slate - teaching student specimens of black Jurassic slate - Unit of 5 specimens
This Jurassic slate forms the core of the Santa Monica Mountains and is a unit of the Santa Monica Formation. It is well exposed at Point Mugu in Ventura County, California. Deposited in a remnant ocean basin during the Jurassic, it is dated using strontium isotopes at 163 + 14 million years (Medial Jurassic). Paleontologically, fossils of Buchia concentrica give a relative date of Late Jurassic (ca. 150 million years)
There is a sequence of increasing compaction from sedimentary mudstone and shale to metamorphic slate. Shales are somewhat compressed and can resemble slates, as they both originated as fine-grained muddy sediments. A shale will often smell strongly muddy when licked. A slate, metamorphosed from shale by greater compaction and/or heating, will either not smell muddy, or will only smell slightly muddy when licked, a good field identification. This slate passes the sniff test, with no muddy smell when wetted. Slate often can be parted in large smooth-surfaced sheets. Large sheets of black slate used as the chalkboard in schools are the origin of the term blackboard.
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