The park is located near the San Andreas fault, which had a hand in creating the unique formations the park protects. The Pinnacles are part of the Neenach Volcano which erupted 23 million years ago near present-day Lancaster. The movement of the Pacific Plate along the San Andreas fault split a section of rock off from the main body of the volcano and moved it to the northwest. It is believed that the pinnacles came from this particular volcano because of the unique breccias that are only found elsewhere in the Neenach Volcano formations. Differential erosion and weathering of the exposed rock created the Pinnacles that are seen today. The rock formations are andesite and rhyolite, forming a dropped fault block embedded in the Gabilan Range.