GEOLOGY

The Point Reyes Peninsula is a geologic anomaly compared to its neighboring inland landscape. Beneath the spiny, tree covered Inverness Ridge and gradually, sloping rangeland lie a very distinct geology that separate the peninsula from the rest of the nearby California Coastline.

The Peninsula is a geologic island, along with Bodega Head and coastal Santa Barbara County, it is part of the Pacific Plate and is separated from the rest of California by the San Andreas Fault. The fault cuts through Tomales Bay, Olema Valley, Bolinas Lagoon, westerly into the Pacific Ocean, and then returns east and inland just south of San Francisco (Figure 2). The rest of California and the United States are part of the slower, westerly moving North American Plate. These two plates are grinding in different directions which results in periodic seismic activity along the San Andreas Fault. On average, the Pacific Plate moves in a northwesterly direction 1-2 inches per year. However, San Francisco's 1906 earthquake caused the Point Reyes Peninsula to jump 12-13 inches in a matter of seconds (Evens 1993; National Park Service 1994; Shuford & Timossi 1989)!

The most notable geologic difference between the Point Reyes Peninsula and the remainder of the California Coast is the presence of granite rock and the absence of the Franciscan Formation (graywacke, shale, conglomerate and chert) commonly found west of the San Andreas Fault (Figure 2). The Peninsula's granite rock and the Franciscan Formation that dominates much of the coastal "mainland" are both thought to be from the late Jurassic and Cretaceous age. However, the granite on the Peninsula probably originated from much deeper in the earth and several hundred miles south of its current location (Evens 1993; Shuford & Timossi 1989).

Some of the other wide spread geologic formations found on the peninsula include Laird sandstone, Monterey shale and Drake's Bay formation (sand). With the exception of the granite rock, most of the Peninsula's rock formations are from the Tertiary Period and are considered to be younger than many other formations found along the California Coast (Evens 1993; Shuford & Timossi 1989). How might the geology represented on the Peninsula affect the plant communities? How might the plant communities found here be similar or different than other plants in the surrounding San Francisco Bay Area? Or California? Why?