaluation in the UK, involves preliminary examination by magnetic gradiometry, and optionally by resistivity survey, to determine the distribution of features, and then partial or complete coverage by MS survey follows where deemed appropriate. Two methods for MS survey are in regular use: (1) volume magnetic susceptibility (vMS) is logged by Bartington MS2 meter with D-head combination (occasionally the MS2 with F head) at 15, 10 (tbe most common), or occasionally at 5m intervals, either producing a complete mesh of points, or just selected transects; (2) samples excavated at depth are removed from points on transects, or as spot samples from features, for off-site laboratory assay to determine vMS or mMS (mass magnetic susceptibility). Data are then used to examine any correlation between MS and established structures or surface scatters.

However, despite such uneven arable surfaces (ploughed, harrowed, or stubble) at Margidunum, Notts. (Johnson 2003) and West Deeping, Lincs. (Johnson 1994), the very large scale of MS survey undertaken with the D-head (TABLE 1), together with subsequent smoothing of data, compensate for such difficulties sufficiently to produce well-defined general distributions over the sites. These correspond fairly well with underlying structures seen clearly by magnetic gradiometry, to give at least an initial, very useful impression of MS over the landscape. At Margidunum a clear increase in MS marked the area of the Roman town, with clear but unexplained foci within it, and the sites of a Roman building, villa, and enclosures were visible as discrete MS anomalies, their origin established by magnetic gradiometry and scatters of surface finds. Similarly, at West Deeping, clear foci of MS marked an Iron Age enclosure, Romano-British farmstead, and Roman villa, all adjacent and within the dispersed spread of enhanced sediment.

A representative sample of published MS surveys, carried out at a range of archaeological sites, grouped according to different substrates, using both methods, is summarised in TABLE 1, to illustrate current practice. The information content of many such MS surveys appears generally low, especially at smaller-scale coverage (TABLE 1, column 'concl'). In method 1 (see above), corruption of data by conditions of poor surface contact, inevitable in using the D-head over rough ground is, however, only very occasionally noted. Uncertainty is, however, often expressed on the origin and validity of relatively incoherent MS anomalies lying beyond any obvious maxima, as to whether they are natural, archaeological, or relate to recent land use. Only at more extensive scale and closer sampling intervals, under improved data-logging conditions of shallow topsoil and close-cropped surface, with archaeological features well-defined structurally and in terms of date, and given clear zonation of the original MS-enhancing activities, does the method make a more significant contribution to interpretation. Use of a closer sampling interval alone is not the solution, which also lies in improving contact with, and establishing closer and more standardised proximity to surviving archaeological features.

In order to provide more robust large data sets in such generalised, exploratory prospection, and certainly for detailed analytical work to examine finer variations in MS over sites, poor contact should be avoided. Its effects can be mitigated by scale and smoothing of data, and largely eliminated by routine use of a ground-insertable probe. Similarly high work-rates can be achieved for both D-head and probe, and the improved data more than repay any additional effort required (see: 'Methods of survey and presentation of data'). Despite the higher quality of values provided, the more labour-intensive nature of method 2 (see above), laboratory analysis of removed samples, means that often too few data are available for more robust statements about the site, especially if this is large, complex, and multi-period. Data from this source is therefore often an adjunct to survey at the surface using the D-head, providing some calibration and more detailed confirmation of stratigraphy.

In smaller-scale published surveys, statistical measures of MS data such as mean and standard deviation, assuming the distribution to be a normal one, are often poorly reported, or are not meaningful because of the low numbers of data logged. Maxima and minima are of little use, the former

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MS over Iron Age enclosures TABLE 1: A sample of existing MS surveys METHOD 1 Using the Bartington D-head to measure vMS directly at the surface smaller scale: coverage 1000 vMS: units SI), with two adjacent areas of particular enhancement, each at about 1.5 hectares, lying on the hilltop near the head of a stream valley to the W of the Neolithic long barrow and ?Bronze Age round barrow(s). The northern peak of MS appears to be unitary, whilst the southern peak contains three constituent maxima. The arable, which covers the entire survey area, has produced a general, light scatter of flint-work, plus fire-cracked pebbles and quern fragments of prehistoric type, but no obvious evidence for settlement of Roman date. This whole hilltop seems best interpreted as a core area of intensive, long-term prehistoric settlement and agriculture, firings associated with the latter generating most of the observed pattern of MS. The known burial monuments appear to have been placed at the

Although most areas of higher ground tend to be of increased MS, many such comparable hilltops remain at reduced MS, indicating that the pattern of MS is not primarily a by-product of topography, nor of geology. Adjacent hills of the same rock and soil-type can produce very different levels of MS. In contrast to upland areas, the mid and lower slopes of valleys, and their bases, tend to be of 'low' MS and,

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MS over the landscape Areas for cultivation must be kept sufficiently open for tillage and planting, and areas of rough grazing are best kept cleared back, to provide varied conditions suitable for a range of stock, from browsers on shorter vegetation, such as sheep and goats, to larger animals, such as cattle and horses requiring lusher foliage.

margins of this activity zone. Separation between areas of increased MS and direct settlement is seen at the Roel hillfort (FIG 13c), and is also supported by surveys at ditched enclosures of Iron Age date (PAPER 1). On this basis direct settlement at Humblebee might be expected in between the barrows and these two principal peaks of MS, in areas where lower and less well defined patches of enhancement occur.

Initial clearance of larger trees, and especially removal of roots, would present a challenge to individuals restricted to using only small and easily blunted hand-tools, even of iron, and particularly of stone or bronze. Partial felling, after removal of smaller branches for fodder and useful timber, would leaving an intractable trunk, or ground-fast stump. The easiest way to remove this obstacle would be to burn it out, by setting a fire around it, and allowing it to smoulder until the remains of trunk and upper roots were more amenable to cutting and digging out. This process of easier removal could be done systematically over a matter of months or longer, from the initial killing of a tree by ring-barking or 'girdling', to setting a fire, perhaps fed by its dead timber, to weathering and removal of the residual root-bole. Treating a series of trees in this way, feeding scrub into the fires, could produce a cleared tract efficiently, within a season, with much reduced effort. The effectiveness of fire in clearing mature woodland is provided by an example from West Africa of a 44m high tree, previously killed by ring-barking, remaining resistant to axe-work, but readily felled by basal firing over 60 hours, with only 6 man-hours of active involvement (Shaw 1969).

-Cotehay (FIG 13c) The entire area surveyed is again of 'high' to 'extreme' MS, with a patch of particular enhancement covering about 1.5 hectare, and a minor one adjacent to it. A round barrow lies at the edge of the main area of MS, overlooking the head of a small stream valley. A thin scatter of flint-work and domestic stone-work debris lies over the area, with occasional Iron Age and Roman sherds suggesting peripheral scatter from adjacent settlement. This pattern is similar to that observed at Humblebee, and could be the result of protracted prehistoric settlement and agricultural firing, with marginal placement of the main funerary area. -Roel (FIG 13a) Much of the area surveyed to the SE of the Iron Age hillfort and the main flint scatter adjacent to it, possibly indicating Neolithic to earlier Bronze Age settlement and activity, is also of 'high' to 'extreme' MS. A single patch of particular MS enhancement covers about 5 hectares, and is fairly unitary in structure but contains some internal maxima. The separation between areas of direct settlement and this peak zone of MS is very clear, with occupation inside the hillfort and over the area of the flint scatter only resulting in minor patches of relatively low MS.

Cleared tracts for future use could be produced well in advance, to provide alternative venues for cultivation whilst the current area lay fallow. A sequence of primary clearance of tracts of virgin land, their use for productive agriculture, then fallowing, leading into a cycle of re-clearance of existing areas, can be envisaged. In addition to setting specific fires for tree-clearance, more generalised, smaller-scale firings of the land could have taken place, from burning off crop stubble, dead grass, and weeds on fields, to torching of marginal scrub-land in order to provide newer foliage for browsing animals. In addition to periodic passive fallowing of the land, addition of some sort of fertiliser would have been beneficial, helping to maintain the fertility of shallow upland soils, of which manuring, both incidental and by deliberate spreading, would have been one source. Wood-ash could have formed a separate and highly important supplementary source of fertiliser, being rich in minerals such as potash. Before artificial fertilisers became available, fire was one of the most widespread methods of fertilisation (Pyne 1997).

Certainly, in these and in all such areas of MS surveyed in detail by the author, the distribution and scale of MS observed is capable of realistic interpretation in terms of settlement-based activities, and relationships with extant sites and monuments, where these are known in the area, can be suggested. This strongly indicates that the general distribution of MS over the landscape is also the result of human economic activity.

GENERAL DISCUSSION The pattern of MS determined in the survey area can be discussed in relation to key aspects of settlement and environmental change, within and beyond the region, over the five millennia from the Neolithic to medieval period. The origin of high-MS blocks, and their interpretation Any tract of suitable agricultural land needs to be cleared at the outset, and maintained in productive condition by repeated clearance of re-growth.

Some wood-ash would have accumulated from routine burning, but more perhaps was added by deliberately locating bonfires on fields to increase

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MS over the landscape would inevitably lead to gradual degradation and decreased fertility of thinner soils on the upland limestone over areas of traditional settlement in the region, requiring relocation of arable areas, at least until partial recovery of the original zone.

their ash level, in a process here termed 'ash-bedding'. Distinct blocks of increased MS, as seen in this survey, could be generated over millennia by repetition of this cycle of small-scale clearance and cultivation, around a localised group of settlements, all taking advantage of a common tract of useful land, individually cleared areas gradually merging to form a more open landscape. Such larger agricultural zones may not have been entirely formed from a mosaic of individual sites and their associated land, but parts of the area may have been held jointly for a common purpose, such as grazing. Location of settlements towards the edge of high-MS blocks might suggest the existence of a more central tract, accessible to all sites, less settled, and therefore more convenient for mass firing, generating widespread and increased levels of MS.

Pressure from decreasing fertility of cultivated areas is a feature of many discussions of prehistoric land-use, a question addressed experimentally at Butser Ancient Farm. Here however, repeat cultivation of experimental plots over an 8 year period saw no evidence for soil exhaustion, as measured by stable levels of organics and key inorganic nutrients and by the lack of any significant decrease in cereal yields, variability being primarily a function of weather (Reynolds 1980: table 2/ p.13, and 1985, table 2/ p. 400). This suggests that land under prehistoric cropping may be fairly resistant to exhaustion, this only becoming important over a longer period of perhaps decades, and given possible mismanagement of resources.

The contribution of the burning process to soil fertility depends on its management. A smouldering pyrolysis of biomass by 'slash-and-char' to produce 'bio-' or 'agri-char' makes a greater net beneficial contribution to the soil than fiercer 'slash-and-burn', which yields a far lower proportion of organics, correspondingly increased ashy inorganic residues, and results in higher denaturation of sediment by burning. Porous particulate carbon from both processes helps to retain water, reduces the leaching of key nutrients such as nitrogen, potassium, phosphorus, calcium, zinc and manganese, and increases soil pH, all acting to improve quality and tilth of soil and to maintain essential biota. Both types of burning also help to reduce pests and weed infestation.

Zones of highest MS in the survey area may therefore represent or contain areas where excessive agricultural use over millennia eventually resulted in deterioration of soils, with decreased productivity providing the impetus for general expansion or relocation of activity onto the less cultivated land of the valleys. However, given the capacity of limestone-based soils to regenerate if rested, full abandonment could be avoided by good management, mitigated by manuring, fallowing, and folding of stock on resting land, with new land forming a supplement rather than a replacement. Some indication of the intensity of clearance sustained by these upland blocks of increased MS is given by comparing the levels of MS seen there against the far lower levels found in upland stream valleys, and over areas of gravel in the Severn-Avon or upper Thames valleys. The MS-enhanced upland blocks, on thin limestone soils, have sustained clearance to support pastoral and other agricultural usage since the mesolithic period, with a notable increase in cultivation beginning during the Iron Age. During the same interval, settlement and clearance of the adjacent valley areas was of smaller scale than that on the upland, although a parallel and significant increase is also apparent. Nevertheless, in these valleys, levels of MS only occasionally reach 'medium' grade, and here around known sites, usually of Iron Age and Roman type. Even around those villages known to have been active agrarian communities since the early medieval period, surrounded by historically documented zones of intensive clearance and agriculture, levels of MS rarely exceed 'low'. Using these examples of low-MS situations as a base-line, it is possible to infer that the blocks of highest MS over the upland represent areas of very intensive, long-term clearance, with burning-off playing a major part.

'Slash-and-burn' often forms part of a shifting pattern of cultivation and livestock management involving mixed arable, grassland, and woodland, and may be cyclical within a restricted area, or progressive if communities are more mobile and move on after a few seasons when land becomes less fertile. Soils in the survey area show no surviving evidence for incorporation of supplementary charcoal, being mainly rendzinas with currently low organic content, and are nutrient-poor, even for available Ca, despite being on a limestone substrate. Over the broader landscape in the survey area, judging by the increased levels of MS encountered, it seems likely that the main input from clearance was ashy detritus from 'slash-and-burn' and burnt sediment, with any organics added by manuring and passive fallowing. The effects of burning on soil fertility would have been in part positive, in terms of its indirect effect of adding essential minerals. Burning would be negative in its direct effect on soils, denaturation of structure by oxidation of organic content, this latter offset by replacement organics from manuring and rubbish dumping. Failure to balance these effects

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MS over the landscape Spring Hill block. It is possible that over this entire period the Spring Hill block formed a large zone of fairly communal agricultural usage, separate from the main area of habitation, cleared less for arable than for the pastoral activities to which it is well suited.

MS and phases of settlement in the survey area Three major phases of settlement can be distinguished in the area for the purposes of general analysis. This division is a broad one, of necessity, because in the absence of sufficient data from excavation, many sites remain essentially unspecified as to date and type, but can still, in most cases, be attributed to one of these general phases with a fair degree of certainty. For instance, many cropmark enclosures producing a scatter of essentially Roman finds are also likely to have Iron Age origins, requiring placement in a combined category.

-Iron Age to Roman (FIG 7) The Iron Age to Roman period, about a millennium long, from about 600 BC to 400 AD, is marked in the area by scattered hillforts, ranging in size from less than a hectare to almost 50, with a few even larger dyke systems, and also by many settlement enclosures, the latter ranging from small singular farmsteads to larger complexes. This phase is marked by a distinct increase in the level of settlement, and by diversification in its location. Greater density is seen within the traditional core-areas of Neolithic and Bronze Age activity over the uplands, with further expansion into dip-slope valleys, and onto the river gravels of the adjacent Severn-Avon and upper Thames. Many of the well-established agrarian settlements of Iron Age date continued in Romanised form, with additional development of other sites, some of richer 'villa'-type, and a few, such as Wycomb, of 'small town' status. Evidence suggests that the area was one of extensive clearance for planned commercial arable cultivation and pastoral use, with its many settlement sites producing clear evidence for an active, mixed economy.

-Neolithic to Bronze Age (FIG 6) The Neolithic to Bronze Age phase, covering about 2.5 millennia, from about 3500 to 1000BC, is partly characterised in the area by the distribution of numerous long and especially round barrows. Little is known about the location and nature of occupation sites beyond scattered evidence from excavation and a series of flint scatters which indicate areas of activity if not always direct habitation. Available evidence suggests the existence of relatively scattered units of settlement and associated agriculture, with preference for the higher ground of spurs and hilltops adjacent to stream valleys, with an especial concentration around the headwaters of the River Windrush (Marshall 1985). The upland, and especially the mid to upper dip-slope, appears to have been an area of relatively high activity. On present evidence it was more intensively utilised over a longer period than the lower reaches of the Windrush, and adjacent areas of the upper Thames, although the gravels there have produced evidence for significant early activity (Benson and Miles 1974; Robinson and Lambrick 1984; Lambrick 1988).

A series of Iron Age hillforts are strung out along the main Cotswold scarp, at similar intervals, perhaps suggesting individual control over specific sectors of upland, and of settlements on the adjacent gravels of the Severn-Avon valley. Most of these hillforts lie at the margin of a discrete block of MS on the upland, as for instance Willersey Hill at the northern end of the Spring Hill major block, with Shenberrow, Burhill, Beckbury, Roel-?Salter's Hill, at the edges of the separate blocks which take their names. Several sites, such as Nottingham Hill, Cleeve Hill, Dowdeswell, and Salmonsbury are isolated in areas of 'low' MS, although blocks of higher MS do occur within a few kilometres. Such apparent detachment from intensive cultivation may indicate that these sites had a more pastoral emphasis, perhaps as centres controlling other economic resources, and were able to secure arable products more remotely.

Barrows and flint scatters tend to lie towards the margins of upland blocks of higher MS, as can be well seen for instance around the West Down, Roel, Westfield, and Salperton blocks. The Eyeford major block contains a particular concentration of sites, including a henge, and here too there is a tendency for more marginal location of settlement and activity areas, but with an increased spread towards central areas of the block. It is notable that the Spring Hill major block contains very few barrow and flint scatter sites, suggesting that this less well watered upland area may have remained more peripheral to the major focus for settlement and agriculture to its south. The Spring Hill major block is of more densely concentrated 'medium' to 'high' MS than the Eyeford major block, but it must be noted that this is its final condition, with these levels probably accruing mainly after the Neolithic-Bronze Age, during the Iron Age and Roman periods, the evidence for which is discussed later. During both this early and in later phases settlement appears concentrated in the Eyeford block, with little penetration apparent for the

Analysis of the entire distribution of sites suggests a pattern in which one or more hillforts, and numerous, possibly dependent 'open' settlements lie closer to accessible water-supplies, at the edges of an extensive area of upland. These tracts were ideal for cultivation and pastoralism, and through repeated clearance came to be marked by increased levels of MS. The Roel MS block, fairly typical of others in the area,

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MS over the landscape and more for stock, with sheep-rearing of growing regional importance during the medieval period. According to Domesday, arable seems conspicuously absent from the Eyeford-Spring Hill major blocks, much of which may have reverted to semi-dereliction, pastoralism and hunting.

gives the clearest indication of this basic model proposed for settlement (FIG 12). This block is similar in pattern to the adjacent Temple Guiting block, where small Iron Age farmstead and stronghold enclosures are located around its edge, with sites at The Park-Bowsings, Middle Ground, and The Paddocks subject to detailed investigation (Marshall 2007c; PAPER 1).

MS and environmental change in the survey area The existence of blocks of high MS in the area implies extensive clearance of land for agriculture over a lengthy period, on a scale likely to lead to considerable environmental change, both within the area, and beyond it. This contrasts with the suggestion that settlement and agriculture in the Cotswolds during the Iron Age was of smaller scale, and largely pastoral (Lambrick 1988). Changes caused by agriculture to ground-cover over the hills and valley slopes, converting them from wooded areas to open grassland and tillage, would increase the rate of drainage and soil erosion in the catchment, and affect the hydrology of areas downstream, in this case parts of the upper Thames valley (FIG 2).

There appears to be slightly more penetration of the Spring Hill major block by settlement than seen during the preceding Neolithic and Bronze Age, although this remains anomalously low. More sites in this under-explored area may await discovery. During the Roman period, expansion of settlement generally in the area continued, with clear evidence for the increased settlement and cultivation of lower valley slopes first noticeable during the mid Iron Age (FIGS 9-11). Clearance and ploughing of heavier valley soils would have been aided by greater availability of iron tools, and may have been necessitated by over-exploitation of core upland areas of traditional agriculture in repeated use during the preceding millennia, since the Neolithic period. The highest levels of MS shown to mark such areas may indicate literal 'burn-out' of the thin soils, with consequent decrease in fertility.

Information on the impact of clearance and agriculture on the landscape in the survey area, over the prehistoric and into the Roman period, can be obtained from several sources:

-early medieval (FIG 8) The early medieval period, from about 700 to 1400 AD, less than a millennium, is marked by the progressive development of village-type settlements, which lie almost entirely in the valleys which cut through the upland limestone, and which exploited considerable tracts of surrounding land for agriculture. Here the archaeological record is supplemented by data from the Domesday Book of 1086 (Moore 1982) which, within certain limits, can provide useful information on land-use during the later Saxon and early Norman periods. Domesday entries for named locations include, along with other tenurial and economic information, an indication of the area under arable, as a number of hides. The actual extent of a hide is debatable, and may well be variable, but the consensus value of about 120 acres, some 49 hectares, is taken here.

-sedimentary profiling within the survey area ..local streamlet valleys (FIG 11) Data from hill-wash in stream valleys on the dip-slope reflect smaller-scale changes at the local level, particularly useful if accompanied by dating evidence at key points. For instance, soil sections at The Paddocks and Middle Ground Iron Age enclosures indicate increasing levels of MS during the Iron Age, as enhanced sediment gravitated down from agricultural areas on the valley slope above (PAPER 1). ..barrow and enclosure ditches (FIGS 9-11) Barrow ditches can provide excellent data on environmental change in the area since they are widely distributed over upland and valley locations, and were collecting unbroken records of sediment and its inclusions from the surrounding landscape over several millennia. Long barrows would provide extended sequences from the Neolithic onwards, and round barrows from the early Bronze Age until the post-medieval period.

Unfortunately, no further details are given for the precise location of arable associated with settlements listed in Domesday, and so it is only possible to represent them uniformly ranged around the site, to give a general impression of relative size (FIG 8). Information on pastoral land is even more difficult to obtain for Domesday settlements. Even with these constraints, and others involved in analysis of this documentary source, a key aspect of land-use does seem clear: that arable was concentrated in valleys, and over their slopes. Upland areas, possibly still damaged from earlier over-use, may have been utilised less as plough-land

Analysis of MS, snail fauna, and carbonised plant material in the fill of the quarry pit at Guiting Power 1, and the ring-ditch at Guiting Power 3, both early Bronze Age round barrows (FIGS 9, 10), and in the ditches of Iron Age enclosures at The Park-Bowsings, and at The Paddocks (FIG 11) indicates a marked increase in land clearance during the Iron Age and Roman period, and provides some

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MS over the landscape from the mid Iron Age onwards, settlement began to increase rapidly, with widespread proliferation of agrarian sites, increased land clearance and utilisation, and development of a more open, organised agricultural landscape, a process which continued throughout the Roman period. This general sequence is similar to that indicated by evidence from the Cotswolds, and is typical of much of southern Britain (Evans 1975; Hingley and Miles 1984; Robson 1984; Lambrick 1992, 1992a; Bell 1996; Lambrick and Allen 2004).

chronology for discussion of distributions of MS over the landscape (Marshall 2007a-c). -sedimentary profiling beyond the survey area Land clearance over the Cotswolds has consequences for hydrology and sedimentary processes in both of the flanking major river valleys, but more so for the upper Thames than for the Severn-Avon. The Severn-Avon valley, along the NW side of the Cotswolds, receives some relatively minor drainage from the restricted area of the scarp, and changes there would tend to have a lower, relatively localised impact. On the other hand, events on the more extensive dip-slope, with its many stream valleys, are likely to produce far greater changes in hydrology, sedimentation, and the pattern of settlement in the upper Thames valley, to which it drains.

One sector of the upper Thames valley subject to particular archaeological investigation, and producing specific evidence for what was probably a valley-wide sequence of environmental change, has been that around the intersection between the Windrush valley and the Thames itself. Many environmental processes in this area are linked to those in the area of MS survey, part of its catchment.

Major hydrological changes in the upper Thames valley have already been linked to increased levels of land clearance in its water-catchment area, which extends over large areas of the Cotswolds to the north, with a lesser contribution from the chalk-lands to the south (Robinson and Lambrick 1984). Similar studies of linked upland and valley systems have been carried out elsewhere in southern Britain (Bell 1983; Scaife and Burrin 1983).

The Windrush valley forms an important element in the highly dendritic drainage system of the upper Thames valley, here defined as that sector upstream from Oxford (FIG 2). Its catchment area covers some 500 km2, about 60% of which consists of land around a fan of headwater streams, which drain a substantial wedge of the upland and upper dip-slope of the Cotswolds, each stream-system merging to a single course in its more restricted mid and lower valley. The Cotswold dip-slope/Thames valley presents a well-linked system of upland 'source' and valley 'sink' areas, clearly suited to investigation of environmental change, both natural and economically-induced, and its wider ecological consequences. The Windrush system also has an advantage for such investigation in that its headwaters penetrate an area of particularly dense early settlement and agriculture, and so levels of environmental impact here are likely to be more pronounced, and processes easier to detect.

..the lower Severn valley Studies relating sedimentary change in the Severn valley to patterns of land use and vegetational change in adjacent areas indicate extensive clearance during the later Bronze Age to Iron Age, with associated increases in alluviation (Shotton 1978; Hazledean and Jarvis 1979; Brown 1983; Brown and Barber 1985). ..the upper Thames valley The upper Thames valley contains extensive areas of gravel, which form a well-drained substrate, far more amenable to early settlement and agriculture than the less tractable clays which underlie and surround them. Scattered settlement took place on these gravels during the Neolithic and Bronze Age, as shown by barrow sites, numerous in places, and there is some evidence for habitation and other activity. Relatively localised clearance of the land took place, with progressive development of larger open tracts. Environmental evidence indicates that significant areas of woodland nevertheless survived into the early Bronze Age, at a time when the future flood-plain remained relatively dry. Evidence for changing land use in the upper Thames area during the subsequent Bronze and Iron Age is supported by pollen analysis, which demonstrates declining oak, lime, and hazel, and increasing cereal and weed pollen associated with arable cultivation, such increased clearance accompanied by a rising water table. During the first millennium BC, and especially

In contrast to the preceding Neolithic and Bronze Age, during the period between the late Bronze Age and middle Iron Age, parts of the upper Thames valley became more prone to extensive flooding, as shown by data from excavated sites and associated sedimentological investigation on the flood-plain. By the mid Iron Age, fuller occupation at certain sites which did push onto the flood-plain itself may have been restricted to intervals of the dry season, with cultivation of cereals not possible, being replaced by more practical horse- and cattle-rearing (Lambrick and Robinson 1979; Allen 1990; Allen and Robinson 1993). During the Roman period, prevalence of wetter ground-conditions may have been increasing: for instance, at Farmoor settlement moved from the

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MS over the landscape survey outlined in this paper. However, there may also be a contribution to flooding from generally increased levels of rainfall during this period, with deteriorating weather conditions noted during the mid Iron Age (Lamb 1995). That such flooding continued into the Roman period, despite marked climatic improvement, argues against rainfall being the major contributing factor, and in favour of aggressive land clearance and cultivation, which certainly continued.

flood-plain up onto the adjacent gravel terrace (Lambrick and Robinson 1979). Although the effects of flooding have been detected during both Iron Age and Roman periods, deposition of alluvium in the valley only became more significant during the latter. During the Iron Age, land clearance in the catchment area of the upper Windrush appears to have been sufficient to allow increased seepage of water into the main river, raising the water table and causing flooding in its lowest reaches. However, only during the Roman period were there sufficient areas under intensive cultivation to provide run-off with enough sediment-load to cause alluvial deposition in the main valley itself (Robinson and Lambrick 1984; Limbrey 1983, pp. 204-206).

This change in hydrology noted in the upper Thames basin during the later prehistoric period should not be seen entirely in negative terms. Although flooding and alluviation would have rendered much of the immediate flood-plain (FIG 2) unsustainable as pasture for long period in winter, and have limited the areas available for practical cultivation, it would have improved the productivity of grassland in summer. In the more extensive areas beyond the flood-plain, a rise in the water-table, and a consequent increase in general hydration of soils over the higher ground of the gravel terraces, the location for much settlement, would have been favourable to crop-growth.

A complex series of factors affects the hydrodynamics of catchment areas and the balance between flow overland and through-flow within sediment and rock. These include the nature of soil and underlying bedrock, agricultural practices, the condition of surfaces, and type of vegetation cover (Kirkby 1978). A major increase in active plough-land, especially on the sloping sides of valleys, which were certainly becoming more settled during the Roman period, provided unstable ground-surfaces open to increased erosion over the autumn to spring wet season, and this would explain the onset of significant alluviation. Sediment from the mid and upper levels of the various sub-catchments would have made the greatest contribution to alluvial deposits, with inputs from clearance of the valley gravels fairly minor, since these are fairly level and free draining.

Examining the pattern of drainage in the upper Thames area (FIG 2) it is clear that the area around the lower Windrush is a hydrologically sensitive one, exposed as it is to inputs of water not only from the Windrush system itself, but also from the entire dendritic system of the Thames upstream from the Windrush-Thames junction. Even minor variations of input from these catchments would tend to cause noticeable change in this 'sink' area around the junction. Further constriction of sudden flow is likely to have been caused by marshland and by extensive meanders, such as seen in the great loop of the river at Oxford, where the course changes from E to S (Robinson and Lambrick 1984, fig.1). Such physical constraints, coupled with extensive wetland vegetation, would act to slow and spread river-flow, encouraging settling of alluvium.

A change in the rate and nature of sedimentation in the upper Thames valley is clearly indicated where gravel or an organic sediment comes to be overlain quite suddenly by a layer of alluvial clay. This allogenic change is typical of locations where products of land clearance accumulate downstream. One such location, in the lower Windrush valley, just S of Witney, produced wood from the alluvial interface dated to the earlier 1st millennium BC (Hazeldean and Jarvis 1979). Here, the radiocarbon age 2660 +/-85 BP [laboratory reference I-9337], can be calibrated by Oxcal software as 970-760 BC at 1 s.d., and 1020-510 BC at 2 s.d. Although some reservations about detailed stratigraphy have been expressed (Robinson and Lambrick 1984, p. 809), a general date in the later Bronze Age to early Iron Age seems clear.

In view of this hydrological sensitivity in the 'sink' area, it is perhaps better not to over-estimate the gravity of environmental changes occurring in the 'source' area which contributed to the observed flooding and alluviation. The picture of the Cotswold upland as one of wholesale mismanagement of a very open, cleared, near-terminal landscape, subject to erosion from greatly increased run-off, seems unrealistic. Interpretation as an area which was indeed intensively used, was subject to deterioration, perhaps locally fairly severe, but which was maintained as an economically-viable ecosystem, seems inherently more likely, supported by clear evidence for a productive rural economy in the area throughout the Roman period.

The evidence for higher water tables and seasonal flooding in the lower reaches of the Windrush accords well with evidence for complementary processes of land clearance from the upper parts of the valley, as indicated by archaeological and environmental evidence, and supported by the MS

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MS over the landscape settlement, and its Romanised successor in the area contributing (FIG 12). A round barrow (Guiting Power 1: Marshall 2007a), and flint scatter, both also at the margins of the block, as if adjacent to a territory, indicate earlier exploitation during the later Neolithic and earlier Bronze Age. MS profiling of sediment in the quarry pit of this round barrow indicates that higher levels of MS only started to develop during the Iron Age and Roman periods (FIGS 9-10).

VISUALISING EARLY LAND CLEARANCE Quantitative modelling Detailed modelling of land clearance and consequent enhancement of MS within a settlement area would be very difficult to achieve in a meaningful way without clear information on the origin, nature, and extent of contributions to the system and their phasing. Precise details of the agricultural cycle, and the role of burning within it, would also be required.

The following question therefore emerges, to be addressed by any model of the process: could such a basic unit of settlement have generated the observed levels of MS within such an area over the known period of its occupation? In order to examine this problem further, the process of land clearance must be outlined in more detail:

Any abstract model would, therefore, best be applied first to a small-scale, relatively isolated settlement within a territory which could be fairly well limited topographically. Ideally, to simplify the analysis, such a site would have been established on uncleared, unburnt land, with abandonment of the settlement area after a well-defined period of occupation. Even if an observed area of MS enhancement could be reasonably well identified with such a singular site, a great deal of guess-work would still be involved in providing estimates of area for associated arable, semi-cleared fallow, and for the pastoral zone, all subject to periodic burning off.

-types of burning Burnt, MS-enhanced sediment in such a notional unit settlement area would come from five main sources: ..within the arable zone: type a...an initial grand clearance of larger trees from original woodland, carried out once, or repeated very few times after protracted reversion of land to very mature woodland; b...annual post-harvest light burning of stubble, grass, and other weeds; c...semi-annual inter-harvest 'ash-bedding' of active arable; d...major burning off of fallow after a period of several years to provide fresh arable;

The realities within one detailed study area, which forms part of the major MS survey, further illustrate some of the obstacles to quantifying the problem, and suggest a way forward. The Temple Guiting block of MS lies fairly well separated from other adjacent blocks: the Guiting Power, Roel, and Beckbury blocks (FIG 12; Marshall 2007 a-c). Around its margins are ranged 5 or 6 known small Iron Age and Roman settlements, possibly each exploiting individual areas of the subdivided block, together with other areas of it in common, to form a composite territory of about 2.3km2, or 230 hectares. Just to its S lies the smaller, discrete Guiting Power block, about 0.5km2, or 50 hectares in maximum extent, with one such known settlement at its margin, providing a very clear candidate for a basic unit area of settlement and adjacent MS-related clearance. Such a basic unit would fit into the composite Temple Guiting block about 5 times, suggesting that this larger zone may consist of about 5 similar units of individual settlement plus land. This is the number of sites currently observed. It should be noted that the MS block would represent the more heavily exploited core area of the territory, which would have extended beyond to include a significant marginal zone.

..within the pastoral zone: e... periodic burning back of scrub to allow fresh re-growth, possibly on an annual autumnal basis. -an arable rotational system Consider a simplified system of 3 plots comprising the arable zone of a small agricultural settlement of the type outlined above, in a notional rotation of cultivation where the inter-fallow period (F) for each plot was 3 years (TABLE 1). Burning would take place on each plot as follows: an initial grand 'type a' clearance, then 3 'type b and c' burnings whilst active arable, followed by one 'type d' burning at the end of fallowing to clear the years of scrub re-growth. The fallow would probably be subject to grazing to keep scrub growth down and help replenish organic content by manuring. After a set number of cycles (C) the land might become less viable through reduced fertility or infestation. If C were 3 then the arable zone would shift every 27 years. The values adopted here are notional, but seem reasonable.

Within the Guiting Power block the development of higher MS in soils can be clearly related to the Iron Age and Roman period from stratigraphic studies (FIGS 9-10), with the adjacent Manor Farm Iron Age

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MS over the landscape TABLE 1: Assessing the extent of cumulative burning taking place on long-term arable land number of rotations (R) 1 2

BURNING ON EACH PLOT for each intercumulative fallow period (F)

a + 3(b+c) 3(b+c)+d

Generalising this gives: R

a + a +

3(b+c) 6(b+c)+ d

a + RF(b+c)+ (R-1)d

Where the complete cycle C is 3 rotations, taking 27 years: 3 3(b+c)+d a + 9(b+c)+ 2d Allowing abandonment for 10 years before re-clearance might allow about 3 such complete cycles per century: 3a + 27(b+c)+ 6d and hence over the 500 year known occupation of certain iron age enclosures in the survey area, about 15 cycles, the extent of burning might be: 15a + 135(b+c)+ 30d the Guiting Power MS block, and would have perhaps best been kept somewhat separate from the cultivated zone. Such projections for arable and pastoral land would therefore account for about 80% of the MS block, a reasonable fit between settlement and areas of increased MS.

Assigning arbitrary low MS values to the variables thus: a=3, b=0.5, c=1, and d=1.5, gives a cumulative value for MS as 293, well into the 'medium' level encountered widely over the sample area. It should be noted that this final figure for MS is particularly sensitive to changes in constants b and c. Raising them by only 0.25 would, for instance, give a final value of 405, which is approaching the 'high' level. The value of constants 'a' to 'd' could be determined experimentally, but in the meantime this basic model, over-simplified though it is, certainly suggests that the cumulative burning likely to have been sustained by arable surfaces over centuries and millennia could account for the levels of increased MS observed over the landscape.

The existence of discrete areas of repeated clearance adjacent to settlement areas is suggested by MS surveys at Humblebee, Cotehay, and Roel (FIG 13). The clearest example is adjacent to the hillfort at Roel, where a patch of high enhancement covers some 5 hectares, the core area of which would be sufficient to accommodate a system of arable rotation. This may perhaps be one of several such local blocks which together formed the entire arable area for the site.

Refinement of this model to reflect the actual situation more closely would be difficult, given the general absence of detailed information on actual agricultural practices. For instance, in the absence of clearly associated field systems, calculation of arable area, an important ingredient in the discussion, can only be attempted indirectly. Analysis of data from the small agrarian settlement complex at The Park-Bowsings (Marshall 2007c; PAPER 1) suggests that, on the basis of modelling grain storage from volumes of known pits of storage type, the area of land under direct cultivation of grain may have been 1.5-3 hectares. Allowing another 2 hectares for other crops would give a maximum total of about 5 hectares, about 10% of the Guiting Power MS block. On this basis a 3-plot rotational system would therefore cover 15 hectares, 30% of the block.

It is possible to simulate exploitation of such a territory by allowing these frames of arable and pastoral use to move around the block and additional margins according to the simple rules outlined above. Notional inputs of MS for each type of burning can be allocated to assess the cumulative effect over a defined period, with about 800 years suggested by stratigraphic data from the main Iron Age-Roman period (Marshall 2007a-c). Even with assumption of very low inputs of MS per burning event the level of cumulative MS in the area reaches the 'medium' level shown in FIG 12. This suggests that development of the pattern of MS over the landscape is certainly capable of being generated by known settlement of the Iron Age and Roman period. The model on which this is based however remains speculative, with many of the processes involved and values adopted for variables

The area in use for more intensive grazing, and therefore subject to 'type e' burning, can be estimated roughly as about 25 hectares, about half of

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MS over the landscape surface contact, would produce far more ground-ward radiation to enhance MS in agricultural soils toward currently observed levels. Analysis of examples of modern reversion of arable fields to scrub-land in the area can help visualise the clearance process, and provide some context for considering fire-based maintenance of the arable zone during the prehistoric period.

necessarily remaining guesswork, albeit reasonable, and supported in places by key data. Insights from observation of modern reversion of arable and processes of clearance (PLATES 1-3) In considering build up of MS in soils it is also important to assess whether scrub regeneration over the intervals outlined above for the notional clearance cycle would produce sufficient standing fuel to generate significant magnetic enhancement of underlying sediments. Mass in-situ flash-firing, probably relatively occasional, is likely to produce more upward radiation from the canopy than intense baking of sediment. However, many individual clearance fires, well aerated and protracted, in direct

With this in mind two representative areas of reversion were examined: area 1, an ungrazed area of abandoned arable on a sheltered spur, and area 2, an area of uncleared long-term pasture routinely grazed by sheep on an exposed hilltop (TABLE 2):

TABLE 2: Sample areas of modern scrub-regeneration used in discussion of ancient land-clearance Locn

Name

NGR

1

Postlip

SP 009 266

2

Cleeve Hill SO 99 25

alt (m) 170

topography spur

300-325

hilltop

agricultural condition former arable; 20 years fallow; left ungrazed.

main scrub species

Hawthorn (Crataegus monogyna) mean spacing: 1.3m mean height: 3.1m ancient pasture; Gorse uncultivated; (Ulex europaeus) well grazed. mean spacing: 2.1m mean height: 1.5m

PLATE 1-2

3

The extensive development and spread of highly MS-enhanced sediment can be gauged from a modern clearance fire (PLATES 2c,d). Two sample fires required to clear surplus non-useful timber from 0.1ha of mature coppice generated ash-beds containing highly baked sediment, each some 7m in diameter, 20cm maximum depth over the centre, and of 0.12m mean thickness. Together they produced about 9m3 of burnt sediment of mean vMS 448 (units: SI), a 14-fold enhancement of the natural unburnt background level of 32, spreading it over 77m2. Taking this as part of a cycle of clearance repeated many times, the levels of actual MS enhancement noted here seem sufficient to explain the observed distribution over the landscape from ancient sources.

-location 1 After 20-year reversion, area 1 became covered by a dense growth of hawthorn (PLATE 1), with mature trees of 13 year growth at 6m high with upright boles 41cm in basal girth (PLATE 1d). This area was taken to represent areas of longer-term ungrazed regeneration, marginal to the main arable and pastoral zones, of a type on which fresh arable might be resumed after prolonged resting as part of the fallowing cycle. Such levels of scrub growth might be expected during the 27 year cycle discussed above (TABLE 1). Although mature trees present more of a challenge to clearance, the boles of younger stands (PLATE 1d) are far more easily axed, or ring-barked, then semi-felled and burnt. Even after removal of any useful timber, the calorific value of remaining growth, some perhaps ring-barked and dead, would be considerable, and a source for many clearance fires capable of causing intense localised baking (PLATE 2b-d). Promotion of firing by partially felling trees to compress them, thereby reducing partial burn-out (PLATE 2a), would cause more ground-ward radiation and baking of top sediment than in an in-situ canopy fire. It would also leave fewer residual charred trunks and roots, hence providing a more effective method of clearance. A minor contribution to magnetic enhancement, by flash-scorching of surface sediment, would result from firing grass and other smaller ground cover within more open areas of regeneration (PLATE 3).

-location 2 This area contains lighter scrub, with persistent grazing by sheep and cattle supressing larger woody species, and would be more representative of pastoral reuse of fallow land lying adjacent to active arable. The resulting growth would be difficult to clear by axe-work given the predominant gorse, with its dense, squat, thorny, grazer-resistant habit. Wholesale torching of such areas during late summer or autumn, when much lighter vegetation would be dying back to provide good starter-fuel, would seem the easier option. Given the lighter vegetation cover, inputs of MS to the soil are likely to have been smaller than for landscapes represented by location 1, but still sufficient to cause significant build-up over

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MS over the landscape and marking core-areas of traditional settlement, with roots in the Neolithic and Bronze Age. Distinct blocks of higher MS appear to link some settlement sites into topographically-coherent territories, perhaps of common exploitation. Such evidence for organised and increasing land-clearance provides further explanation for certain environmental changes noted beyond the area, downstream in the upper Thames valley.

centuries, if not millennia, to the levels observed.

CONCLUSIONS This analysis indicates the importance of large-scale MS survey for discussion of aspects of landscape history and organisation of early agricultural settlement. Within the study area, extensive areas of long-term land-clearance by burning, marked by distinctly higher levels of MS are evident, increasing in intensity during the Iron Age and Roman periods,

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SUPPLEMENTARY INFORMATION Appendix 1: location of sites mentioned in the text Barrow sites are numbered according to the system of O'Neil and Grinsell 1960. Site Parish NGR (SP-) Ref. ------------------------------------------------------------Bronze Age round barrows Cotehay Sudeley 025 232 PAPER 4 Guiting Power 1 Guiting Power 08443 24461 Marshall 2007a Guiting Power 3 Guiting Power 09574 24660 Marshall 2007b Humblebee Sudeley 023 250 PAPER 4 Temple Guiting 2/3 Temple Guiting 08313 29186 PAPER 4 Temple Guiting 8 Temple Guiting 10835 28540 PAPER 4 Upper Slaughter 1 Upper Slaughter 13025 24565 PAPER 4 Iron Age hillforts Beckbury Burhill Cleeve Hill Dowdeswell Nottingham Hill Roel Gate Salmonsbury ?Salter's Hill Shenberrow Hill Willersey Hill

Temple Guiting Buckland Southam S0 Dowdeswell SO Gotherington SO Sudeley Bourton Winchcombe Stanton Willersey

Iron Age enclosures -upper Windrush valley The Bowsings Guiting Power The Park Guiting Power Middle Ground Temple Guiting The Paddocks Temple Guiting Lot's Barn Temple Guiting Wharton's Furlong Cold Aston

0640 2990 085 363 985 255 998 191 983 284 0465 2435 173 209 045 286 081 334 117 383

PAPER PAPER RCHME RCHME PAPER PAPER RCHME RCHME RCHME

2 2 1976 1976 2 2 1976

08580 08325 09180 094 11320 13000

PAPER PAPER PAPER PAPER PAPER PAPER

1 1 1 1 1 1

25865 25865 27505 272 27860 21580

-lower Windrush valley Farmoor Farmoor 444 056 1979 Mingies Ditch Hardwick/Yelford 391 059 Watkins Farm Northmoor 426 035 Roman sites Millhampost Wycomb

Stanway Andoversford

041 308 027 200

1976 1976

Lambrick and Robinson Allen and Robinson 1993 Allen 1990 PAPER 3 PAPER 3

More extensive area surveys of MS by the author (AJM) Cotehay Sudeley 025 232 PAPER 4 Humblebee Sudeley 023 250 PAPER 4 Roel Gate Sudeley 047 244 PAPER 2 Mingies Ditch, Hardwick-with-Yelford, Oxon. Thames Valley landscapes: The Windrush valley, volume 2. Oxford: Oxford Archaeological Unit/ Oxford University Committee for Archaeology.

BIBLIOGRAPHY Allen, T.G. 1990. An Iron Age settlement and Romano-British enclosed settlement at Watkins Farm, Northmoor, Oxon. Thames Valley landscapes: The Windrush valley, volume 1. Oxford: Oxford University Committee for Archaeology.

Brown, A.G. 1983. Floodplain deposits and accelerated sedimentation in the lower Severn basin. In K.J. Gregory (ed.). Background to palaeohydrology: a perspective. John Wiley. pp. 375-397.

Allen T.G., and Robinson, M.A. 1993. The prehistoric landscape and Iron Age enclosed settlement at

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MS over the landscape of London. pp. 78-105.

Brown, A.G., and Barber, K.E. 1985. Late Holocene palaeoecology and sediment history of a small lowland catchment in central England. Quaternary Research 24, 87-102.

Lambrick, G. 1992a. Alluvial archaeology of the Holocene in the Upper Thames Basin 1971-1991: a review. In S. Needham and M.G. Macklin (eds.). Alluvial archaeology in Britain. Monograph 27. Oxford: Oxbow Books. pp. 209-226.

Bell, M. 1983. Valley sediments as evidence of prehistoric land use on the South Downs. Proceedings of the Prehistoric Society 49, 119-150.

Lambrick, G., and Allen, T. 2004. Gravelly Guy, Stanton Harcourt: the development of a prehistoric and Romano-British community. Thames Valley Landscapes. Monograph 21. Oxford: Oxford Archaeology.

Bell, M. 1996. Environments in the 1st millennium BC. In T.C. Chapman and J.R. Collis (eds.). The Iron Age in Britain and Ireland: recent trends. pp. 5-6. Benson, D., and Miles, D. 1974. The Upper Thames valley: an archaeological survey of the river gravels. Survey 2. Oxford: Oxford Archaeological Unit.

Lambrick, G., and Robinson, M. 1979. Iron Age and Roman riverside settlements at Farmoor, Oxfordshire. Research Report 32. Oxford: Oxford Archaeological Unit and Council for British Archaeology.

Dalan, R.A., and Goodman, D. 2007. Imaging buried landforms using down-hole susceptibility data and three-dimensional GPR visualisation software. Archaeological Prospection 14(4), 273-280.

Limbrey, S. 1983. Archaeology and palaeohydrology. In K.J. Gregory (ed.). Background to palaeohydrology: a perspective. John Wiley.

Evans, J.G. 1975. The environment of early man in the British Isles. London: Elek.

Marshall, A.J. 1985. Neolithic and earlier Bronze Age settlement in the northern Cotswolds: a preliminary outline based on the distribution of surface scatters and funerary areas. Transactions of the Bristol and Gloucestershire Archaeological Society 103, 23-54.

Hazleden, J., and Jarvis, M.G. 1979. Age and significance of alluvium in the Windrush valley, Oxfordshire. Nature 282, 291-292. Hingley, R. and Miles, D. 1984. Aspects of Iron Age settlement in the upper Thames valley. In B. Cunliffe and D. Miles (eds.) Aspects of the Iron Age in central southern Britain. Monograph 2. University of Oxford: Committee for Archaeology. pp. 52-71.

Marshall, A.J. 2007a. Analysis of an early Bronze Age round barrow: a case study at Guiting Power 1, Glos. (UK). Archaeological Report 1. Cheltenham: Guiting Manor Amenity Trust. ISSN 0960-197X.

Johnson, A.E. 1994. Rectory Farm, West Deeping, Lincolnshire: topsoil magnetic susceptibility and gradiometer survey. Unpublished report (survey reference 0321093/REL/TEM). Oxford: Oxford Archaeotechnics.

Marshall, A.J. 2007b. Interpretation of an early Bronze Age round barrow: excavation of the monument at Guiting Power 3, Glos. (UK). Cheltenham: Guiting Manor Amenity Trust. Archaeological Report 2. ISSN 0960-197X.

Johnson, A.E. 2003. A46 Newark to Widermere improvements: geophysical survey. Unpublished report (survey reference 2730403/MAW/TPAT). Oxford: Oxford Archaeotechnics.

Marshall, A.J. 2007c. Farmstead and stronghold: development of an Iron Age and Roman settlement complex at The Park-Bowsings, near Guiting Power, Glos. (UK). Archaeological Report 4. Cheltenham: Guiting Manor Amenity Trust. ISSN 0960-197X.

Kirkby, M.J. 1978. Hillslope hydrology. John Wiley.

Moore, J.S. 1982. Domesday Book: Gloucestershire. Chichester: Phillimore.

Lamb, H.H. 1995. Climate, history and the modern world. 2nd edn. Routledge.

15,

O'Neil, H., and Grinsell, L.V. 1960. Gloucestershire barrows. Transactions of the Bristol and Gloucestershire Archaeological Society 79, 1-149.

Lambrick, G. 1988. The Rollright Stones: megaliths, monuments, and settlement in the prehistoric landscape. London: Historic Buildings and Monuments Commission for England.

Palmer, R. 1984. Danebury: an Iron Age hillfort in Hampshire: an aerial photographic interpretation of its environs. Supplementary Series 6. Royal Commission on Historical Monuments: England. London: HMSO.

Lambrick, G. 1992. The development of late prehistoric and Roman farming on the Thames gravels. In M. Fulford and E. Nichols (eds.). Developing landscapes of lowland Britain. The archaeology of the British gravels: a review. Occasional Paper 14. London: Society of Antiquaries

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MS over the landscape Pyne, S.J. 1997. Vestal Fire: An environmental history, told through fire, of Europe and Europe's Encounter with the World. Seattle and London: University of Washington Press. ISBN 0-295-97596-2.

FIGURE 2 Topography and hydrology of the northern Cotswolds, the upper Thames valley, and its tributaries:. FIGURE 3 Topography and general geology of the area surveyed for magnetic susceptibility (MS) in the northern Cotswolds.

RCHME 1976. Ancient and historical monuments in the county of Gloucestershire. 1: Iron Age and Romano-British monuments in the Gloucestershire Cotswolds. Royal Commission on Historical Monuments: England. London: HMSO.

FIGURE 4 Distribution of magnetic susceptibility (MS) in basal topsoil over the survey area in the northern Cotswolds. Inset: definition of individual blocks of higher MS.

Robinson, M.A., and Lambrick, G.H. 1984. Holocene alluviation and hydrology in the Upper Thames basin. Nature 308, 809-814.

FIGURE 5 Place-names used in the survey area of the northern Cotswolds for individual sites, and for blocks of higher magnetic susceptibility (MS).

Reynolds, P.J. 1980. The working agroscape of the Iron Age. Landscape History: Journal of the Society for Landscape Studies 2, 1-18. ISSN 0143-3768.

FIGURE 6 Distribution of magnetic susceptibility (MS) in basal topsoil over the survey area in the northern Cotswolds: relationship to Neolithic and Bronze Age sites. Inset: detailed MS surveys at specific sites: refer to 'Location of sites mentioned in the text'.

Reynolds, P.J. 1985. Carbonised seed, crop yield, weed infestation and harvesting techniques of the Iron Age. Les techniques de conservation des grains a long terme, fasc. 2. Paris: CNRS.

FIGURE 7 Distribution of magnetic susceptibility (MS) in basal topsoil over the survey area in the northern Cotswolds: relationship to Iron Age and Roman sites. Inset: detailed MS surveys at specific sites: refer to 'Location of sites mentioned in the text'.

Robson, M. 1984. Landscape and environment of central southern Britain. In B. Cunliffe, and D. Miles (eds.). Aspects of the Iron Age in central southern Britain. Monograph 2. Oxford: University of Oxford Committee for Archaeology. pp. 1-11. Scaife, R.G., and Burrin, P. 1983. Floodplain development and vegetational history of the Sussex High Weald and some archaeological implications. Sussex Archaeological Collections 121, 1-10.

FIGURE 8 Distribution of magnetic susceptibility (MS) in basal topsoil over the survey area in the northern Cotswolds: relationship to early medieval sites listed in the Domesday book.

Schrufer-Kolb, I. 2004. Roman iron production in Britain: technological and socio-economic landscape development along the Jurassic Ridge. British Archaeological Reports British Series 380. Oxford: BAR Publishing.

FIGURE 9 Changing levels of magnetic susceptibility (MS) from the early Bronze Age to post-Roman period, as shown by stratigraphy in ditches of round barrows at Guiting Power 1 and 3, sites in the survey area.

Shaw, T. 1969. Tree-felling by fire. Antiquity 43, 52.

FIGURE 10 The rate of change of MS through the fill of barrow ditches at Guiting Power 1 and 3 round barrows from the early bronze age to the post Roman period. The rate is calculated from the graph of MS against depth through the fill, as the tangent to the line. The depth of sedimentary columns for both sites have been brought to the same relative scale.

Shotton, F.W. 1978. Archaeological inferences from the study of alluvium in the lower Severn-Avon valleys. In S. Limbrey and J.G. Evans (eds.). The effects of Man on the landscape: the Lowland Zone. Research Report 21. London: Council for British Archaeology.

FIGURE 11 Changing levels of magnetic susceptibility (MS), from the early-mid Iron Age to post-Roman period, as shown by stratigraphy in ditches of Iron Age enclosures at The Park, Bowsings, and Paddocks, sites in the survey area.

CAPTIONS FOR FIGURES FIGURE 1 Location of the survey area (A) in the northern Cotswolds (upper Windrush valley). A hydrologically-related area (B), in the lower Windrush valley, which has been subject to relevant archaeological and environmental investigation is also shown.

FIGURE 12 The Guiting Power area: distribution of MS.

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A sample of scrub semi-felled and layered as for firing; scale: 2m; B the fuel-stack of a small clearance fire; scale: 2m; C the burnt out ash-bed of a clearance fire; scale: 1m; D a section through the above ash-bed; scale: 20cm.

CAPTIONS FOR PLATES PLATE 1 Postlip: location 1/ shrub re-growth. The age in years for individual trees is marked in B and C.

PLATE 3 Cleeve Hill: location 2/ shrub re-growth. General and detailed views of rough grass with spreads of gorse (Ulex europeus) interspersed with minor tree growth, mainly hawthorn (Crataegus monogyna), wth occasional ash (Fraxinus excelsior) and sycamore (Acer pseudoplatanus); scale: 2m.

A the entire area of hawthorn scrub viewed from SP 012 262 looking NNW; B the edge of a densely packed mature stand of hawthorn; scale: 2m; C younger growth; scale: 2m; D a mature hawthorn bole, 13 years old; scale 20cm.

Ethnographic parallel: slash-and-burn in progress at Eno, Finland, 1892 (Copyright: Wikipedia 2010).

PLATE 2 Postlip: location 1/ shrub clearance. The age in years for individual trees is marked in A

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ITEMS COMMON TO ALL PAPERS ACKNOWLEDGEMENTS All project planning, geophysical and other survey, analysis of data, and final preparation of text and figures were carried out by Alistair Marshall.

Thanks are due to Tony Johnson (Oxford Archaeotechnics) for access to unpublished data on his large-scale MS surveys, also to Dr Paul Linford (English Heritage) and Dr Chris Gaffney (University of Bradford) for general information and comments.

The probe was developed as an experimental prototype, and later as a commercial product by Geoff Bartington of Bartington Instruments (Oxford OX8 7GE, UK), to whom great thanks are due for its loan and periodic repair.

Co-operation of the many landowners who gave permission to carry out project work on their property is gratefully acknowledged.

Software used to present the results of geophysical survey (Geoplot) was provided by Roger Walker (Geoscan Instruments, Bradford BD14 6AE, UK), to whom again many thanks.

COPYRIGHT The author, A.J. Marshall produced all of the figures and plates and is the copyright holder, unless otherwise stated. Where material from other sources has been used as a basis for figures this is clearly acknowledged (after: [author, publication]).

The author is most grateful to Dr Andrew David (English Heritage) who read the final text and provided many useful suggestions.

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