Oh, the answer is geological and may not make any sense to you, but it's a lump of ophiolite (ancient seafloor) in a surrounding of Franciscan formation, which is softer. It correlates with the rock under Piedmont, Point Richmond, and parts of Novato in Marin County. The Coyote Hills down in Fremont is another example. Bay Area bedrock geology is very difficult to unravel, but imagine it like this: a nice neat layer of snow and freezing rain has covered a road. A snowplow comes along and the stuff stacks up in slices on the plow, including a few pieces of the asphalt, then spills off the edge along the roadside. Using your rock hammer, study the roadside and reconstruct the story in detail, including the date and time of each event.
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