This is a Ribbon Chert

My buddy Tim Horner a Geology Professor here at Calfironia State University, Sacramento states re: my photo of rocks

This is a ribbon chert. It forms in the deep ocean, often in upwelling zones. Plankton feed on the nutrients in an upwelling zone, and when they die their silica-rich shells are so common a cherty (silica-rich) layer forms. It is probably Cretaceous in age (more than 65 million This is good stuff. I didn't know that there was such a good exposure in S.F. There are similar outcrops in Point Reyes and near Bodega Bay.