Marin Headlands - sedimentary rock (chert)

Some sedimentary rocks are made up mostly of fossils. One such fossil-rich sedimentary rock is the chert of the Franciscan Complex (shown as KJfc on the geologic map). Excellent outcrops of this rock are seen at Marin Headlands on the north side of the Golden Gate.

Chert is a sedimentary rock. Sedimentary rocks are formed when accumulations of sediment (small bits of rocks and minerals produced by weathering and erosion, often with some plant or animal remains mixed in) are buried, compacted, cemented, and slowly converted to rock. Plant and animal remains are incorporated into the rock as fossils. In certain environments, sediment accumulations can be formed almost entirely of plant or animal remains. Chert is formed from silica ooze that results from the accumulation of countless tiny remains of certain kinds of plankton that collect in the deep ocean.

Thin beds of red chert at Marin Headlands, Marin County.

An electron microscope image of a radiolarian fossil. Radiolarians are tiny plankton that form intricate "skeletons" of silica. These "skeletons" accumulate at the bottom of the deep ocean into layers of silica ooze that can solidify into chert.

Learn more about the geology of Marin Headlands (PDF ) .