Franciscan chert

Franciscan chert is the colorful, fine-grained stone of California's Coast Ranges. It began as sediment on the floor of a deep ocean, where the glassy skeletons of tiny radiolaria piled up with very little mud to dilute them. The National Park Service describes the red and green chert of the Marin Headlands, laid down roughly 200 to 100 million years ago, with its classic folded beds along Conzelman Drive at the north end of the Golden Gate Bridge. Tectonic squeezing scraped the beds off the sinking Pacific plate and folded them. The red colour reflects how much oxygen was present when the rock formed. An archaeological report for the Sites Reservoir project calls Franciscan chert the signature toolstone rock of the central and eastern Franciscan belts, with higher-quality chert in the northern Coast Ranges.

Bedded chert formed from the altered shells of microscopic radiolaria on the deep ocean floor, in the Franciscan Complex. In the Marin Headlands terrane it was deposited from about 200 to 100 million years ago. The beds were scraped off the subducting Pacific plate, accreted to the North American margin and highly folded. The older, more uplifted central and eastern belts contain the chert; higher-quality, denser chert occurs in the northern North Coast Ranges.