Ordovician period ôrdəvĭsh´ən [key] [from the Ordovices, ancient tribe of N Wales], second period of the Paleozoic era of geologic time (see Geologic Timescale, table) from 505 to 438 million years ago. It was similar to the preceding Cambrian period, with shallow seas spread for most of the time over the British Isles, Scandinavia, the Baltic region, the Mediterranean region, a large part of Siberia, and much of North America. The Ordovician rocks are chiefly sedimentary. Because of the restricted area and low elevation of the solid land, which set limits to erosion, marine sediments that make up a large part of the Ordovician system consist chiefly of limestone; shale and sandstone are less conspicuous. The Ordovician of North America can best be studied in New York state. In the Early, or Lower, Ordovician epoch, also called the Canadian epoch, the waters spread over the Appalachian area and deposited the Beekmantown limestone, then withdrew generally, to return and deposit the Chazy limestone of the lower Middle Ordovician, also known as the Champlainian epoch. In the interval between Beekmantown time and Chazy time, large areas, chiefly outside New York, were apparently covered with wind-blown sand which became the St. Peter sandstone. In the Middle Ordovician the sea spread over North America to a greater extent than in any other period and laid down the Trenton limestone, which in its eastern section is overlaid or intercalated with the Utica mud shale. In the east, increased erosion of the land subsequently led to the deposition of other shales, which became more and more sandy toward the end of the period. The close of the Ordovician was marked by more general earth disturbances than the close of the Cambrian. The Taconian disturbance created a chain of fold mountains extending from Newfoundland to New Jersey and was accompanied by volcanic activity. The later start of the Acadian-Caledonian uplift may have also been the start of the proto-Atlantic Ocean. Among the economic resources of the Ordovician strata are oil, natural gas, the lead and zinc of Wisconsin, Iowa, and Illinois, the
Portland cement rockof Pennsylvania and New Jersey, Vermont marble, and the calcium phosphate of the Tennessee limestone. The Ordovician seas were rich in animal life. The most characteristic invertebrates were minute graptolites, other numerous forms being brachiopods, bryozoans, and trilobites. Some cystoids and crinoids appeared; there were a few corals and many cephalopods. Especially noteworthy was the appearance of a few primitive, fishlike vertebrates (jawless fishes) and tiny land plants resembling liverworts.
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