Hi Don:

It is hard to id any rock without hammering on it, feeling it, peering at it and tasting it (there are parts of the Miocene Monterey that even smell differently!) But this is a fascinating outcrop, and I dunno...would guess a very weathered limey sandstone just on appearance alone. But don't hold me to that!

The appearance of serpentine can be misleading. The stuff is often found with the Franciscan Formation, which is a true "melange" of all kinds of things, including mudstones, sandstones, limestones, etc.
Later....

Now--have consulted my wonderful text on "Geology of California." Per this, the New Idria district is known for a big serpentinite diapir (an intrusion), about 13 miles long, possibly emplaced in the Jurassic/Cretaceous and exposed to erosion during the Miocene. This has created a several large folds in the region, which display rocks of the Paleocene, Eocene and Miocene and are filled with later Plio and Pleisto gravels. This area is also the source of California's state gem, benitoite, which is a barium-titanium-silicate. Sounds like a fascinating place to visit!

Best, PennyB