California Division of Mines and Geology Bulletin 202 Geology of the Point Reyes Peninsula, Marin County, California

Topography

The peninsula is dominated by Inverness Ridge, a line of wooded hills which extends in a southeast-northwest direction along the eastern edge of the area. The highest point on this ridge is Mt. Wittenburg (elevation 1407 feet), about 2 miles west of Olema. The second highest peak, Point Reyes Hill (elevation 1336 feet), is west of Point Reyes Station. Inverness Ridge slopes steeply eastward into the Olema Valley and less steeply westward to the ocean. The ridge is being actively eroded by streams, which form deep, steep-sided canyons. At the top of the ridge, remnants of rounded, mature land forms of earlier erosion cycles can be seen.

Inverness Ridge ends to the northwest in the granitic bluffs of Tomales Point. On the southeast it merges into Bolinas Mesa, an uplifted, wave-cut platform of Monterey Shale. The ridge is cut through from north to south near Olema by Bear Valley, a low pass of less than 400 feet elevation.

West of the ridge in the latitude of Inverness, the country becomes lower, open, and rolling and is dominated by Drakes and Limantour Esteros (estuaries), a system of drowned valleys invaded from the south by the sea. At the west edge of this low country, Point Reyes Beach, a nearly straight, sandy beach 12 miles long, connects Tomales Point and Point Reyes.

Point Reyes, the southern anchor of this beach, is one of the most remarkable features of the peninsula. Running east-west, across the prevailing northwest structural grain of the terrain, it stands majestically at the south end of the low dairy-farming country (photo 2).

The eastern boundary of the peninsula, the long straight depression in which lie Bolinas Lagoon, Tomales Bay, and Olema Valley, is the most striking topographic feature (photo 1). The early geologists recognized that this depression was the expression of a major fault, but it was not until the San Francisco earthquake of 1906 that the geological significance of this fault was fully realized. After that earthquake took place, the greatest recorded lateral movement along the fault was near Point Reyes Station, where the ground moved 20 feet northwestward on the west side of the fault, relative to the ground on the east side of the fault. This dramatically drew attention to the fact that the long depression was the San Andreas fault zone. The rupture in the soil could be traced all the way from Bolinas through Olema Valley to the south end of Tomales Bay. Many text-book examples of fault topography, including ridges, sags, hollows, and ponds could be identified along the fault trace.

On the west coast of the peninsula about 6 miles northwest of Duxbury Point, several small freshwater lakes occupy a series of basins formed in landslides by rotation of large blocks of Monterey Shale which slid downslope toward the sea.

Mud Lake, at the crest of the ridge, west of Five Brooks, is probably in a large fault sag, as it lies between two prominent fracture traces. Gilbert (1908, p. 76) reported that this lake emptied quickly at the time of the 1906 earthquake. The water apparently emerged on the east side of the ridge, three-quarters of a mile away, presumably along a fault crack. Clague (1969) suggests that Mud Lake could also be of a landslide origin.

Drainage

The main drainage of the area is determined by the dominant Inverness Ridge. Small streams descend the east slope of this ridge into Olema Valley, where they flow either northwest into Tomales Bay or southeast into Bolinas Lagoon. On the west side of the ridge larger streams, such as Arroya Honda and Bear Valley Creek, descend to the ocean.

The drainage pattern is generally dendritic with consequent main streams. As would be expected in an area with so much recent earth movement, however, there are many drainage anomalies. The most striking of these are in the San Andreas fault zone. In the southern part of the fault zone, 6 miles northwest of Bolinas, Pine Gulch Creek and Olema Creek run side-by-side for 2 miles at nearly the same elevation on parallel courses about a quarter mile apart, but they flow in opposite directions. Each of these streams has eroded its course in an old fault trace—Pine Gulch Creek on the southwest side of the San Andreas fault zone and Olema Creek on the northeast side.

Farther to the south, Pine Gulch Creek follows a remarkable course. Near Woodville it leaves the San Andreas fault zone which would seem to be its natural location. The creek cuts southwestward into the hills, where it describes an elongated arc turning northeastward to debouche into Bolinas Lagoon halfway down its western side. It seems most likely that the anomalous course is due to capture by a stream that originally occupied the lower part of the present course and that was rejuvenated by uplift of the land or fall in sea level.

The short streams which flow from the Inverness Ridge into the San Andreas fault zone are, for the most part, approximately normal to the ridge, but there are some anomalous exceptions which flow southeast instead of northeast. These are associated with extensive fracture traces, visible in aerial photographs, which are considered to be branches of the San Andreas fault traversing the peninsula to the west. These are discussed in the section on "Faulting" and are shown on plate 1.

Several of the main streams, which flow from the south half of Inverness Ridge to the ocean, follow a north-south trending course, instead of a southwest course that would be expected from the topography and the general structural dip. Examples of these stream courses are the Arroya Honda, Alamea Creek which meets the ocean just north of Double Point, and the central portion of Bear Valley Creek. The reason for these anomalous directions is not clear. However, the stream courses in question are associated with the older land forms present, for instance, in the central part of Bear Valley. Further study of the old topography might show that these are antecedent or superimposed stream courses.

Many of the streams flow the year round in spite of the rainless summer. During the dry season they are fed by the summer fogs, which are intercepted by the forest trees. After a thick fog, the ground under a large tree in the forest is as wet as though it had been raining; where there are no trees, the ground will remain dry.

In the low, rolling dairyfarm lands, west of the Inverness Ridge and north of Point Reyes, there are no streams of any consequence.

Climate

The climate of Point Reyes Peninsula is characterized by cool, dry, foggy summers and cool, rainy winters. The fogs cool the area in the summer well below the temperatures a few miles inland. Inverness Ridge forms a barrier to the fog, and often in the summer the area west of the ridge will be shrouded in thick fog, while Olema Valley is sunny.

The proximity of the Pacific Ocean tends to reduce the average seasonal range of temperature. At Point Reyes Lighthouse the difference between the mean daily January and September atmospheric temperatures is less than 7° F. At Petaluma, 25 miles inland, the difference between the mean daily January and September temperatures is 19.3° F (table 1; figure 2). The temperature of the ocean shows very little variation over the year. Table 2 compares surface water temperatures at North Farallon Island with those at the Golden Gate.

Figure 2. Mean daily atmospheric temperature by months, Point Reyes Lighthouse and Petaluma.

Table 1. Mean daily atmospheric temperature in degrees Fahrenheit by months, Point Reyes Lighthouse and Petaluma¹.

Table 2. Surface water temperatures in degrees Fahrenheit, North Farallon Island and Golden Gate¹.

Rainfall is greatly affected by the topography; for instance, at Point Reyes Lighthouse, the average annual rainfall over the past 64 years is only 19.55 inches, while at Point Reyes Station east of Inverness Ridge, the average annual rainfall between 1923 and 1936 was 29.90 inches (table 3).

Table 3. Mean monthly precipitation in inches, Point Reyes Lighthouse, Point Reyes Station, and Petaluma.¹

Vegetation

Inverness Ridge is densely forested on its higher slopes, with the Douglas fir (Pseudotsuga menziesii) dominant at the south end, and the Bishop pine (Pinus muricata) dominant at the north end. On the slopes below these forests, coast live oak Quercus agrifolia), California laurel (Umbellularia californica), and buckeye (Aesculus californica) are found in the drier areas, and tanbark oak (Lithocarpus densiflorus) and madrone (Arbutus menziesii) are found in the moister areas. Redwoods (Sequoia sempervirens) are not found in the Point Reyes Peninsula west of the San Andreas fault zone. This anomaly seems to be connected with the relative dryness of the soil formed on the Monterey Shale.

Poison Oak (Rhus diversiloba), coyote brush (Baccharis pilularis), sticky monkey flower (Mimulus aurantiacus), wild lilac (Ceanothus thrysiflorus), California sagebrush (Artemisia californica), and coffee berry (Rhamnus californica) form a dense, brush cover on the lower slopes. Wild currant (Ribes sanguineum) is found in moist places.

On the windswept ocean bluffs, many of these coastal brush plants have a depressed, windblown form, and some have assumed a low or creeping habit. This is particularly noticeable at Point Reyes headland where the coyote brush, which inland may be a large shrub five to ten feet high, forms a low mat only a few inches high.

The flat coastal areas form grasslands, as do many of the flatter hilltops near the coast. These grassy areas cleared for agricultural purposes revert to brushland when not grazed or regularly cleared.

The vegetation of the peninsula has been described in detail by Howell (1949), in a comprehensive work which includes some excellent photographs of Point Reyes terrane and flora. Howell describes 61 species of plants from the Point Reyes Peninsula which are not found elsewhere in Marin County. This is attributed to the very different geologic histories of Point Reyes and the rest of Marin County.