Pleistocene epoch

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.

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