Three Rocks at Baker Beach, San Francisco

Baker Beach is just west of the Golden Gate Bridge and exposes a wide assortment of rocks of the Franciscan complex. Tectonic forces have squeezed and mixed rocks that formed in widely separated places. Here are the three main rock types of the Franciscan, exposed just a few meters apart. Serpentinite, in the foreground, is deep seafloor rock (peridotite) that was altered into blue-green minerals. Chert, in the middle, formed from clay particles and silica skeletons of floating plankton, thousands of kilometers from land. Graywacke, in the back, is the product of nearshore sand, eroded from rising land and dumped into the sea in avalanches. All of these formed at roughly the same time, about 100 million years ago, and were telescoped together by the tectonic processes of subduction and, more recently, transcurrent faulting along the San Andreas fault.