Goat Rock Beach - Geology

Geology

Goat Rock Beach is subject to continuing marine erosion as well as windborne erosion, thus creating a situation where an average of one to three feet per year of land mass is lost, except for the hardest of outcrop formations . In winters of heavy storms this value can be yet higher. Over the last geologic epoch the land has been subject to uplift, a process combined with marine erosion, which has created a marine terrace above the entire extent of the beach. This marine terrace elevation varies approximately 30 to 150 feet (10 to 50 meters) above mean sea level, which results in a steep bluff directly above the littoral zone, and a succession of terrace levels.

The sea stacks along the coast at Goat Rock Beach consist of rocks from the Franciscan Complex, formed within an era of plate collision along the western coast of North America. From about 200 to 30 million years ago, the North American Plate was in continual collision with the Farallon Plate. A variety of rock types resulted from this collision, including pillow basalt, chert, and marine sandstone. During the plate collisions, these rocks were considerably faulted and crushed into melange, which is a mixture of ground-up matrix and resistant pockets of rock floating within. When melange is eroded by wave action, the softer part of the matrix is washed away, leaving the more resistant blocks exposed in the ocean as sea stacks. Goat Rock is such a flat topped sea stack comprised by a block of resistant greywacke.

Vertical sea stack formations, a geological hallmark of this shoreline, appear standing out of the water or on the beach resembling sculptures. Occasionally these stacks appear on the marine terrace, indicating their ancient genesis on the sea floor prior to uplift. These rock formations are characteristically composed of sandstone with layers of quartz.

The active San Andreas Fault runs roughly parallel and near to the coastline of Goat Rock Beach. Soils within the site are classified as coastal beach sands (where rocky shoreline is not evident) and escarpment group soils on the marine terrace; typically soils above the marine terrace are in the Rohnerville loam group. Most of the beach sands consist of a medium coarse brown to gray sandy materials, reflecting the high rate of erosion of escarpment soils into the ocean; however, there are patches of dark gray smooth pebble beach such as the approximately one hundred meter stretch lying immediately north of Goat Rock.The beach is changed every year.