The Headlands in SF?

Blocks of the Marin Headlands terrane in the mélange.

Looking towards the ocean, the promontory in front of you is made of bedded red chert and graywacke sandstone. These are pieces of a much larger block of ancient sea floor that forms the Marin Headlands across the Golden Gate. To the south of this point until Lands End, the Marin Headlands block also underlies San Francisco. These large blocks, known as "terranes" to geologists, are large pieces of sea floor that ended up being plastered on to the edge of North America, rather than sliding beneath the continent.

Chert is a glassy rock made up of Radiolaria, a big word for ocean faring single-celled organisms. These little guys are smaller than a grain of sand and come in a variety of symmetrical shapes that survive for hundreds of millions of years embedded in the rocks as fossilized mineral skeletons. They tell us the age of the rocks and the environment in which they lived. Radiolaria in these rocks lived somewhere in the central Pacific between 200 and 100 million years ago.

The graywacke sandstone formed at the edge of North America during submarine landslides that dumped sand into the deep-ocean subduction trench about 100 million years ago. These rocks sometimes contain fossilized shells of marine mollusks.