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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." 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.
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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.
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