The Claremont Chert is a distinct rock unit that I described a bit in an earlier post. This is my official hand specimen of it. You’ll find it halfway up Claremont Canyon and along both Skyline and Grizzly Peak boulevards until they intersect near Round Top, where it leaves Oakland territory. Canyon sits on it, and it underlies the east side of Upper San Leandro Reservoir farther south. To the north it pinches out just north of Claremont Canyon, so it really is an Oakland stone.
It’s got some clay content in it, making it look chalky rather than flinty. I like its fine layering. Until just a couple years ago, the spectacular exposures along Skyline approaching Grizzly Peak Boulevard were in deep shade and shrouded in brush. Since the upper catchment of Temescal Creek was opened up, removing dense stands of eucalyptus (and using poison to keep them removed, thank goodness), the chert has become dazzlingly exposed with its contortions laid out in plain sight.