Evolution of the Landscape of the San Francisco Bay Region [Reprint 2020 ed.] 9780520315235

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California Natural History Guides: 7

EVOLUTION OF THE

LANDSCAPE OF THE SAN FRANCISCO BAY REGION

BY

ARTHUR DAVID HOWARD

UNIVERSITY OF CALIFORNIA PRESS BERKELEY, LOS ANGELES, LONDON

UNIVERSITY O F CALIFORNIA PRESS BERKELEY AND LOS ANGELES, CALIFORNIA UNIVERSITY O F CALIFORNIA PRESS LTD. LONDON, ENGLAND © 1 9 6 2 BY T H E REGENTS O F T H E UNIVERSITY O F CALIFORNIA FOURTH PRINTING, 1 9 7 4

ISBN:

0-520-00577-5

LIBRARY O F CONGRESS CATALOG CARD N U M B E R :

62-17535

PRINTED I N T H E UNITED STATES O F A M E R I C A

CONTENTS

INTRODUCTION

5

T H E MODERN LANDSCAPE

7

T H E REGIONAL SETTING

7

T H E LOCAL SETTING

8

PROBING THE P A S T

13

T H E P R O B L E M OF THE BEGINNING

22

T H E G R E A T INUNDATION: THE MIOCENE E P O C H

28

END-MIOCENE

MOUNTAIN-MAKING

T H E L E S S E R INUNDATION: THE PLIOCENE E P O C H

43 47

L A T E - P L I O C E N E MOUNTAIN-MAKING

53

T H E G R E A T DENUDATION

57

T H E PLEISTOCENE E P O C H

59

T H E FIRST STAGE

59

T H E SECOND STAGE

64

G E N E R A L REFERENCES

72

I L L U S T R A T I O N ON COVER:

Lover's L e a p : a volcanic neck. (Photograph by Arthur C. Smith)

INTRODUCTION The majestic setting of the San Francisco Bay Region, with its picturesque land-locked harbor, its rugged mountain border, and its lovely valleys, represents the culmination of a remarkable series of natural events that had its beginning many millions of years ago. Few realize that on a number of different occasions within the recent geologic past, mountains were upheaved here only to be later worn to their roots; that the waters of the Pacific repeatedly flooded the area even to the site of the present Sierra; that generations of volcanoes came into existence, spread lava and ash over the landscape, and then disappeared; that titanic forces cracked the surface of the Earth and jostled great blocks about like cordwood; that the climate at times was far less pleasant than now, sometimes much warmer, sometimes much colder. The landscape is not immobile, nor permanently set in stone. On the contrary, it is continually changing. Its appearance at any one time represents a fleeting episode in an endless struggle—a struggle in which powerful forces from within the Earth are arrayed against forces from without. Tl>e internal forces crumple and break the outer shell of the Earth and heave it into mountains. Their activity is generally accompanied by volcanic activity and jarring earthquakes. We must not suppose, however, that the rise of mountains is cataclysmic; from the human standpoint it is painfully slow. It is true that in some regions, like the western United States, an entire mountain range may break loose from its surroundings and snap upward a few feet, but these movements rarely occur more than once a century. Thus, the average rate of growth of such mountains is only a few inches a year. Other ranges rise without perceptible movement, by an infinitesimally slow buckling of the ground. Precise sur[5]

veys, however, when periodically repeated over long spans of time, reveal these faint changes in elevation. The cumulative effects of even these imperceptible movements are the lofty mountains of the Earth. This explains why we find entombed in the rocks of many high peaks the fossilized shells of creatures that once lived in the sea, thousands of feet below. Against the powerful internal forces are arrayed others, much less spectacular, that operate more subtly and insidiously and pass largely unnoticed. These are the forces of weathering and erosion which slowly but relentlessly nibble away at the highlands created by the forces from within. When the internal forces dominate, mountains stand high and dot the surface of the Earth; when the internal forces are dormant, the processes of weathering and erosion dominate and carry the substance of the mountains into the sea. There have been times in the past when the internal forces lay dormant for such long periods of time that the mountains in some regions were completely obliterated and the landscape was reduced to a low plain. The landscape, then, changes constantly at the whim of the forces that produce it. The present landscape may be likened to a single frame in a timeless motion picture—a motion picture run at an inconceivably slow rate. It is our intent in this little book to rerun a small part of this film—a part amounting to a fraction of one per cent of the great span of Earth history—only that part in which the unusual past of the Bay Region is involved. Before delving into this story, however, let us consider briefly the characteristics of the present landscape, for it is this landscape that we seek to explain.

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THE MODERN LANDSCAPE The over-all landscape of middle California, from the latitude of Cape Mendocino in the north to the Tehachapi Mountains in the south, is relatively simple. It consists of three major topographic units: the Sierra Nevada, the Great Valley of California, and the Coast Ranges. T H E REGIONAL SETTING

The Sierra Nevada, one of the world's great mountain ranges, forms a 400-mile-long barrier between the arid lands of Nevada and the Great Valley of California. Trap-door-like in profile, the range presents a precipitous escarpment on the east—a rugged wall that must have dismayed the early pioneers seeking to reach the California gold fields. The crest of the escarpment is topped by a line of lofty peaks of which Mt. Whitney, the highest peak in the United States exclusive of Alaska, is perhaps the best known. The descent from the High Sierra westward to the Great Valley is gentle, encompassing almost the entire width of the range. This gentle slope is scarred by a number of deep canyons, of which Yosemite Valley is justly famous. The Great Valley of California, comparable to the Sierra Nevada in geographic dimensions, is an almost level plain lying at or close to sea level. Actually, in the delta area east of Suisun Bay the surface is as much as 17 feet below sea level and is protected from the inroads of the sea and from river floods by an impressive network of levees and dikes. The landscape is strikingly reminiscent of many parts of Holland. The floor of the valley rises gently toward the flanking mountains where it reaches altitudes of several hundred feet. The only landscape features of any prominence within the valley itself are the Marysville Buttes, a group of ancient volcanoes about 50 miles north of Sacramento. [7]

The northern half of the Great Valley is drained by the Sacramento River, which rises in the Klamath Mountains far to the north and flows south into San Francisco Bay. The southern half of the valley is drained by the San Joaquin River, which rises in the southern Sierra not far from the source of Yosemite's Merced River. The San Joaquin joins the Sacramento just east of San Francisco Bay. The Coast Ranges are a great natural barrier between the Great Valley and the Pacific Ocean. The individual ranges vary considerably in size and shape. Some are poorly defined, sprawling masses like the mountainous wilderness stretching north from the latitude of Clear Lake. Others are large, well-defined units separated by broad valleys. The Coast Range belt varies in width from as little as 40 miles in the vicinity of San Francisco Bay to as much as 90 miles in the far north. The belt trends about 30 degrees west of north, roughly parallel to the coast, but many of the individual ranges trend obliquely across the belt and terminate abruptly at the coast on the west, and less abruptly at the Great Valley on the east. Interspersed among the ranges are numerous long, linear lowlands such as Petaluma Valley and the San Francisco Bay lowland, and a number of irregular basins such as Livermore Basin and the basin of Clear Lake. The Coast Ranges are not particularly high, averaging between 2000 and 4000 feet, with only a few peaks rising higher. Until now we have been surveying the California landscape as though from the fringes of space. Let us descend now to lower altitudes and bring the Bay Region into sharper focus. T H E LOCAL SETTING

The essential characteristics of the Bay Region landscape are pictorially shown in Figure 1. Notice that San Francisco Bay and its companions, San Pablo and [8]

Suisun bays, split the Coast Range belt in two, and that the mountain ranges to the north and south extend toward it like stubby fingers. Notice, too, that the ranges south of the bay are much larger than those immediately to the north. To the south, the Coast Range belt consists of three major mountain blocks. These are, from east to west: the Diablo Range, terminating northward in the Contra Costa Hills and the Diablo Hills; the Santa CruzGabilan mountain tract; and the Santa Lucia Range along the coast. Between the Diablo Range and the Santa Cruz-Gabilan tract is a long lowland, flooded by San Francisco Bay in the north. For simplicity's sake we shall refer to this as the San Francisco Bay-Santa Clara lowland. The lowland between the Santa CruzGabilan tract and the Santa Lucia Mountains we shall refer to as the Salinas lowland, after the river that drains it. This simple picture of large mountain ranges and broad intervening lowlands does not apply north of San Francisco Bay. It is true that for 45 or 50 miles to the north there are linear mountains separated by lowlands, but they are small and irregular compared to those to the south. These small ranges, from east to west, are the Vaca, Howell, Mayacmas, Sonoma, and Marin. The intervening valleys, named in the same order, are the Berryessa-Suisun, Napa, Sonoma-Rincon, and Petaluma. The Berryessa-Suisun and SonomaRincon are strings of depressions rather than continuous valleys. These northern ranges and lowlands lose their individuality within 45 or 50 miles of San Francisco Bay. Beyond that point they more or less merge into a broad, dissected upland, the Mendocino Plateau, the crest of which descends gradually seaward. Only the long depression occupied by Petaluma Creek and the Russian River penetrates northward into the plateau. Four peaks dominate the Bay Region landscape. All [9]

are accessible by road and afford superb views of the surrounding territory. These are Mt. Hamilton in the Diablo Range, 4206 feet high and site of the famous Lick Observatory; Mt. Diablo, 3849 feet high, at the northeastern extremity of the Diablo Range; Mt. Tamalpais, 2606 feet high, in the Marin Mountains north of the Golden Gate; and Mt. St. Helena, 4336 feet high, in the Mayacmas Mountains. This, then, is the Bay Region landscape for which we seek explanation. In any discussion of geologic history, a question that frequently arises is, "How do you know that such an event took place?" A very reasonable question, inasmuch as we are dealing with events that took place millions of years ago. Actually, the unraveling of ancient geologic history requires a special kind of training. The man so trained is a geologist. The geologist is in reality a detective, because from obscure clues scattered about the landscape he deduces events to which there have been no witnesses. In a sense, then, he emulates the fictional Sherlock Holmes. Let us see how he does it.

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PROBING THE PAST One of the fundamental truths of Nature is that the processes that are shaping the landscape today have acted in similar fashion throughout the long course of Earth history. From the record preserved in the rocks we know that streams eroded as relentlessly in the distant past as they do now; glaciers ground and gouged as do the Alpine and Antarctic glaciers of today; winds swept sand about, etched the faces of the rocks, and created fields of dunes; the waves of the sea battered ancient shorelines as they do the present California coast; volcanoes spewed lava and ashes as they do today; and the all-powerful internal forces heaved up mountains and caused earthquakes even as now. Not only have the processes remained the same, but the products of their activity—the beds of clay, sand, and gravel, and the layers of lava and volcanic ash—are essentially the same, whether one or 100 million years old. It is this similarity of process and product throughout time that enables the geologist, the student of Earth history, to ferret out the story of the Earth. Rocks are the products of events. Could one read the rocks, the whole fascinating history of the Earth would be laid bare. To read the past, however, requires an understanding of the present; the present is the key to the past. The layers of sediment that are spread over the lowlands of the Earth by streams, winds, and glacial ice, and those that are laid down on the sea floor by waves and currents, are like the pages of a history book. The sedimentary layer at the bottom of any sequence is, of course, the oldest and is covered in turn by successively younger ones. Thus, the layers of sediment in the walls of deep canyons, such as Niles Canyon between the San Francisco Bay lowland and Livermore Basin, provide a record of the past going back many millions of years.

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What have the rocks to teach us? First, they tell us of the ancient patterns of land and sea, of the geographies of ancient times. For example, sediments that are being deposited today beneath the sea entomb the remains of marine organisms, of clams and oysters, starfish and sea urchins, and myriad other marine creatures. In contrast, the sediments that are laid down on dry land, the continental sediments, entomb the remains of land plants and animals. Suppose, then, that our geological detective traces one of the rock layers that is widely exposed in the Coast Ranges. Suppose that in the eastern part of the Coast Range belt this formation contains the remains of land plants and animals, whereas in the western part the fossils are those of marine organisms. Obviously then, the ancient shoreline lay somewhere between. If this procedure is repeated for each of the succeeding layers exposed in the Coast Ranges, a complete record of the shifting shorelines of the past emerges. It is even possible to determine, within limits, the temperature of the water of the ancient seas. This is done first by analogy—by observing the temperatures favored by the modern descendents of the marine creatures entombed in the ancient formations. The second method is by chemical analysis of the fossil shell material, because the exact composition depends on the temperature of the water in which the animal lived. In practice, the deciphering of ancient geographies is a difficult, time-consuming procedure because of the vast size of the pages of our history book and because parts or whole pages have been removed by erosion. The deciphering of a single page, even in a relatively limited area, requires the investigative effort of hundreds of geologists and may take generations. We might inquire next as to how the topography of the past is determined—that is, how are ancient mountains and lowlands located? Again, relying on the concept that the present is the key to the past, if we examine the sediments that are being eroded from [14]

present mountains and spread out over the adjacent lowlands, we come to some interesting conclusions. One is that as the mountain streams reach the lowlands they lose velocity and are forced to deposit much of their sedimentary burden. Naturally the largest fragments, the boulders and cobbles, are dropped first, and the sand, silt, and clay are carried farther. Thus, by examining the variations in particle size in an ancient sedimentary layer, the direction of the source of the sediment is easily determined. The volume of the sediment eroded from the mountains may provide an approximation of the size of the mountain mass; and the composition of the pebbles, cobbles, and boulders provide information on the rocks that composed it. In many areas the geologist can even determine the source of the pressures that crumpled up ancient mountain ranges. This is rather difficult in the Coast Ranges because of the complicated geology. It is much simpler in the Appalachian Mountains of the eastern United States, where the layers of rock are crumpled into folds like wrinkles in a loose rug. The rock folds of the Appalachians become smaller and smaller to the west and eventually die out completely. Thus, the pressure must have come from the east. One can achieve the same result experimentally by moistening a piece of tissue paper on a glass plate and pushing one side toward the other. The wrinkles die out away from the source of the pressure. So it is frequently a simple matter to deduce the source of the pressures that crumpled up ancient mountain ranges. How these powerful forces were set in motion, however, is still a mystery. Perhaps there is Fig. 2 Syncline

Anticline

something to the old idea that the Earth's crust is shrinking over a cooling interior and wrinkling like the skin of a dried apple. There are other possible explanations, of course, but the problem is beyond the scope of our present story. The growth of mountains is accompanied by many earthquakes. Some are weak and are recorded only by delicate instruments known as seismographs. Others, however, are readily perceptible: they may shake the ground gently beneath our feet, crack plaster or topple chimneys, or even wreck entire cities as they did San Francisco in 1906. What is an earthquake and what causes it? Because earthquakes are of almost daily occurrence in the Bay Region it is appropriate to seek answers to these questions. An earthquake is a shaking or quaking of the ground caused by the sudden shifting of blocks of the Earth's crust. We must confess at the outset that the origin of the powerful forces that cause these dislocations is still a mystery. That the Bay Region is even now under the influence of these forces, however, is evident from measurements being made by the U.S. Coast and Geodetic Survey. Engineers of this organization have precisely surveyed many points throughout the Bay Region on both sides of the long fracture known as the San Andreas Fault. At each such point they have set up a marker. Periodic re-surveys of these markers since the San Francisco earthquake of 1906 reveal that the ground west of the fault is moving northwestward at an average rate of about 2 inches a year. That this movement has been going on for a long time is also evident from the bending of stream valleys where they cross the fault. Rocks, however, cannot bend indefinitely. Like a rubber eraser they will bend up to a certain point known as the elastic limit, but if flexed beyond this point they will break. If the bending forces are removed before the elastic limit is reached, they will spring back to their original positions.

[16]

r

Let us imagine an area crossed by a road trending east and west as in Figure 3. Now suppose that this / \ area is subject to forces acting in the directions of the arrows. The left side of the Fig. 3. Elastic-rebound block will move north, the theory of earthquakes right side south, and the rocks in the central area will be flexed. The flexing will extend over a broad zone and will distort the road as shown in position 2 of the diagram. This amount of bending may represent the limit for these rocks; if the forces continue to act, the rocks will split along the fracture labeled F-F. When the fracture develops, the severed ends of the road, no longer tied to each other, spring back elastically to their new displaced positions (3-3). Because of the elasticity of the rocks, however, momentum carries the end of the road past point 3 and then, like the tines of a tuning fork, the severed ends of the road and the rocks beneath it, vibrate back and forth along the fault. It is this vibration, resulting from the snapping of the rock, that we know as an earthquake. The fracturing, or faulting as geologists prefer to call it, temporarily relieves the strain on the rocks, but the responsible forces still remain and continue to deform the rock. The fault surface previously formed is not a smooth plane along which sliding can take place continuously. On the contrary, it is very irregular and offers considerable resistance to further movement. In spite of its presence, therefore, the rocks once again begin to bend and continue to do so until the limit of frictional resistance along the fault is reached. A new sudden movement then takes place. These repeated movements generally occur at irregular intervals, making earthquake prediction very difficult. To add to the difficulty, there are so many faults in the Bay Region that the strains set up in the rock are sometimes relieved

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by displacement on one fault and sometimes on another. As yet, no way has been found to predict on which fault the next movement will take place. Figure 4 is a map of the larger faults of the Bay Region. Most of the faults, like the San Andreas Fault, are characterized by horizontal shifting of the ground. Many of these horizontal faults, however, started out as vertical faults—that is, faults in which one block moved vertically past the other. Only in recent geologic time has the movement changed to horizontal. The reason for this change is another unsolved mystery. A geologist can also identify areas of past volcanic activity and locate the sites of long-vanished volcanoes. The lavas that poured out on the surface in ancient times, and the ash and cinders (pyroclastic materials) that were exploded into the air, were no different from those of today. Thus recognition of areas of past volcanic activity is merely a matter of being able to recognize ancient lavas and explosion products. The location of the volcanoes themselves is another matter. The lava that is ejected from volcanoes comes from great depth. The volcano itself is simply a cone of volcanic rocks built up at the surface end of a long pipe leading from the depths. Sometimes during the life of a volcano explosions will crack the sides of the cone; the cracks

Fig. 4. Major faults of San Francisco Bay

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Region

formed will radiate outward from the center. Later, molten rock from the depths may squeeze its way into these fractures and solidify, forming features known as dikes. When the volcano becomes extinct and its flanks are deeply eroded, the rocks in these fractures may stand up like radiating ribs, and the central pipe will rear upward as a great monolith, a huge isolated spire known as a volcanic neck or plug. Thus, the central monolith and the radiating ribs constitute the skeleton of the volcano. There are several examples of volcanic necks in the Bay Region. The black spire known Fig- 5 as Lover's Leap, through which Highway 152 passes east of Bell Station on its way to Pacheco Pass, is a fine example. Ancient lavas are also widespread in the Bay Region. One is exposed in the quarries back of Stanford University where the rock is excavated for use in road building. The entire crest of the Sonoma Range consists of old lava flows and pyroclastic deposits. We know now how the geological detective determines the geography and topography of the past, and how he locates centers of volcanic activity. Let us inquire next how he deciphers ancient climates. One easy way is by studying the fossils entombed in the sediments. We know, for example, that the different climatic zones of the Earth are characterized by different assemblages of plants and animals. When we find fossil palms and the remains of alligators in a sedimentary [19]

formation, we are justified in assuming that the sediments in which they are entombed were laid down in a warm, moist, sub-tropical climate, for such is the climatic environment of these organisms today. In contrast, fossils of redwood, elm, ash, walnut, and similar trees indicate a moist temperate climate. And the remains of the woolly rhinoceros and woolly mammoth indicate glacial climates, even in regions that now are warm. Ancient climates are often indicated, too, by the composition of the sediments and certain inherent structures. The presence of salt layers, for example, indicates intensive evaporation, presumably in a dry climate. Dry climates are also indicated by the rounded shapes and frosted appearances of the sand grains in certain sandstone formations, and by the presence of a peculiar curved layering known as eolian cross-bedding. As for dating the past, the geological detective now relies largely on methods developed during the atomic age—methods based on the phenomenon known as radioactivity. A radioactive element is one that loses part of its substance by radiation and changes into progressively lighter elements at a steady, unchangeable rate. Uranium, for example, after changing from one element to another, finally ends up as non-radioactive lead. One of the elements in the uranium-to-lead sequence is radium, a substance that is used in painting luminous watch dials. The actual disintegration of radium atoms may be observed in a darkened room by viewing the luminous hands or numbers of a watch or clock under a strong magnifying lens. Each flash of light is a miniature explosion, and each one represents the disintegration of one radium atom. The time for half of a given amount of uranium to pass through the complete sequence of changes to lead is very long. Since the rate of change is known, one can determine how long ago a uranium mineral came into existence by comparing the amount of the remaining uranium with

[20]

the amount of surrounding lead into which the missing uranium disintegrated. This method has been used successfully in dating rocks as old as 3 billion years. The uranium-lead method, however, is useful only for dating very distant events involving many millions of years. Other radioactive elements such as potassium and carbon are used for shorter spans of time. The granite of Montara Mountain on the San Francisco Peninsula and the granite of the Farallon Islands have been shown by the potassium method to be about 90 million years old, whereas the granites of Pt. Reyes and the Santa Lucia Range are only 84 and 82 million years old respectively. The radioactive carbon method is useful for dating within the last 40,000 years. By measurement of the radioactive carbon in fossil sea shells, it is now known that the lowest of the marine terraces along this part of the California coast was formed more than 40,000 years ago, when sea level stood much higher than now. Inasmuch as other evidence indicates that the last high stand of the sea was about 100,000 years ago, it is believed that the terrace is probably that old. We see, then, that there is sound logic behind many of the sweeping statements made by geologists. For example, we can accept as essentially true the following statements relating to the Sierra Nevada: A great mass of molten rock invaded the core of a lofty mountain range where the Sierra now lie; this molten mass solidified far below the surface as granite; during the ensuing eons this ancient range was eroded to a low plain, exposing the granite, after which the modern Sierra Nevada was lifted up like a giant trapdoor above the plain. With this background into the clues used by the geological detective, we may proceed to our story of the Bay Region landscape.

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THE PROBLEM OF THE BEGINNING Where shall our story begin? Latest estimates place the age of the Earth at approximately 5 billion years. Geologists have divided this vast span of time into subdivisions, much as the year is divided into months, weeks, and days. The major subdivisions of this geologic time scale or calendar are shown in the accompanying table. Note that the first 4Vz billion years of Earth history are grouped collectively as the Precambrian. So little is known of these ancient rocks that subdivision is practically impossible. There are several reasons why these rocks provide so little information about the Earth's infancy and adolescence. One is that over much of the Earth these rocks are deeply buried under other rocks and are not visible to us. We see them generally where great canyons like the Grand Canyon of the Colorado River have eroded through the covering rocks, or where the ancient rocks have been punched up in the cores of mountain ranges and exposed by erosion. Secondly, these ancient rocks have been crushed and broken so many times during the long and turbulent history of the Earth that their original characteristics have long since been obliterated. Hence they tell us little of the conditions at the time of their origin. These ancient rocks, too, were formed when life was exceedingly scarce on the Earth. The almost complete absence of fossils deprives us of some of our best clues to ancient environments. The past 500 million years of Earth history is much better known, because many of these rocks have been changed very little from their original condition. It has not only been possible to divide the past 500 million years into major subdivisions called eras, but even to subdivide the eras into smaller divisions known as periods, and these in turn into smaller units known as epochs. The hills and mountains of the Bay Region are carved [22]

out of rocks of many different ages. Rocks of undoubted Precambrian age, however, are unknown. We have no information on what happened here that far back in the past. Actually, we are not even sure that rocks of early Paleozoic age are represented. The oldest reasonably well-dated rocks in the region are of late Paleozoic age. These are widely exposed in the Santa Lucia Range but occur only as small patches in the Gabilan and Santa Cruz ranges. The remnants are so small and so badly altered by the ravages of time that they tell us little about the conditions that prevailed when they were deposited. Rocks of Mesozoic age, formed when dinosaurs ruled the Earth, underlie the greater part of the Bay Region from the ocean to the Great Valley. They form the foundations of much of San Francisco, the eastern foothills of the Santa Cruz Range, and large areas in the Diablo Range, including Mt. Diablo itself. North of the Bay these rocks extend from the Golden Gate to the Mendocino Range, and from the Pacific Ocean to the Sacramento Valley. Although the rocks are largely sedimentary, they include some igneous rocks. Some of these are dark; others are light. The lighter ones are granite, the same kind of rock in which Yosemite Valley is carved. In the Bay Region the granites are exposed in Inverness Ridge, Pt. Reyes, the Farallon Islands, Montara Mountain, Ben Lomond Mountain, and the Gabilan Range. Early Tertiary rocks are not so abundant as the Mesozoic rocks and are more widely scattered throughout the Bay Region. They form some of the ridges that encircle Mt. Diablo. The thinly banded rocks in the vicinity of the Broadway Tunnel in Oakland and at San Pedro Point on the Coast Highway south of San Francisco are also early Tertiary in age. Both the Mesozoic and early Tertiary rocks contribute to the details of the landscape about us largely because of their variable resistance to weathering and erosion, but they are not responsible for the gross forms

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of the modern ranges. During the long span of time when these rocks accumulated, some under the sea, some on dry land, some by volcanic eruption, and some by molten injection below the surface, generations of mountain ranges were upheaved only to be later worn to their roots. The topography that existed when these older rocks were laid down has long since disappeared. Where in this repetitious sequence of events shall we start our story? A logical place would be the last of the great invasions of the sea, during the Miocene epoch, which began about 28 million years ago and lasted until about 12 million years ago. Delightful as the landscape of that ancient day must have been— with the sea interlaced among numerous large islands and the gentle slopes covered with forests—it had none of the rugged beauty of today. No San Francisco Peninsula then, no Mt. Tamalpais, no Tómales Bay or Monterey Bay, no Mt. Diablo, and no Mt. Hamilton for Lick Observatory to perch on. And far to the east, where the Sierra now lie, was only a broad welt in the landscape, no higher than the present Coast Ranges. In terms of human history 28 million years seems fantastically long, but relative to the great age of the Earth it is short indeed. As an analogy, let us assume that the height of one of the towers of the Golden Gate Bridge, 750 feet high, represents the 5 billion years of Earth history. On this scale the upper 4 feet would represent the 28 million years we are concerned with, and the thickness of a dime atop the tower would represent more than all recorded human history. Now we face another problem: how to simplify a story that is exceedingly complex. Our story would have been simple if each event in the past, such as invasion by the sea, upheaval of mountains, and volcanic eruption, had affected the entire region simultaneously rather than piecemeal. We would merely have to catalogue the successive events in chronologic order. But each event affected only part of the region.

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While some localities were being invaded by the sea, others were being crumpled into mountains or being buried under sedimentary or volcanic accumulations. In other words, each separate area had a history of its own which may have been entirely unrelated to the history of the immediately adjacent areas. Even if we knew all the details of the complex history of the Bay Region, which we do not, any attempt to describe every small episode in the creation of the present landscape during the past 28 million years would involve us in such a welter of detail that the essential elements of the story would be lost. For the sake of clarity, then, we shall select only the major episodes in the evolution of the landscape. To simplify the story still further we shall telescope events; that is, we shall consider as having happened simultaneously, events that were actually separated in time. To help us visualize the major events more clearly, a sequence of diagrams—Figures 6-12— is presented. Each landscape represents a composite of the major landscape features that prevailed during the indicated period of geologic time. And so, to our story.

[27]

THE GREAT INUNDATION: THE MIOCENE EPOCH About 28 million years ago, at the start of the Miocene epoch, the Pacific Ocean began to flood slowly across the western continental margin. The flood reached its culmination some 15 or 16 million years later with the shoreline far inland, within what is now the Great Valley. The invasion was not uninterrupted, however, nor was it uniform throughout the entire area. At times, arms of the sea probed ahead along favorably disposed lowlands, while elsewhere the sea was forced back by slight upwarpings of the land. But groping ahead, first here, then there, the sea spread inland, eventually inundating large portions of what is now the Great Valley. In that distant day the landscape probably looked something like that shown in Figure 6. It is unlikely that the sea covered so large an area at any one time, but the distribution of Miocene marine sediments indicates that at one time or another the sea covered most or all of the area indicated. Nor should we assume that the size and configuration of the land areas were precisely as shown, or that they all existed concurrently throughout the Miocene epoch. They were undoubtedly larger at some times and smaller at others. Furthermore, there may have been other large islands seaward of the present shoreline, but if so, the evidence is now submerged beneath the sea. The landscape shown in Figure 6 is thus a composite picture of a landscape that was changing constantly during this long episode of geologic time. Let us examine this landscape in somewhat more detail, for the origins of some of our scenic showplaces date from this early episode in Earth history. Note first that even at that distant date there were land masses where some of our modern mountains lie. Far to the southeast, in the lower right corner of the

[28]

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diagram below Pt. Sur, we see the northern end of an earlier version of the Santa Lucia Range. Note that it straddled the coastline at that time and extended neither as far inland nor as far north as the present range. The sediments that were washed into the sea around the margins of the reduced range were later elevated and may be seen in many places today above sea level. In the city of Monterey, for example, at the north end of the Santa Lucia Range, and near Carmel Mission along Highway 1, road cuts reveal inclined beds of marine sandstone, known as the Monterey formation, resting on granite. These beds, of course, were originally laid down horizontally in the Miocene sea; the tilting resulted from later mountain-making. But the very presence of these marine beds indicates that at that distant date the northern part of the present Santa Lucia Range lay beneath the sea. The presence of the Monterey formation along the east flank of the range proves that this part of the range was also submerged at that time. Eastward across the broad seaway that occupied the site of the present Salinas Valley rose another long island where the Gabilan Range now stands. On this isolated island a succession of interesting events took place. There emerged from the depths, by way of great cracks in the Earth's crust, large volumes of porridgelike lava that spread over the surface, building up a long dome several thousand feet high. On the crest of this dome a string of volcanoes came into existence. At times these volcanoes erupted violently, scattering enormous quantities of broken rock over the area; at other times they erupted quietly, emitting great floods of lava. From this beginning the scenic Pinnacles National Monument was eventually to develop. Farther to the northwest along this ancient coast was another long, low island. Its western margin coincided with the San Andreas Fault. The entire area to the west had earlier subsided beneath the sea. In the San [30]

Francisco Peninsula the western slopes of this island covered the site of what are now the rolling eastern foothills of the Santa Cruz Range. Of special interest was the presence of an active volcano in the La Honda area about 10 miles southwest of Stanford University. Lava from this volcano flowed westward into the sea and is now found buried among the marine sediments that were accumulating beyond the shoreline. Cinders in the sediments indicate that the volcano at times erupted explosively. Later another volcano came into existence about 2 miles south of Stanford University. During violent eruptions cinders from this volcano also rained down in the nearby sea. At one time during the life of this volcano, a stream of dark lava flowed northwest for about 3 miles. Near the site of the volcano the lava was about 400 feet thick; where it is penetrated by a well in western Menlo Park it is only 20 feet thick. Much of the molten material that rose from the depths at this time never broke through to the surface, but instead sandwiched itself in among the layers of sediment that were accumulating and hardening on the sea floor. Thanks to later uplift and erosion of the Santa Cruz Mountains, we can see these ancient rocks, known as diabase, in many road cuts and stream valleys from about the latitude of Half Moon Bay in San Mateo County to a point about 4 miles east of Boulder Creek in Santa Cruz County. The road down to Half Moon Bay from Skyline Drive passes through exposures of these rocks in the early part of the descent. The best exposures, however, are at Langley and Mindego Hills near La Honda, about 12 miles southeast of the town of Half Moon Bay, in the heart of the Santa Cruz Range. The Monterey sediments that were deposited in the sea around the flanks of the Santa Cruz Range are now above water in many places. Near Santa Cruz itself they have been battered by the sea and carved into narrow promontories—thin partitions between adjacent coves. In places the promontories have been worn [31]

through to create arches and natural bridges. This is the site of Natural Bridges Beach State Park. South and east of the present site of the San Francisco Bay lowland lay a broad sprawling upland area. This was an early ancestor of the Diablo Range, but it extended only from the lower end of the present bay to about the latitude of the Pinnacles. The entire north end of the Diablo Range was not then in existence; the waves of the Pacific washed across the area where now lie the Contra Costa and Diablo hills. The ancestral Diablo Range, unlike the land areas previously described, was not an island; it was tied to the hinterland by river-deposited sediments carried across the site of the present Great Valley by streams from the ancestral Sierra Nevada, then only a few thousand feet high. Volcanoes were active in scattered parts of the ancestral Diablo Range. The largest volcanic area was east of Hollister where Cathedral Peak and many other peaks in an area of about 180 square miles are carved out of a complex of ancient lava flows and intrusive rocks. North of the present site of San Francisco Bay the entire Pt. Reyes Peninsula lay under water, and sediments of the Monterey formation similar to those that were deposited elsewhere in this ancient sea were laid down. The sea probably extended inland as far as the Sonoma Range, because remnants of these marine sediments are found along Carneros Creek 5 miles west of Petaluma, and oil seeps in the Sonoma Mountains are believed to have their source in deeply buried Monterey sediments. The sediments that were spread over the submerged Pt. Reyes Peninsula consisted largely of shale, a rock formed from the compaction of mud. Many of the shales are white, due to the presence of innumerable remains of microscopic marine plants called diatoms. It was the resemblance of these cliffs to the white cliffs of Dover that led Sir Francis Drake in [32]

San Andreas Lake on the San Francisco Peninsula: the lake occupies a valley eroded along the San Andreas Fault. San Bruno Mountain in the distance is an eroded fault block.

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1579 to name this new land Nova (New) Albion, Albion being the ancient name of Britain. The configuration of the shoreline of the ancient Miocene sea north of San Francisco Bay is not precisely known. Marine Miocene sediments are absent north of the inundated area shown in Figure 6, but they could have been eroded away after deposition. There were centers of volcanic activity north of San Francisco Bay as well as to the south. As a matter of fact, volcanic activity was more widespread during the Miocene than at any time since. Scattered volcanoes north of the bay erupted intermittently and distributed volcanic ash over the adjacent seas. Ash from these northern volcanoes is found in the marine sediments as far south as Mt. Diablo. And far to the north at Pt. Arena more lava poured out, but whether this lava came from a long-vanished volcano or from a great crack in the Earth's crust is unknown. The volcano shown there in Figure 6 is not completely authenticated. The shoreline of the Miocene sea lay beyond the eastern margin of the Coast Ranges within the Great Valley itself. Stretching inland from the shoreline was a smooth alluvial plain rising gradually toward the fledgling Sierra far to the east. The land areas of Miocene time were, of course, subject to continuous erosion, and the sediment-laden streams carried the debris down into the sea. By the close of Miocene time the land areas were worn nearly to sea level. Even the volcanoes that dotted the region were eventually worn away. During much of the 16-million-year span of Miocene time a large part of the Bay Region lay beneath the sea, and the ancestors of some of our modern ranges were mere islands rising above the flood. Even then, however, the ranges trended northwesterly as they do today.

[41]

END-MIOCENE MOUNTAIN-MAKING For some obscure reason, long-dormant subterranean forces began to bestir themselves toward the close of the Miocene epoch and caused relatively rapid changes in the landscape. The changes were so rapid, from the geologists' point of view, as to warrant application of the term "revolution" to this episode of crustal unrest. In actuality, however, the changes were extremely slow and probably passed unnoticed by the creatures that lived at that time. Through the centuries the subterranean forces continued to exert pressure on the Bay Region, and great welts began to appear in the topography. In time these grew into giant folds, like wrinkles in a loose carpet. Earthquakes were common, the result of the shifting about of blocks of the Earth's crust. Most of the major faults of the region, such as the San Andreas Fault, were already in existence, but whereas today displacement along these faults is horizontal, in those days it was vertical. New volcanoes came into being because faults offer easy upward passage for molten material from the depths. The earthquakes and volcanic activity were the growing pains of the rising mountains. This was a time of upheaval, of crustal unrest. The sediments that had been deposited previously in horizontal layers on the Miocene sea floor, and those that had been laid down in interior lowlands, were crumpled into folds several thousand feet high and many miles across. Locally the folded rocks were heaved up bodily along great faults. And these great changes in the landscape took place to the accompaniment of violent volcanic eruptions. The landscape at the culmination of this activity was probably something like that shown in Figure 7. During this crustal revolution the internal forces again crumpled and shattered the Santa Lucia Range and added to its size by plastering against it the crum[43]

that had been laid down on the Miocene sea floor, are high above sea level on the flanks of the range. The Gabilan Range across the Salinas Valley was subjected to similar treatment and also grew in size. Part of its additional height resulted from upheaval along faults, particularly the San Andreas Fault on its east side. An especially interesting episode in the history of the Pinnacles area took place at this time. Great cracks developed across the volcanic dome that had been built here earlier, and a large segment of the dome 2 or 3 miles wide and more than 6 miles long subsided hundreds of feet below the surface. A block of the Earth's crust that drops down between faults is known as a graben (also plural form). The valley of the Dead Sea and the Rhine Valley in its most picturesque portion are modern examples on a grand scale. Had the Pinnacles graben not formed here 10 million years ago, there would be no Pinnacles National Monument today. Because of this subsidence the volcanic rocks out of which the spire-like Pinnacles are carved were dropped far enough below the surface to escape complete removal in the long period of erosion that followed. It was during the End-Miocene mountain-making that the Santa Cruz Range was raised out of the sea. The sea bottom was first thrown into broad folds and then raised high in the air. The eastern margin was elevated the highest along the San Andreas Fault. The [44]

Monterey Bay lowland in much its present form was created at this time. Between the Gabilan and Santa Cruz ranges on the west and the Diablo Range on the east was a long, newly formed trough whose configuration was almost identical to that of the San Francisco Bay-Santa Clara lowland of today. In the north, however, the trough narrowed considerably and was confined to the western half of the present lowland. The Diablo Range also grew in size by folding and uplift not only of part of the surrounding sea floor but of the margin of the vast alluvial plain that extended east to the Sierra. The steep western margin of the Diablo Range was the result of uplift along another great fracture known as the Hayward Fault. Thus, the long trough between the Diablo Range on the east and the Gabilan and Santa Cruz ranges on the west is another large graben. The Diablo Range at that time narrowed to the north and continued across the site of the present bay to connect with the mountains beyond. It formed a barrier that was to bar the sea from the interior during the entire succeeding geologic epoch. North of the bay the Pt. Reyes Peninsula was compressed and elevated, the central area being raised the least. Across Tomales Bay from the peninsula the crustal block out of which the Marin Mountains and the Mendocino Range were subsequently formed was uplifted bodily above the sea. The upheaval of the western part of this block took place along the San Andreas Fault. The entire belt of country between the MarinMendocino block and the Sacramento Valley was thrown into long broad folds and cracked by faults. One of the faults is shown in Figure 7, northeast of Petaluma, along the western front of the present Sonoma Range. This was the topography at the culmination of the End-Miocene orogeny some 12 million years ago. As [45]

the internal forces grew feebler, the mountains ceased to rise, but the rivers and streams continued to erode the newly risen highlands. They persisted in this task until much of the area was reduced to a low plain prior to the beginning of Pliocene time. The Pliocene epoch was heralded in by an uneven warping of this lowland plain, which permitted the seas to invade parts of the coastal area.

[46]

THE LESSER INUNDATION: THE PLIOCENE EPOCH The erosion of the End-Miocene mountains, and warping of parts of the erosion surface thus formed, created the landscape shown in Figure 9. The Santa Lucia Range was now a comparatively low ridge, higher and more rugged in the noith than in the south. The Gabilan Range, too, was lower, and the block of volcanic rocks in the Pinnacles area that had been dropped down during the End-Miocene orogeny now lay flush with the surface on either side. The Santa Cruz Range was a shrunken remnant of its former self, and its base lay submerged beneath the waves. East of the Gabilan and Santa Cruz ranges, however, across the long lowland where San Francisco Bay and the Santa Clara Valley now lie, there persisted the remarkably long ridge that had been created in the EndMiocene mountain-making. This ridge extended some hundred and twenty-five miles south from San Pablo Bay. Like a great dike it kept the seas from much of the interior; only in the far south were the waters able to bypass the ridge and gain entrance to the Great Valley. For reasons to be seen shortly, we shall refer to this long ridge as the Orinda-Merced Axis. Because geologists have given the name Purisima to the sediments that were deposited in the southern half of the flooded area, we shall refer to this part of the sea as the Purisima Sea. It covered the present Monterey Bay lowland, the Salinas Valley, the Santa Clara-San Benito lowland, and a portion of the San Joaquin Valley inland from Priest Valley. To the north of where San Francisco Bay now lies, the Pt. Reyes Peninsula had changed little from its EndMiocene appearance. To the east, however, over the site of the present Marin Mountains the waters of an inland sea stretched eastward as far as the Sonoma Range and as far northward as the Mendocino Range, [47]

whose southern foothills then lay north of the Russian River. We shall refer to this as the Merced Sea. East of the Mendocino Range the folds that had been crumpled up in the End-Miocene mountain-making were worn to their roots, and the general level of the country was probably not very different from that of the Mendocino Range to the west. East of these ancient Coast Ranges was a broad plain that extended to the foothills of the Sierra. North of the bay the western margin of the plain was about where it is today, but to the south it lay much farther west, at the foot of the Orinda-Merced Axis. No Berkeley Hills existed then, and no Mt. Diablo. The sediments that were eroded from the Orinda-Merced Axis were spread eastward like a giant apron at least as far as the present site of Mt. Diablo. These sediments, named the Orinda formation after a town of that name in the hills east of Berkeley, were deposited on dry land. They entomb the remains of land plants and animals, including the bones of an ancestral three-toed horse. In places plants grew profusely, so much so that eventually they formed beds of coal. Within the Orinda sediments, too, are layers of tuff, a rock formed from volcanic ash. It is quite clear that there were active volcanoes in the vicinity. The fact that the volume of ash diminishes southward suggests that the site of most intense volcanic activity was north of the bay. However, there was volcanic activity to the south as well. Where the Berkeley Hills now stand, lava came from the depths on several occasions, probably from great fractures but possibly also from volcanoes. Some of the lava is light in color as that at Northbrae on the west slope of the Berkeley Hills north of Berkeley. Lava also came to the surface a short distance south of Berkeley and flowed southeastward for 21 miles to the vicinity of Decoto. This lava, well exposed at Leona Heights in the hills east of Alameda where it covers several square miles, is light bluish-green where freshly ex[49]

posed but yellowish or brownish where weathered. Other lavas that poured out at this time were almost black in color, like those that cap Bald Peak and Grizzly Peak in the hills back of Berkeley. There were apparently one or more volcanoes in the San Jose area, for the Orinda sediments east of San Jose include volcanic debris, but the exact site of these volcanoes is unknown. The largest area of volcanic activity was north of San Francisco Bay and east of the Petaluma-Santa Rosa lowland. At times enormous volumes of lava emerged from volcanoes or deep fractures and inundated the countryside; at other times large quantities of ash, cinders, and rock fragments were exploded over the landscape, contributing to its burial. The volcanic activity was not continuous, however. Sometimes the volcanoes lay dormant for long periods of time, and soils formed on the lavas and ash beds. Forests grew in these soils only to be buried under subsequent floods of lava and ash. The trees of the Petrified Forest between Santa Rosa and Calistoga grew in one of these quiet periods. But during the eruption of a nearby volcano, heavy rains from moisture-laden clouds saturated the loose ash on the slopes of the volcano and created rivers of mud. One of these flowed down into an adjacent lowland at the site of the present Petrified Forest. The fossil trees of the Petrified Forest, including huge redwoods and firs, have all been toppled to the southwest, proving that the flow of mud came from the northeast. Much lava flowed down into the Merced Sea, and large amounts of ash settled into its depths. These were buried under the sediments collecting on the sea floor and which we know today as the Merced formation. The end result of this prolonged period of volcanic activity was the accumulation of several thousand feet of volcanic rocks covering an area of more than 350 square miles. Toward the close, of this episode of volcanic activity hundreds of feet of bluish-gray lava [50]

poured out on the surface. Today this lava caps the very crest of Mt. St. Helena. The Pt. Reyes area was apparently still above sea level, for the marine Merced sediments are not found there. Bolinas and Tomales bays were under water, however, as indicated by the presence of Merced clays and sands in road cuts within this lowland for some 5 miles north of Bolinas. Merced sediments are also exposed over a large triangular area from Dillon Beach and Petaluma on the south to the northward bend of the Russian River on the north. About two miles west of Tomales on the road to Dillon Beach the Merced formation, here a pebbly sandstone, forms imposing bluffs. Although now 500 feet or more above sea level, these sands once lay on the floor of the Merced Sea.

[51]

LATE-PLIOCENE MOUNTAIN-MAKING The comparative quiet of the Pliocene, broken only now and then by volcanic eruptions in widely scattered localities, came to an abrupt end about 2.5 million years ago. Once again, powerful forces that had lain dormant within the Earth's crust began to bestir themselves, and once again great wrinkles, like waves, began to rise above the surface as these forces pressed upon the continent from the southwest. As the wrinkles grew higher and broader the entire region rose—even those areas that had once been beneath the sea—and the shoreline was driven even farther seaward than now. Actually the mountain-making involved more than just crumpling and emergence of the region; at many places the crust cracked and great blocks were heaved up along faults. The jarring gave rise to frequent earthquakes. Many ranges were both crumpled and faulted. The landscape at the height of this activity probably looked something like that portrayed in Figure 10. Although the ancient landscape did not very closely resemble that of today and was destined for still further change, the rocks themselves at this tim3 suffered the last of the deformations they were destined to undergo. Henceforth they were never again to be crushed into tight folds, although the region as a whole was to experience still another episode of fracturing. Let us consider briefly some of the interesting details of this million-year-old landscape. The Santa Lucia Range in the south was now higher, broader, and more rugged than formerly. No longer did the sea invade the Salinas Valley; the floor of that lowland was above sea level, and Monterey Bay was no more than a slight embayment—a shallow, inundated area between the Santa Lucia Range and the Santa Cruz Range to the north. Across the Salinas Valley lay the Cabilan Range, now comparable in size to the present range, largely [53]

because of upheaval by faulting particularly on the east side. Near its south end was still preserved the block of lava at the site of Pinnacles National Monument. To the north and connected to the Gabilan Range was a greatly expanded Santa Cruz Range. It, too, had been tilted upward on its east side, but much of the western slope was crumpled into folds. The Santa Cruz Range did not then terminate at the present coastline. Instead it continued across what is now open water to the Pt. Reyes Peninsula and terminated near the mouth of the Russian River. Throughout its great length its eastern edge lay along the San Andreas Fault. Its western slopes met the sea west of the present shoreline. Where the bay lowland now is, the sediments that had been deposited earlier in the Merced Sea were warped into shallow folds. Across the San Francisco-Santa Clara lowland the Diablo Range was again crumpled and shattered by faults. A great part of the upheaval, at least of the western portion of the range, resulted from renewed uplift along the Hayward Fault. Between Suisun Bay and Livermore a large fold, like the back of a giant whale, rose slowly above the lowlands. Eventually the crest of the fold cracked and shattered, and a huge plug of ancient rock broke through to the surface. Thus, for the first time did the peak of Mt. Diablo appear in the landscape. We have already noted that the Santa Cruz Range extended across what is now open water to join the Pt. Reyes Peninsula. East of the Pt. Reyes Peninsula, across the San Andreas Fault, the mountainous Marin Peninsula was lifted out of the sea as was the Mendocino Range farther north. The area between was lifted up a lesser amount, forming a broad shallow sag in the topography. Thanks to this uplift, the former sea bottom sediments are now above sea level and, as indicated earlier, are well exposed along the road between the town of Tomales and Dillon Beach. It is quite probable that the lower course of the Russian River, the part [54]

that drains westward into the ocean, had its origin at this time. Its long headwaters upstream from Santa Rosa, however, were not then in existence. East of the Marin and Mendocino highlands the entire region was thrown into shallow folds and broken by faults. Even the thick sequence of lavas, ash, and cinders that had accumulated during the preceding volcanic episode were crumpled into broad waves. The evidence of this is to be seen in many of the road cuts in the Sonoma Mountains, in the western Mayacmas Mountains, and on the western flank of the Howell Mountains, where the volcanic layers are now inclined rather than horizontal. This was the landscape 2.5 million years ago-a land scape in turmoil, crumpled into giant folds and shattered by faults. There was no Golden Gate at this time, and no Monterey Bay. But even this landscape was not to endure for long.

[55]

THE GREAT DENUDATION During the late Pliocene the landscape was high and rugged and the climate was moist. This was due in part to the presence of high mountains which deflected the moisture-laden winds into colder levels of the atmosphere where the moisture condensed and fell as rain. Part of the increased rainfall of these times, however, was due to a change in climate toward cooler and wetter conditions. The countless streams that rushed down the newly steepened slopes rapidly eroded their beds and carried the substance of the hills down into the lowlands and eventually to the sea. Through the years the mountains were worn lower and lower until eventually, either at the close of the Pliocene or the beginning of the Pleistocene, the landscape was reduced to the undulating plain shown in Figure 11. The central portions of the ranges were not completely destroyed, however, but remained as lines of low hills. These included the ridges of which Mt. Hamilton, Mt. Tamalpais, and Mt. St. Helena are part. The bay lowland which had formerly been isolated from the sea was now filled with sediment washed from the surrounding hills, and the low folds that had previously existed here were buried under this sediment. North of the bay the earlier uplift of the Marin and Mendocino highlands had so stimulated erosion that the Merced formation, which had once extended over much of these highlands, was stripped away. It was preserved only in the depressed area between the Russian River and the Marin Mountains. The Mt. Diablo dome was almost leveled at this time, but the hard core of Mt. Diablo itself still stood above the plain. The drainage of the Bay Region was established at this time. The combined waters of the Sacramento and San Joaquin rivers flowed across the site of the present bay and emptied into the ocean some distance west of [57]

the present shoreline. To the north the Russian River still flowed westward into the sea, but—like an expanding gully—it had extended its valley to the northwest along a zone of rock that had been crushed by repeated faulting. Remnants of the ancient rolling plain are preserved on the crests of many of the modern ridges. They appear as level or gently undulating areas high above sea level, as in the Diablo Range surrounding Mt. Hamilton. Even where no such remnants exist, the former presence of this widespread plain is indicated by the remarkable accordance in elevation of the ridge crests. This suggests that these ridges were once part of a continous plain and that their present separation is due merely to erosion. From this plain the modern landscape took form within less than 2.5 million years. The modern aspect is primarily due to another remarkable episode of mountain-making, but this time the mountains resulted primarily from uplift along faults.

[58]

THE PLEISTOCENE EPOCH The past 2.5 million years, known to geologists as the Pleistocene epoch, witnessed the creation of the major elements of our modern landscape from the featureless plain that preceded it. The Pleistocene epoch was also the time of the Great Ice Age. At present about 6 million square miles, or 10 per cent of the land area of the Earth, is covered by glacial ice. During the Great Ice Age, glaciers covered more than twice that area. A huge glacier, as large as that which covers Antarctica and 2 or more miles thick, covered the northern half of North America, extending as far south as St. Louis in the interior of the continent. At this time the high Sierra were buried under ice. One of the great tongues of this highland ice flowed down to lower levels to carve the beautiful valley of the Yosemite. The Coast Ranges in the Bay Region were not high enough to support glaciers, but there were small glaciers in the Trinity Alps in northern California. Although the Bay Region itself was not glaciated, it felt the effects of the Ice Age indirectly. Floods of glacially eroded sediments were carried down from the Sierra by meltwater streams and dumped into the Great Valley, much of it reached the Bay Region, and some even the sea. And fluctuations of sea level, resulting both from the extraction of water to form the glacial ice and from the return of the waters when the ice melted, caused the sea at times to withdraw from the continental margin and at other times to inundate it. In the discussion that follows, only the effect of the last inundation will be considered, for little is known of the earlier floods. T H E FIRST STAGE

Sometime during the first half or first stage of this 2.5million-year period, the entire Bay Region began to [59]

crack and break up, like the breakup of ice on a frozen river during the spring thaw. Some blocks rose vertically, others dropped, and many were tilted like giant trapdoors. The steep trapdoor side of most of the tilted blocks along the coast faces the east, like small replicas of the Sierra Nevada. It is as though the whole coastal region were sliding westward into the ocean depths and breaking up in the process (Fig. 12). In the far south strange things were happening to the Santa Lucia Range. The range as a whole was bodily uplifted along faults on both the inland and seaward sides, and the interior of the range was shattered by other faults. But even more interesting is the fact that a long narrow segment along the eastern margin began to break away from the main mass. The segment settled along a fault on its west side and rose along another great fracture on its east side. We know this long trapdoor today as the Sierra de Salinas. Across the Salinas Valley the Gabilan Range was lifted up again, higher on the east side, along the San Andreas Fault. Remember that at the south end of the range, beneath the surface, lay the volcanic rocks that were soon to be eroded into the novel features that are today included in Pinnacles National Monument. The Gabilan Range was now connected to the Santa Cruz Range to the north. The latter, too, was shattered by faults. It apparently started out as another giant trapdoor, but during the upheaval its back was broken in several places, giving rise to the lesser trapdoors of Montara Mountain, Butano Ridge, and Ben Lomond Mountain. Across the bay lowland the Diablo Range was lifted up vertically, presenting a precipitous slope overlooking the lowland. Here and there relatively small areas subsided below the upland level to form intermontane basins. Livennore Basin was one of these. Between the Gabilan and Santa Cruz ranges on the west and the Diablo Range on the east, the San Fran-

[61]

cisco Bay-Santa Clara lowland foundered into the depths. In doing so, however, the floor of the lowland broke into long, narrow blocks that are now almost completely buried under sediment washed from the surrounding mountains. The top of one of these blocks, not completely buried, sticks up above the surface of the lowland just east of the south end of the bay. The eroded crest of this block is known as the Coyote Hills. Faulting on a grand scale also took place north of San Francisco Bay. Both the Marin Mountains and the Mendocino Range were bodily uplifted along the San Andreas Fault. Across the fault to the west the Pt. Reyes Peninsula was uplifted once again, and the base of the peninsula next to the San Andreas Fault may have been tilted up as another trapdoor to form Inverness Ridge. The Farallon Islands far off the coast may represent the eroded crest of another trapdoor uplift. East of the Marin Highlands across the Santa RosaPetaluma lowland, the Sonoma Range was uplifted along the northern extension of the Hayward Fault— the same fault that follows the western foot of the Diablo Range. Still farther east the Mayacmas Range also rose along a major fracture. North of the Marin Mountains and the Sonoma Range the surface subsided from the crest of the Mendocino Range eastward to the foot of the Mayacmas Range. The subsidence carried the Merced formation several thousand feet below sea level. Other faults cracked the surface back of the scarp of the Mayacmas Range. One of these along Big Sulphur Creek extended down to a body of hot rock far below—perhaps the same heated mass that had caused the volcanic activity at the site of the Sonoma Range much earlier. Heated vapors rising along this fracture converted much of the ground water to steam, and steam vents sprang up along the trace of the fault. In this way the geysers north of Healdsburg came into existence. Actually the steam vents are not geysers; [62]

they do not erupt like Old Faithful in Yellowstone Park; they are more correctly referred to as fumaroles or gas vents. Between the Mayacmas Mountains and the Great Valley the area north of the bay was upheaved bodily and broken locally by long fractures. At about this time there began the violent episode of volcanic activity that created the picturesque landscape of the Clear Lake region and gave rise to Clear Lake itself. Volcanoes sprang up over this region, pouring out flows of light and dark colored lavas and spewing great volumes of ash, cinders, and broken rock fragments. Mt. Konocti, which rises nearly 3000 feet above Clear Lake, is one of these ancient volcanoes, or rather a composite volcano, with the remnants of several craters still visible. That the volcano became extinct a long time ago is indicated by the great amount of erosion it has undergone. During the height of the volcanic activity several thick lava flows advanced toward each other just southwest of Clear Lake. They failed to make contact, however, and left a deep basin between them —the present site of Thurston Lake. On the opposite side of Clear Lake a south-draining valley was blocked by lava to create the basin of Borax Lake. Volcanic activity continued in this area until recent times, as indicated by the fresh appearance of Roundtop Mountain, a cinder cone about one mile west of the southern tip of the lake. Actually, volcanic activity has not yet entirely died out. Hot vapors are even now being emitted at Borax Lake and Sulphur Bank. Before there was a Clear Lake, the basin was apparently drained by two streams whose headwaters were separated by a low divide at the present narrows of the lake. Cold Creek drained the northern and larger part of the basin, leading the waters west into the Russian River; and Cache Creek drained the smaller southern basin toward the east. A flow of lava a short distance east of the present outlet of the lake dammed [63]

Cache Creek and impounded a lake in the lower lake basin. The lake finally rose high enough to spill over the divide into the headwaters of Cold Creek. And then, to complicate matters, a huge landslide in the valley of Cold Creek, below where the Blue Lakes now are, dammed Cold Creek and impounded its waters. The waters of this lake rose higher and higher and eventually merged with those on the south side of the divide. This large lake, modern Clear Lake, once again drains eastward through Cache Creek. In spite of the upheaval of block mountains all over the Bay Region, the rate of uplift was so slow that the Sacramento River was able to maintain its path through the rising blocks and continue on to the sea. Geologists refer to streams that are able to accomplish this feat as "antecedent," because their paths are already established before the mountain barriers through which they flow come into being. T H E SECOND STAGE

We must not suppose that mountain-making ceased with the development of the landscape shown in Figure 12. On the contrary, it is still going on, as attested to by the many earthquakes that beset the region and by the elevated and warped marine terraces along the coast. The circular group of hills known as Montezuma Hills east of Suisun Bay rose from the flatlands of the Great Valley, and for all we know they may still be rising. As this dome-like feature rose from the plains, it deflected the Sacramento River into a sweeping curve around it. These few examples illustrate that while mountainmaking activity reached its culmination in the first stage of the Pleistocene, it did not terminate there. During the period of a million years or so of the second stage the great fault scarps formed earlier were weathered and eroded and their sharp edges blunted. And the rejuvenated streams that rushed down from [64]

the uplifted blocks eroded a complex of deep, narrow valleys to create the present rugged landscape. Only where the streams have not yet sent their tentacles into the heart of the ranges are broad level remnants of the earlier plain preserved. In all previous episodes of mountain-making, when the layers of rock were crumpled into folds or broken by faults, the folds and faults trended approximately parallel to the coast. The folded rocks included some layers that were weak and easily eroded, and others that were durable and resisted erosion. The zones of crushed rock along the faults were also susceptible to rapid erosion. It will be recalled that at the time of the great denudation the entire area had been worn down to a plain almost at sea level. Because of the extremely gentle gradients of the streams and their inability to erode lower than the level of the sea, the belts of weak rock were not at that time eroded significantly lower than the areas of resistant rock on either side. But the uplift of large segments of the plain during the first half of the Pleistocene provided the streams with steep gradients which rejuvenated them. With their newfound vigor they began to attack the uplifted rocks, eroding the weak rocks more rapidly than the resistant rocks on either side. The crushed zones along the faults were especially susceptible to erosion, and many of the major valleys of today coincide with major faults. A classic example of this is the valley, or rather, the series of aligned valleys that are etched out along the San Andreas Fault. South of San Francisco in the Santa Cruz Range the crush zone has been eroded into a long, straight valley now occupied in part by San Andreas and Crystal Springs lakes. The crush zone here is more than a half mile wide. The material in the crush zone can be examined first-hand in the road cuts a short distance up the grade from the east end of the causeway across Crystal Springs Lake. Incidentally, at the time of the San Francisco earthquake of 1906, the causeway [65]

across the lake was displaced about 12 feet, the western side having moved north. North of the bay the crushed material was eroded to form the remarkably straight valley occupied in the south by Bolinas Bay and farther north by Tomales Bay. With interruptions, the line of the San Andreas Fault can be followed by its topographic expression from one end of the Bay Region to the other, a distance of nearly 250 miles. Its total trace, including the areas outside the Bay Region, is more than 600 miles. The San Andreas Fault is a type known as a lateral or transcurrent fault, in which the ground moves horizontally rather than vertically. Not all the major valleys are stream-eroded. As we have seen, the San Francisco Bay-Santa Clara lowland is a graben that foundered between major faults on both sides of the valley. It is similar in origin to the basin of the Dead Sea between Israel and Jordan, and to the Rhine Valley between the Vosges Mountains on the west and the Black Forest on the east. Other examples are the so-called "Rift Valleys" of East Africa, occupied by Lakes Nyassa, Tanganyika, and others. Actually a number of tilt blocks were formed contemporaneously within the San Francisco Bay-Santa Clara graben. Although these were subsequently buried in sediment, their existence has been demonstrated by special geophysical techniques. The end result of the erosion of the belts of weak rock, of the foundering of graben, and the downtilting of other blocks, was the present Bay Region landscape with its parallel ranges and valleys approximately parallel to the coast. Erosion by streams also added detail to the landscape. It will be recalled that Mt. Diablo, which had been created in the late Pliocene mountain-making, had been obliterated by erosion, except for the central core, at the time of the great denudation. The rejuvenation of the streams following the uplift pictured in Figure 12 [66]

enabled the streams to dissect the area anew. The weaker rocks were rapidly eroded, leaving the resistant once again standing as ridges encircling the central core. The effects of erosion were similar throughout the Bay Region; valleys were etched out on weak rocks, leaving the resistant ones standing as ridges. Only in those areas where all the younger sedimentary rocks had been eroded away during the great denudation, leaving only the hard, more or less uniformly resistant ancient rocks, was there no opportunity for selective etching of the landscape. In these areas, except for valleys etched out along faults, there is little parallelism of ridges and valleys. The Mendocino Range, the Marin Mountains, and the Diablo Range south of the Livermore Basin are examples of such irregularity dissected areas. It will also be recalled that far to the south, east of Soledad, a mass of volcanic rocks had earlier been dropped down into the older rocks of the Gabilan Range and thereby preserved from removal during the great denudation. Now, thanks to the most recent upheaval, invigorated streams attacked these weak and fractured volcanic rocks, etching them into a filligree of spires and pinnacles of such unusual beauty that the area has been set aside as the Pinnacles National Monument. The modern landscape has been influenced by another factor. The steep slopes remaining from the faulting of the first stage of the Pleistocene, and the steep slopes created by the rapidly eroding streams of the second stage, were particularly susceptible to landslides. Much of the difficulty experienced in this region with buildings sliding downhill or roads being blocked by landslides can be blamed on the swiftly flowing streams created by the rise of mountains more than half a million years ago. San Francisco Bay is a relatively recent feature of the landscape. When the ice of the last glacial stage

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began to melt away, a large amount of water was returned to the oceans. Sea level rose and flooded in among the mountain blocks of the Bay Region. A slight down buckling of the surface contributed to the flooding. This is evident from the fact that coastal terraces descend toward San Francisco Bay from both the north and south. The subsidence was local, however, because neither the valley of the Russian River 50 miles north of the bay nor that of the Pajaro River 50 miles to the south is flooded. Between these two rivers the mouths of all valleys are submerged; the valleys are drowned. Bolinas and Tomales bays lie at opposite ends of the straight valley eroded along the San Andreas Fault, yet the ends of this valley are now under water. Drakes Bay on the Pt. Reyes Peninsula is another clear example of submergence by the sea. The finger-like character of the bay is precisely what one would expect if a branching stream valley were to be inundated. It is interesting to note, however, that the Pt. Reyes Peninsula also provides evidence of emergence of the land. There is a clearly defined, wave-cut terrace at the south end of the peninsula, indicating emergence of the land from the sea, while alongside are the drowned valleys of Drakes, Bolinas, and Tomales bays, indicating submergence. Obviously the geologic history of the region is more complicated than indicated in this brief survey. The complications stem from the fact that changes in sea level and changes in the level of the lands often take place independently of each other, and often in opposite directions. The flooding of the San Francisco Bay Region may have continued up to the time of human occupation of the area, because Indian shell heaps are now found below sea level at Emeryville and elsewhere. During the Pleistocene Ice Age the fluctuations of sea level amounted to hundreds of feet. At each level at which the sea remained fixed for some time, the waves carved a shoreline in the flanks of the coastal

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mountains. Thus, each elevated shoreline that we see today means either that the sea once stood that high or that the coast rose out of the sea to that height. Possibly both events took place. The origin of the terraces becomes clear if we examine what is going on along the coast today. The waves are battering the shore, driving a sea cliff inland and leaving behind a smooth rock surface under water. Should sea level drop or the land rise, we would see, out of water, a broad flat, like the broad terrace followed by the coast highway south of Half Moon Bay or along the Sonoma coast, or like those carved into the mountain slopes up to a thousand feet above sea level. The sea has also busily engaged itself in the construction of beaches, sand spits, and bars along the coast. The main oceanic current along this part of the Pacific Coast is toward the south, but irregularities of the shore create countereddies which transport sand northward. One of these is responsible for the northward drift of sand, derived from cliffs many miles south of the Golden Gate, that has led to the fashioning of Ocean Beach at Golden Gate Park. Northerly currents have also built bars across the mouths of many of the valleys that were drowned during the flooding of this area. Merced Lake, now in part artificially contained, occupies one of these valleys shut off from the sea. Many smaller land-locked bays have been completely filled with sediment and now appear as low, flat, often swampy areas back of sand ridges. To the north of Golden Gate the drifting sands were carried across the mouth of Bolinas Bay nearly isolating it from the ocean. On the south side of Pt. Reyes Peninsula the former valley system now occupied by Drakes Bay was also isolated from the sea by the growth of sand spits. Before the San Francisco Peninsula was densely settled, the vigorous winds swept the sands of Ocean Beach inland creating extensive fields of sand dunes. [69]

The reclamation of a large part of this sand dune area in the creation of Golden Gate Park was a notable achievement. Few of us are probably aware of the presence of a large semicircular, submarine bar 5 or 6 miles off the Golden Gate. This bar, much of which is barely 35 feet below the surface, may represent the terminus of the delta of the Sacramento River before the bay lowland was flooded by the sea, or it may have been formed from sediment carried out of the bay on ebb tides. Another possibility is that it represents sand carried northward along the coast by the same current that built Ocean Beach—and kept well offshore by the strong ebb tides. Perhaps even fewer of us are aware of the presence, off the Monterey coast, of a submarine canyon that rivals the Grand Canyon of the Colorado River in grandeur. This canyon, which extends to depths of more than 10,000 feet, is outlined in Figure 1. Underwater canyons such as this are found along the margins of all continents. Some geologists believe that they were eroded by rivers when the level of the sea stood two miles lower than now. Others object to this suggestion because there seems to be no reasonable way to account for a two-mile drop in sea level, followed by a similar rise after the canyons were eroded. Others believe that the waters of the sea, locally made heavy by clouds of sediment introduced by rivers or submarine landslides, flow down the submerged continental margins to erode the canyons. According to this hypothesis, submarine canyons are forming even now. if the Monterey canyon is stream-eroded as some suggest, it seems strange that it does not lie opposite a large river on land. In contrast, there is no submarine canyon opposite the Golden Gate where the waters of the Sacramento River reach the sea. We can understand, therefore, why the suggestion was long ago made that the Sacramento River once flowed southward down [70]

the San Francisco Bay-Santa Clara lowland to empty into Monterey Bay opposite the head of the submarine canyon. It is also possible that the drainage of the Great Valley once flowed north up Salinas Valley and emptied into Monterey Bay. As a matter of fact we do not have enough evidence at present to decipher exactly what did take place here in the past. Maybe there is a submarine canyon off the Golden Gate, buried under the floods of sediment that are drifted out of the bay or washed along the coast and across the mouth of the bay. Or perhaps a submarine canyon that once formed outside the Golden Gate has been shifted far to the north by continued movements along the San Andreas Fault. What the future has in store for the Bay Region is difficult to say. Ordinarily we might expect that the bay would be filled with sediment in a relatively short time, much as a reservoir becomes filled with in-washed sediment. Even if this filling were to go on uninterruptedly, however, our beautiful bay will be with us for many, many centuries. The filling will almost certainly not continue uninterruptedly, however; there is no assurance that the Ice Age is over. Sea level may continue to rise and fall, at times withdrawing from the bay lowland, at other times flooding it. At times the bay may increase in size, flooding much of the adjacent lowlands; at other times it may shrink, exposing parts of the bay floor. To add to the confusion, it is equally certain that the Bay Region has not seen the last of upheaval and depression. Whenever the land rises, the shoreline will recede seaward; whenever it subsides, the shoreline will encroach into the interior. Thus, the bay will react delicately to changing geologic conditions, enlarging and contracting like the lens opening of a camera. In the light of these unpredictable conditions, it is futile to speculate on what the future has in store. The crystal ball is too clouded. [71]

GENERAL REFERENCES Branner, J. C., J. F. Newsom, and Ralph Arnold. Geologic Atlas of the Santa Cruz Quadrangle, California. U.S. Geological Survey, Folio 163, 1909, 11 pages, maps. California Division of Mines. An Excursion Through. the San Francisco Bay Area. Mineral Information Service, vol. 2, no. 2, Feb., 1949, pp. 6-9, 12-13. Gilliam, Harold. San Francisco Bay. New York, Doubleday, 1957. Hinds, N. E. A. Evolution of the California Landscape. California Division of Mines, Bulletin 158, 1952. See section on Coast Ranges, pp. 157-181. Howard, A. D. Development of the Landscape of the San Francisco Bay Counties. California Division of Mines, Bulletin 154, 1951, pp. 95-106. Lawson, A. C. Geologic Atlas of the San Francisco District, California. U.S. Geological Survey, Folio 193, 1914, 24 pages, maps.

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CALIFORNIA NATURAL HISTORY GUIDES Arthur C. Smith, General Editor "In these days of rising prices and declining values, it's always good to read a new California Natural History Guide; these little books are small enough to fit into a pocket, inexpensive and authoritative." — Sunset "If you're vacationing in California this summer — camping out or just taking day trips to some of our great parks and primitive areas — some of the nicest things to pack along are the California Natural History Guides, published by the University of California Press. They're compact (in paperback), scientifically precise but interestingly presented, and written so even elementary school children can read them with relative ease. They're illustrated with quite accurate line drawings and color plates and, best of all, they're relatively inexpensive." — San Francisco Chronicle "This entire series . . . constitutes a library in miniature of beautifully designed and illustrated, authoritatively written, and cheaply priced pocket books aimed at familiarizing readers with the natural world around them." — Westways "A series of excellent pocket books, carefully researched, clearly written and handsomely illustrated." — Los Angeles Times

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