The oldest rocks on the Point Reyes Peninsula are small patches of mica schist, quartzite, and crystalline limestone which are scattered through the outcrop of granitic rocks along Inverness Ridge from Mt. Wittenburg to Tomales Point (photo 3). These are clearly the remnants of a sedimentary series into which the granitic rocks were intruded. Most of the outcrop areas are too small to be outlined on the geologic map (plate 1), but the more important localities are indicated by symbol. No metamorphic rocks were found in the granitic outcrop at Point Reyes proper.
According to Weaver (1949a, p. 18), the metamorphic rocks closely resemble the Sur Series of the Santa Lucia (Monterey and San Luis Obispo Counties), Gabilan (Monterey and San Benito Counties), and Santa Cruz (Santa Cruz County) Ranges (Trask, 1926).
These pre-intrusive rocks of the Point Reyes Peninsula are not well exposed due to deep weathering and the dense vegetation cover on most of the outcrop area. It seems likely that the pre-intrusive sediments of the Point Reyes Peninsula were originally at least as extensive as the present outcrop of granitic rocks. Erosion subsequent to the granitic intrusion has been very deep so that only scattered patches of metamorphic rock remain. A study of the distribution of these patches of metamorphic rock on the eroded surface of the granitic pluton does not reveal any significant pattern.
For purposes of description, the metamorphic rocks are divided into two groups: schist and limestone. In the field the two occur together in places, but the limestone is distinctive enough to merit separate discussion. The amount of quartzite exposed is insignificant relative to the schist and limestone.
Numerous small outcrops of schist are scattered through the granitic rock of Inverness Ridge, and the more important localities are indicated on the map by a symbol (sch). Interbedded in the schists are minor amounts of fine-grained biotitic quartzites. Sillimanite, garnet, wollastonite, and diopside are found in several outcrops where calcium-rich schists are interlayered with the more common biotite schists.
The schists are closely related to the surrounding granitic rocks and usually contain injections and stringers of granitic material. Often there is no clear boundary between the schists and the surrounding plutonic rock; the schist gradually merges into the surrounding granite, leaving traces in the form of unusual minerals enclosed in the plutonic rock.
Some unusual types of schists from Shell Beach on Tomales Bay include calc-schists containing quartz, wollastonite, diopside, and scapolite, and quartz biotite schists containing quartz, biotite, diopside, and amphibole (F.J. Turner, personal communication). These rocks were probably derived from interbedded pelitic sediments (and) impure limestones. The presence of oligoclase in the biotite schists indicates an advanced grade of metamorphism in the almandine zone of the greenschist facies (Charles Chesterman, personal communication).
Compton (1966, p. 286) concluded that the metamorphic rocks represent platform-type sedimentary rocks. This and the abundance of dolomitic rocks indicate that they are not metamorphosed rocks of the Franciscan Formation.
Crystalline limestone occurs at several places in the granitic rocks of Inverness Ridge. It often shows distinct bedding and is interbedded with schist, demonstrating its sedimentary origin. All gradations, from fine-grained to coarsely crystalline types, are found; flakes and crystals of graphite are common. The principal limestone occurrences are described below from south to north.
The Mt. Wittenburg exposure, on the southeast flank of Mt. Wittenburg at an elevation of about 600 feet, is mentioned by Anderson (1899, p. 133), who regarded it as Franciscan foraminiferal limestone similar to that found in the fault zone and on the San Francisco peninsulaCalera limestone. However, the rock resembles the other metamorphic limestone occurrences and does not have the typical Calera foraminifera, so that, in view of its position within the plutonic rocks, it seems logical to regard it as part of the pre-intrusive metamorphic rocks. Thin sections of rock from this exposure consist of calc-silicate rock, complexly veined and variable in grain size and texture, containing wollastonite and diopside as well as calcite.
The Noren, Haggerty Gulch, and Lockhart exposures may be contiguous, but the thick cover of vegetation makes it impossible to connect them with any certainty. Therefore, they are shown separately on the geologic map. Weaver's map (1949a, plate 9) shows these exposures forming a single outcrop about a mile long and 1000 feet wide, but no doubt he meant to indicate only that this was a general area in which limestone outcrops were to be found. Outcrops in all three areas consist of relatively thin beds of crystalline limestone interbedded with schist and impregnated with stringers of aplite and granitic rock. The Noren exposure is described in some detail by Ver Planck (1955, p. 265-266) with particular emphasis on the scheelite which is present here with the limestone. The limestone is medium-grained and in thin section is seen to consist largely of calcite with grains of quartz, sphene, chalcedony, and phlogopite.
On the north side of Haggerty Gulch, 2000 feet west of Sir Francis Drake Highway at Inverness Park, exposures of limestone are interbedded with schists and permeated with pegmatite and granitic stringers. These exposures are probably continuous with those at the Lockhart Ranch (described below) but, due to the ground cover, they cannot be connected in the field and are, therefore, shown separately on the map. The limestone is coarsely crystalline in part, with veins of clear white calcite in a groundmass of darker material. It shows streaks of color resembling bedding.
The Lockhart exposure of limestone is at the summit of Drake's Summit Road leading west from Inverness Park to the Laguna Ranch, the Lockhart exposure of crystalline limestone is found. There are remains of old limekilns here, as well as at Inverness Park, and this is believed to be the exposure referred to by Whitney (1865) and Blake (1856 and 1857). It was also described by Ver Planck (1955, p. 259) and mentioned in Eckel (1933, p. 353), Anderson (1899, p. 131) and Weaver (1949a, p. 19; 1949b, p. 16, 89). The largest exposure at the Lockhart locality is about 40 feet long and 20 feet high, but the total area covered by limestone boulders is about 30 acres. The limestone is coarsely crystalline and banded, with some of the dark bands containing scales of biotite. Scales of graphite are common. In thin section the rock consists largely of equigranular calcite and includes grains of quartz, hornblende, orthoclase and limonite (after pyrite).
The Bender exposure is about 2500 feet west of the Sir Francis Drake Highway on the creek which flows into Tomales Bay at Willow Point. Scheelite, which was described by Ver Planck (1955, p. 265-266), occurs in the exposure. Ver Planck described the exposure as "A roof pendant composed of coarsely crystalline white limestone and biotite schist....in intrusive contact with quartz diorite. The limestone, which contains sparsely distributed flakes of graphite, lies on the east side of the roof pendant and schist is on the west side."
The metamorphosed limestone in this locality may have economic value and is discussed further under "Economic Geology." These metamorphosed limestone exposures are an integral part of the series of metamorphosed sediments that includes the schists and quartzites; they are intimately associated with schists and the whole is permeated with material from the intrusive granitic pluton.
Thickness of the metamorphic beds cannot be estimated because the outcrops consist of isolated patches surrounded by plutonic rock. The patches of metamorphic rocks represent what originally must have been a thick and widespread sedimentary formation into which the plutonic rock was intruded.
The metamorphic rocks must be older than the pluton which has intruded them. The age of the granitic rocks at Point Reyes Coast Guard Lifeboat Station has been determined as early Upper Cretaceous, or 83.9 million years (Curtis and others, 1958). If the pluton of Inverness Ridge is of the same age, as seems likely, the sediments represented here by the metamorphic rocks are older than early Upper Cretaceous; no fossils have yet been found in them.
Weaver (1949a, p. 18) has correlated these rocks with the pre-Cretaceous Sur Series of the Santa Lucia, Gabilan, and Santa Cruz Ranges (Trask, 1926). The only evidence for this correlation lies in their general lithologic similarity and the fact that both sets of metamorphic rocks are engulfed in plutons of roughly the same age (Curtis and others, 1958). The Sur Series is believed by W.S. Durham (personal communication, 1964) to be of Paleozoic age, based on the presence of crinoid plates in some of the limestones. Bowen and Gray (1959) record the finding of "indistinct forms suggestive of cup corals" and "rocks suggestive of crinoidal calcarenities" in the similar limestones of the Northern Gabilan Range of California.
The term Bodega diorite was applied by Osmond (1904) to the granitic rocks of Point Reyes, Tomales Point, and Bodega Head. On the basis of priority it would appear that this name should be applied to the granitic rocks exposed in the Point Reyes Peninsula. However, because these latter exposures are separated by the ocean from the Bodega exposure, and because they are not diorites, but range in composition from quartz diorite to adamellite, the name seems inappropriate. Curtis and others (1958) referred to a rock from Point Reyes as Point Reyes granodiorite, but this does not qualify as a formal name. No new name is proposed here for these rocks; they will be referred to in general as the granitic rocks of the Point Reyes Peninsula.
Granitic rocks form the backbone of the Point Reyes Peninsula, being exposed along the north half of Inverness Ridge from Tomales Point as far south as Mt. Wittenburg, with an inlier to the south in Bear Valley, a total distance of nearly 20 miles. They also occur in a separate area at Point Reyes. Granitic rocks of Point Reyes extend northward under the Pliocene cover. Four miles north of Point Reyes the granite is 1370 feet below sea level as indicated by core hole borings. No information is available on the subsurface extent westward of the main Inverness Ridge body of granitic rock. The nature of the connection, if any, between these two similar bodies of plutonic rock is unknown. However, it seems reasonable to assume they are both part of the same pluton.
The granitic rocks, when fresh, are usually gray, medium-grained, and form rounded and sometimes craggy rock outcrops. On the west flank of Mt. Vision some more resistant zones of the granite stand out conspicuously in linear fashion. Generally, unweathered material is found only in the seacliffs. Weathering on the land surface usually has been very deep, leaving a granitic, sandy residue that is hard to distinguish in the field from the basal Miocene sand, which is also granitic. Many road cuts show more than 20 feet of weathered granite, and one core hole drilled near the top of Inverness Ridge encountered weathered granitic rock at a depth of 60 feet.
As a result of this deep weathering and the fact that most of the area is under cultivation or is covered with brush and trees, it is impossible to get a true statistical idea of the constituents of the pluton. The geologist in the field can only examine specimens from the few localities where good fresh rock can be obtained in the field. The results of this work, therefore, may not be representative of the whole pluton.
The plutonic body underlying Tomales Point and Invernes Ridge ranges in composition from quartz diorite through granodiorite to adamellite. Granodiorite and adamellite are also exposed at Point Reyes. Good exposures are insufficient to justify an estimate of the relative proportions of quartz diorite, granodiorite, and adamellite.
Many variations of these rock types occur in the outcrops, including pegmatites and aplites. Thin pegmatite veins are common. A leucocratic variety of adamellite, consisting of about 65 percent feldspar and 30 percent quartz, is fairly common. When deeply weathered, it is sometimes quarried locally as gravel for road surfacing.
Hand specimens and thin sections of the granitic rocks were examined and described by Charles W. Chesterman (written communication, 1965).
Biotite quartz diorite, one of the more common types of plutonic rocks exposed on the Point Reyes Peninsula, has a hypidiomorphic-granular texture, is medium-grained, of a medium gray with a color index less than 30. It is composed of 40 percent acid andesine, 35 to 40 percent quartz, 15 percent biotite, and 5 percent accessory minerals, including sphene, zircon, magnetite and apatite. The andesine (An30-35) occurs as twinned anhedral and subhedral grains that show slight alteration to sericite. and range in size from a fraction of a millimeter to 7 millimeters. The quartz has rounded grains that exhibit wavy extinction and contain needle-like inclusions of black tourmaline (?). The biotite is deep brown, strongly pleochroic with pleochroic halos around zircon inclusions, and slightly altered to green chlorite. Dark green hornblende generally occurs in amounts subordinate to the biotite and is entirely absent from some quartz diorite. Orthoclase is rare and is generally altered to sericite. Minor accessory minerals include sphene, zircon, and magnetite.
The adamellites on the Point Reyes Peninsula differ from the quartz diorites by containing more quartz and potash feldspar but still have a similar color index of less than 30. The texture is hypidiomorphic-granular and the grain size ranges from a few millimeters to 5 millimeters. The plagioclase is intermediate to basic oligoclase in the composition ranges of An20 to An25. It is generally fresh and occurs in subhedral and anhedral grains that show multiple twinning. Zoning is not common, but oligoclase (An20-25) grains in one specimen contain rims of clear, twinned plagioclase in the composition range of An5-10. This albitic rim can be observed also on a few microcline grains, and this peculiar relationship has been interpreted as a rapakivi texture and is attributed to the assimilation of basic rock by the crystallizing adamellite magma. Microcline is fresh and occurs in subhedral and anhedral grains. Quartz occurs in rounded grains and shows weak strain features.
Biotite is a common characterizing mineral, and one specimen contained an appreciable amount of muscovite. The biotite is brownish-black and exhibits characteristic pleochroic colors. Locally, the biotite is altered to chlorite and commonly contains pleochroic halos around small zircon crystals. Hornblende, rare on the adamellites, exhibited dark green subhedral prisms in one specimen. Magnetite, apatite, and zircon constitute the minor accessory minerals.
Granodiorite has a hypidiomorphic-granular texture and is fairly even-grained. The plagioclase is zoned oligoclase with a core composition of An25-30, and a rim composition of An15-20. It occurs in anhedral and subhedral grains, slightly altered to sericite, and contains small inclusions of biotite. Microcline occurs in anhedral and irregular grains that show the well-developed plaid structure. Alteration to sericite is slight. Quartz exhibits a wide range in grain size and constitutes about 35 percent of the rock. Biotite occurs in strongly pleochroic, anhedral grains that contain inclusions of zircon.
Chesterman concluded that the assemblage of plutonic rocks exposed on the Point Reyes Peninsula comprises a pluton of limited compositional range from quartz diorite as the basic member through granodiorite to adamellite, the acid member. Rocks of similar composition (Darrow, 1963, p. 9 and 11) crop out at Montara Mountain, about 44 miles southeasterly from Point Reyes Peninsula. Quartz diorite (Hanna, 1952, p. 302) has been identified from the Farallon Islands, about 21 miles south of Point Reyes Station. Rocks of comparable composition, though not in place, were dredged from the continental shelf off the coast of San Francisco (Chesterman, 1952, p. 360).
From the moderately wide distribution of plutonic rocks of similar compositional range, it is reasonable to assume that this part of California is underlain by a batholith which has been deroofed in a limited area only. It can be assumed, also, that this batholith extends as far south as Santa Cruz and includes the plutonic rocks that are exposed in and around Ben Lomond Mountain (Fitch, 1931, p. 2).
Spotts (1962) reaches similar conclusions from his studies of the zircons and other accessory minerals of the plutonic rocks of Point Reyes, Bodega Head, Farallon Islands, Montara Mountain (San Mateo County), and Ben Lomond. The rocks vary considerably in texture and in relative abundance of constituent minerals. Textures are typically hypidiomorphic to allotriomorphic granular. According to Spotts, the general similarity in heavy-mineral assemblages is evidence of the development of the isolated plutonic masses of the Coast Range batholith from closely related magmas.
The highest exposure of granitic rock on Point Reyes Hill is more than 1300 feet above the lowest exposure on Tomales Bay. Although the block of plutonic rock has been tilted southwestward, it seems valid to assume that the true vertical thickness of granitic rock exposed here is on the order of 1000 feet. More than 500 feet of granitic rock are exposed at Point Reyes. The plutonic mass intruded the sedimentary rocks and metamorphosed them. The plutonic mass is overlain at Point Reyes by the Paleocene conglomerate and elsewhere by younger beds.
Curtis and others (1958), using the potassiumargon (KAr) method, determined the age of a specimen of granodiorite collected by Anderson (1899) from a quarry near the old Government landings at Point Reyes. (A map of 1854, now in the Bancroft Library at the University of California, shows the granite quarry to have been at the site of the present Coast Guard Lifeboat Station.) The age of this rock was determined lobe 83.9 million years, or early Upper Cretaceous. Approximately the same age was obtained for a specimen of similar rock from Gabilan Mesa in the southern Coast Ranges. (Note that the work of Kistler and others, 1963, suggests that biotite KAr ages in some circumstances can be too young).
Compton (personal communication, 1965) noted the close similarity between these rocks and those of Montara Mountain, the Santa Lucia Range (Monterey and San Luis Obispo Counties), and even La Panza Range (San Luis Obispo County) to the south. The age relationships of the different types of granitic rocks are undoubtedly complex; however, the exposures are insufficient to permit detailed analysis.
It was the original intention of the writer to exclude the Franciscan rocks from this report, because the formation is characteristic of the mainland, as distinguished from the Point Reyes Peninsula, the San Andreas fault zone separating the two. However, as the work progressed, it became desirable to map the San Andreas fault zone, as it was clear that the fault zone in this area is developed in the Franciscan Formation, either exposed or covered by younger rocks. The following remarks, therefore, pertain mainly to the Franciscan Formation of the fault zone.
The name Franciscan was first given to sandstones, cherts, and limestones exposed in the San Francisco and Marin Peninsulas (Lawson, 1895).
Franciscan rocks underlie the San Andreas fault zone and the terrain immediately east of the San Andreas fault zone, from the north end of Tomales Bay to the south end of Bolinas Lagoon. Within the fault zone itself, the Franciscan rocks are exposed in the topographically high area near Five Brooks. To the south, the Franciscan rocks become progressively covered by the Merced Formation and the waters of Bolinas Lagoon. To the north, they are overlain by the fresh-water deposits of the Olema Creek Formation and by terrace deposits, alluvium, and the waters of Tomales Bay. Farther northwest, the Franciscan rocks crop out at Hog Island and at Tom's Point, in the middle of the fault zone. At Tom's Point, the Franciscan rocks are overlain by beds of the Millerton Formation.
According to Daetwyler (1965, p. 115), Franciscan rocks also are present 2 miles southeast of Hog Island on the floor of Tomales Bay. Apparently Franciscan rocks occupy the whole width of the fault zone all the way from Tomales Point to Bolinas Lagoon, being covered by younger beds at the north and south ends of the zone and being exposed at the surface in the central portion.
The Franciscan rocks of the Coast Ranges, described in detail by Bailey and others (1964), consist of a varied assemblage of graywackes, shales, cherts, limestones, conglomerates, and pillow basalts, with frequent serpentine intrusions. Graywackes predominate in the outcrop areas.
The Franciscan rocks immediately east of the San Andreas fault zone in western Marin County have been described by Gluskoter (1969). For a distance of 1-1/2 to 2 miles east of the fault zone, they consist exclusively of graywackes, with subordinate shale, dipping homoclinally eastward.
In contrast, the Franciscan in the fault zone itself includes blocks of limestone, conglomerate, chert, serpentine, and green stone, as well as graywacke and shale. Exposures are very poor, since the fault zone forms a topographically low and moist area which is largely covered with vegetation. Individual rocks often stand out as rounded boulders in grassy meadows. These boulders are usually very hard, and commonly those lying side by side will be composed of entirely different or contrasting rocks. It seems that these scattered hard boulders are tectonic inclusions which have been moved along the San Andreas fault and have become separated from the original outcrops. The most conspicuous of these tectonic inclusions is the large block of Calera limestone described separately below. Other types of boulders observed in the fields along the fault zone include bronzite, conglomerate, sandstone, and both black and white limestones.
In the southern part of the fault zone where the streams have cut through the overlying Merced Formation, the Franciscan rocks are mainly greenish-blue or bluish clayey material which is probably decomposed ultrabasic igneous rock.
The largest and best known of the tectonic inclusions in the fault zone is the block of Calera limestone near Five Brooks. The history of the so-called Russian lime kilns built alongside this block has been fully described by Treganza (1951), and the geology of the deposit was described by Eckel (1933). The total quantity of limestone present in this tectonic inclusion is too small to be commercially exploited (see "Economic Geology"). It does not appear to extend far laterally and is not found in the nearby stream cut. Nearby outcrops in the fault zone appear to be mostly decomposed ultrabasic igneous rocks, represented by bluish clayey material. The limestone is clearly an isolated block that has travelled along the San Andreas fault zone. The nearest large outcrop of Calera limestone reported in the literature is in San Mateo County, more than 30 miles to the south. Possibly the Calera limestone is present under water off the Golden Gate.
Following clues provided by local residents, the writer found another group of lime kilns on the Texeira Ranch about 2 miles south of the "Russian" kilns. The limestone at the Texeira Ranch is very similar to that at Five Brooks. Road grading operations near this second group of kilns uncovered fragmentary limestone, suggesting that there is another block of Calera limestone in this vicinity, which is now covered by vegetation and soil but which has been exposed sometime during the last 100 years.
In the San Andreas fault zone, the exposures of Franciscan rocks are too poor and too discontinuous to permit determination of structure or thickness. Topography suggests that the Franciscan rocks of the fault zone are cut into narrow elongated "slivers", some as large as several hundred feet long and tens of feet wide, bounded on either side by lateral faults subparallel to the fault zone.
To the east of the fault zone, the structure is relatively simple, consisting of a homocline dipping northeasterly at 35 degrees to 60 degrees. The base however is not exposed, so that no estimate of thickness is possible. Weaver (1949a) estimated the thickness of this formation east of Tomales Bay to be greater than 10,000 feet.
The origin of the rocks of the Franciscan rocks has been a subject of controversy for many years and is still unsettled. Presently it is believed that the greywackes and shales were deposited by turbidity currents in deep ocean water (Bailey, and others, 1964). The mineral grains show little chemical weathering, indicating rapid erosion and deposition. The cherts are usually associated with the volcanic rocks, suggesting a generic relationship, and other considerations suggest their formation in deep water. The volcanic rocks are often pillow lavas, typical of submarine eruptions. The limestones appear to be largely chemical precipitate, genetically associated with volcanism.
The serpentines are intrusive into the other Franciscan rocks and often appear in the form of sills, but they seem to have been emplaced as serpentine rather than fluid magma and often appear to be intruded along faults. It seems reasonable to conclude that their origin was from the peridotite of the mantle below the ocean floor.
The over-all picture, therefore, is of a succession of rocks formed in deep oceanic water and subsequently elevated orogenically to their present position, perhaps as a result of plate tectonic action along the east edge of the Pacific Ocean. The source of the sediments was the continent lying to the east of the area of accumulation; the igneous rocks have strong affinities with the ocean floor.
In the Point Reyes area, the Franciscan rocks are overlain by the Pliocene Merced Formation and the Pleistocene Millerton Formation. The base of the formation is nowhere seen. Franciscan rocks are not found west of the San Andreas fault zoneand likewise the Pliocene and Pleistocene sediments and granitic and metamorphic rocks are not found east of the San Andreas fault zone in the area adjacent to that covered by this report. Rocks ascribed to the Merced Formation by Weaver (1949a) and other authors rest on Franciscan rocks between the mouth of Tomales Bay and Petaluma, but they are quite distinct in lithologic facies from the Merced Formation found at Bolinas and apparently were laid down in a different sedimentary basin.
The Franciscan rocks of the northern Coast Ranges include rocks which may range in age from Upper Jurassic to Upper Cretaceous (Bailey and others, 1964). No fossil evidence has been obtained to date the Franciscan rocks in the area of this report, except for the Calera limestone which occurs near Five Brooks. Foraminifera in this limestone were described by Thalmann (1943) as of Turonian age (Upper Cretaceous) and identical with the fauna of the type Calera limestone. Long before that, the Calera limestone was described as foraminiferal by Anderson (1899, p. 134) and Lawson (1895).
Point Reyes Conglomerate
Point Reyes is designated in this report as the type area of the Point Reyes Conglomerate, which forms the craggy, rocky outcrops which give the area a great deal of its scenic character. The rocks of this formation crop out only in the extreme western part of the peninsula, overlying the granitic rocks of Point Reyes proper. Anderson (1899) considered them to be the basal conglomerate of the Miocene. Weaver (1949a,b) mapped the outcrop as part of the Laird Sandstone, a basal Miocene sandstone of the Tomales Point area. Taliaferro (1951) and Bowen (1951) pointed out that these beds at Point Reyes could well be Eocene in age. Since then, sufficient microfossil evidence has been obtained (see below) to justify the conclusion that the beds are Paleocene in age.
The Paleocene outcrops occupy three separate areas on the higher portion of Point Reyes, the three outcrops being separated by outcrops of granitic rocks from which the Paleocene rocks have presumably been eroded. The position of these outcrops is determined by cross-faulting. The beds generally dip steeply (35 to 45 degrees) to the northwest. By reason of their relative hardness, they tend to occupy the higher points of the Point Reyes ridge, where they often form craggy or stack-like outcrops emphasized by prominent vertical jointing. The beds extend discontinuously from the extreme west end of the Point to the extreme east end.
For the purpose of definition of this new rock-stratigraphic unit, the westernmost fault block (at the Point Reyes light house) is designated the type section.
Paleocene rocks have not been found to crop out elsewhere on Point Reyes Peninsula. However, an exploratory oil well, Standard (Lockhart) Tevis #1, drilled at North Double Point 10 miles to the southeast, encountered beds which may be Paleocene. In contrast to the conglomeratic sandy facies observed in the outcrop at Point Reyes, the beds encountered in this well (from a depth of about 5010 feet to about 6587 feet) consisted of fine-grained material described as hard, slickensided shale and siltstone, which contained microfauna identified as Paleocene in age.
The Paleocene beds exposed at Point Reyes consist for the most part of a coarse, sandy conglomerate. The largest clasts in the conglomerate (up to several feet in longest dimension) consist of subangular or only slightly rounded boulders of granitic rocks similar to the suite of granitic rocks exposed in the vicinity. Granitic fragments of all sizes occur, and the matrix is largely granitic sand, poorly sorted and generally not well cemented. Along with these angular fragments of the local granitic rocks, the conglomerate contains well-rounded pebbles (up to several inches in size) composed of very hard rocks, such as volcanic porphyries and cherts. One of these rocks, a porphyry with cream to pinkish feldspar crystals in a dark purple groundmass, is particularly distinctive and can be recognized on the present beaches in Drakes Bay, in the basal Pliocene conglomerate, and in the raised beach material at the south end of Point Reyes Beach. The well-rounded and even polished appearance of these pebbles suggests that they might have been derived from some pre-existing sedimentary formation. However, since no such formation occurs in outcrop in the vicinity, the provenance of these well-rounded pebbles is not known.
The conglomerate beds are separated by strata of laminated silty micaceous, fine sand, with an occasional thin shale layer (photo 4). Carbonized plant remains are common in the silty micaceous sand layers, and Foraminifera are found in the shaley layers. The conglomerate and sand layers are distinctly cross-bedded.
Anderson (1899) notes the occurrence of a fine-grained sandy shale layer 50 to 60 feet thick near the bottom of the series in the westernmost fault block. This shows up clearly in oblique aerial photographs of the cliffs but is accessible in the field only where it extends to the top of the cliff.
The thickness of the Paleocene beds exposed at Point Reyes, based on outcrop measurements, is about 700 feet. However, the top portion has been eroded away, and the original thickness must have been greater. The shaley beds of probable Paleocene age found in the Standard (Lockhart) Tevis #1 exploratory well at Double Point were over 1500 feet in apparent thickness, and the bottom was not reached. Steep dips in this formation were reported from the cores recovered from this well, so that the true thickness penetrated is less than 1500 feet.
The source of the material comprising the Point Reyes Conglomerate was to a large extent the local granitic complex which is exposed today. The well-rounded pebbles of exotic rocks, however, come from some unknown source. Their extremely well-rounded condition and unusual hardness suggest that they may have been derived from some older conglomerate.
The conglomerate beds are graded by grain size in units which may be tens of feet thick. The sequence of beds suggests an initial onrush of extremely coarse material down a submarine slope, accompanied by coarse sand, and followed by minor flows of micaceous sand with occasional pebbles. The upper portion of these relatively fine-grained beds are often truncated by additional conglomeratic material, suggesting renewed uplift of the source beds or instability due to accumulation of coarse material at the top of the slope, with the downward movement triggered perhaps by earthquakes. Probably both causes were at work.
The almost straight east-west alignment of the south-facing cliff of Point Reyes suggests that it is an eroded fault-line scarp downthrown to the south. The presence of such a fault is also suggested by regional gravity anomalies (Chapman and Bishop, 1968).
The exposures near the Point Reyes Lighthouse illustrate graded bedding and many other sedimentary structures. Most of the exposures are coarse-grained at this locality, with very little clayey material present. These rocks, therefore, were probably deposited under water near land, on a steep slope where landslides and turbidity currents were the chief agents of deposition. Similar rocks occur in the Carmelo Formation (Paleocene) at Point Lobos in Monterey County (Lawson, 1893; Bowen, 1965). Similar pebbles have also been dredged from Cordell Bank, 20 miles west of Point Reyes in the Pacific Ocean (Hanna, 1952).
The Point Reyes Conglomerate lies unconformably on the granitic basement at Point Reyes. It is overlain unconformably by the basal glauconitic sand of the Drakes Bay Formation of early Pliocene age. In the eastern part of the Point Reyes Peninsula, beds of Miocene age lie directly on the granitic basement, and the Paleocene Point Reyes Conglomerate is not present.
The Point Reyes Conglomerate contains a foraminiferal fauna of Paleocene age. Foraminifera from clayey beds near the base of the Point Reyes Conglomerate include the following:
Carbonized plant remains and seeds also have been found in these beds.
Foraminifera from the Paleocene shales in the well Standard "Tevis" #1 include:
The Miocene rocks of the Point Reyes Peninsula are difficult to study because of the poor exposures and the scarcity of fossils.
Anderson (1899) divided the Miocene rocks west of Bear Valley into three units as follows:
In his basal unit 1, Anderson included the Paleocene Point Reyes Conglomerate as well as the basal conglomerate of the Miocene. His middle unit 2 comprised the basal (Laird) sandstone of the Miocene, while upper unit 3 included all the shales and mudstones which he grouped together as white Miocene shale of the Monterey series.
Weaver (1949) divided the Point Reyes Miocene beds north of Double Point into 3 units:
Weaver's basal unit 1 as mapped included the Paleocene Point Reyes Conglomerate as well as the basal sands and conglomerate of the Miocene and of the Drakes Bay Formation. He termed units 2 and 3 Monterey Shale and added that they "grade into one another and cannot be separated."
Taliaferro (1951, p. 140) recognized the unconformity of the beds exposed at Drakes Bay upon the underlying cherts. He divided the succession as follows (unit 1 is the oldest):
Taliaferro did not apply the name Monterey Shale to any part of the section.
In this report a modification of Taliaferro's succession has been adopted as follows (unit 1 is the oldest):
This sandstone lies on the granitic basement and below the Monterey Shale. The name Laird was first used by Weaver (1949) for the sandstone that is well exposed west of Laird's Landing on Tomales Bay. Weaver included in the Laird Sandstone (1) the conglomerates of Point Reyes herein assigned to the Paleocene and (2) the basal glauconitic sandstones of the Drakes Bay Formation. Taliaferro (1951) used the terms Laird breccia and Laird conglomerate for the Point Reyes Conglomerate but pointed out that those beds might be Eocene in age. In this report, the name Laird will be restricted to the sandstones at the base of the Monterey Shale typically found west of Laird's Landing.
The Laird Sandstone (restricted) crops out in bluffs beside the road between Abbotts Lagoon and Pierce Ranch. It is also well exposed in the sea-cliff on the west coast of the peninsula west of Laird's Landing, at the north end of Point Reyes Beach. Here it lies on the granitic rocks with a few feet of granitic conglomerate at the base and is overlain by, and interbedded with, white siliceous shale (photo 5).
It is also exposed less prominently in the central area of the peninsula, lying between the siliceous shales and the granitic basement. A road recently constructed by the National Park Service overlooking Haggerty Gulch traverses an extensive outcrop of Laird Sandstone between Mt. Wittenburg and the Drakes Summit Road and provides good exposures in the road cuts.
A sand which probably is correlative with the Laird Sandstone was found at a depth of about 4700 feet in the Standard Oil Company (Lockhart) Tevis #1 exploratory well drilled at North Double Point several miles south of its most southerly surface outcrop in Bear Valley.
In the outcrops west of Laird's Landing, the Laird Sandstone is typically a massive medium- to coarse-grained, fairly friable sandstone with little cementation. Between the subangular quartz grains, there is usually a substantial quantity of silty material. Grains of white opaque feldspar and flakes of biotite are common. At its base, the sandstone is usually pebbly or conglomeratic, with clasts of granitic rocks.
At the top the sandstone becomes interstratified with white, silty thin-bedded, laminated siliceous shales typical of Monterey Shale; these shales soon replace the underlying sandstone but are often sandy or silty for some distance above the top of the sandstone. The color of the sandstone in field exposures is usually light brown; but at White Gulch, and in a few other exposures, the weathered sandstone appears almost white. The only fossils reported from this sandstone are casts of a few small marine pelecypods resembling Cardium.
The thickness of the sandstone in the type area is over 200 feet; but farther south, where the Laird Sandstone is overlain by Monterey Shale and underlain by granitic rocks of the basement complex, the sandstone seems to be much thinner in many places, although exposures are too poor for accurate measurements. It thickens again on the north flank of Mt. Wittenburg. Its apparent thickness in the exploratory well (Standard (Lockhart) Tevis #1) at North Double Point is about 100 feet.
This quartzose feldspathic sand was clearly derived in the type area from weathering of the granitic basement. In the northern part of the peninsula, the Laird Sandstone can be seen lying on the irregular surface of the granitic rocks. It seems to have been laid down in a quiet transgressive sea, advancing gently across an old irregular eroded granitic surface.
The Laird Sandstone, wherever seen in outcrop, lies unconformably on the granitic basement; but it lies above Paleocene beds in the exploratory well at North Double Point (Standard (Lockhart) Tevis #1). At Point Reyes it is not exposed, and the overlying Drakes Bay Formation lies directly on the granitic and Paleocene rocks. The upper part of the Laird Sandstone grades by interbedding into the Monterey Shale, subordinate beds of shale appearing with increasing frequency higher in the section until the section is predominantly siliceous shale with subordinate sand beds.
The youngest beds on which the Laird Sandstone rests are Paleocene in age. The oldest beds found above it are Miocene in age, assigned to the Relizian stage, but in the type area it is overlain by beds of Mohnian age. No fossils of any diagnostic value have been found in the Laird Sandstone. The fact that the Laird Sandstone passes upward conformably into middle Monterey Shale by interstratification leads to the conclusion that it is Miocene in age. Since it was deposited by a transgressive sea, it may well vary in age from place to place.
The Monterey Shale was first described by Blake (1856) from Monterey, California. The formation in the type area includes the Luisian, Mohnian, and Delmontian stages (Kleinpell, 1938; Bowen, 1965); and all of these stages are found on the Point Reyes Peninsula. Both Anderson (1899) and Weaver (1949) refer to these rocks as Monterey Shale, although Taliaferro (1951) does not use this name. It seems logical to use this well-known name for these rocks at Point Reyes, since they are lithologically similar and of the same age.
These shales cover most of the peninsula area included in the Bolinas and Double Point quadrangles, and the outcrop extends northwest between the outcrops of the granitic rocks and the Drakes Bay Formation as far north as the Kehoe Ranch in the Tomales quadrangle. The outcrop area is about 3 miles wide at the latitude of Bear Valley, narrowing rapidly northward until in the vicinity of the Home Ranch it is only 1/2 mile wide. North of the Home Ranch, it increases again to a width of 2 miles before feathering out onto the granitic high of Tomales Point.
The outcrop surface is normally smooth and covered with vegetation; but, where the shale is predominantly cherty, a number of striking crags and pinnacles have been formed by differential erosion (photo 6).
As pointed out by Taliaferro (1951), the Monterey Shale includes cherts, porcelanites, organic shales, and thin hard sandstones, with every variation between these types. Some shales are silty and fissile, others hard and massive, and some display a conchoidal fracture. Altogether they comprise a bewildering variety of rather similar appearing rocks, but some general groupings of lithologic types can be made.
Going from north to south in the Tomales quadrangle, nearly all the shales are thin-bedded, laminated, micaceous, and white weathering. They contain fish scales and fish remains along with molds of Foraminifera and diatoms. At the south end of this quadrangle, the shales tend to be hard and flinty and commonly blocky and flagstone-like. In the Drakes Bay quadrangle between Abbotts Lagoon and the Home Ranch, they become harder and more like flagstones. Microfaunal evidence suggests that all of the above rocks are of Mohnian age.
Immediately southeast of the Home Ranch, shales of Luisian age overlie granitic rocks in a small area between Home Ranch Creek and Glenbrook Creek. These shales are laminated, porcelaneous, and silty and do not differ markedly from the Mohnian shales, although their microfauna is unequivocally of the Luisian age. In the field they appear to be conformable with the overlying Drakes Bay Formation, though this conformity can only be apparent in view of the difference in age.
In the Inverness quadrangle southeast of Glenbrook Creek, a long outcrop of hard siliceous cherts about a mile wide extends as far southeast as Bear Valley and the Double Point quadrangle. These very hard, brittle cherts are dark grey-brown and often laminated with thin shaley or sandy interlayers. They commonly exhibit the banded "spheroids" described by Taliaferro (1934). The silica necessary to form the cherts probably was derived by solution of diatoms and other common siliceous organisms in the shales (Bramlette, 1946). The cherts are usually very contorted in the outcrop, suggesting that the whole mass of beds slid down the continental shelf while in a relatively pliable condition (Curray, 1965, figure 3). As a result of these contortions, it is impossible to map the regional attitude of these beds with any reliability (photos 7, 8).
Underlying the cherts on the slopes of Mt. Wittenburg and extending southeastward as far as Bear Valley is a bed of low-density fissile shales with plant remains, molds of Foraminifera, fish remains, and sandy interlayers near the base. This shale bed appears in the vicinity of Laguna Ranch Creek and extends southeastward, gradually widening until at Bear Valley it is about a mile wide. Anderson (1899, p. 137) took special note of these shales, as he was puzzled by the interlayers of sand in the very fine-grained shale in the outcrop on Mt. Wittenburg.
These fissile shales are MohnianDelmontian age, based on the foraminiferal fauna, and accordingly, the overlying cherts must be of this age or younger. The presence of MohnianDelmontian beds overlying the granitic rocks of Mt. Wittenburg indicates that in this vicinity the MohnianDelmontian beds overlap the underlying Luisian beds onto the granitic rocks. In this connection, Kleinpell (1938) states that the Mohnian lies unconformably on wide expanses of preLuisian rocks in a number of areas, including Point Reyes.
In the wedge-shaped area between Bear Valley and the San Andreas fault, a distinctive type of shale occurs in which no diagnostic fossils have been found. This shale is dark blue-grey, hard, micaceous, and fine-grained. In most of the outcrop area, the shale dips northward or northeast toward the San Andreas fault, suggesting that these beds are at least as young as the beds at the top of Mt. Wittenburg, which are of MohnianDelmontian age.
The large landslide area of the Lake Ranch and Wildcat Ranch is located to the southeast of the chert outcrop. Most of the shales are affected by the landslides; they overlie the cherts except in a few instances, where both cherts and shales are involved in the landslides, and are, therefore, at least as young as MohnianDelmontian. In lithology and appearance, the shales of the landslide area resemble the shales at Duxbury Point to the southeast.
In the Bolinas quadrangle, southeast of the Lake Ranch landslides, nearly all of the outcrop area is occupied by shales similar to those well exposed at Duxbury Point, from which Foraminifera of Delmontian age have been recovered. These thin-bedded siliceous shales and claystones are grey, lavender, or chocolate, commonly have conchoidal fracture, and in places have laminae of fine silt. They weather to grey-white. A durable member of these shales forms Duxbury Reef, which extends a mile out to sea along the south-east strike and which has been a graveyard for sailing ships (photo 9). These shales are more argillaceous than the previously described varieties and jointing is prominent. The shales commonly contain yellow-buff calcareous concretions, often flattened or elongated along the bedding planes. The shales make most of the south end of the Point Reyes Peninsula and can be traced from Duxbury Point northward along the coastal cliff section as far as the Palomarin Ranch.
At the Palomarin Ranch, a number of thick sandstone beds occur in an overturned syncline, which suggests that most of the Duxbury Point shales are older than the sandstones. These beds include a richly glauconitic member which may well be equivalent to the basal glauconitic sand of the Drakes Bay Formation to the north. Although it is impossible to trace the bed through the extensive landslides north of the Palomarin Ranch, displaced exposures of the glauconitic unit in the landslide area indicate that it originally extended across this area.
The total thickness of the Monterey Shale is difficult to determine since the complete section is not exposed. However, some idea of the minimum thickness can be obtained. On the coast between Duxbury Point and Bolinas Point, approximately 3500 feet of Monterey Shale is exposed in a southwesterly dipping homocline. Near the base of this section is the top of an abandoned exploratory well, Lockhart RCA 31, reported to be bottomed in Monterey Shale of Relizian age at a depth of 8409 feet. The true thickness of Monterey Shale represented by the sum of these two measurements is probably on the order of 8000 to 9000 feet.
The Monterey Shale thins northward due to erosion and overlap, until in the south part of the Tomales quadrangle only a thin section of upper Mohnian age overlies the Laird Sandstone, and further north it is completely absent. At the southwest extremity of the area on the Point Reyes promontory, it does not appear from under the overlapping Drakes Bay Formation, which lies on the Paleocene and the granitic basement.
The basin in which the Monterey Shale was laid down probably deepened southeastward, judging by the evidence of two deep exploratory wells, one at North Double Point and the other at Bolinas Mesa. It can be expected, therefore, that the thickness of the Monterey Shale increases in a southerly direction. Chapman and Bishop (1968) point out that the negative gravity anomaly between the Farallon Islands and the Marin Peninsula could be caused by a prism of sedimentary rocks about 2.5 miles (or 13,000 feet) thick. The anomaly extends southeastward to include the La Honda basin (San Mateo County) of Cummings and others (1962), suggesting that the Point Reyes sedimentary area is a northerly extension of the La Honda basin.
When Anderson (1899) wrote the first geological report on the Point Reyes Peninsula, the origin of the siliceous Monterey shales, at Monterey and elsewhere, was a matter of active speculation, as evidenced by the space devoted to a discussion of the origin of the shales of Point Reyes. Nearly fifty years later, Bramlette (1946) devoted an entire U.S. Geological Survey Professional Paper to the origin of the Monterey Shale. Bramlette's conclusions are applicable to the comparable shales of the Point Reyes area. The more important conclusions are summarized as follows:
(1) Most of the porcelaneous and cherty rocks of the Monterey Formation were formed through an alteration that consisted largely of a rearrangement of the silica of originally diatomaceous deposits.
(2) The thin rhythmic bedding or lamination was evidently formed at depths below that affected by appreciable wave or current action.
(3) Few lithologic zones are persistent enough to be of much value in the general correlation.
(4) Most of the foraminiferal faunules indicate depths between the upper limits of the neritic zone and the edge of the continental shelf.
(5) The evidence available seems, on the whole, to indicate conditions of temperature and rainfall not markedly different from those now existing along the California coast.
(6) The deeper basins and shallow water divides off the present coast in the region of the Channel Islands constitute such conditions of bottom topography as ore postulated for the Monterey seas.
The Monterey Shale lies conformably on the Laird Sandstone with which it merges by interbedding. In most of the area, the Laird Sandstone in turn lies on the granitic basement rocks; but in exploratory well Standard (Lockhart) Tevis #1 at North Double Point, it lies on shale of Paleocene age.
Overlying the Monterey Shale is the Drakes Bay Formation of Pliocene age. There is a pronounced angular unconformity between the Drakes Bay Formation and the Monterey cherts near the URanch in the northern part of the area; but at the Home Ranch, nearer the axis of the Point Reyes syncline, the two formations appear to be conformable with the base of the Drakes Bay Formation marked only by a glauconitic sand. This conformity is, however, only apparent, since at the Home Ranch the shale is of Luisian age and the Mohnian and Delmontian are not present and the Drakes Bay Formation lies directly on beds containing fauna of the Luisian age.
Deposition of Monterey-type shales seems to have continued for a short period after the glauconitic sand at the base of the Drakes Bay Formation was laid down. These shales immediately above the glauconitic sand are lithologically very similar to the MohnianDelmontian shales of Duxbury Point, but must be younger.
In the overturned syncline at the Palomarin Ranch, the glauconitic sand is exposed in the outcrop, and the beds above and below it appear to be conformable. However, observations in this area are rendered uncertain by the landslides. Thus the unconformity at the base of the glauconitic sand seems to disappear to the south as the Miocene-Pliocene basin thickens.
The oldest Miocene foraminiferal fauna in the Point Reyes Peninsula outcrop of the Monterey Shale is found in beds exposed 2000 feet southeast of the Home Ranch headquarters. This is approximately Weaver's locality No. 2180 (1949, p. 76-77). Foraminifera collected include:
In addition diatoms of the genera Coscinodiscus and Isthmia were found.
Mollusca collected include:
Anadara cf. A. montereyana Osmont
Weaver collected the following mollusca from this locality:
Anadara devincta Conrad
At another locality (A.J.G. 974) 6000 feet southeast from Weaver's locality and on strike with it, a similar foraminiferal microfauna is found with the additional species:
Also the diatoms Actinoptychus and Coscinodiscus and the radiolarian Cenodiscus were found. These fossils indicate a Luisian age for these beds. Similar beds containing Pecten peckhami and the Foraminifera Nonion, Buliminella, and Robulus are found in the center of the anticlinal structure which extends through the head of Schooner Bay.
Elsewhere the Foraminifera of the Monterey Shale generally are consistent with a Mohnian age. For example, the following middle to upper Miocene Foraminifera were found near the J. McClure Ranch headquarters (north of the Home Ranch):
At the north end of Abbotts Lagoon (north of the Home Ranch), the following microfauna of probable upper Mohnian age were found:
Near Mud Lake and also at McCormick Creek inland from Duxbury Point, the following microfauna were found, which suggest a MohnianDelmontian age:
At the top of Mt. Wittenburg, the shales yielded the following, which again suggest a Delmontian or Mohnian age:
Cyclammina cf. C. constrictimargo
Foraminifera of lower Mohnian age have been reported in the Monterey Shale near the granite contact along the road between Inverness and Point Reyes lighthouse. A skeleton of the fish Eclipes, common in the late Miocene, and impressions of the small decapod crab Pinnixa galliheri Rathburn (photo 10) were found in this vicinity.
In the vicinity of Duxbury Point, the following have been found:
Callianassa goniophthalma (mud shrimp)
Foraminifera of MohnianDelmontian age have been found here also, including:
The foregoing evidence shows that a central inlier of middle Miocene (Luisian) shale at the Home Ranch is bordered to the northwest and southeast by Monterey Shale variously determined as of Mohnian or MohnianDelmontian age. In deep exploratory wells at North Double Point and on Bolinas Mesa, middle Miocene shales are overlain by substantial thicknesses of upper Miocene shales. Thus, it appears that the Mohnian shales overlap the Luisian shales over most of the peninsula. The outcrop of Luisian stage rocks in the inlier at the Home Ranch is probably due to local uplift; however, it must be assumed that they are present under the rocks of Mohnian age in most of the area southeast of the Home Ranch.
Other Miocene Rock Types
Dikes composed of medium- to fine-grained sandstone are commonly found in the cherty shales of the Monterey Shale, northwest of the mouth of Bear Valley Creek, at north Wildcat Beach, in the chaotic landslide shales of south Wildcat Beach, at Double Point, and in the Duxbury-type shales at the mouth of Arroya Honda. In a number of cases, the locus of the clastic dike emplacement is the axis of a small anticlinal fold. Dikes at Wildcat Beach, Double Point, and Arroya Honda are often strongly bituminous. Some dikes pass into sills, and it is difficult to distinguish them from normal sedimentary sandstone beds.
It seems likely that these clastic dikes were injected into the shale as a slurry of sand suspended in water or oil. Large earthquakes, accompanied by landsliding, are the probable immediate cause of the disturbances which formed the dikes.
A particularly impressive bituminous sandstone dike is to be seen in the cliff on Wildcat Beach, about 5000 feet south of Wildcat Military Reservation, Double Point quadrangle (photo 11).
Shales containing blebs, lenses, and laminae of phosphatic material are common in the Miocene of the Coast Ranges of California (Dickert, 1966), particularly in the beds of Luisian age. A search was made for phosphatic material in the siliceous shales of the Point Reyes Peninsula without success. Blebs of white material, similar to the phosphate blebs illustrated by Dickert, are fairly common in the beds southeast of the Home Ranch which contain a Luisian fauna; but the phosphatic material, if such it originally was, appears to have been replaced by silica, and no unequivocal phosphate was found.
Gower and Madsen (1964) reported the occurrence of phosphate nodules from the upper Miocene of the Point Reyes Peninsula; this occurrence is believed to be in the basal glauconitic sand of the Drakes Bay Formation, which contains many bone fragments as well as glauconite.
Extrusive Igneous Rocks
Basaltic and pyroclastic rocks are common in the Miocene of California. In the Point Reyes Peninsula, only two occurrences are reported:
(1) In the exploratory well (Standard (Lockhart) Tevis #1) at North Double Point volcanic material was reported at a depth between 4900 to 5000 feet.
(2) Basalt and obsidian fragments were also reported from the sea cliff between Duxbury Point and Bolinas, close to the faulted Miocene Merced Formation contact near Bolinas. The stratigraphic position of this sample is uncertain since the beds are adjacent to the San Andreas fault zone and apparently are standing vertically due to structural deformation.
According to KIeinpell (1938), volcanic material is most commonly encountered in the Miocene near the RelizianLuisian contact. This is approximately the stratigraphic position of the material encountered in the Standard (Lockhart) Tevis #1 well. He also reports small lava flows near the coast southwest of Mt. Wittenburg, but this has not been confirmed.
The beds, which are here formally designated the Drakes Bay Formation, form the white cliffs of Drakes Bay which Sir Francis Drake saw in 1579 in the course of his circumnavigation of the globe. They reminded him of the white Cretaceous chalk cliffs of southeastern England, and he called the newly discovered land New Albion.
Drakes Bay Formation
Anderson (1899) and Weaver (1949) did not distinguish the Drakes Bay Formation but included these beds under the general term Monterey Shale. Taliaferro (1951) identified the formation clearly in a cross-section (p. 128, plate 1, cross-section IV). In the report (p. 140), Taliaferro stated that, although the age of the beds was unknown, they were probably upper Miocene, The present writer first termed the beds upper Miocene in 1962, but later evidence pointed to their being early Pliocene in age (Gallo way, 1966).
The type area and locality of the Drakes Bay Formation is the syncline lying between the granitic ridges of Inverness Ridge and Point Reyes; the Drakes Bay sea-cliff extending from Drakes Estero to the granitic outcrop at the eastern end of the Point Reyes ridge is designated as the type section (figure 3).
The Drakes Bay Formation occupies a synclinal area bounded on the west by the granitic promontory of Point Reyes and the Pacific Ocean, on the south by Drakes Bay, and on the east by a line from Abbotts Lagoon southeast to the URanch. Discontinuous patches of the formation extend southeasterly as far as Double Point. Remnants of weathered deposits on hilltops suggest that the Drakes Bay Formation once extended over most of the Point Reyes Peninsula, and it is still the most extensive of the formations exposed on the Peninsula.
The main outcrop area forms the rolling windswept pastures that are seen from the Sir Francis Drake Highway between the head of Drakes Estero and the Point Reyes Lighthouse. Few trees grow in the outcrop area, in contrast to the thickly forested granitic outcrop of Inverness Ridge. The absence of tree cover may be partly due to the lower rainfall on the lowlands and partly to clearing by man. Exposures, except for the seacliff section, are few and small and are mostly in creek beds and road cuts. The softness of the siltstones and mudstones leads to a general rounding and softening of the contours of the hills, a feature which would also have reminded Sir Francis Drake of the Chalk Downs of southeastern England. The sandstones at the base of the formation, however, weather into resistant ledges and form prominent topographic features between the URanch and the Home Ranch. The weathered sandstone is commonly pitted like weathered limestone, although it is generally not calcareous.
At the base of the Drakes Bay Formation is a striking glauconite-bearing greensand. In many places, the concentration of glauconite is so great as to give the rock a brilliant green color. In the outcrop at east Point Reyes where the greensand lies unconformably on the granitic rocks, it has stained them a bright grass-green. The sandstone has this same color at the opposite side of the syncline, where it lies unconformably on the Miocene cherts on the beach near the URanch; and a number of the inland exposures also show this unusual concentration of green glauconite.
The greensand is thickest (about 100 feet) near the Laguna Ranch; at this locality, the green color is diluted to greenish-gray by the presence of much quartzose sand. At Point Reyes it is only about 25 feet thick, and at its inland outcrop it is often thinner or represented by thicker sands containing only scattered glauconite.
The greensand, in addition to abundant glauconite, contains biotite, quartz grains, and shale pebbles, which make it harsh and rough to the touch in hand specimen. At Point Reyes it contains numerous calcite veins and is quite calcareous. Remains of fish vertebrae, bones of marine mammals, and pieces of carbonized wood have been found in the formation. At Point Reyes the Drakes Bay Formation has a thin conglomerate at the base which contains large granitic clasts and boulders of the Paleocene conglomerate. East of the Point Reyes syncline, the green sand lies on the Monterey Shale and contains shale and chert fragments derived from the underlying rocks. The glauconite of the greensand oxidizes to a distinctive rusty brown.
Overlying the greensand are thin-bedded hard chocolate-brown shales 50 to 100 feet thick, which grade upward through light-colored laminated shales interbedded with sand into the overlying tan to white siltstones and mudstones. The shale immediately overlying the greensand is in places very similar to the Monterey Shale exposed at Duxbury Point.
The main body of the Drakes Bay Formation, overlying the greensand and associated shales, consists of fine-grained, tough, compact, cream to brown siltstones interbedded with massive silty mudstones that are grey to yellowish. In general these siltstones and mudstones resemble those of the Monterey Shale, but they lack the hard siliceous shale and the rhythmic bedding of the Monterey Shale. The Drakes Bay Formation contains large rounded calcareous concretions at various levels, with the concretions being particularly numerous on the beach east of the Mendoza Ranch. Layers of fine grey to brown sand are occasionally present. The contact between mudstone below and siltstone above is often of knife-edge sharpness.
In the outcrops along the hilltops southeast of the URanch, the Drakes Bay Formation is represented mostly by the basal glauconitic sand and the sandy beds overlying it; the soft siltstones and mudstones overlying the sands have apparently been largely lost by erosion. Thus the beds southeast of the URanch are distinctly coarser in grain than those to the northwest.
On the geologic map, a division is made between the Drakes Bay basal sands and the overlying mudstones and siltstones; however, it must be remembered that this division is arbitrary and inexact since the two types grade into one another.
The Drakes Bay Formation occupies a syncline and feathers out to zero thickness on the two flanks of the syncline through erosion of the overlying beds. The thickest known section is that encountered by a Standard Oil Company of California core hole (Molseed #1) drilled near the junction of the road to Drakes Bay County Park and the Sir Francis Drake Highway. This core hole is reported to have encountered granite at a depth of 1620 feet; it appears to have been in the Drakes Bay Formation from the surface to a depth of at least 1543 feet, so that an apparent thickness of more than 1500 feet is present here. Since this hole is close to the axis of the syncline, dips are low and the reported thickness is believed to be close to the true thickness. The top of the formation is not present so that its original thickness is not known. Apparently a minimum thickness of about 1600 feet should be credited to it, since the core hole (Molseed #1) started at a relatively low elevation.
The basal sands of the Drakes Bay Formation increase in thickness in an easterly direction, indicating that the sands were derived from the granitic outcrop at Inverness Ridge. The claystones and mudstones could readily have been derived from the nearby Monterey Shale.
The conditions under which glauconite is formed have been the subject of much discussion. According to Emery (1960), glauconite is presently found under the sea off southern California in a general environment of oxidizing conditions, such as on submarine bank tops, ridge crests, hills that rise above shelves, and on some slopes. Glauconite accumulates very slowly in areas of detrital sediments. In the basal member of the Drakes Bay Formation, the accumulation of glauconite probably indicates shallow water conditions with very little sedimentation. The scattered glauconite in the thicker gray-green sands may be reworked, and these sands undoubtedly accumulated in shallow water.
The mudstone-siltstone series is so similar to the Monterey Shale (except for the absence of rhythmic bedding) that it probably accumulated under very similar circumstances in offshore basins of relatively deep water. The relative scarcity of fossils also suggests this type of environment.
The Drakes Bay Formation lies unconformably on the Paleocene conglomerate and the granitic basement at Point Reyes on the west side of the Point Reyes syncline, the Monterey Shale being absent in the outcrop there (photo 12). On the east side of the Point Reyes syncline, the Drakes Bay Formation lies on the Monterey cherts. A marked unconformity between the two units is exposed on the beach near the URanchthe greensand with a thin basal conglomerate dipping about 7° to the west and the underlying Monterey cherts dipping 45° to the northeast (photo 13). Near the Home Ranch there is little angular unconformity apparent; and yet here the beds underlying the Drakes Bay Formation are middle Miocene in age, so that a considerable hiatus in time is indicated.
Farther southeast along the coast, as far as Bear Valley, the basal glauconitic sandstones occur as outliers on the hilltops and again are clearly unconformable with the underlying Monterey cherts. South of Bear Valley the Drakes Bay Formation has been removed by erosion or is obscured by landslides.
Clague (1969) concludes that the Drakes Bay Formation originally extended southeastward from Bear Valley across the Lake Ranch landslide area, where he found discontinuous remnants of the basal glauconitic sand. At the Palomarin Ranch, south of the landslide area, the glauconitic sand crops out in a syncline and appears to be conformable with the underlying Monterey Shale of the MohnianDelmontian age. Lying on the Monterey Shale at several hill-top points in the Bolinas quadrangle are remnants of deposits which may have been the basal conglomerate of the Drakes Bay Formation; this also suggests that possibly the Drakes Bay Formation originally extended much more widely over the Point Reyes Peninsula than it now does.
The youngest beds on which the Drakes Bay Formation lies are late Miocene in age; and it is overlain only by sand dunes, beach deposits, and marine terrace deposits of Pleistocene to Holocene age.
Many fossils have been collected by the writer and others from the Drakes Bay Formation. These are listed below:
Regarding the diatoms, Hanna (personal communication, 1969) stated:
Near the head of Creamery Bay, on the east side at water level, a number of carbonized cones of Pinus lawsoniana Axelrod, which are very similar to those of the living Monterey pine (Pinus radiata), have been found. The beds in which the fossil pine cones occur closely resemble the Drakes Bay Formation but could possibly be part of a younger mudflow or mud slide overlying the Drakes Bay Formation proper. These fossil pine cones are, therefore, the same age as the Drakes Bay Formation or younger. Unfortunately, none of the above fossils is particularly diagnostic as to age. Most of them could equally well be from Miocene or Pliocene beds, and some range up into the Holocene. The diatoms, according to Hanna and Wornardt (personal communications) are more indicative of upper Miocene than of lower Pliocene.
A sample of the glauconite bed at the base of the Drakes Bay Formation at Point Reyes was submitted for dating by the K/Ar method, and an absolute age of 9.3 ± 0.5 million years (m.y.) was determined. The reliability of K/Ar dating based on glauconitic sediments may result in dates that are too young (Evernden and others, 1960; Obradovich, 1965; and Wetherill, 1965). It seems unlikely that the Drakes Bay Formation has ever been deeply buried; it is now overlain only by Pleistocene or Holocene sediments. The presence of reworked glauconite, or incomplete glauconitization, will result in too high a figure, but no determination of this possibility has been made.
Table 4. Species of diatoms found in the Drakes Bay Formation at localities 1234 and 37674 (California Academy of Sciences). See photos 14 and 15.
Table 5. Species of diatoms found in the Drakes Bay Formation at locality 409721 (California Academy of Sciences). See photos 14 and 15.
If the 9.3 m.y. age is approximately correct, it may be compared with the following dates for the beginning of the Pliocene:
Even allowing for the fact that glauconite K/Ar ages have a tendency to be 10 to 20 percent too young, it appears that the base of the Drakes Bay Formation is approximately at the base of the Pliocene. The formation is, therefore, younger than the Monterey Shale.
It is interesting to note that Clark (1966, p. 139) reports a K/Ar date of 6.7 ± 0.5 m.y. for glauconite at the base of the Purisima Formation in the Santa Cruz area, suggesting that the Drakes Bay Formation is older than the Purisima Formation.
Cummings and others (1962, p. 195) described an unnamed member of the Monterey Formation from the La Honda basin in the Santa Cruz Mountains which "consists principally of alternating beds of siliceous and diatomaceous mudstone, and sandy siltstone or very fine-grained sandstone", brownish in color. "At the base of the member there is a light gray to greenish-gray, coarse-grained, feldspathic sandstone that is typically glauconitic". These and other particulars suggest that this unnamed formation is very similar to the Drakes Bay Formation.
Its age is believed to be late Miocene or possibly early Pliocene and may be of Delmontian age. It is thought to be unconformably on the Woodham's Shale member of the Monterey Formation, and it is overlain conformably by the Purisima Formation.
Farther south in the FeltonSanta Cruz area, Clark (1966) has described a sequence of organic mudstone beds of Relizian to Luisian age as belonging to the Monterey Formation. These mudstones are overlain unconformably by the Santa Margarita Sandstone, which in turn is overlain conformably by the Santa Cruz Mudstone. He believes the Santa Margarita Sandstone to be of Mohnian age or younger. The Santa Margarita Sandstone in this area with the overlying Santa Cruz Mudstone also resembles the Drakes Bay Formation in age and stratigraphic position. The following correlations between the Point Reyes area, the La Honda basin and the Santa CruzFelton area are suggested. It is noticeable that, while shales of the Mohnian stage are widespread in the Point Reyes Peninsula, being apparently transgressive over earlier Monterey shales, in the La Honda basin and in the Santa CruzFelton area, beds of the Mohnian stage are either absent or represented by the Santa Margarita Sandstone. According to Clark (personal communication), west of Ben Lomond Mountain (Santa Cruz County), the lower part of the Santa Cruz Mudstone may be of the Mohnian stage.
The Merced Formation was named by Lawson (1893) and the type section is exposed in the cliffs at Seven Mile Beach, south of San Francisco. The presence near Bolinas of the Merced Formation is first mentioned briefly by Lawson (1908, p. 29-30) in the State Earthquake Investigation Commission report which was issued after the San Francisco earthquake of 1906. The Merced Formation at Bolinas is again mentioned by Lawson (1914) in the San Francisco folio of the United States Geological Survey and by Martin (1914, 1916). Martin (1916) gives a comprehensive review of the Pliocene of middle and northern California, in which the Merced Formation at Bolinas is included. He concludes that the section at Bolinas very closely resembles the type section at Seven Mile Beach, correlating it with the lower Merced Formation of the type section.
The Merced Formation of the Point Reyes Peninsula is confined to the San Andreas fault zone near Bolinas (Higgins, 1961) and extends for about 7 miles to the northwest. The Merced Formation is not found at the Tomales Bay end of the Olema Valley, nor is it found west of the fault zone. The Merced Formation of a rather different facies occurs in the vicinity of Dillon's Beach north of Tomales Bay, where it lies on the Franciscan rocks east of the fault zone, but not in the fault zone itself.
The similarity of the lithology and fauna of the Merced Formation at Bolinas to the type section leads to the conclusion that the Bolinas section must have been laid down in the same basin as the type section at Seven Mile Beach. It is probable, therefore, that the outcrop is continuous under the ocean between Seven Mile Beach and Bolinas. The section at Bolinas is, however, only a few hundred feet thick compared with more than 5000 feet in thickness at the type section.
The Merced Formation at Bolinas, consisting mostly of soft sands and silts, forms low-relief rounded topographic features. The sea cliffs at Bolinas, formed by the Merced Formation, are being rapidly eroded by wave action. (See section on "Erosion")
The Merced Formation at Bolinas consists of blue-gray, soft siltstones and fine friable sandstones in the lower part, with coarser sandstones, pebble beds, and gravels, usually weathered to brown colors, in the upper part.
In view of the accessibility of the Merced Formation exposure in the sea cliff at Bolinas, it seems worthwhile to describe it in a little more detail (photo 17). The cliff section is Lshaped; the western part of the section faces south toward the ocean, and the line of cliffs trends nearly east-west. This part of the section is nearly a dip section, the beds dipping eastward at 10 degrees or less. At the channel leading into Bolinas Lagoon, the cliff line turns almost due north and faces east. Since the beds are dipping easterly, the trend of the cliff is nearly parallel to the strike. The cliffs are 100 to 140 feet high.
The lower part of the section, exposed in the ocean-facing cliffs, is more than 100 feet thick, and the base of the formation is not exposed. The cliffs consist of gray-blue siltstones with some fine, clayey sandstones, with occasional layers of shale fragments and scattered calcareous concretions, many of which contain fossils. The beds display some crossbedding. The base of these siltstones is not seen; the top forms a sharp contact with the overlying sandy beds. The siltstones contain abundant Foraminifera and occasional pieces of carbonized wood. Water seeps out at the contact between the siltstones and the more permeable beds which overlie them.
Above the siltstones, in the cliff facing the channel, are found about 50 feet of silty fine sands, with occasional layers of small rounded pebbles (up to about 1 inch diameter) and scattered layers of shell fragments. These sands are also crossbedded and are usually weathered tan to brown in contrast with the blue-grey siltstones on which they rest. These sands are overlain by about 80 feet of coarser weathered material, including medium to coarse sands and gravels with bands of pebbles.
Summarizing, the cliff section contains:
The thickness of the Merced Formation exposed in the cliff section is 230 feet, with the base not exposed. Water wells drilled in the vicinity of Bolinas have penetrated up to 300 feet of Merced Formation, starting at some point below the highest beds exposed in the seacliff. It seems that the total thickness of the Merced Formation at Bolinas is more than 300 feet and not over 500 feet. This is remarkably thin compared with the 5000 feet exposed at the type section only some 15 to 20 miles to the southeast.
The lithologic features and fossil content of the lower part of the Merced Formation at Bolinas indicate deposition under shallow water, neritic conditions, in a sea probably less than 400 feet deep. Higher in the section, the lithology indicates deposition in still shallower water until, at the top of the section, the beds resemble back-beach deposits. The coarser pebbly layers contain abundant Monterey Shale pebbles, derived from west of the San Andreas fault zone. Material was also contributed from east of the fault zone as indicated by pebbles of Franciscan chert.
Reed (1933, p. 246) points out that the Merced silts of Bolinas Bay contain large numbers of Elphidiella hannai a foram which is now an important part of the small living fauna of such embayments as San Francisco and Tomales Bays. We can therefore envisage the Merced silts of Bolinas as being deposited in a somewhat restricted bay, connected on the southeast with the type area of the Merced Formation. Movement on the San Andreas fault zone has to be taken into account in imagining what the situation was in Pliocene times. Assuming that right-lateral movement on this fault has continued since before Pliocene times, the Point Reyes Peninsula, at the time of deposition of the Merced Formation, will have occupied a position west of the Golden Gate, forming an enclosed shallow embayment between the peninsula block and the mainland, in which the Merced beds of the type section and also those of Bolinas were laid down.
The Merced Formation in the Point Reyes Peninsula is confined to the south end of the San Andreas fault zone near Bolinas. On the west side of the zone, it is in contact with the Monterey Shale. This contact has been described by earlier writers as depositional, but the present writer could find no evidence of this; it seems more likely that the contact, which is practically a straight line, is a fault, the west boundary fault of the San Andreas fault zone. (The boundary could also be due to a fault line scarp forming a topographic feature in Merced time.) (Gluskoter, 1962, also concludes that the MercedMonterey contact is a fault contact). The Merced beds lie unconformably on the Franciscan rocks of the fault zone facies, a relationship which is clearly exposed in the San Andreas fault zone northwest of Bolinas.
In the eastern part of the fault zone, the Merced Formation also seems to be delimited by a fault-line scarp, in this case the east boundary fault as determined by the writer. The Merced Formation is not found on the graywacke facies of the Franciscan that lies east of the fault zone. Only beaches and sandspits overlie the Merced Formation in the Bolinas area.
The type section of the Merced Formation at Seven Mile Beach south of San Francisco is regarded by Glen (1959) as ranging from middle Pliocene to lower Pleistocene. The Merced Formation at Bolinas is correlated with the type section on the basis of lithologic and faunal similarity, although the Bolinas section is only one-tenth as thick as the type section. Martin (1916) concludes that "the Merced of Bolinas Bay... (and) of Seven Mile Beach...are... of the same age", that is, younger than the Etchegoin of the Sargent oil field and older than his "upper Merced", which he concludes is Pleistocene in age. The Merced beds at Bolinas are, therefore, middle to upper Pliocene in age, according to Martin.
The fossil marine invertebrates recorded by Martin (1916, p. 230) from Bolinas, including the most characteristic, occur in the Merced Formation at the type locality and show the close connection between the two sections. A list of fossils from these beds, including those listed by Martin (1916) and those recorded by the writer and other workers, is presented in table 6. In addition, sponge spicules, radiolaria, fish and plant remains, carbonized wood, and molds of diatoms are common.
Table 6. Fossils from the Merced Formation at Bolinas.
These fossiliferous sand, clay, and gravel beds were named and described by Dickerson (1922) as the Millerton and Tomales Formations. Subsequently Mason (1934) and Weaver (1949) also described the beds; each of these authors thought Dickerson's two formations should be combined. Mason called the combined formations the Tomales Formation, and Weaver called them the Millerton Formation. The term Millerton Formation, following the U.S. Geological Survey nomenclature, will be used in this report.
The Millerton Formation consists of discontinuous marine and nonmarine deposits, found on headlands on the northeast shore of Tomales Bay, entirely confined to the San Andreas fault zone. It is analogous in setting to the Olema Creek Formation near Olema and to the Merced Formation near Bolinas. Since only remnants of the formation are to be found, it is difficult to determine the original form except that it is clearly related to the fault zone. It was probably laid down in an inlet similar to the present Tomales Bay.
Patches of the Millerton Formation are to be seen at Millerton Point (opposite Inverness), the point 1-1/2 miles northwest of Millerton Point, 2 miles south of the town of Hamlet, and at Tom's Point. Hog Island, in the middle of Tomales Bay near White Gulch, consists of Franciscan graywacke overlain by 10 to 30 feet of incoherent tan sand, which may belong to the Millerton Formation but which has yielded no fossils. All of these patches of sediment are, in general, similar in lithology but cannot be correlated in detail.
The Millerton Formation includes both marine and fresh-water clays, silts, sands, gravels, and conglomerates (Johnson, 1962) which are generally deeply weathered and poorly consolidated. These beds are not continuous over any great distance, and they cannot be correlated in detail between the different exposures.
The formation is very thin as would be expected from what is probably an old terrace deposit. The maximum thickness is about 60 feet. The Millerton Formation was deposited in an environment not unlike that of Tomales Bay today, judging by the presence of both fresh-water and marine beds. According to Dickerson (1922), the fossil invertebrate marine fauna lived in warmer water than that of the present Tomales Bay and could be compared with molluscan fauna living today in the latitude of San Diego. The fossil floral evidence indicates a foggy climate very similar to the present climate at Tomales Bay. The two determinations are not necessarily contradictory, since the range of climatic conditions in the fog belt between Tomales Bay and San Diego is not very great. The invertebrate marine fauna may have lived in a warm enclosed bay, while a fog-belt climate like that of Tomales Bay today prevailed above water level.
The Millerton Formation is overlain only by alluvium and terrace deposits. Where the base can be seen, it is lying unconformably on rocks of the Franciscan Formation.
The Millerton Formation is very fossiliferous. Johnson's (1962) faunal list is included as table 7, and Mason's (1934) list is included as table 8. The same fauna and flora are found throughout the formation, and they indicate a Pleistocene age for the formation. Axelrod (1967, p. 121) mentions that the "Tomales" formation, in the sense used by Mason (1934), has been dated by the C14 method as about 30,000 years old, while Richards and Thurber (1966) have dated the "Millerton" beds in the sense used by Dickerson (1922) as most probably more than 50,000 years old. This evidence suggests that these beds are of late Pleistocene age, and they probably can be correlated with the Olema Creek Formation.
Table 7. Fossil fauna of the Millerton Formation (Johnson, 1962).
Table 8. Fossil flora of the Millerton Formation (Mason, 1934).
Olema Creek Formation
The name Olema Creek Formation has been given to previously unrecognized siltstone and claystone beds which crop out in Olema Creek between the Boyd Stewart Ranch and the Vedanta retreat. This outcrop has been designated as the type section.
The Olema Creek Formation is of limited distribution, being confined to the central part of the San Andreas fault zone between Five Brooks and Olema. North of Olema, if it is present, it is concealed by terrace material or alluvium. A fresh-water type, blue-grey silty clay has been observed underlying the fluvial terrace on which the town of Point Reyes Station is situated. It is not known whether the occurrence represents the Olema Creek Formation.
This formation does not extend east of the "east boundary fault" of the San Andreas fault zone, nor is it found west of the 1906 fault trace. Since the beds are soft and incompetent, there is no topographic expression, and outcrops are limited to stream banks and road cuts.
The Olema Creek Formation consists chiefly of light blue-grey, thinly bedded and laminated clayey siltstone or claystone interbedded with coarse granitic gravel. Layers of silt and peat occur irregularly, and organic material is abundant throughout the formation (photos 17 and 18). The siltstone is dark brownish-grey and often micaceous, and the whole is somewhat current-bedded.
The base of the formation is not exposed, and the total thickness preserved is not known. The thickness exposed is on the order of 700 feet, based on the outcrops which dip generally westward or northward at angles up to 45 degrees.
A number of tree trunks (one in growing position) and much woody and peaty material have been found in the Olema Creek Formation. The formation also contains numerous fresh-water diatoms. The granitic gravel in the beds indicates that part of the material came from the west. It can be concluded that it was laid down in a fresh-water lake occupying part of the fault zone between Five Brooks and Olema.
It can be inferred from field relationships that the Olema Creek Formation lies unconformably on the serpentine and related Franciscan rocks of the San Andreas fault zone. It is overlain by Quaternary alluvium and stream terrace material. To the east, it is probably in fault contact with the Franciscan graywackes of the mainland. On the west, it appears to terminate against the serpentine phase of the Franciscan of the fault zone.
No definite age can be determined for the beds of the Olema Creek Formation from their geological relationships, except that they are younger than Franciscan rocks and older than the stream terraces of the Olema Valley. A C14 measurement on the fossil wood resulted in a determination that the wood is 38,700 years old ± 2000 years. This was confirmed by an independent determination of 38,000 years.
The fresh-water diatoms present in the formation are all Holocene species, indicating that the beds are relatively quite young. Their appearance and lithification also suggests that they are quite young. The only evidence suggesting that they might be older than, perhaps, Pleistocene is that they have been tilted by earth movements which have resulted in dips as great as 45 degrees. However, the fact that they are situated in the middle of the San Andreas fault zone makes such attitudes compatible with a Pleistocene age.
A list of diatoms from these beds identified by Hanna is shown in table 9. Some of the species of fresh-water diatoms found in these beds are illustrated in photo 19. According to Hanna (per sonal communication):
Table 9. Freshwater species of diatoms found in the Olema Creek Formation at locality 38377 (CAS) = 41988 (CAS). See photo 19.
Fresh-water deposits occur at the base and at the top of the Pleistocene Millerton Formation (Mason, 1934) on the east side of Tomales Bay, about 6 miles northwest of Olema in the San Andreas fault zone. Possibly the Olema Creek Formation is correlative with these beds. The age of the Millerton Formation has been determined as probably greater than 50,000 years by Richards and Thurber (1966), but it is worth noting that C14. age determinations on shell fragments from this formation yielded ages of 34,500 ± 3000 years and > 37,000 years in the two instances reported, thus being about the same age as the fossil wood from the Olema Creek Formation. It is concluded that the Olema Creek Formation is Pleistocene in age and probably contemporaneous, at least in part, with the Millerton Formation.
One of the most striking features of the Point Reyes Peninsula is the prominent marine terrace which extends from Bolinas to Limantour Estero (photo 20). It is interrupted by the big landslide at the Lake Ranch between Palomarin and Bear Valley, but there can be little doubt that it originally extended continuously the whole distance. For most of its extent, it consists of a flat-topped terrestrial deposit of Monterey Shale fragments lying on top of an uplifted flat wave-cut platform formed in Monterey Shale.
At the south end of the peninsula in the vicinity of Bolinas, there is no terrace deposit on the broad, old wave-cut platform, called the Mesa, which is at an elevation of 120 to 200 feet. A few miles to the north, a thin non-marine terrace deposit is present on top of the wave-cut platform and the deposit thickens northward irregularly until at the URanch it is nearly 100 feet thick. This non-marine deposit consists almost entirely of unsorted fragments of Monterey Shale and chert. It displays strong internal unconformities, suggesting intermittent deposition. At its base, what appears to be a thin soil (a few feet thick) and some well rounded pebbles can occasionally be seen. In this report, the terrace will be called informally the BolinasDrakes Bay terrace.
The upper surface of the terrace deposit remains fairly level at an elevation of 100 to 120 feet between Bear Valley and the URanch; but the wave-cut platform of Miocene shale on which it rests loses elevation northward until at the URanch it descends to sea level and disappears under the Holocene beach. The cliff here consists entirely of terrace material, showing that the terrace deposit thickens northward as the wave-cut platform on which it rests decreases in elevation. In Drakes Bay proper, the terrace deposit is absent.
Durst (1915) was unable to find evidence to correlate the BolinasDrakes Bay terrace described above with those he studied between Santa Cruz and San Francisco. Nevertheless the similarity in appearance of the BolinasDrakes Bay terrace to those farther south suggests that they may be of about the same age. Bradley and Addicott (1968) considered the age of the first marine terrace near Santa Cruz to be between 68,000 and 100,000 years. Hoskins (1957) concluded that the low terrace from Halfmoon Bay south was 70,000 to 90,000 years old. So it seems likely that the age of the BolinasDrakes Bay terrace is roughly the same as these; namely 70,000 to 100,000 years. Vestiges of probable wave-cut terraces, apparently devoid of deposits, are also seen in the hills facing the ocean behind the Palomarin Ranch, at various elevations higher than the dominant Bolinas Drakes Bay terrace. These terraces are mentioned by Douglas and Rhoades (1915). The top surface of Tomales Point at an elevation of 400 feet is also markedly flat north of the Upper Pierce Ranch, suggesting marine planation.
The land between the URanch, Point Reyes, and Abbotts Lagoon, underlain by the Drakes Bay Formation, displays a remarkably smooth upper surface (photo 21) which has been dissected by numerous valleys that originally contained streams flowing into Drakes Estero. This surface can be observed from the Point Reyes Lighthouse road a few miles north of the lighthouse.
At first sight the land surface seems very similar to the BolinasDrakes Bay terrace in appearance and elevation, but it must be significantly older, since the north extremity of the BolinasDrakes Bay terrace dips under the present beach just west of the URanch. Therefore, the BolinasDrakes Bay terrace is lower and younger than the flat surface on top of the nearby hills. This latter surface is gently warped by the Mendoza anticline and syncline and may be warped by the Point Reyes syncline, although the evidence for this is less clear. Is this flat surface a wave-cut platform or an old land surface? Minard (1964, p. 29) reported older marine terrace sands and gravels as cappings on some of the low hills (200 to 300 feet high) east of the southern part of Point Reyes Beach. These localities are west of Creamery Bay and Barries Bay. Lawson (1894, p. 245) also reported a marine terrace in this vicinity, at an elevation of about 300 to 330 feet. These observers, therefore, suggest a marine origin for this distinctive surface, although it seems quite plausible that it repreents an old low-relief land surface formed at a period of high sea level.
The open, flat central portion of Bear Valley between elevations 280 to 360 feet is occupied by an old alluvial or terrace deposit which slopes to the south. Drainage from the south end of this area flows south to the Pacific Ocean. The north end of the deposit is being eroded by the headwaters of an aggressive north-flowing stream which empties into the Olema Valley west of Olema. The deposit is largely composed of Monterey Shale fragments.
Another deposit very similar to the Bear Valley terrace or alluvial deposit is on the southeast flank of hill 1043, 10,500 feet due south of the bench mark on the summit of Mt. Wittenburg. This deposit is at an elevation of nearly 800 feet above sea level and has been considerably eroded by the headwaters of an active stream which flows south into the ocean at the Pig Ranch. This deposit, composed largely of Monterey Shale debris, must represent the alluviated floor of an old valley which has now been almost entirely destroyed by erosion. Both of these alluvial deposits belong to a cycle of erosion much older than the present. The aggressive streams which are eroding back into Inverness Ridge from both sides, approximately at right angles to the ridge; belong to the present erosional cycle. The Bear Valley deposit and the deposit at hill 1043 belong to an earlier erosional cycle, which was much more mature than the present one. The same earlier erosional cycle probably formed the mature rounded contours which prevail at the top of Inverness Ridge. It is worth noting that this older portion of Bear Valley has a nearly north-south alignment in contrast to the northeast-southwest alignment of the more recent streams.
Numerous terrace deposits are found in the long, straight valley which is occupied by the San Andreas fault zone. Some terrace deposits near Bolinas have been described by Higgins (1961) and by Gluskoter (1962). The latter refers to them as older alluvial deposits. These deposits rest on the Merced Formation which occupies the fault zone and also on the Franciscan graywacke of the mainland to the east of the fault zone. They consist of poorly consolidated coarse sandstones, gravels, and conglomerates composed of Franciscan Formation and Monterey Shale debris, and are very similar in character to the recent valley fill of the existing small valleys which enter Bolinas Lagoon and the fault zone from the east. They have been deformed to some extent by earth movement in the fault zone, showing easterly dips up to 15 degrees where exposed in road cuts at the head of Bolinas Lagoon (photo 22).
Other stream terrace deposits are found in the vicinity of the National Seashore Headquarters, previously the old Bear Valley Ranch or Skinner's Ranch (Gilbert, 1908). Between the Headquarters buildings and the forest-covered Monterey Shale outcrop to the west is a conspicuous flat terrace at an elevation of 120 to 160 feet, on which the ranch residence was built. The terrace appears to be associated with Bear Valley Creek, which debouches from the hills just south of this terrace. The southern part of this terrace deposit consists mostly of fragments of Monterey Shale. The northern part of the terrace contains a large proportion of granitic material, reflecting the exposure of the granitic basement to the north.
To the southeast of the mouth of Bear Valley Creek, a corresponding portion of terrace deposit survives at the same elevation. This portion has been affected by a branch of the San Andreas fault, which heads through it in a northwesterly direction. The terrace is underlain by faulted slivers of Franciscan Formation and of Monterey Shale. The deposit is composed largely of Monterey Shale fragments, but granitic material appears anomalously at its northern termination. Weaver (1949) correlates these deposits tentatively with the Pleistocene Montezuma Formation of the Coast Ranges, but there seems little basis for, or benefit in, this long-distance correlation.
Remains of old terrace deposits are found at a number of the higher points of Inverness Ridge where traces of an older, more mature topography are preserved. One of the most striking of these deposits is at the crest of Pablo Point, about 5000 feet west of Woodville in the Bolinas quadrangle, at an elevation of about 900 feet. Here are found cobbles of hard Franciscan chert and also of granitic rocks, apparently the hardest remnants of a nearly eroded terrace deposit resting on a surface of Monterey Shale. Similar boulders, cobbles, and pebbles are found on the southerly slopes of Stewart Point (Douglas and Rhoades, 1915).
To be in their present position, the Franciscan chert of these rocks must have been derived from the mainland east of the San Andreas fault zone. This implies that the mainland, at some post-Monterey time, must have been higher in elevation than the Point Reyes Peninsula and that the OlemaBolinas valley was not yet eroded. Since the late Pliocene Merced Formation was apparently deposited within the fault zone depression after the valley was eroded, the deposits of which these boulders are remnants must be post-Monterey and pre-Merced in age, or early Pliocene. It is possible, therefore, that the original deposits were as old as the Drakes Bay Formation, which was then much more widespread than its present outcrop area. Rounded pebbles are also found high on Inverness Ridge at Mount Vision, and deeply weathered granitic boulders are found on Mt. Wittenburg (elevation 1400 feet). No doubt all of these occurrences are related to the period of formations of the mature topography which existed before the peninsula was incised by the present streams.
Boulders at Clam Patch
The Clam Patch is a portion of the Holocene wave-cut platform in the Monterey Shale at Bolinas which is dry at lowest tides. This area of beach, about 2000 feet southwest of the Coast Guard Mast, is much enjoyed at low tides by clam diggers; at high tides the surfboard enthusiasts take advantage of the ocean swell which breaks far out on the shallow Clam Patch.
Concealed by the waves except at low tides are numerous large, rounded boulders lying on the wave-cut platform. Some boulders are composed of the underlying Monterey Shale, but many consist of hard sandstone, conglomerate, calcareous sandstone, and occasional granitic rocks, each with a red-stained exterior. These boulders are scattered over the Clam Patch surface at random (photo 23); a few are found on the beach in the direction of Duxbury Point. Similar boulders are seen in a raised beach deposit near the top of the Bolinas cliff about 2500 feet southwest of the Coast Guard Mast. This raised beach deposit consists largely of poorly sorted yellow and iron-stained sands enclosing large boulders of various hard rocks showing a reddish "rind", or cortex, like those at the Clam Patch. Similar boulders are scattered over the east side of the Bolinas Mesa, an old wave-cut platform in Monterey Shale, now standing at an elevation of 120 to 200 feet, part of the BolinasDrakes Bay marine terrace. In the hills to the north, red-coated pebbles and cobbles of similar material are found on the Texeira Ranch west of the ranch house on the surface of the Monterey Shale.
It is easy to conclude that the red-stained boulders of the Clam Patch came from the old wave-cut platform of the Bolinas Mesa as the cliffs were eroded back by the ocean. But how did they reach that position?
Since no granitic rocks are now exposed closer than Mt. Wittenburg, it is necessary to assume that the granitic boulders came from there, from Point Reyes, or from some former outcrop west of the fault zone now covered by the ocean.
Some of the boulders of the Clam Patch resemble a Pliocene calcareous sandstone exposed at hill 1034 (6000 feet north of the west end of Bear Valley); possibly, therefore, the Clam Patch boulders indicate that the Pliocene Drakes Bay Formation originally extended as far south as Bolinas, which certainly seems probable judging by its present distribution.
Older Beach Deposits
East of the present Point Reyes Beach, there is a long stretch of older beach deposits which are essentially horizontal, composed of reddish-brown sand with occasional white shale pebbles, and attain a maximum thickness of about 100 feet (photo 24). These deposits crop out discontinuously in the sea cliff all the way from west Point Reyes to McClures Beach. Locally they are covered by blown sand. These beds are referred to briefly by Anderson (1899), and in more detail by Cherry (1964), Minard (1964), and Cooper (1967). Minard refers to them as "older, ferruginously cemented dune sands," but the writer agrees with Cooper that they are not dune sands but are essentially beach sands.
According to Minard, these older beach deposits are derived from the Franciscan Formation and are associated with an earlier cycle of erosion and deposition. The beds have been affected by earth movement, being structurally low at Abbotts Lagoon where they cross the Point Reyes syncline, and also low west of the Mendoza ranch which lies in the Mendoza syncline. They gradually rise toward the west end of Point Reyes until they show a thickness of more than 60 feet; but, instead of extending over the top of the Point, they terminate abruptly against an old cliff facing northwest on the north side of the west end of the point. North of McClures Beach, they reach an elevation of 200 feet (photo 25).
The land immediately east of the 12-mile-long west-facing Point Reyes Beach is occupied by a belt of sand dunes which has been described by Cooper (1967). The outline of these dunes shown on plate 1 was delineated by the U.S. Geological Survey in 1952 when the Drakes Bay 7-1/2minute quadrangle was surveyed. A field check suggests that there has been little change in outline since that time.
According to Cooper (1967, p. 40), the main belt of dunes "is made up of scores of overlapping tongues and narrow parabolas pointing southeast." One interesting long tongue crosses the highway near Bench Mark 166 at about the latitude of the D Ranch. Another tongue, strikingly illustrating the persistance of the prevailing northwesterly wind, has climbed from the beach up the north-facing slope of western Point Reyes and crosses the lighthouse road not far from the RCA radio tower. This tongue is not shown on the geological map, since its thickness is not great. East of the belt of dunes in the area north of the RCA installation, the soil is concealed under a thin layer of blown sand for a considerable distance inland.
Last Updated: 28-Nov-2007