Article

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." Late Wisconsin glaciation occurred during late Wisconsin time. During 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 late Wisconsin glacial limit on the map of surficial deposits and materials is the southernmost extent of glacial deposits attributed to the late Wisconsin glaciation. The ice sheet margin fluctuated (retreated and readvanced) during net southward advance and net northward retreat, and the extents of readvances 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 readvance 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.

The time interval between 310,000 and 128,000 years ago is referred to informally as "Illinoian time." Illinoian glaciation in the Eastern and Central United States occurred during Illinoian time. The Illinoian glacial limit on the map of surficial deposits and materials is the southernmost extent of glacial deposits attributed to Illinoian glaciation. The Illinoian glacial limit is not everywhere the same age. Illinoian glaciation was represented by fluctuations of the ice sheet margin, and the glacial limit in some regions is younger than the limit in other regions.

The age of pre-late Wisconsin glacial deposits in some regions is conjectural. In those regions, the southern limit of the pre-Wisconsin deposits is referred to as either Illinoian or pre-Illinoian in age, implying only that the limit and the associated glacial deposits are older than 128,000 years.

The time prior to Illinoian time (more than 310,000 years age) is referred to informally as "pre-Illinoian time." Several pre-Illinoian glaciations occurred during pre-Illinoian time. The pre-Illinoian glacial limit on the map of surficial deposits and materials delineates the southernmost extent of glacial deposits of pre-Illinoian age, without reference to specific glaciations.