PLEISTOCENE PERIOD (from Gk.rNeicrros, pleistos, most, superlative of roX6s, pops, much ma my scurf's, kainos, new). The name duced by Sir Charles Lyell to designate that period of geologic time which intervened between the end of the Tertiary and the beginning of the historic period. Within the last few years no other of the earth's crust perhaps has received so much attention as the strata included under this name, and this is particularly true in America. The term Pleistocene is commonly used as synonymous with Quaternary, although some geologists tend to make two divisions of the Quaternary, namely, Pleistocene and recent. Other terms which are used are Post-Tertiary, Glacial Period, and Ice Age. One of the most marked features of the Pleistocene was its cold climate and the great development of continental glaciers which formed and spread over a large portion of the globe as a result of these climatic conditions. The faunal and floral characters of the Pleistocene were not sufficiently • different from those of the Pliocene to have permitted making it a separate division of geologic time on this account, for in those regions which were not covered by the ice-sheet we find that plant and animal life apparently continued on uninter ruptedly from the Pliocene into the' Pleistocene.
Aceording to Geikie, there were six glacial and five interglacial stages in Europe, while in the United States Chamberlain considers that there were five glacial and four interglacial stages. The thickness of the continental ice-sheet must no doubt have been enormous, and in some of the mountainous districts in the east we find evi dence of its presence on the mountains to a height of several thousand feet. The deposits formed by the ice at different localities and tinder different conditions are very varied, and are known under a variety of names, such as oskars, Eames, over-wash plains, moraines. etc. The term drift is a general one applied to the deposits laid down either by the ice or by the waters flowing from it. The classification pro posed by Chamberlain for the drift deposits is as follows: Wisconsin till sheets (earlier and later).
Interglacial deposits (Toronto?).
Iowan till sheets, Interglacial deposits.
Illinois till sheets, Interglacial deposits (Buchanan).
Kansan till sheets, Interglacial deposits (Aftonian).
Albertan drift sheets.
In the coastal plain region of the Atlantic States there occurs a great series of Pleistocene gravels and sand. known as the Columbian forma tion. These are the brick clays still extensively worked around Philadelphia, Baltimore, and Washington.
The Pleistocene period was marked by a depres sion of the land along the Atlantic coast and a temporary drowning of many of the river valleys. such as those of the Hudson River and of the Saint Lawrence. The subsequent elevation gave rise to many lines of raised beaches which are represented in the terraces in the Hudson Valley and along Lake Champlain. A similar submer gence occurred along the Pacific coast.
The animal and plant life of the Pleistocene was in some cases decidedly Arctic in its nature, but many species existed which are still living. The mammals were especially abundant. Those of North America included mastodons, elephants, tapirs, horses, and sabre-toothed tigers, as well as a giant beaver; while in South America the sloths were extraordinarily developed, some being equal in size to an elephant. In Europe the remains of the mammoth, hairy rhinoceros, the great cave-bear, and the Irish deer have been found.
BIBLIOGRAPHY. Stone, "The Glacial Gravels of Bibliography. Stone, "The Glacial Gravels of Maine and their Associated Deposits." United States Geological Survey, Bulletin 33 (Washing ton, 1891) ; Leverett, "The Illinois Glacial Lobe.' united States Geological Surrey, Bulletin 3S (Washington, 1899) ; Chamberlain, "Preliminary Paper on the Terminal Moraine of the Second Glacial Epoch," United States Geological Surrey, Third Annual Report (Washington, 1883). See also various articles in the volumes of the American Geologist, Journal of Geology, Ameri can Journal of Science, Bulletin of the Geological Society of America, etc. See GLACIAL PERIOD.