Geologic Trips, San Francisco
The Cliff House (right) is built on thick sandstone beds of the San Bruno Mountain terrane, shown in this photo. The sand in these rocks was derived from weathering of granitic rocks in the ancestral Sierra Nevada. The sand was carried from the ancestral Sierra Nevada by ancient rivers and then deposited in a deep oceanic trough that had formed along the Franciscan subduction zone during late Cretaceous time. These sandstones appear somewhat similar to the Alcatraz terrane, but the Alcatraz sandstones are older and were formed during an earlier stage of subduction.