Glacial lakes may be divided into three classes. Those found in the "kettle holes" of the terminal or medial moraines, and those that are formed by the deposition of the glacial drift, as, for instance, Devil's Lake, and those that are caused by ice forming dams across the valley of a river that lasted only during the ice age. In some lakes of the second class erosion undoubtedly entered into their formation as well as the piling up of glacial drift. In order, however, that we may understand more fully the formation of these greater lakes it will be necessary for us to go back and examine the conditions that seem to have existed before the glacial period. It is a fact well known that continents have periods of elevation and depression. There is abundant evidence that the northern portion of the North American continent was elevated to a much higher level in preglacial times than it occupies now. This is evidenced in very many ways by sounding the depths of old river beds now filled with glacial débris. The old beds show unmistakable evidences of having been worn down to their present level by the action of running water. They also prove to be many feet below the present sea-level. This fact seems to be sufficient to prove the theory of a higher elevation of the North American continent in preglacial times. It should be said here that undoubtedly the constant filling up of the ocean with the drift carried down by the rivers has somewhat raised its level, but hardly to the extent indicated by the old river beds. The question naturally arises, Where did all the dirt come from to fill up these great river beds and change the whole topography of the northern half of the continent? Dr. Wright estimates that there is not less than 1,000,000 square miles of territory in North America covered with glacial débris to an average depth of 50 feet. Of course, the depth varies in different places from a few inches to several hundred feet. Of the carrying power of these great glaciers we will speak more fully in a future chapter. In preglacial times the watershed of the Mississippi and of the great rivers east of the Alleghany Mountains, the Susquehanna and Hudson, extended probably farther north than it does to-day. The larger portion of the drainage area that now finds an outlet through the River St. Lawrence at one time undoubtedly drained off through the Mississippi Valley into the Gulf and the Valley of the Mohawk into that of the Hudson.