shale - teaching hand specimen of tan diatomaceous shale from the Monterey Formation, Ventura County, California.

This Miocene shale is from the Monterey Formation in Ventura County, California. These specimens are fairly soft and light weight, being composed of fine grained sediments and diatoms, plankton that forms a siliceous exoskeleton. Some specimens show evidence of turbidity flow, splitting on a surface that appears hashed. Diatoms contain a droplet of oil when living. When they die and rain to the deep sea floor and are buried, the oil eventually becomes petroleum or natural gas and the siliceous skeletons become diatomaceous earth.  The Monterey shale contains a considerable amount of petroleum, and much of California's oil production comes from the Monterey. This shale was collected in Balcom Canyon which roughly separates South Mountain from Oak Ridge in Ventura County, California. Oak Ridge is the surface expression of an anticline. In the South Mountain Oil Field, petroleum is produced from the Oligocene Sespe and Pliocene Pico Formation. Oil in the Pico likely migrated upward from