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San Mateo County Profile 4.2.6 Geology The San Francisco Peninsula is a relatively narrow band of rock at the north end of the Santa Cruz Mountains separating the Pacific Ocean from San Francisco Bay. It represents one mountain range in a series of northwesterly-aligned mountains forming the Coast Ranges geomorphic province, which stretches from the Oregon border nearly to Point Conception. In the San Francisco Bay area, most of the Coast Ranges have developed on a basement of tectonically mixed Cretaceous- and Jurassic-age (70- to 200-million years old) rocks of the Franciscan Complex. These basement rocks are capped locally by younger sedimentary and volcanic rocks. Most of the Coast Ranges are covered by younger surficial deposits that reflect geologic conditions for about the last million years (City of San Mateo, 2004). The major fault in the region is the San Andreas Fault. Lateral and vertical movement on the many splays of the San Andreas Fault system and other secondary faults has produced a dominant northwest-oriented topographic trend throughout the Coast Ranges. This trend reflects the boundary between the North American plate to the east and the Pacific plate to the west. The San Andreas Fault system is about 40 miles wide in the Bay Area and extends from the San Gregorio fault at the coastline to the Coast Ranges-Central Valley blind thrust at the western edge of the Great Central Valley. The San Andreas Fault is the dominant structure in the system, nearly spanning the length of California, and capable of producing the highest magnitude earthquakes. Many other subparallel or branch faults within the San Andreas system are equally active and capable of generating large earthquakes. Right-lateral movement dominates on these faults, but an increasingly large amount of thrust faulting resulting from compression across the system is now being identified (City of San Mateo, 2004). 4.2.7 Soils Uplands comprise about 80 percent of the planning area. The following four soil associations have been mapped and described in the uplands (U.S. Soil Conservation Service, 1961):  Hugo-Butano—Steep and very steep, brownish, moderately deep and deep soils on sedimentary rocks under coniferous forest.  Miramar-Sheridan—Steep and very steep, dark-colored, shallow to deep soils on acid igneous rocks under shrubs and forest.  Sweeney-Mindego—Sloping to very steep, dark-colored, moderately deep soils on basic igneous rocks under grass or forest.  Lobitos-Santa Lucia-Gazos—Sloping to very steep, grayish-brown, very shallow to deep soils on sedimentary rocks under shrubs and grass with some trees. Soils of the marine terraces, alluvial fans, and floodplains comprise less than 20 percent of the planning area, but they contain most of the agricultural land and many of the home sites of the survey area. Three soil associations have been mapped in these lower areas (U.S. Soil Conservation Service, 1961):  Tierra-Colma—Gently sloping to steep, dark-colored, shallow to deep soils on high, dissected marine terraces; composed of weathered sedimentary rocks or alluvium from them; under grass and shrubs.  Watsonville-Elkhorn—Nearly level to sloping, grayish, shallow to deep soils formed on low marine terraces composed of alluvium from sedimentary rocks or mixed sources; under grass.  Tunitas-Lockwood—Nearly level to sloping, grayish or brownish, deep soils on fans and floodplains composed of alluvium from various rocks; under grass with some shrubs and trees. 4-5