Pliocene, in geology, the upper of the three epochs of the tertiary or mammalian age. The term was introduced by Sir Charles Lyell, and is derived from the Greek , more, and recent, because more than half of the fossils found in it belong to existing species. The quaternary age, or age of man, is next above it. The pliocene epoch is divided into older and newer pliocene, the latter called also by Lyell pleistocene (Gr. , most), because nearly all the fossils in it belong to existing species. The pliocene formation has been more carefully studied in England than anywhere else, particularly the lower pliocene in Suffolk, which is the only place where it occurs on the island. It there covers the upper beds of the London clay, and its upper and lower divisions have received the local names of red crag and the coralline crag, each about 50 ft. thick. The red crag consists of beds of quartz-ose sand and gravel with a mixture of shells, the whole deposit being strongly ochreous. The fossils are chiefly mollusca, but there are also bones and teeth of large sharks, skates, and other fish, and the ear bones of one or more true whales. The coralline crag is calcareous and marly, consisting of mollusks, echin-oderms, and other marine animals, separated in places by thin layers of hard limestone and coral-like masses. It is easily distinguished from red crag by its whiteness; it was formed at a greater depth, and in an ocean having a higher temperature.
Pliocene deposits have been found near Antwerp and on the banks of the Scheldt, from which over 200 species of shells have been taken, more than half of which are recent species found in the northern seas, and a few still living in the Mediterranean. Similar deposits occur in Normandy, and in Italy between the Apennines and the sea, on either side; and the marine strata of the seven hills of Rome are of the same age. In the United States pliocene beds occur in North and South Carolina, extending as far south as the Edisto river. In them have been found the remains of a mastodon and a stag, and they contain from 40 to 60 per cent, of living species of shells. The beds are soft, either clay, loam, or sand, and lie in depressions of the older tertiary and cretaceous formations. The equivalents of these beds in Virginia and New Jersey are not clearly made out. In the upper Missouri region, the great cemetery of the pliocene, the White river group is overlaid by other fresh-water tertiary beds 300 to 400 ft. thick, called by Meek and Hayden the Loup river group.
They contain in their upper part the remains of numerous extinct mammals, including camels, rhinoceroses as large as the Indian species, elephants, five species of the horse family, a wolf larger than any living species, a tiger as large as the Bengal tiger, and a porcupine, besides land and freshwater shells, which are probably of recent species. These beds occur on the Loup fork of the Platte, and stretch north to the Niobrara and south beyond the Platte.