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California Coast

Field Trip Stop #4:

The Red Cherts of the Marin Headlands

North of the Golden Gate Bridge, we find the red cherts of the Franciscan Formation - sedimentary rocks that were originally deposited flat in the oceans during Jurassic-Cretaceous times (about 100 million years ago). They are made of up clays and the skeletons of microscopic plankton (radiolaria), which fell to the sea floor and accumulated there.

an example of a radiolarian skeleton

Geologic History:

1. Layering

The layering is still controversial. One hypothesis is that these represent original alternations between radiolarian-dominated deposition and clay-rich deposition (from storms?). An alternative idea holds that the layers are formed through heat-driven density differentiation.
2. Folding

The folding and bending of these strata represent soft sediment deformation on the sea floor before the layers had become hardened into rock (otherwise, they would be broken or faulted, not bent). After the sediments became cemented, the plate that they had been deposited on became subducted—but these sediments were not subducted and instead became accreted to the edge of the overlying continent. These processes may also have caused deformation to the rocks (in addition to the early soft sediment deformation).

3. Uplift, Weathering and Erosion

These rocks then became uplifted during the last 30 million years or so during the formation of the Coast Ranges. Weathering and erosion have acted on these rocks since then. One can observe piles of loose sediment around the base of the outcrop indicated ongoing modern erosion caused by both constant atmospheric changes and the root systems of these plants that have managed to take hold.

Additional Resources:

NAGT guidebook:
Stoffer, Philip W. and Gordon, Leslie C. 2001 (eds.). Geology and Natural History of the San Francisco Bay Area, A Field-Trip Guidebook. U.S. Geological Survey, Bulletin 2188,

Travel on to Tying it all Together.

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