Mammoth Rocks and the Geology of the Sonoma Coast

Remnant sea stacks on an uplifted, Pleistocene, wave cut terrace, on the north coast of Sonoma County, north of San Francisco California, have localized mirror-like polish on their surfaces which extends over areas up to 4 meters high. Scanning electron microscope (SEM) and atomic force microscope (AFM) images of this polish, and comparison with wave polish from the cliffs below the terrace, and known elephant rubs, indicate that the polished surface was most probably made by large Pleistocene mammals rubbing themselves against the rock. The AFM images demonstrate the extreme planar nature of the mirror surfaces.