shale - siliceous McLure Shale member of the Miocene Monterey Formation - unit of 5 student specimens

The McLure Shale member of the Monterey Formation is upper Miocene in  age. This shale is siliceous, white weathering, and hard. It was collected from the summit of the Temblor Range, just east of the trace of the San Andreas Fault in California. Diatomite in the Monterey Formation is the source rock for most of the petroleum produced in California, though it is pumped from overlying rocks where it has become trapped, either by faulting or by impervious shale layers.

This shale make an interesting comparison with diatomaceous shale from the same formation. Students should realize that not all shales are hard, and that what defines a shale is its fine grains and platy nature.

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 muddy when wet, but it's not likely a student will call this "slate."

The field photo shows the blocky nature of the outcrop in a roadcut. The typical slope of shale flakes that appears below most shale outcrops is removed here by Caltrans, since the road is narrow. The vertical surfaces of this much fractured outcrop are dark, but flat surfaces of this shale weather white.

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