Pliocene epoch (plĪˈəsēn) [key], fifth epoch of the Cenozoic era of geologic time (see Geologic Timescale, table), from 5.1 to 2 million years ago. By the beginning of the Pliocene, the outlines of North America were almost the same as in recent time. Encroachments by the sea were limited to a narrow strip along the coasts of the Carolinas, Georgia, Florida, and the Gulf Coast states, and an embayment, smaller than that of the preceding Miocene epoch, in California. The Pliocene formations on the Atlantic coast are chiefly marine marls; on the Gulf they are nonmarine sediments resulting from erosion. In California they contain much volcanic ash and some are oil-bearing. The Pliocene formations of the western interior are small and scattered. In western interior North America and on the west coast, volcanic activity continued into the Pliocene from the Miocene. The close of the Pliocene was marked in North America by the Cascadian revolution, in the course of which the Sierra Nevada was elevated and tilted to the west. The Cascades, Rockies, Appalachians, and the Colorado plateau were uplifted, and there was activity in the mountains of Alaska and in the Great Basin ranges of Nevada and Utah. In Europe the Pliocene sea covered small parts of the northwest of the continent and a large area around the present Mediterranean; a number of volcanoes were active, among them Vesuvius and Etna. There was considerable mountain building, including the folding and thrusting of the Alps. The climate of the Pliocene was markedly cooler and drier than that of the Miocene and foreshadowed the glacial climates of the Pleistocene epoch. The life of the Pliocene was notable for its modern appearance; the Pliocene marked the climax, and perhaps the initial decline, of the supremacy of the mammals.
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