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Continental ice sheets expanded southward from Canada into the Eastern and Central United States ten or more times during the past 2.5 million years. The area covered by ice varied from one glaciation to another. The glaciations were geologic events, and the events were recorded by glacial deposits.

The time interval between 35,000 and 11,150 calendar years ago is referred to informally as "late Wisconsin time." During the late Wisconsin glaciation, the ice sheet margin reached its maximum southern extent in many parts of the United States between 24,700 and 23,600 years ago, and it had retreated into Canada by about 11,400 years ago. The ice sheet margin's fluctuated (retreated and re-advanced) during southward advance and northward retreat so the extents of re-advances and retreats varied in different regions. Consequently, the mapped limit is not the same age everywhere. In some regions, the late Wisconsin glacial limit was formed about 23,600 years ago or earlier. In other regions, an earlier limit was overridden during a later re-advance and the mapped limit represents an overlap of younger glacial deposits over older glacial deposits. In central Iowa, the late Wisconsin glacial limit is only about 16,500 years old.
(Source: United States Department of the Interior: URL: http://nationalatlas.gov/article_glacial.html)

There is no evidence that the Black Hills area was glaciated during theWisconsin glacial maximum. Then nearest glaciated regions were the Bighorn Mountains in Wyoming 155 miles (250 km) to the west, and to the east, the area north of the Missouri River, South Dakota 155 miles (250 km).

A Rare Glimpse

The following is a partial list of invertebrate remains recovered from the Mammoth Site.

Each small fossil provides a rare glimpse of middle Wisconsin environmental conditions in the northern Great Plains and Black Hills southwest of the Laurentide ice sheet. Terrestrial and aquatic varieties of mollusks were recovered during the course of excavation by washing the clay-rich sediment, directly associated with the mammoth remains, through sieves of 1mm mesh.