Rockridge got its name from its outcrops. Although the location of the original Rock Ridge is not quite clear, the Bilger quarry, now home to the Rockridge Shopping Center, exposes some of the bedrock of Rockridge. This is the Franciscan sandstone that makes up the western end of the quarry. I think it was probably considered overburden, something to be removed or maybe used for cheap landfill. There was surely a big demand for cheap fill as Oakland and the other Bay cities encroached into the wetlands. The eastern end, where the pond is today, yielded a higher grade of traprock, a light-colored fine-grained diorite that weathers with rusty stains. There are boulders of it all over the neighborhood; a good specimen is in front of the Quarry Ridge Apartments in the first block of Gilbert Street, built next to where the rail line from the quarry once ran.
In between the two rock types is a fracture zone with a lot of this black, greasy-looking stuff. Grinding along the fault zone and deep-seated alteration has turned the dark minerals into serpentine. That’s roughly where the broken wall is that I showed in the previous post.