Ribbon Chert - Highly folded sedimentary chert exposed in a road cut along Conzelman Road in the Marin Headlands. The cherty layers formed from original radiolarian mud deposited in the deep sea during the Jurassic Period. Radiolarians are single-celled zooplankton with siliceous shells. In this image the chert layers are folded into a syncline (person 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. Within 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 (Wahrhaftig and Murchey, 1987; Elder, 2001; NPS, [2010b]).