Pliocene Purisima Formation, Drakes Beach
Point Reyes National Seashore, California

ID 2997

The Pliocene Purisima Formation consists of mudstone and siltstone beds that were deposited on a continental shelf in an oxygen-rich environment that supported an abundant community of animals. The most common fossils include single-celled algae (diatoms), radiolarians, and siliceous sponge spicules. The light color of these sediments is caused, in part, by the abundance of silica from the shells of these single-celled organisms. In addition, fish, seal, and whalebones are abundant in the cliff faces. Mollusk and echinoids are also rare.

The light-colored rocks reminded Sir Francis Drake of the chalk cliffs of England when he landed here in 1579.

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