shale - teaching hand specimen of light tan diatomaceous shale from the Monterey Formation, Santa Barbara Co., California

The Miocene age Monterey Formation is the source of much of the petroleum produced in California, thanks to the diatoms. Each contains a tiny droplet of oil. A constant rain of diatom exoskeletons falls to the sea floor in colder waters, with each containing a droplet of oil. With burial, compression and heating, the oil is converted first to kerogen, then with additional heating, to petroleum and then to.natural gas.

This shale is light in weight and color. The specimens will stand up to student examination if they are careful. It is a good contrast with much older green and gray shales we have in stock, as those are much harder.

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.

These specimens were collected near Lompoc in Santa Barbara County, California, from outcrops along Sweeney Road, well worth a visit if you are in the area. The two photos of folds in this shale were taken at that outcrop. Click to select, then click again to enlarge.

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