The Mountain's Geology


Greywacke on San Bruno Mountain. The foundation of the San Bruno Mountain landscape began to develop more than 130 million years ago in the Cretaceous Period.  What is now the California Coast was undersea during this period.  For millions of years, layers of sediment were deposited in deep ocean trenches by landslides and erosion of evolving volcanic mountain ranges.  The resulting sedimentary rock, called graywacke, contains quartz, feldspars and volcanic rock, as well as silt and clay. The volcanic rock fragments give graywacke its greenish-gray color.

San Bruno Mountain is an elevated fault block, created by the buckling of the earth’s crust starting about 1 million years ago.  The San Francisco Peninsula and Marin are composed of a series of northwest-trending earth blocks that moved relative to each other along fault lines.  The San Bruno Mountains are bordered on the north by the City College Fault and on the south by the Hillside Fault.  These earth blocks, or terranes, form part of the Franciscan Complex and each block is now known to have a unique sedimentary rock composition.

Since the San Bruno Mountains are composed mostly of the Franciscan graywacke, with minor beds of shale, there is little variation in types of soil.  A thin, rocky loam is found on steeper slopes and a thicker, clay loam is found on more gradual slopes.  However soil depth varies considerably with the degree of the slope.  Small outcroppings of serpentine are found along the Hillside Fault near Serbian Ravine, but there are no unique plant communities associated with this serpentine.