Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Pleistocene
Contents
Overview
Defining and Dating the Pleistocene Boundary
Pleistocene Glacial Events
Pleistocene Ecosystems and Extinctions
Human Evolution during the Pleistocene
Evidence
Skeleton of the woolly mammoth
learn more
sample
Walrus
Saber-toothed cat
Human ancestor
references and links
Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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Glossary Credits Email Us


Pleistocene Glacial Events
The Pleistocene geological record gives evidence of 20 cycles of advancing and retreating continental glaciers, though during most of the Pleistocene glaciers were far more extensive than they are today. Much of this glaciation occurred at high latitudes and high altitudes, especially in the Northern Hemisphere. Up to 30% of the Earth's surface was glaciated periodically during the Pleistocene. Large portions of Europe, North America (including Greenland), South America, all of Antarctica, and small sections of Asia were entirely covered by ice. In North America during the peak of the Wisconsinan glaciation approximately 18,000 years ago, there were two massive yet independent ice sheets. Both the eastern Laurentide and the western Cordilleran ice sheets were over 3900 meters thick. In Europe, ice covered Scandinavia, extended south and east across Germany and western Russia, and southwest to the British Isles. Another ice sheet covered most of Siberia. In South America, Patagonia and the southern Andes mountains were beneath part of the Antarctic ice sheet. Because so much water was taken up as ice, global sea level dropped approximately 140 meters.

The causes of the Pleistocene cycle of glacial and interglacial episodes are still being debated. It appears that continental positions, oceanic circulation, solar-energy fluctuations, and Earth's orbital cycles combined to generate these glacial conditions, so perhaps it is inappropriate to pinpoint any single cause. Some scientists have calculated that changes in the concentration of greenhouse gases were a partial reason for large (5-7° C) global temperature swings between the ice ages and interglacial periods.

Two scientists greatly influenced how Pleistocene glaciations were interpreted. In the 1800s, geologists were studying widespread surface deposits called diluvium. This archaic term referred to deposits that could not be explained by the normal action of rivers and seas, but instead were believed to have been produced by extraordinary floods of vast extent. Louis Agassiz, a Swiss geologist who initially worked on fossil fish, demonstrated that diluvium was actually a ground moraine formed by continental glaciation. The other influential figure, the Yugoslav mathematician M. Milankovitch, showed that variation in Earth's orbital motions could explain periodic climate changes, including continental glaciation.

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Overview | Defining and Dating the Pleistocene Boundary | Pleistocene Glacial Events
Pleistocene Ecosystems and Extinctions | Human Evolution during the Pleistocene



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