Ribbon Chert - Highly folded sedimentary chert exposed in a roadcut along Conzelman Road in the Marin Headlands. The chert layers formed from original radiolarian mud deposited in the deep sea during the Jurassic Period. Radiolarians are zooplankton (single-celled animals) with silicious shells. In this image the chert layers are folded into a syncline (Eleyne Phillips for scale). At the time this mud accumulated, this part of the sea floor was perhaps hundreds to thousands of miles seaward of the coast. Over the past 200 million years plate tectonic forces moved the ocean crust bearing these deposits (now converted to chert) to their present location on the Californian coast.