Along Lake Chabot, almost all the way to the park at the south end, the path rises high and passes this cut into a rotten light-colored rock, part of the San Leandro Gabbro.
Gabbro, you learn in geology school, is a dark coarse-grained rock that forms deep in the ground with the same minerals as basalt. But its formal definition ignores the dark minerals, and this light rock is mostly plagioclase feldspar with almost no quartz or alkali feldspar—a white gabbro. But the rock is thoroughly shattered and crumpled here and in most places I’ve seen it. For all I know (and I don’t know much), it is deeply altered from its original appearance. It is of Jurassic age (about 150 million years) and has been faulted several times since its formation, most recently by the Hayward fault.
The San Leandro Gabbro is perhaps the oldest rock in Oakland. I have yet to find a decent outcrop of it in Oakland, although it’s mapped in several places including the hill north of the zoo across I-580 (see the end of this post).