slate - teaching hand specimen of black Jurassic slate from the core of the Santa Monica Mountains
This Jurassic slate forms the core of the Santa Monica Mountains. 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 a faint or no muddy smell when wetted, though it varies. Slate often can be parted in large smooth-surfaced sheets. Black slate used as the chalkboard in schools is the origin of the term blackboard.
This slate has been saw-trimmed on one or two or three sides to eliminate sharp edges and to fit into a 3 3/4" x 5 1/4" unit tray. It ships in a small flat rate box unless combined with other purchases. Click > About Shipping < for shipping rates. Use back button to return to this page.
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