shale - hand specimen of a tan siliceous Miocene shale - the Altamira Shale - of California
This is the Altamira Shale, the oldest subunit of the Monterey Shale, Middle Miocene in age. These specimens are a fairly hard shale, with a fairly high silica content from included volcanic ash, so they will not crumble during student examination as will more fissile shales. Fissile refers to the behavior of shales that weather out as a pile of loose flat flakes. The Monterey shale contains a considerable amount of petroleum and is now drawing considerable new interest. This unit, the Altamira Shale, underlies a large area of the Palos Verdes Peninsula in Los Angeles County. The orange iron staining is typical in outcrops of this shale. On a cut or broken surface, it's light gray to white.
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 fairly hard and should stand up to student examinations.
Some shales closely resemble slate, metamorphosed from shale. To distinguish a shale from a slate, the test is to lick it. The shale will smell muddy. The slate won't. This shale smells slightly muddy when wet, but it's not likely a student will call this "slate."
Some of these specimens are a set of two thinner plates, useful to illustrate the way shales typically weather into flat plates.
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