Wallace Creek

The Carrizo Plain is perhaps best known for its spectacular preservation of landforms associated with the San Andreas Fault. This view is looking east along Wallace Creek toward the Temblor Range. The location of the photograph is situated nearly directly on the active trace of the San Andreas Fault. Bends in the stream uphill from this location are probably the location of additional faults associated with the San Andreas Fault zone. Outcrops in the distance are part of the Monterey Formation (deposited 20 to 9 million years ago--within the Miocene Epoch) (Dibblee, 1972 and 1973).

Being in the rain shadow of the central Coastal and Western Transverse Ranges, the overall low amount of precipitation results in slow erosion rates relative to the amount of tectonic motion along the San Andreas Fault. As a result, the surficial features associated with the fault are relatively "fresh" in appearance. Features include offset, beheaded, and abandoned stream channels, fault scarps, sag ponds, linear ridges, and shutter ridges that display recent strike-slip motion (click here for an illustration of these features). Major earthquakes produced much of this offset motion as demonstrated by the last major earthquake in the region, the magnitude 8.2 Fort Tejón earthquake of 1857. As much as 22 feet (7 meters) of right lateral- displacement occurred in the Carrizo Plain area during that great earthquake (Grant and Sieh, 1993).

The San Andreas Fault extends from the Salton Sea in southern California northward for nearly 800 miles were it extends out to sea at Cape Mendocino. The fault system began to develop nearly 30 million years ago when orientation and relative motion of boundaries between the North American and Pacific crustal plates formed a great strike-slip fault system. Geologic evidence suggests that rocks have moved hundreds of miles northward along the west side of the San Andreas Fault relative to the eastern side. Although a great deal of geologic and seismological research has been conducted in the region, perhaps the most significant story of the fault is told by the displacement of rocks of known ages across the San Andreas Fault zone. The Carrizo Plain lies roughly midway between the Pinnacles Volcanics (Pinnacles National Park) and its geologic equivalent, the Neenach Volcanics located along the San Andreas Fault in Los Angeles County (Dibblee, 1973). Pinnacles and Neenach Volcanics began to form along the young San Andreas Fault system about 24 million years ago (Early Miocene). These rocks are now separated by a distance of 196 miles (315 km) along the San Andreas Fault (Sims, 1993).

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Last modified: 1/12/2011