Vertical joints (fracture in the bedrock) serve as conduits for water into the bedrock. Weathering is most intense ajacent 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 sheroidally-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).