Marin County Serpentinite

In the hills of western Marin County you'll often come upon exposures of slick-looking, greenish serpentine rock. This is a signature lithology of the Franciscan complex, an informal name for a suite of exotic rocks derived from deep ocean crust and associated volcanics. Serpentinite, California's state rock, is a common roadside sight from the country south of San Luis Obispo all the way up past Fort Bragg. But Marin has the most of it.