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.
Oakland Geology 
7 April 2008 at 6:19 am
Have you ever taken a thin section of it? some times the relict texture can be preserved, including microfossils.
7 April 2008 at 9:00 am
Sorry this is unrelated, but I wanted to make sure you saw it.
I would be really interested to see you talk about the geology and history of the spot where the “old tunnel” is/was above the caldecott tunnel. It’s at the junction between Skyline and Old Tunnel roads. As far as I can tell, the old tunnel appears gone. But, the location is very interesting, with an old flagpole, a sign for the “intercounty tunnel” and what look like some suggestive depressions in the land.
Love the blog. Keep it up!
7 April 2008 at 1:04 pm
Thin section? Why, I’d need a thin-section machine; and if I could do that I’d need a microscope next, and then a lab, and there’d be no end to the trouble and expense. But I’ll bet that there’s plenty of original detail still in these rocks. The unit is mostly turned vertical, but with the shear confined to distinct joints every 5-10 centimeters there would be little disturbance in something like this hand specimen.
7 April 2008 at 3:21 pm
Johnny, I understand that the west side of the tunnel is destroyed beyond repair for safety reasons, but that the east end is somewhat intact, although inaccessible. But soon I’ll put something up about it.
2 May 2008 at 10:35 pm
I’m sure it is destroyed, but I still think it’s very interesting to know where the actual location is. Also, I dont know if the eastern portal of it would even be accessible.