If we accept that an ice age is a geological epoch during which a much greater part of the earth's surface was covered with ice than is now the case, then we can say that during the last 1.7 million years -- the Pleistocene Epoch of time -- four ice ages have occurred. Each of these ice ages produced continent-size glaciers. The first and oldest ice sheet is known as the Nebraskan, then came the Kansan, then the Illinoian, and the last one was the Wisconsin. A good place on the Web to start learning all about the Pleistocene is at the Pleistocene page of the Berkely University web site.
Dating the three older ice ages has not been easy, since after about 70,000 years so little Carbon-14, or C14, remains in fossils that results become questionable. The earlier three glaciers occurred before 70,000 years ago. However, the last glacial period, the Wisconsin, occurred during a time range in which C14 dating works well. The thing is, the vast majority of loess deposited in the lower Mississippi Valley was deposited in association with the Wisconsin ice sheet.
Geologists have divided the Wisconsin glacial epoch into subdivisions, each with its own name. C14 dating correlates loess in the lower Mississippi Valley with a loess type known as Peoria Loess in the upper Mississippi Valley, where glaciation has been studied longer and in more detail than with us. Our Peoria Loess is between approximately 17,850 years and 22,000 years old. In some places in our area, an older loess, named Farmdale Loess, is found beneath the Peoria. Farmdale is older than about 22,000 years. Both of these loess units appeared during Wisconsin times. There may be even earlier loess units in our area, but they have not been clearly identified. According to the Millsaps College team, most Mississippi-loess deposits are more than 90% Peoria loess.
The glacier associated with the Wisconsin ice age began melting about 20,000 years ago. At its greatest penetration south, its southern boundary passed through central Ohio, south-central Indiana, central Illinois, and central Iowa. Its retreat was slow, not withdrawing from the Great Lakes region until about 10,000 years ago.
It's easy to believe that weather in the Lower Mississippi Valley was much different 20,000 years ago than it is now. In fact, we have proof that such was the case. Fossils of an impressive number of prehistoric animal species have been found beneath the loess in and around Natchez, Mississippi, and other places. You can view a number of such fossil remains on our Fossils Page.
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