• East view from Crissy Field overlook with old Coast Guard station on left and city on right

    Presidio of San Francisco

    California

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Chert

Chert exposure

Red and green chert in the Marin Headlands Terrane of the Franciscan Complex was deposited from about 200 million years ago to 100 million years ago. The red chert photo is from classic exposures in the Marin Headlands along Conzelman Drive, north end of the Golden Gate Bridge.

Will Elder, NPS

Highly folded bedded chert is a characteristic rock of the Franciscan Complex. The folding is largely due to deformation produced as the oceanic sediments were scraped off the subducting Pacific Plate and accreted to the North American continental margin at the subduction zone. In the Franciscan Complex, chert is a silica-rich rock formed from the altered shells of microscopic radiolaria, which slowly rained down onto the ocean bottom. Bedded chert is only developed in deep ocean settings, where few mud particles from the continents are present to dilute the silica. Chert is also typically formed under areas of high productivity in the oceans, which promotes the growth of radiolaria relative to other types of plankton. The thin beds or layers may be due to changes in oceanic silica productivity associated with the Earth's orbital cycles. The red color reflects the how much oxygen was present when the rock formed.

Learn more about chert.

 
Shells of radiolaria
Franciscan cherts are formed from the tiny
(0.5 to 1.5 mm) silica shells of radiolaria. Many of
these radiolaria are tropical species indicating that
the sediments were deposited near the equator and
were later transported northeastward by plate movements.

Did You Know?

The Officers' Club at the Presidio

The modern-day Officers' Club was part of the original Spanish Presidio. In the front section of the building, excavations have uncovered adobe walls dating from the 1790's.