296 AISTNUAL EEPORT SMITHSONIAN INSTITUTION, 1912. 



basins; and, fourth, by differential elevation of the land during and 

 after the ice occupation, the maximum elevation occurring in the 

 north and producing several changes of outlet. 



The combmed effect of these and other less important factors pro- 

 duced a complex history, not only as expressed by the distribution 

 and relations of the various drift forms, but also by the remarkable 

 effects of the ice sheet upon the associated drainage. It determined 

 the location of great rivers which flowed only temporarily from the 

 ice or along its border and produced remarkable shiftmgs of theu' 

 courses. Its most noteworthy effect, however, was the production of 

 a complex succession of shiftmg and changing lakes — enJarging, fall- 

 ing, shrmking, combinmg, dividmg, and rismg lakes — of large extent, 

 with frequent changes of outlet, and commg at last to the lakes as we 

 find them to-day. As investigation has inclined more and more to 

 details and has covered an area of increasing extent, it has been found 

 that the succession of changes involved in the later lake history is 

 much more complex than was formerly supposed. 



THE SHRINKAGE OF THE ICE SHEET INTO THE LAKE BASINS. 



In one of its earlier epochs (the Illinoian) the ice sheet covered the 

 entire region of the Great Lakes, the only exception being the well- 

 known driftless area which lies chiefly in western V/isconsm. This 

 area is in the angle between Lake IMichigan and Lake Superior, but 

 does not comprise any part of the dramage basin of either one of them. 

 As the ice sheet moved southward the lake basins naturally offered 

 the easiest lines of flow and the high lands between the basins were 

 areas of greater resistance and slower flow. But when the ice 

 attamed its maximum, reaching nearly to Cairo, 111., some 20 miles 

 across the Ohio River at Cincinnati, and to Beaver Falls, Pa., the 

 lake basins became relatively unimportant m their effect upon the 

 ice movement, for at that time the ice overwhelmed them ail, mclud- 

 ing even the high lands between them. As it retreated, however, 

 the relative importance of the lake basins m controlling the ice flow 

 increased rapidly and by the time the ice front had withdrawn to the 

 most southerly pomts of the watershed of the lake basms it had 

 taken on lobate forms of a most pronounced type. In the latest or 

 Wisconsin epoch of glaciation the farthest extension of the ice did not 

 reach so far south in the region west of central Ohio as it had before. 

 In Illinois it reached only about half way from the shore of Lake 

 ]\Iichigan to the IVlississippi and Ohio Rivers. Beyond this the older 

 drift shows now only occasional ridgings suggestmg terminal morahies. 

 The great moraine system wliich is conspicuously related to the basins 

 of the Great Lakes belongs to the Wisconsin or latest glaciation. As 

 the ice drew back, each lake basm, at the time of most pronounced 

 lobation, had its ice lobe which conformed to the outlmes of its 



