GEOLOGY OF THE POINT REYES PENINSULA, MARIN COUNTY, CALIFORNIA
ALAN J. GALLOWAY1
The isolated triangle of land called the Point Reyes Peninsula is one of the most picturesque and dramatic spots in California. High rugged cliffs pounded by the sea; secluded coves populated by sea lions, seals, and seabirds; a long, exposed wind-swept beach; quiet sheltered beaches and estuaries; gently rolling, grassy hills; a high forested ridge; and the long, narrow, partly submerged trough of the great San Andreas fault, all combine to make the Point Reyes Peninsula a magnificent natural region with great scenic, historic, scientific, and recreational value. Although summers in this geological island may be foggy and windy, a visit to Point Reyes on a warm sunny day in April or May, made more beautiful with spring wildflowers, or on a bright fall day will be long remembered. The major portion of this unique area is now preserved as the Point Reyes National Seashore, a 64,000 acre park within sight of San Francisco.
This book describes the geology of the Point Reyes Pensinsula, and it is designed to be both a comprehensive report for the geologist and a source of information for the interested layman, who can more fully enjoy the area today by knowing how it came to be. This introduction is a generalized summary of the main report.
The Point Reyes Peninsula can be divided conveniently into four areas:
(1) The long northwest-trending depression occupied by Tomales Bay and Bolinas Lagoon and the strip of land between them.
(2) The high forested Inverness Ridge and its extensions from Tomales Point to Bolinas Mesa.
(3) The rolling grassy pastureland west of the ridge, including the major sand beaches.
(4) The rugged ridge of Point Reyes itself.
Each of these landscapes is a product of its geological composition (the rocks) and its geological history; the main features of the peninsula are directly related to the geology.
The Point Reyes Peninsula is divided from the Coast Ranges of the mainland by a long straight depression, called here the Olema trough, extending from Tomales Bay to Bolinas Lagoon. This trough serves as a natural highway diverting traffic northwest and southeast, while the hills to the west of it and the two arms of the sea form barriers to western travel.
At the shock of the great San Francisco earthquake of April 18, 1906, a crack in the ground was formed all the way up the valley from Bolinas Lagoon to Tomales Bay. This dramatically drew attention to the fact that this long depression is the site of the great San Andreas fault.
In this earthquake, the land on the west side of the fault moved northwestward relative to the land on the east side. There was little or no vertical movement. The greatest measured horizontal movement took place not near San Francisco, but in the Point Reyes Peninsula near the little town of Point Reyes Station, where the displacement amounted to 20 feet.
The 1906 earthquake crack can still be identified today. Along its course can be seen the curious topographic forms caused by the fault; little ponds perched on the tops or sides of hills, valleys without streams, and long straight low hills. As one studies the long valleys, however, it soon becomes evident that there have been many earlier earthquake cracks like that produced in the 1906 quake. In some places, the belt of fault topography is as much as a mile wide. The remains of the old cracks are too indistinct to be counted, but it is clear that there has been a tremendous amount of recurrent movement along this zone of weakness. Shattering of the rocks by continuous earth movements has weakened their resistance to erosion by rain, stream, and ocean, resulting in the formation of the long straight depression. Highway 1 traverses the San Andreas fault zone from Stinson Beach to the north end of Tomales Bay where the fault runs out to sea.
The dramatic evidence of faulting in the Olema trough leads to speculation as to how far the block of the earth's crust carrying the Point Reyes Peninsula has moved northward relative to the mainland during geologic times. Geologists hold widely varying views as to the amount of cumulative displacement on the San Andreas fault; some holding that it has been only of the order of a few miles, while others (with whom the writer is inclined to agree) suggest that it may have been of the order of 100 miles or more.
One reason for this latter view is the fact that the Point Reyes Peninsula is a little geological "island" of its own. On the adjacent "mainland" the rocks are "Franciscan", the familiar buff sandstones, reddish cherts, and green serpentines that are to be seen in San Francisco and along Highway 1 to Stinson Beach. These rocks are entirely different in appearance and age from the younger granite and white shales which make up most of the Point Reyes Peninsula; there is just no similarity between the two, and the discontinuity occurs along the fault. Another reason is the evidence for repeated faulting to be seen in the long depression described above.
The available evidence suggests that in the past 25 million years the land west of the San Andreas fault has been moving northward at a rate averaging 1/2 inch per year, which means that, in the 65 million years since the oldest sedimentary rocks were deposited on the Point Reyes Peninsula, the movement on the fault could have amounted to hundreds of mites. When the rocks of the white cliffs of Drakes Bay were being deposited under the sea, the Point Reyes Peninsula probably was in the ocean looking like one of the Channel Islands and was many miles south of where it now is located, relative to the mainland.
The rocks exposed in the Olema trough (San Andreas fault zone) are mostly Franciscan sandstones, cherts, and serpentines of the Coast Ranges which have been broken and shattered by the faulting. Overlying these rocks are much younger sands and gravels that were deposited in streams and ponds or at the bay margin in the fault zone or were deposited in the sea at the south end of the trough. These latter marine deposits, known as the Merced Formation, extend along the north and west sides of Bolinas Lagoon with the ocean.
To the visitor descending the Sir Francis Drake Highway from the east toward Olema, the Point Reyes Peninsula rises up as a dark forested ridge across his line of travel. This is Inverness Ridge, which extends from Tomales Point in the north to Bolinas Mesa in the south. The outstanding topographic feature of the Inverness Ridge is Mount Wittenburg (elevation 1407 feet), highest point in the Point Reyes Peninsula.
The backbone of this high ridge is formed of granite which on the surface extends from Tomales Point as far south as Mount Wittenburg, but over a large part of this area the granite is difficult to see because of the thick cover of soil and vegetation. (The term granite, as used in this introduction for simplicity, includes several varieties of granitic rocks described later.) The granite can be seen in the road cut of the Sir Francis Drake Highway immediately south of Inverness, also on the north side of the highway between Inverness and Tomales Bay State Park, and on top of Mount Vision. The granite is most grandly exposed in the ocean-facing cliffs of Tomales Point and Point Reyes. Here the hardness and toughness of the rock defends the land against the erosion of the breakers, but it is being gradually broken down as is evident from the many isolated rocks, stacks, and islets that can be seen in the surf. Good places to see the battle between the granite and ocean are the south-central and eastern parts of Point Reyes and McClures Beach on Tomales Point. The granite of Inverness Ridge and the similar granite of Point Reyes are about 80 million years old.
The oldest rocks in the Point Reyes Peninsula are schist, quartzite, and crystalline limestone (marble) that are found in small isolated patches surrounded by the granite of Inverness Ridge. These rocks were originally shale, sandstone, and limestone, respectively, that were engulfed by the molten granite about 80 million years ago. Since the original rocks were invaded by the molten granite, they must of course have been older originally than 80 million years, but how much older we do not know. The heat and pressure caused by the granite intrusion changed, or metamorphosed, the original shales and clays into schists, the sandstones into quartzite, and the calcareous sediments into crystalline limestone or marble. Being embedded in the mass of granite, these metamorphic rocks are harder to see than the other rocks of the Peninsula. Crystalline limestone may be seen in the field to the north of the LimantourInverness Park Road, west of the church but on the north side of the road. The limestone is harder than the surrounding rock and stands out in the field as white boulders. There are old lime kilns here too. This is private land at the time of writing; permission is needed to go on the land. [The well-known so-called "Russian" lime kilns are in the San Andreas fault zone; the limestone rock there belongs to the "Franciscan" group of rocks typical of the mainland Coast Ranges and not found on the peninsula]. Schists can conveniently be seen at the south end of McClures Beach; they do not make prominent outcrops because they are generally softer than the surrounding granite rock.
Southward from Mount Wittenburg the granite of Inverness Ridge is covered by an increasing thickness of younger, light-colored, marine shales that extend to Bolinas Mesa and Duxbury Reef. Similar shales and sandstone cover the lower western slopes of the ridge and extend onto the northern part of the ridge in the vicinity of the Kehoe Ranch. Ten to 15 million years ago, these sedimentary rocks, called the Monterey Shale and the Laird Sandstone, entirely covered the granite of Inverness Ridge but were later eroded from the northern part. The Laird Sandstone is a thin unit that was deposited directly on the granite; the Monterey Shale was subsequently deposited over the sandstone. The Laird Sandstone can be seen west of the road between Inverness and McClures Beach, 3/5 mile south of Kehoe Ranch. These soft sandstones are composed mainly of eroded fragments of granite; the sandstones resemble weathered granite, for which they are readily mistaken. The Monterey Shale is seen in the cliffs west of Bolinas, at Duxbury Reef, and in the sea cliffs northward to Drakes Bay. To the north, these shales become very flinty and cherty; the flinty rocks may be seen in the cliff at the Pig Ranch. In the cliffs and hills around Double Point, the Monterey shales are involved in a huge landslide nearly 4 miles long and at least a mile wide. The landscape of the slide area differs considerably from that developed elsewhere on the Monterey Shale. Extremely large blocks of shale have broken off from the main body of rock, have been tilted, rotated, and contorted, and are slowly sliding down into the sea. The tilting of the shale blocks has impounded a number of small lakes, an uncommon sight in coastal California.
On the west side of the forested Inverness Ridge, the traveller comes down to open windswept pastureland that stretches southward to the rocky promontory of Point Reyes. The smooth contours of this area can be attributed to the relative softness of the underlying light-colored siltstones and claystones. These rocks are the Drakes Bay Formation, which is younger than, and overlies, the Monterey Shale; the Drakes Bay Formation is 5 to 10 million years old. Very little in the way of rock is to be seen while driving across these grassy slopes, but the gently rolling topography ends abruptly at the famous "white banckes and cliffes" of Drakes Bay which Sir Francis Drake saw in 1579 and likened to the chalk cliffs of England. The cliffs are made of claystones and siltstones of the Drakes Bay Formation, which may be readily examined by walking along the beach in either direction from the parking lot at Drakes Beach. These sedimentary rocks overlying the granite have been folded into a broad shallow elongate bowl extending from Inverness Ridge to Point Reyes. The centerline of the bowl runs northwest-southwest through the western part of Drakes Estero.
The smooth pastureland has been cut into by the many arms of Drakes Estero and the Estero de Limantour. These are drowned river valleys that were flooded by the sea as the land slowly sank and by the rise of sea level following the melting of the glaciers of the last ice age. The long sand bar of Limantour Spit has been extended across the mouths of the esteros by the slow westward drift of sand along the shore of Drakes Bay. Another, much smaller, spit extends eastward from the west side of the mouth of Drakes Estero. These shifting sand bars are continually modifying the passage into the esteros.
The west margin of the pasturelands is the great, 10-mile-long Point Reyes Beach, a magnificent place to visit, but a very dangerous place for swimming. The wide, sandy beach is open to the sweep of the prevailing northwest winds and a strong surf and undertow are usually present. Behind the beach are high sand dunes, partly stabilized by vegetation, but in places blowing well inland. A mile south of the turn-off to Drakes Beach, the Sir Francis Drake Highway crosses an old dune ridge, and at Point Reyes the blowing sand has been heaped high against the north flank of the ridge.
At the seaward end of the Point Reyes Peninsula is the rugged rock promontory of Point Reyes itself, the most dramatic feature of the peninsula. The high, rugged ridge is due partly to the hardness of its constituent granite and conglomerate and partly to uplift along a fault under the ocean. Point Reyes is principally a mass of granite similar to the granite of Inverness Ridge. Overlying parts of the granite are large patches of conglomerate, a hard sedimentary rock made up largely of rounded cobbles and pebbles cemented together. A spectacular conglomerate is beautifully exposed at the parking lot above the Point Reyes lighthouse and all the way down past the lighthouse to the foaming surf. [Coast Guard permission required to enter this area] While most of the rounded pebbles are about egg-sized, some large blocks of granite several feet in diameter are to be seen, indicating that granite cliffs were not far away when the conglomerate was laid down. The Point Reyes Conglomerate is younger than the granite but older than the Laird Sandstone and Monterey Formation. The conglomerate is found at Point Reyes and nowhere else in the peninsula.
We have described the main areas of the Point Reyes Peninsula and the kinds of rocks found in each area. The history of these rocks is a complex story that must be reconstructed from fragmentary evidence.
The oldest rocks in the Point Reyes Peninsula are the crystalline limestones, quartzites, and schists found embedded in the granite between Mount Wittenburg and Inverness. These rocks were deposited as layers of sand and clay and soft lime, just as sands and muds accumulate in shallow seas today. There must have been a great thickness of these deposits, sufficient to bury deeply the rocks that now remain. [For more detail, please see figure 5 in the section titled "Geologic History."]
Millions of years later, the sediments had become compacted and perhaps had been uplifted and folded by great earth movements. They were invaded from beneath by a mass of molten rock that solidified into granite. The heat and pressure caused by this granite intrusion changed, or "metamorphosed", the old sedimentary rocks into the hard quartzites and crystalline schists and crystalline limestones of which we now see remnants. Their age is unknown. They must be older than the molten granite that later engulfed them, which is about 80 million years old, so the limestones, quartzites, and schists are older than 80 million years, which makes them of quite a respectable age even on the geological time scale. The granite, a familiar hard gray rock made up of interlocking small crystals, today is exposed continuously between Tomales Point and Mount Wittenburg near Olema, and it forms the prominent cliffs at Point Reyes.
The solidified granite and its covering of metamorphosed sediments were lifted up to form mountains, which were then eroded by wind and rain. Probably most of the metamorphosed sediments and part of the granite had been eroded away when the sea started to encroach again on the land some 60 million years ago. Conglomerate of about this age lies on the Point Reyes granite. The sea invaded further, and some 15 to 5 million years ago the widespread sands and shales were laid down that now are found at the surface west of Inverness Ridge to Point Reyes, and south from Mount Wittenburg to near Bolinas.
Later the sea retreated and sands and gravels of about 5 million years in age seem to have been deposited only in the vicinity of Bolinas. Since that time, most of the area has been above the sea; and wind, waves, and rain are again at work, eroding the rocks grain by grain and transporting the grains into the sea to form new layers of sediment.
For tens of millions of years, while these sedimentary rocks were being deposited, the area that is now the Point Reyes Peninsula was moving slowly northward along the great San Andreas fault. Close to the coastline, one can see evidences of fairly recent vertical movements in the marine terraces and the drowned valleys. These earth movements have continued until just yesterday, geologically speaking, in the Point Reyes Peninsula; and they are still going on, invisibly, slowly, today.
The Point Reyes Peninsula, a roughly triangular piece of land that extends westward into the Pacific Ocean, lies about 30 miles north of San Francisco. The peninsula is joined to the mainland by the linear, trough-like Olema Valley, the drowned extensions of which are occupied by Tomales Bay and Bolinas Lagoon. The long northeast side of the triangle, from the town of Bolinas to Tomales Point, is nearly 30 miles as the crow flies. From Inverness to the west end of Point Reyes, the width of the triangle is about 12 miles. The triangular peninsula is approximately 100 square miles in area (figure 1 and photo 1). Five United States Geological Survey 7-1/2minute topographic quadrangles, Bolinas, Double Point, Inverness, Tomales, and Drakes Bay, extending from latitude 37°52'30" N. to latitude 38°15'00" N., and from longitude 122°37'30" W. to longitude 123°00'00" W., cover the area.
This unique area encompasses the long, narrow, partly submerged trough of the great San Andreas fault, the high forested Inverness Ridge, the rolling grassy pastureland west of the ridge with the long exposed wind-swept beach on the west and the sheltered beaches and estuaries on the south, and the rugged promontory of Point Reyes with its high rugged cliffs. Each of these landscape forms is a product of its geological composition (the rocks) and its geological history. The major portion of this scenic area, 64,000 acres, is now preserved as the Point Reyes National Seashore.
The principal settlement in the area is Point Reyes Station, at the head of Tomales Bay. Other settlements are Olema, Inveress, and Bolinas, summer resorts with small permanent populations. These towns are served by the Sir Francis Drake Highway, from San Anselmo to Point Reyes, and by State Highway 1, from San Francisco to Olema and Point Reyes Station. The area can also be reached by the Panoramic Highway from San Francisco across the south slope of Mount Tamalpais to Stinson Beach.
The area mapped and described in this report includes the Point Reyes Peninsula and the adjacent portion of the San Andreas fault zone. The Franciscan terrane that lies east of the natural boundary caused by the fault zone is not covered in this report.
Point Reyes Peninsula is distinguished from the other coastal areas west of the San Andreas fault largely by being a distinct, nearly sea-girt, tract of land separated from the mainland by the valley created by the fault zone, and by having a varied geology entirely different from that of the mainland. These factors have resulted in the peninsula having a distinct character of its own, geologically, botanically, and agriculturally.
Geologically, Point Reyes Peninsula is distinctive because:
(1) Together with Bodega Head it is the last northward appearance of the granitic-metamorphic core complex (Page, 1966).
(2) It is an isolated area where, because of the almost complete absence of Tertiary cover on the rocks east of the San Andreas fault zone, the granitic-metamorphic and the Franciscan eugeosynclinal (associated with volcanism) core complexes are juxtaposed in dramatic contrast.
(3) It is the locale of the maximum known movement on the San Andreas fault in the 1906 earthquake.
Two hundred and thirty days of field work were completed during the years 1959-1965, using as a base the U.S. Geological Survey topographic quadrangles on the scale of 1:24,000. Complete vertical aerial photo coverage was obtained from 1952 aerial photographs of the U.S. Geological Survey, series GSUX. These photographs were studied stereoscopically and used for locating field observations. Oblique aerial photographs were used to study the cliff faces. Rock samples were studied under binocular microscope, and thin sections were made of selected igneous and metamorphic rocks for petrological study. Numerous rock samples were washed and examined for microfaunal content.
The original land survey of the area was based on the Spanish ranchos. There is no surveyed grid of sections, townships, and ranges. Consequently many localities described by the writer and earlier writers are identified only by reference to nearby ranches. Table 14 presents the latitude, longitude, and earlier names of many ranches and localities and identifies the ranches mentioned by Gilbert (1908).
The first scientific mention of the geology of the Point Reyes Peninsula is found in the "Report of the Superintendent of the Coast Survey" for 1855 by Blake (1856). This report included a geologic map and cross section of Point Reyes and vicinity. Blake also wrote a geological report for the Pacific Railroad Exploration and Surveys of 1857, and in both reports he mentions the granite and limestone of Tomales Bay.
Whitney (1865) mentions the area briefly in Volume 1 of "Geological Survey of California", and Brewer (1930) in "Up and Down California in 1860-1864" describes his visit to the peninsula while working for Whitney.
In 1899 F. M. Anderson published a paper entitled, "The Geology of the Point Reyes Peninsula" in the University of California Bulletin of the Department of Geology. This was the first description of the general geology, but his work did not extend east or south of Bear Valley. Nevertheless, it is of great value to later geologists, and the hand specimens he collected in the course of his field work can still be seen at the University of California at Berkeley.
The 1906 San Francisco earthquake caused a fracture and offset in the surface of the ground along the whole course of the San Andreas fault through Olema Valley. Consequently, the area figures prominently in the "Report of the State Earthquake Investigation Commission" (Lawson, and others, 1908). In this report G. K. Gilbert vividly described the phenomena visible along the Olema Valley.
A portion of the area south of latitude 38° N. is briefly described by A.C. Lawson in the San Francisco Folio published by the U.S. Geological Survey in 1914. Charles E. Weaver published "Geology of the Coast Ranges immediately north of the San Francisco Bay Region" in 1949, covering the area north of latitude 38° N.
Bulletin 154 of the California State Division of Mines, "Geology of the San Francisco Bay Counties", published in 1951, includes articles by Oliver E. Bowen, Jr., which summarize the information available at that time and add original observations. Bulletin 154 also contains a report by N.L. Taliaferro in which the geology of the peninsula is discussed along with that of the rest of the Bay Area.
For future students of the geology of the Point Reyes Peninsula, it seems desirable to mention some of the more important geological maps of the area which have been published. The first reliable geological map (scale 1:125,000) was included in "The Geology of the Point Reyes Peninsula" by Anderson (1899). The map, which covers only the area north of latitude 38° N., is a remarkable piece of work considering that no accurate topographic base was available.
Map No. 4 in the Atlas of the State Earthquake Investigation Commission Report (1908) shows the trace of the 1906 rupture through Marin County as well as San Francisco and San Mateo Counties.
Lawson (1914) included a geological map of the Tamalpais quadrangle on the scale of 1:62,500 in the San Francisco Folio of the U.S. Geological Survey. This map included the southern portion of the Point Reyes Peninsula as far north as latitude 38° N. and as far west as longitude 122°45' W. On this scale not much detail is shown, but the accompanying text in Lawson's inimitable style is well worth study.
California Division of Mines Bulletin 118 (Douglas, 1943, p. 621) contains a small map showing fairly detailed geology around Bolinas. This informative map is derived from Douglas and Rhoades (1915).
Weaver (1949) includes a geologic map on a scale of 1:62,500 using the Point Reyes 15-minute quadrangle as a base. This map extends only a short distance south of latitude 38° N.
A map derived from Harold J. Gluskoter's 1962 Ph.D. thesis at the University of California was published in 1969 by the California Division of Mines and Geology as Map Sheet 11, on a scale of about 1:40,000. This is an excellent detailed map, but the Point Reyes Peninsula portion does not extend north of latitude 38° N. or west of longitude 122°45' W.
So many people have assisted the writer in different ways in the preparation of this report that it would be impossible to list them all; indeed it is with much diffidence that any names are mentioned for fear of omitting some important contributor.
The scientific staff of the California Academy of Sciences has been consistently helpful; in particular, the writer is much in debted to Dr. G Dallas Hanna, Dr. Leo Hertlein and Dr. Peter U. Rodda. Mrs. Robert Neff and Mrs. Enid Cook kindly transcribed the field notes and typed the manuscript, two tedious tasks.
Much assistance has come from members of the staff of the California Division of Mines and Geology and from a number of geologists with the U.S. Geological Survey at Menlo Park, California.
The Texas Company and Standard Oil Company of California have been generous with information; Shell Oil Company, the writer's former employer, supplied much information; E. H. Stinemeyer of that Company contributed invaluable micropaleontologic data.
Many members of the faculty of the University of California, Berkeley, and Stanford University have contributed. In particular, the writer is indebted to Dr. Robert Compton of Stanford University for assistance in the study of igneous and metamorphc rocks. To Mr. Charles Chesterman of the State Division of Mines and Geology, he is indebted for descriptions of numerous thin sections of igneous rocks from the area. Several collectors supplied fossils found in the area; in particular the writer is grateful to Mr. C. L. Richard of San Francisco, who brought to attention the interesting callianassids from the Monterey Shale at Agate Beach near Bolinas. These fossils were identified by Mr. Dustin Chivers of the California Academy of Sciences.
Dr. W. S. Cooper assisted the writer in studying the sand dunes and old beach deposits of Point Reyes beach and Mr. Michael Lampen was of invaluable assistance in compiling the bibliography.
To numerous land owners of the Point Reyes Peninsula, the writer is grateful for permission to enter their properties. The National Park Service and the Vedanta Society have been most cooperative in this respect.
The writer takes this opportunity to thank everyone who has helped him.
Last Updated: 28-Nov-2007