shale - siliceous McLure Shale member of the Miocene Monterey Formation - hand specimen
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.
Shipping: USPS Ground Advantage. Click > here < for shipping rates. Use back button to return to this page.
Select a specimen: You can select a specimen by telling us what is in the photo with it, a black and silver pen, a black mechanical pencil or one of those plus some number of coins, or you can let us make the selection.
Making multiple purchases? Click on the "combine shipping" button in the shopping cart. We'll send an invoice with combined shipping. A link in that invoice will bring you back to checkout, no hassle.