Geology: The Science of the Earth's Crust

by William J. Miller

Even a cursory examination of a large map of the world reveals the fact that the regions of most numerous lakes are those which were recently occupied by glaciers—either the vast ice sheets of the Glacial epoch or mountain (or valley) glaciers. This is because more lakes of the present time have come into existence as direct or indirect results of glaciation than by any other cause. A considerable number of these lakes occupy rock basins which have been eroded or excavated by the direct action of flowing ice. Small lakes of this sort are commonly found in the upper parts of valleys formerly occupied by mountain or so-called Alpine glaciers, because there the excavating power of such glaciers was especially effective. More rarely rock basins have been scoured out by glaciers farther down their valleys. Many lakes occupy rock basins excavated by ice in the high Sierra Nevada and Cascade Ranges, in the Rocky Mountains from Colorado into Canada, in the Alps, and in the mountains of Norway. Few, if any of them are, however, large or famous. Other lakes, some of very considerable size, occupy rock basins scoured out by the passage of the great ice sheets of the Glacial epoch in North America and Europe, though they are less common than formerly supposed. Some of the many lake basins of Ontario, Canada, are quite certainly of this origin, as might well be expected, because the power of the great ice sheet was there in general notably greater than south of the Great Lakes where the tendency was to unload or deposit the eroded materials as shown by the great accumulations of glacial débris (moraines).

Where the ice walls of certain existing glaciers form dams across valleys, waters are ponded, a small lake of this kind occurring alongside the Great Aletsch Glacier of the Alps, where its wall is slowly moving past a tributary valley. Lakes of this kind also occur in Greenland and in Alaska, but none are of considerable size. During the Great Ice Age, however, literally thousands of large and small lakes were formed, both during the advance and the retreat of the ice, wherever the glacier wall blocked valleys which sloped downward toward the ice. New York State furnishes many fine examples of large and small lakes of this sort. Thus, when the great glacier was melting in northern New York, waters hundreds of feet deep and many miles long were ponded between two ice lobes—one retreating eastward and the other westward from the Mohawk Valley. An ice dam lake was also formed a little later, when an ice wall blocked the northern part of the Black River Valley just west of the Adirondack Mountains and caused a lake covering about 200 square miles. One of the largest of all known ice dam lakes has been called Lake Agassiz, which attained a maximum length of over 700 miles and a width of 250 miles in the Red River of the North region of eastern North Dakota, western and northwestern Minnesota, and northward into Canada, most of its area having been in Canada. It began as a small lake with southward drainage into the Mississippi when the great northward retreating ice sheet formed a dam across the valley of the Red River of the North. The retreating ice continued to block the northward drainage until the vast lake, covering a greater territory than all of the present Great Lakes combined, was developed. Beaches, bars, deltas and the outflow channel of this remarkable lake are wonderfully well preserved. Lake Winnipeg is a mere remnant of great Lake Agassiz.

Many ponds and small lakes occupy basins formed by irregular accumulations of glacial (morainic) materials. Still others lie in depressions which formed by the melting of masses of ice which became wholly or partly buried by ice deposits, or by sediments washed into bodies of water which were held up by ice dams. Depressions of the latter kind are commonly found as pits or so-called “kettle holes” below the general level of sand flats or sand plains of glacial lake origin.

Most common of all lake basins of glacial origin are those formed by accumulation of glacial débris or morainic materials acting as natural dams across valleys. This is, in fact, the most common of all ways by which existing lake basins, some of them very large, have been formed. Most of the thousands of ponds and lakes of Minnesota, Wisconsin, and northern New York belong in this category.

In the Adirondack Mountains, for example, most of the lakes, like the well-known Lake Placid, Saranac Lakes, Long Lake, and Schroon Lake, have their waters ponded by single dams of glacial débris across valleys. In some cases a series of such dams blockades a valley and forms a chain of lakes like the well-known Fulton Chain in the Adirondacks. Less commonly the lake may have its waters ponded by two natural dams of glacial débris, one across a valley at each end of a lake. A very fine, large scale example of the last-named type is the famous Lake George in the southeastern Adirondacks. It is over 30 miles long and from 1 to 2¹/₂ miles wide. It lies in the bottom of a deep, narrow mountain valley, mountain sides rising very steeply from a few hundred feet to 2,000 or more feet above its shores. There are many islands, especially in the so-called “Narrows,” thus greatly enhancing the scenic effect. The valley itself has been produced by a combination of faulting and erosion. There was a preglacial stream divide at the present location of the “Narrows.” This divide was somewhat reduced by ice erosion when the deep, narrow body of ice plowed its way through the valley during the Ice Age. During the retreat of the ice heavy morainic accumulations were left as dams across the valley at each end of the lake.

Another remarkable body of water, similar to Lake George in its origin, is Chautauqua Lake of western New York, famous for its Chautauqua assemblies. It lies 1,338 feet above sea level, with its northern end near the edge of the steep front of the plateau overlooking Lake Erie. Chautauqua Lake really consists of parts of two valleys, one sloping north and the other sloping south, each dammed by glacial deposits.

The famous Alpine lakes—Garda, Como, and Maggiore—have resulted from deposition of glacial morainic materials under conditions different from those above described. In these cases great mountain or valley glaciers once flowed down the valleys and spread out part way upon the Italian plain. Great accumulations of glacial débris took place around the borders of the glacier lobes, and, after retreat of the ice, the glacial deposits acted as dams ponding the waters far back into the mountain valleys.

The origin and history of the Great Lakes constitutes one of the most interesting and remarkable chapters in the recent geological history of North America. Most of the salient points have been well worked out and they may be very briefly summarized, as follows: Before the Ice Age the Great Lakes did not exist, because the region, prior to that time, had been land subjected to erosion for millions of years—a time altogether too long for any lake to survive. Their sites were occupied by broad, low, stream-cut valleys which were quite certainly locally somewhat deepened by ice erosion during the Ice Age. Ice erosion is, however, altogether insufficient to account for the great closed basins. The two most important factors entering into the formation of the basins of the Great Lakes were doubtless the great glacial (morainic) accumulations acting as dams along the south side, and the tilting of the land downward on the north side of the region. In support of this explanation it has been established that the great dumping ground of ice-transported materials from the north was in general along the southern side of the Great Lakes and southward. It has also been well established that, late in the Ice Age, the land on the southern side of the Great Lakes region was lower than at present, as proved by the tilted character of beaches of the well-known extinct glacial lakes which were the ancestors of the present lakes. Such a down-warp of the land must have helped to form the closed basins by tending to stop the southward and southwestward drainage of the region.