shale - hard green Lower Cambrian shale from the Harkless Formation - teaching hand specimen
This shale is part of the Lower Cambrian Harkless Formation, from the White Mountains of California. These specimens are a fairly hard shale, 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 platy flakes. This shale passes the lick test - it smells slightly muddy - see below. I would ask students for the reason for their identification. If they called this a shale because it smells muddy when wet, or if they called it a slate because it does not smell muddy, I would accept either answer. This particular shale exhibits platy parting - it looks like a shale in the outcrop, but its hardness is slightly variable.
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.
To distinguish a shale from a slate, the field test is to lick it. The shale will smell muddy. The slate won't. The field photo shows the shale in the outcrop, where is has been tilted almost vertical.
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