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.