Geography of Ohio 221 



ing this theory to the Great Lakes, it is found that Superior occu- 

 pies such a syncUnal basin; but it is generally understood that 

 the other Great Lakes do not occupy warped basins. 



Glacial erosion. There are verj^ few geologists to-day who do 

 not believe in the potency of glaciers in eroding rock basins. 

 Basins thus produced, how^ever, are almost alwaj's over-deepened 

 portions of preglacial valleys. Concentrated ice erosion, in a por- 

 tion of such a valley, is the explanation offered for the stupendous 

 deepening noted in a few localities. Examples of basins thus pro- 

 duced are Lago Maggiore and other lakes about the Alps area; 

 man J' of the Lochs of Scotland ; Lake Chelan and a few others in 

 the Rocky Mountain region; and part of the Finger Lakes of 

 New York state. It is noted that all of these are long narrow 

 lakes quite unlike the Great Lakes. Nevertheless some geologists 

 have advocated the same theory for the origin of part, at least, of 

 the Great Lakes. Some years before ice erosion was invoked to 

 account for deepened valleys, J. S. Newberry, then connected with 

 the Ohio Geological Survey,'" urged that the rock of the Erie 

 basin was easily carved by ice and had been sufficiently basined 

 to account for the lake. His suggestion was given very little 

 credence at the time. 



When we consider the shape of some of these lake basins, par- 

 ticularly Lake Erie, it does not recjuire very much basining 

 of the shales to produce them. The average depth of Lake Erie 

 is only 70 feet and its greatest depth 204 feet. Cross sections of 

 this basin show how shallow it is, relative to its area. It does not 

 seem at all improbable that the Erie-ice-lobe sufficiently eroded 

 the fissile shales beneath it to make this shallow basin. 



The erosive work of the continental glacier was accomplished 

 almost entirely by the lobes and dependencies along its margin. 

 Back from the front, where the ice was deep, it is not likely that 

 the subjacent rock was very effectively abraded; but at the front, 

 where a lobe or tongue, shod with tools, continued for centuries 

 to wear and grind the rock beneath, some efTect was certainly 

 produced. Furthermore, these marginal forms of the ice sheet 

 received the concentrated motion of the great ice mass in the rear; 

 if the ice is a small lobe or tongue fitting into a valle}^, the side 

 walls and boundary divides of which tend to obstruct the move- 



»» Vol. i, (1873), p. 49. 



