This story is based on a longer article here by former Conservancy president, Martin Holden.
The Claremont Chert, named after our own canyon, is a type of marine sedimentary rock. It consists of hard, fine-grained silica, plus more friable shales and mudstones from ancient siliceous skeletons of microscopic creatures. These tiny skeletons and other materials were deposited over millions of years in off-shore basins—when the shoreline of the California coast looked quite different from today.
The now well-known Chert Wall is part of the University of California’s Ecological Study Area. Geology students can be seen gathered around the wall mid-way up the canyon across from Signpost 29.
According to geologist Martin Holden, chert likely was quarried from this site in the 19th and early 20th centuries and used as roadbed fill under what is now Claremont Avenue. The vertical layers shown in the photo above, now more visible after recent vegetation removal, result from strikeslip and reverse faulting caused by the Hayward and related, smaller faults. Creation of this fascinating geological deformation began in the late Miocene, and is associated with the movement of the North American Plate relative to the Pacific Plate. Needless to say, this process is still occurring.
