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."
Did You Know?
The entire Presidio has been designated a National Historic Landmark. Of the 870 structures on the former post, 470 are considered to have historical significance.