Footprints Of The Great Ice


Although the Great Lakes are the largest glacier-created bodies of water, there are countless other lakes from the Dakotas to Maine and north into Canada. Minnesota alone has many more than enough to justify its tourist-attracting slogan of “Land of Ten Thousand Lakes,” while in Canada they are so numerous and interconnected as to be virtually uncountable, since it is a matter of individual judgment where one ends and another begins.

These are the waters that bore the bark canoes of the Chippewas and Crees, and later of the coureurs de bois. The forests around them have provided many fortunes in fur and timber; a very respectable amount of pelts and lumber still come from among them, but today the lakes are producing an additional bonanza: recreation dollars. The lakes are all creatures of the ice sheet, some gouged out of the earth or even out of rock, others created by glacier-built dams, but most simply formed when ice water filled the hollows and valleys in the churned-up mass of glacial debris.


But the greatest of the glacial lakes is long since gone. Geologists call it Lake Agassiz, after the Swiss-American who pioneered in advancing, and securing acceptance for, the ice-age theory; at its greatest extent it filled the valley of the Red River of the North in Minnesota and North Dakota and covered most of Manitoba—an inland sea larger than all the present Great Lakes combined. It formed when the retreating glacier left the area clear, while still blocking the normal northward drainage of the Red River; the resulting lake grew larger as the ice melted back, until the time came when the glacier withdrew so far that the water could rush out into Hudson Bay.

The only remnants of Lake Agassiz are Lake Winnipeg and some swamps and smaller lakes, but its ancient bed is easy to identify—and valuable as farmland. When the lake lay over the land, the silt brought into it by wind and water slowly settled, adding only a fraction of an inch each year to its bed but continuing century after century until the lake bottom was covered dozens of feet deep and every irregularity was hidden. When the water went at last, it revealed a plain as flat as a floor and stretching to the horizon in every direction. It is a land almost without trees and one completely monotonous to anyone raised among even modest hills, but it is a joy to the farmers who work its deep, rich, stoneless soil.

The effects of the ice on life were profound. It completely annihilated everything unable to get out of its slow-moving way, and when it retreated it left a lifeless desert. However, it is very doubtful that any large expanse of barren debris was exposed at any one time by the melting ice because plants can spread with surprising speed even on unlikely soil, and the retreat of the glacier, only a fraction of a mile a year, was no faster than most plants can follow.

The large mammals associated with the ice age have disappeared, some long ago, a few outlasting all four glaciations. The Kodiak bear of Alaska is a splendid survival but the rest are gone.

One of the most curious of the survivals is an insect, the White Mountain butterfly. It is a creature that can exist only in a cold climate and nourishes today in Labrador. It moved south ahead of the glacier and followed it back north as it melted, all except some which sought refuge from the warming climate by fluttering up mountainsides. Two such colonies survive in the United States. One found a safe haven on the chilly slopes of Mount Washington, New England’s highest peak; the other lives on a mountain in Colorado.

Probably the greatest of all effects of the last glacier was that it brought man to North America. Until that time, the Western hemisphere had been, as far as all evidence available to us indicates, empty of humans; with so much water tied up in the icecaps, ocean levels dropped three to four hundred feet to make the Bering Strait a dry and easy passage for nomads from Asia. The first comers very likely were restricted to the Arctic at first, prevented by the ice from moving south. Strange though it may seem, the northern two thirds of Alaska was free of ice except on the mountains, while at the same time the Arctic Ocean was unfrozen. Though hardly balmy, the region was much milder than it is today, and artifacts found along the northern seashore indicate that man tarried there until the glaciation had passed its peak and a way had opened to the south.

But once started, the immigrants came fast. Charred bones of an extinct bison, associated with arrowheads and indicating a feast after a successful hunt, have been found in Clovis, New Mexico; radioactive carbon dating gives their age as approximately 9,900 years. Much more surprising was the debris from another site, the burned bones of a sloth and an extinct horse uncovered near the Strait of Magellan, about as far from the Bering Strait as it is possible to get. Carbon 14 dating established that this meal was eaten nine thousand years ago. The glacier reached its southern limit eleven thousand years ago, and if we accept the idea that man did not find a way south until somewhat later, it means that human beings made their way over rivers and mountains and through jungles from Alaska to the limits of South America in less than two thousand years.