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| Vertical joints (fracture in the bedrock) serve as conduits for water into the bedrock. Weathering is most intense adjacent to water-filled fractures. As the rock breaks down, erosion at the surface carves into the bedrock faster in weathered bedrock. The result is the formation of spires of spheroidally weathered rock and boulders on the surface. Most of the bedrock in the distant landscape of the Gabilan Range is underlain by nonmarine sedimentary rocks of Miocene Age (younger than the Pinnacles volcano). Farther north, crystalline basement rocks consisting of granitic plutonic rocks, gneiss, schist, and marble crop out in the high peaks of the range. The valley in the upper roughly follows the trace of the San Andreas Fault (Wagner and others, 2002). |
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