On the shore of Tomales Bay, north of Point Reyes in Marin County, this large greenstone boulder sits near the San Andreas fault. It's an echo of very similar rocks some 100 kilometers south in Pacheco Pass, on the other side of the fault. The white veins are aragonite, an alternative form of calcium carbonate that formed in the high-pressure conditions of an ancient subduction zone.
California for much of its geologic history was like Japan, a place of persistent subduction. As plate configurations changed after the Cretaceous, subduction-related rocks like this were sliced sideways and smeared to the north, exposing their tectonic innards in a display unique in North America.