Boulders in Bear Gulch, Pinnacles National Monument, San Benito County, California

This view is of a debris cave in eastern Pinnacles National Monument (San Benito County). Large boulders from rock falls and debris flows cover or fill narrow gorges associated with stream valleys carved in volcanic rock. The rock exposed in the Pinnacles area consists of massive volcanic (pyroclastic) debris and welded tuff that accumulated on the flanks of an ancient volcano that formed along the San Andreas Fault Zone about 23 million years ago. Tectonic forces along the fault zone ripped the volcano apart and moved the Pinnacles portion to its current location. Over time, tectonic uplift and erosion have carved the old volcanic deposits to their current landscape configuration. Seasonal climate (wet winters, dry summers) are conducive to erosion in the form of debris floods or flows in the region during the winter months.

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