Further Reading

Condie, Kent C., and Robert Sloan. Origin and Evolution of Earth, Principles of Historical Geology. Upper Saddle River, N.J.: Prentice Hall, 1997. Erikson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. New York: Facts On File, 2001. Miyashiro, Akiho, Keiti Aki, and A. M. Celal Sengor. Orogeny. Chichester, England: John Wiley & Sons, 1982.

Moores, Eldridge M., and Robert Twiss. Tectonics. New

York: W. H. Freeman, 1995. Strahler, Arthur. Plate Tectonics. Cambridge, Mass.: Geo

Books Publishing, 1998. Windley, Brian F. The Evolving Continents. 3rd ed. Chich-ester, England: John Wiley & Sons, 1995.

Pleistocene The Pleistocene is the older of two epochs of the Quaternary Period, lasting from 1.8 million years ago until 10,000 years ago, at the beginning of the Holocene epoch. Charles Lyell formally proposed the name Pleistocene in 1839, after earlier informal proposals, based on the appearance of species of North Sea mollusks in Mediterranean strata.

The Pleistocene is recognized as an epoch of widespread glaciation, with glaciers advancing through much of Europe and North America and across the southern continents. Glaciers covered about 30 percent of the northern continents, most as huge ice sheets that advanced across Canada, the northern United States, and Eurasia. Smaller alpine glaciers dissected the mountain ranges, forming the glacial landforms visible today, including horns, arĂȘtes, U-shaped valleys, and giant eskers and moraines. Some of the ice sheets were up to two miles (three kilometers) thick, acting as huge bulldozers that removed much of the soil from Canada and scraped the bedrock clean, depositing giant outwash plains in lower latitudes.

The continental ice sheets are known to have advanced and retreated several times during the Pleistocene, based on correlations of moraines, sea surface temperatures deduced from oxygen-isotope analysis of deep-sea cores, and magnetic stratigraphy. Eighteen major glacial expansions and retreats are now recognized from the past 2.4 million years, including four major glacial stages in North America. The Nebraskan glacial maximum peaked at 700,000

Protalus ramparts, dating from a late part of the last major glaciation, along the north base of Sunrise Ridge, northeast of Mount Rainier, Washington. The ramparts form when gravel and other debris fills cracks between the glacier and the canyon wall, leaving the debris behind as an elongate ridge when the glacier melts. (USGS)

years ago, followed by the Kansan, Illinoian, and the Wisconsin maximums. Ice from the Wisconsin glacial maximum retreated from the northern United States and Canada only 11,000 years ago, and it may return in a short amount of geological time.

Many species became extinct or otherwise changed in response to the rapid climate changes during the Pleistocene. Many species lived in the climate zone close to the glacier front, including the woolly mammoth, giant versions of mammals now living in the arctic, rhinoceros, and caribou. Farther from the ice, giant deer, mastodons, dogs, cats, ground sloths, and other mammals were common. Both humans (Homo sapiens) and Neanderthals roamed through Eurasia, but anthropologists do not know the nature of the interaction between these two hominid species. Many of the giant mammals became extinct in the latter part of the Pleistocene, especially between 18,000 and 10,000 years ago. Currently considerable debate exists about the relative roles of climate change and predation by hominids in these extinctions.

See also Neogene; Quaternary; Tertiary.

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