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 the underlying Monterey, a rich petroleum source rock, thanks to the diatoms. 

Students should know the basic sedimentary rocks in order of increasing grain size, from mudstone and shale, siltstone, sandstone to conglomerate and coquina. Shales differ from mudstones in that they are more compressed, form flat plates, and are less likely to disappear in a poof of dust when dropped by a student. This shale is soft and will suffer somewhat from student examinations.

Some shales closely resemble slate, metamorphosed from shale. To distinguish a shale from a slate, the field test is to lick it. The shale will smell muddy. The slate won't. In this case, there is no confusing this shale with a slate.

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