16 LAKES OF NORTH AJSIERICA. 



The lakes occupying depressions on the glacial drift number hundreds of 

 thousands. They vary in size from mere tarns up to sj)lendid water- 

 sheets many square miles in area. In portions of Minnesota, Michigan, 

 and adjacent areas, where the drift is unusually deep, the lakes in irregu- 

 lar depressions on its surface sometimes number a score or more to the 

 square mile. It is estimated that in Minnesota alone, there are not less 

 than ten thoiisand lakes of this class, besides many swamps and marshes 

 marking the sites of former lakes of the same type, which have become 

 choked wdth vegetation. 



Numerous lakes of the same character as those on the drift of the North- 

 eastern States and Canada, occur about the southern margin of Malaspina 

 glacier, Alaska, in depressions in moraines left by the retreat of the ice 

 within the past few years. These very modern basins, some of which are 

 still occupied in part by the ice of the retreating glaciers, are similar in every 

 way to the basins on the moraine-covered surfaces just referred to, and are 

 surrounded by topography of the same character, thus leaving no room for 

 doubting that each of the two series is due to similar agencies. 



When the general sheet of debris left after the retreat of continental 

 glaciers does not completely mask the pre-glacial topography, former 

 valleys are sometimes dammed, and lakes of another type produced. In 

 many instances these lakes are long and narrow, and indicate, to some 

 extent, by their form, the character of the ancient drainage lines they 

 occupy. Again, they may be broad water-bodies, and occupy ancient 

 drainage basins, the outlets of which have been closed. Pre-glacial 

 valleys may be deepened by ice erosions, as well as obstructed, and the 

 two processes may unite to form lakes, as is believed to have been the 

 case in the group of " Finger lakes " in the central part of New York state. ^ 



Still another type of lake basins, due to glacial agencies, is found in 

 unconsolidated water-laid material deposited about the borders of ice- 

 sheets. When the stream-borne debris from a glacier is abundant it 

 forms low alluvial cones and sand and gravel plains, which may surround 

 or cover isolated ice masses. When such buried ice masses finally melt 

 a depression is left, and may be water-filled. The borders of such lakes 

 are of loose material which slides into the depression and forms steep 

 banks. The inclination of the enclosing walls depends upon the nature 

 of the material of which they are composed. Broad tracts of sand and 



1 A. P. Brigham, "The Finger lakes of New York," Geographical Soc. Am., Bull., vol. 

 25, 1893. R. S. Tarr, "Lake Cayuga a rock basin," Geological Soc. Am., Bull., vol. 5, 189-1, 

 pp. 339-356. 



