- 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
The glacier eventually reached its limit when weakening forward movement was just balanced by the rate at which the ice front was melting back, a period of equilibrium that probably lasted for hundreds of years, during which there were only minor fluctuations of the ice front. During this time, the glacier acted as a giant conveyer belt, carrying forward great quantities of boulders, gravel, and clay embedded in the ice, which were exposed and dropped at the melting front of the ice mass. In this way were amassed the great drifts, or moraines, that still trace the forward limit of the glacier.
The most easily identifiable stretch of moraine was laid down in the East, where the glacier stopped with its leading edge a number of miles offshore from the present coast. Much of the tremendous burden it dropped still abides, though considerably worn away by the sea; its largest portions are Long Island, Block Island, Martha’s Vineyard, Nantucket, and Cape Cod. Here, for what comfort it may give some Vermonter or New Hampshireman fighting poverty on a hardscrabble farm, is much of the earth that once clothed his hillsides.
To the west, the ice just buried Manhattan and stopped with an advanced lobe resting on Staten Island. Its line bent to the north in Pennsylvania because the mountains, while unable to stop it, did hold il back somewhat. It reached almost to the Ohio River in Indiana and Ohio, then tended in an irregular line northwestward until it came almost to the Canadian border in North Dakota, and then ran straight westward to the mountains. The western mountains were too high to be overrun, but they proliferated a rich crop of mountain glaciers that joined together into an almost solid icecap which connected with the continental glacier to the east.
There were other glaciers before the one we are concerned with, hundreds of thousands of years earlier. Some of them pushed farther to the south; dim remains of ancient terminal moraines show that ice has reached as far south as Louisville, St. Louis, and Topeka.
After pausing at its farthest-advance line, the glacier began to melt back. It was hardly a precipitous retreat, because a mile-deep ice mass does not fade away in a summer or two. During the period in which it was melting back through New England, the Connecticut Kiver valley was a lake and thus provided a sort of measured course over which we may time the rate of retreat. Each summer a thin layer of sandy sediment settled to the bottom; each winter an even thinner one of fine silt drifted down. Each pair of layers, called a varve, indicates the passage of a year just as plainly as does a ring on a tree. From a painstaking count of these varves, geologists find that it took more than four thousand years for the ice to melt from the present site of Hartford, Connecticut, to that of St. Johnsbury, Vermont, a sluggish average rate of only about one mile in twenty-two years.
But this, like all averages, is misleading because the retreat was not a steady one. Sometimes the ice melted back without interruption and dropped its load of debris in an even layer on the land. At other times the ice front would pause, balanced between its rate of melting and another time of forward movement, so that a ridge or “recession moraine” would be laid down. And occasionally a new forward surge would override and scatter earlier deposits.
So, both coming and going, the ice reshaped the land and left it far different from what it had been. Consider, for example, two of the many rivers it altered and rerouted. At one time the streams that now make up the headwaters of the Ohio flowed into Lake Erie; when the ice dammed the old courses, the water was forced to flow south and west around the glacier front until it found its way to a tributary of the Mississippi anil so formed part of the Ohio River system. In the West, the upper Missouri also followed a different path; it flowed north into Canada until the ice blocked that outlet and made it find a new channel that also led to the Mississippi.
These two river diversions are especially interesting because of their subsequent effects on American history. The first settlers across the Allegheny Mountains found rivers that led toward the setting sun; and they underwent a quick change in economic orientation because all at once it was easier to ship their produce all the way to New Orleans by flatboat than to haul it by wagon a few dozen miles back across the mountains. The Missouri performed an equally important function. It was the only water roule leading from the Mississippi into the western mountains at a point only a short distance across the continental divide from a river that flowed into the Pacific, it thus became the route of Lewis and Clark, and after them the surge of traders and mountain men who crossed to the Columbia River valley and so established the firm American claim to the Pacific Northwest.