The other week I took a hike up Claremont Canyon, one of those parts of Oakland that everyone thinks is in Berkeley. Walking there is slightly dicey because the road is narrow and cars don’t expect pedestrians, but I plan to do it again. The bedrock shifts from nondescript sandstones of the Great Valley Formation to vertical stripes of Claremont Chert, to coarse conglomerate of the Orinda Formation, then basalt of the Moraga Formation near the top. That’s what this is. According to the map, this must be float—loose stones carried downhill in the soil—rather than actual bedrock. The fresh rock is black and the weathered rock is brown. This is the stuff exposed throughout Sibley Volcanic Regional Preserve, but you can’t collect it there. You probably can’t collect this either, because Claremont Canyon is also a reserve, but nobody cares about roadcuts. Still, I try to observe these things because I talk to the public and really need to set the right example. You can collect it in the fantastic exposures along Route 24.
As you continue south on Grizzly Peak Boulevard from the top of Claremont Canyon road, you can look back and follow the basalt along the hillside, over the crest, across Route 24 and up toward Round Top. This is a hard region to map, but the Moraga basalt is easy.