Footprints Of The Great Ice


The last continental glacier is only the latest of four which came and went during the Pleistocene epoch. Geologists sometimes refer to the era as the “last ice age” in recognition that there were other—and more severe—periods of glaciation back through the dim mists of time. The first of the four “recent” glaciations, called the Nebraskan, came an estimated one million years ago, and was followed in turn by the Kansan, Illinoian, and finally, the Wisconsin glaciations. Geologists have named them from the areas where their traces were most clearly identified and studied; the glaciers that occurred at the same time in Europe have completely different appellations. Between the times of ice, there were long interglacial periods when the climate grew mild, and most of the work of the glaciers was erased by a hundred thousand or more years of erosion.

Although scientists have generally agreed on a chronology of the comings and goings of ice, they are the first to admit that the dates are only the best possible estimates. But about one event there is little guesswork; we know almost exactly when the last glacier reached its southern limit. It so happened that a final surge of the ice overran a spruce forest near what is now Two Creeks, Wisconsin, snapping off the big trees and burying them deep under sand and gravel. Nuclear physicists, by a method recently discovered, are able to determine quite accurately the age of a bit of organic matter by finding what degree of radioactivity still remains in the carbon it contains. Complex Geiger counter tests of Two Creeks spruce proved that the forest had died approximately eleven thousand years ago, give or take a century or two. It was a result that confounded many scientists who had placed the peak of the glaciation at least twenty-five thousand years ago, but subsequent radioactive carbon datings from many sources have since corroborated the readings from the Two Creeks wood.

Eleven thousand years is not a long time, geologically speaking. The ice-torn land has mellowed in that time, but the softening is only superficial and our northern landscape is still basically a glaciated one. Just as the ice left the altered Ohio and Missouri rivers to show it had passed that way, it also produced many other monuments to its action.

The most spectacular of these is the Great Lakes. All four glaciers played a part in producing them, each one scooping the basins a bit deeper, until the last, the Wisconsin glacier, left their beds pretty much as they are today. The Great Lakes are the earth’s largest body of fresh water and comprise its greatest inland waterway. From the days of the French explorers they have been an avenue into the heart of the land, one which has become even more important with the opening of the St. Lawrence Seaway. It is the further good fortune of America that the lakes are rimmed with rich deposits of iron and coal, which have been laced together with cheap transportation to form the greatest industrial complex in the world.

When it formed the Great Lakes, the glacier also increased their usefulness by providing good access to them to supplement the St. Lawrence outlet. As it melted back, the glacier not only filled the lake basins but also formed an enormous dam to prevent excess water from draining across the lowlands to the north. The overbrimming water had only one place to go; it burst through to the south in several mighty torrents.

One of these was the great river that tore a channel from Lake Erie to the Hudson River. The Mohawk River now flows in the eastern part of the valley, which has been a strategic pathway between Lake Erie and the Hudson ever since the French and English started squabbling over North America. Its most important period, however, began in 1825 with the opening of the Erie Canal, which was dug in the bottom of the old glacial watercourse and brought a rush of settlers into western New York and onward into the Northwest Territory.

Another overflow channel left Lake Erie at the point where Toledo now stands, and ran into the Ohio. The Maumee and Wabash rivers now follow the same valley, which became the route of the almost forgotten Wabash and Erie Canal, between Toledo and Evansville, Indiana, whose 452 miles made it the longest in the country. Although a financial failure, it had a big part in opening northern Ohio and Indiana to settlement and development.

Still another outlet ran across the site of Chicago and, via the present courses of the Des Plaines and Illinois rivers, to the Mississippi. Between 1836 and 1848 the Illinois and Michigan Canal was dug in this valley to tie Lake Michigan to the Illinois River, and thereby to the Mississippi. Another waterway, usually called the Drainage Canal, was constructed much later along part of the same route to do double duty as a carrier of both Chicago’s sewage and water commerce to the Mississippi, but it did not supersede the old canal, which is still in use, a very rare survival of the heyday of canal building, (The Erie Canal, or its successor, the New York Barge Canal, cannot make the same claim because it was completely rebuilt and much of it relocated during this century.)