Hawk Hill

Related Links

Geologic background: Marin Headlands (pdf)

Geologic trips: Hawk Hill Marin Headlands Terrane Point Bonita Lighthouse

 

Exterior Websites

Golden Gate National Parks Conservancy: Marin Headlands

National Park Service: Marin Headlands

Looking east from Point Bonita across the Marin Headlands.

Geologic Trips, Marin Headlands

Marin Headlands Terrane

 

The Marin Headlands are made up of some unusual Franciscan rocks called the Marin Headlands terrane. These rocks are mainly pillow basalt, red chert and sandstone and were once the ocean floor of the Farallon plate. When these rocks were formed, the Farallon plate was very large, and underlay most of the eastern Pacific Ocean. During the subduction process, almost all of the Farallon plate was devoured as it crept eastward into the Franciscan subduction zone. As these rocks were crammed into the subduction zone, they were broken into numerous thin fault slices, so they now lay scattered around the Marin Headlands like pieces of cucumbers, tomatoes, and cauliflower in a giant salad.