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The Jurassic period is divided into Lower, Middle and Upper. The Lower Jurassic started after the end-Triassic extinction event at 201.3 million years ago (mya). Its rocks form in Europe the Lias group of marine strata consisting mostly of limestone, with some clays and siltstone. It was in this formation on the Dorset Jurassic Coast that Mary Anning did much of her work. The mudstones of the earliest Jurassic (Blue Lias) give way to carbonate rocks as the water deepens in the Middle Jurassic.
The Lower Jurassic in western North America is quite different. It is continental in origin, sandstones laid down in shifting streams and rivers. These sandstones were laid down over a long period from the later Permian to the Lower Jurassic.