Graywacke Sandstone

Graywacke sandstone

Bedded graywacke sandstone at Baker Beach.

Will Elder, NPS

Graywacke sandstone is the product of underwater landslides that carry sand, silt, rock fragments, and plant and animal debris off of the continental margin and rapidly deposit them in deep ocean environments. Graywacke is associated with subduction zones, where sediments are being shed off a continent at high rates. Little sorting of mineral types, rock types, or particle size occurs before deposition of these rocks; hence their other name, "dirty sandstone."

Learn more about graywacke sandstone.

Ammonite Sandstone from Baker Beach in the Presidio

This ammonite (Douvilleiceras mammillatum) was found
in the sandstone near Baker Beach, indicating that these
continental derived sediments of the Marin Headlands Terrane
are about 100 million years old.

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