The "sea stacks" (eroded parts of rock) along the Goat Rock Beach coast are rocks from the Franciscan Complex, rock formations which were made during earthquakes in California. About 200 years to 30 million years ago, the North American Plate collided with the Farallon Plate, causing earthquakes. Different rock types came from these collisions, including pillow basalt, chert, and sandstone. Goat Rock has a flat topped sea stack, made of greywacke. The rock "formations" (groups) are made of sandstone with layers of quartz.