plīˈstəsēn, 6th epoch of the Cenozoic era of geologic time (see Geologic Timescale, table). According to a classification that considered its deposits to have been formed by the biblical great flood, the epoch was originally called the Quaternary. Analyses of the magnetic polarity in deep-sea sediment cores indicated that the Pleistocene began more than 1.8 million years ago—much earlier than had previously been suspected (see glacial periods). Since the interglacial periods of the Pleistocene were of longer duration than the time elapsed since the end of the Pleistocene 11,000 years ago, it is sometimes suggested that the Holocene, or Recent, epoch, which is occurring now, may be merely another such interglacial stage and that the glaciers may return at some future time.

An Ice Age

The Pleistocene is the best-known glacial period (Ice Age) of the earth's history. Its ice sheets at one time covered all of Antarctica, large parts of Europe, North America, and South America, and small areas in Asia. In North America they stretched over Greenland and Canada and over the United States as far south as a line drawn westward from Cape Cod through Long Island, New Jersey, and Pennsylvania, along the line of the Ohio and Missouri rivers to North Dakota, and through N Montana, Idaho, and Washington to the Pacific. The ice sheets of Europe radiated from Scandinavia and covered Finland, NW Russia, N Germany, and the British Isles. Glaciers distinct from the main sheets were formed in the Rockies and the Alps. In South America, Patagonia and the S Andes lay under an extension of the antarctic sheet, while in Asia the Caucasus, the Himalayas, and other mountain regions were glaciated.

The glaciation of the Pleistocene was not continuous but consisted of several glacial advances interrupted by interglacial stages, during which the ice retreated and a comparatively mild climate prevailed. In all probability there were actually only four glacial stages, the Iowan and Bradyan being included in the Wisconsin as one complex stage. Carbon-14 analysis of fossils shows that the last glacial period ended about 11,000 years ago.

Topographic and Climatic Changes during the Pleistocene

The characteristic formation laid down in the glacial stages of the Pleistocene, as in all glacial periods, is the drift. The interglacial stages were marked by the weathering of the till of the drift to form a sticky, heavy soil called the gumbotil and by the deposition of peat and loess. Peat is plentiful in the Aftonian, Yarmouth, and Sangamon interglacial stages in North America.

The Pleistocene glaciers made important alterations in the topography of the glaciated regions, leveling hilly sections to low, rolling plains, both by erosion and by deposition of drift, eroding hollows that later became lakes, and forcing rivers to cut new channels by filling their former beds. Among the characteristic surface features formed in the Pleistocene are the drumlin, kame, esker, and moraine. The retreat of the ice after the Wisconsin glacial stage was followed by the formation, at the edge of the melting glaciers, of lakes, such as the extinct Lake Agassiz and the Great Lakes. The further retreat of the ice led to the flooding by the Atlantic of the NE United States and SE Canada, which had been depressed below sea level by the weight of the ice. In the areas of North America not covered by ice, the Pleistocene was marked chiefly by erosion, with only very slight marine transgressions over the coast.

During the various glacial stages many areas not covered with ice, including the arid and semiarid parts of the W United States, had periods of increased rainfall and lessened evaporation. Called pluvial periods, they were characterized by the spread of vegetation and the formation of many lakes. Heavy precipitation in the West was responsible for two great lakes—Lake Lahontan of Nevada and Lake Bonneville of Utah (which today forms the Great Salt and Utah lakes). During the Pleistocene, volcanic activity and warping of the earth's surface occurred on the Pacific coast. The cutting of the Grand Canyon took place chiefly in Pleistocene time.

Fauna of the Pleistocene

Among the characteristic Pleistocene mammals of North America were at least four species of elephants, including the mastodon and the mammoth, true horses, of the same genus as the domestic horse though not of the same species, saber-tooth carnivores, large wolves, giant armadillos and ground sloths, bisons, camels, and wild pigs. Among the arctic mammals that ranged far south in the glacial stages were the musk ox in North America and the woolly mammoth in Europe. The Pleistocene saw the beginning of the trend toward the extinction of many mammal species, which continued into historic times. The Pleistocene is noted also for the first appearance of modern humans approximately 500,000 years ago and the migration of humans to the American continents.

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