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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
a relatively long stage in the geological history of the earth during which, against the background of a general relative cooling of the climate, there was repeated alternation of very cold intervals of time (glacials), when extensive continental glaciation occurred, and intervals of time with a warmer climate (interglacials), when a significant part of the continental ice melted. Ice ages have been established in the Lower Proterozoic in North America; in the Upper Riphean in Africa and Australia; in the Wend in Europe, Asia, and North America; in the Ordovician of Africa; and at the end of the Carboniferous and the beginning of the Permian on the continent of Gondwana. The ice age of the Pleistocene has been studied most.