- Historic Sites
Footprints Of The Great Ice
The glacier that covered most of North America scarred the land, turned rivers in their courses, and deeply influenced our history
April 1960 | Volume 11, Issue 3
A narrow band of very low, very gentle hills extends across the northern states from Cape Cod to the Rocky Mountains in Montana. In places the winds and rains of thousands of years have worn them down to insignificant undulations; in other places they may be a hundred feet high or more. There is nothing about it to catch the casual eye, but the geologist recognizes this ridge as the terminal moraine of North America’s last continental glacier, the line where the ice ended its long advance and began to melt back.
The glacier that stood on that line would have been a spectacular sight had there been anyone to see it: a great palisade of green and white ice many hundreds of feet high and stretching to the horizon cast and west. For the most part, it probably loomed silently menacing, but from time to time huge sections crumbled off in awesome avalanches. Forests grew, and herds of woolly mammoths and other, less lordly creatures grazed almost up to the ice face, but to the north, atop the glacier, there was only a barren expanse of blizzard-swept ice stretching in absolute desolation toward the Arctic Circle.
The glacier was the last cataclysmic event that helped shape the face of the continent. North of the line of terminal hills, the land was drastically changed. Rock was rasped from mountains till the craggy buttresses were smoothed away and the valleys ground wider and deeper. Soil and gravel were stripped down to bedrock in some places, and the land level dropped as much as hundreds of feet deep in others. Myriads of lakes were gouged into the land, while rivers were dammed, diverted to new beds, and sometimes even shunted bodily from one drainage system to another. The effects of the ice sheet are not only still very apparent, they are often so marked as to have influenced the course of our history and helped to shape the economic pattern of the northern part of the continent. They have even had a strong effect in molding the character of some of our people.
If that last statement sounds extreme, consider that most individualistic type, the New England Yankee, and note well his relationship to his land. When the ice sheet ground its way across New England, it scoured off most of the soil down to the granite ribs of the land—and when it melted back much later, it dropped millions of boulders over the landscape to make the region even less promising. It is not exactly a fat and hospitable land, and the people who elected to live there have had to spend their lives contending with it for a living in a battle so close that they could not help but absorb some of the flintiness of their own fields.
If the New England rocks yielded a scant living, they were an excellent hone against which the wits of the men who lived among them were sharpened to a fine edge, until “Yankee ingenuity” came to denote the ultimate in the ability to improvise and to get much out of little. At the same time, the streams that foamed through the uneven, glaciated hills provided abundant water power to put that ingenuity to work. As a result New England was, for a long time, the industrial heart of the nation.
What was the nature of this phenomenon that sent unbelievable quantities of ice grinding out of the north until more than half of North America was buried a mile or two deep? Contrary to popular belief, the ice did not form around the North Pole and then flow southward. It formed in a number of centers—Canada, Greenland, Europe, Siberia—more or less simultaneously, and spread from each of those places. Nor was the glacial era a period of unusual cold; the essential for a glacier’s formation is only that more snow shall fall during the winter than can melt in summer.
And times have come when the winter snow has outlasted the summer sun, not just for a few seasons but for thousands of years, piling up inch by inch, foot by foot, and packing into ice under its own weight until it came to be probably two miles or more thick at the center and hundreds of miles across. It was a burden so massive that it pushed the rock foundations of the land down into the molten magma on which they float. In many places the land is still springing back as the displaced magma flows back in currents of unbelievable ponderousness. The land in the Hudson Bay region, where the ice first started to accumulate and disappeared last, still has a long way to rise; it is estimated that eventually the bottom of the bay will emerge as dry land-provided another glacier does not come first.
As the ice became thousands of feet deep, the enormous pressure began to force the great mass outward at the edges. Meanwhile, the unending snows kept falling and turning to ice, always maintaining the pressure and keeping the glacier pushing forward, grinding rocks to pebbles and pebbles to clay and sand, and moving incredible quantities of rock and earth over the face of the land.