maximum extent of Pleistocene glaciation in North AmericaIf 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. The glacier associated with the last ice age, the Wisconsin, began melting about 20,000 years ago. Its retreat was slow, not withdrawing from the Great Lakes region until about 10,000 years ago.The map above shows the maximum extent of all four glaciers into the US.

North America's surface geology has been profoundly altered in many places by glaciers. In some places glaciers plowed away soil leaving bare rock with long scratches showing which way the glaciers moved. In other places the big ice sheets dumped piles of jumbled dirt, sand and gravel. When the ice sheets melted, vast outwash plains formed. I live in southern Mississippi and even here Ice-Age events profoundly altered the landscape because of a special kind of dust, called loess, that was deposited here at the end of the Ice Age, in depths of up to 200 feet along the Mississippi River. If we want to interpret the surface geology of our local areas, most of us simply can't ignore the Ice Age! You may be interested in a Web site I produce just about The Loess Hills of the Lower Mississippi Valley.

If you live in an area affected by glaciers, you may be able to find some absolutely fascinating land features to interpret. There are ground moraines, end moraines, drumlins, kettle lakes, eskers, kames and kame terraces, and many other obscure and interesting features. If you live close to the Great Lakes, New York's Finger Lakes or Long Island, or the Ohio River, you should learn the details of how these were formed. A good place on the Web to start learning all about the Ice Age, also known as the Pleistocene, is at the Pleistocene page of the Berkeley University web site.