Pinnacles National Monument gets its name from rock spires and crags that are remnants of an ancient volcano. The volcano eroded over millions of years as it moved northward along the San Andreas Fault. Rock debris in the form of boulders has weathered and settled, leaving behind spires of volcanic rock and talus caves.
Pinnacles National Monument began as a volcanic field that originated about 195 miles south of its present location. It has traveled northward along the San Andreas Fault, and currently moves at a rate of about 3 – 6 centimeters per year.
More on the website of the National Park Service:
http://www.nps.gov/pinn/index.htm
Tags: California, National Park