222 Frank Canieij 



ment of the glacier, the great field of ice in the rear uses the valley 

 tongue as an outlet, and consequently lines of movement con- 

 verge towards the axis of this tongue. Such a tongue, or depend- 

 ency, becomes a strong erosive agent, provided it is carrying 

 the proper tools. In a similar manner, but in a slighter degree, 

 a lobe of ice recewes the onward impulse of a greater area in its 

 rear; but the lobe is larger and is not occupying so confined a 

 basin, consequently the erosive work done is not comparable to 

 that accomplished in narrower valleys. Through the work of an 

 ice lobe, a shallow basin like that of Lake Erie may easily be 

 produced; under the concentrated work of a valley tongue, 

 the deep Lochs of Scotland and the grossly overdeepened Ticino 

 valley of Italy are natural. 



While ice erosion was a factor in developing the basin of Lake 

 Erie, at the same time it is apparent that land tilting has had 

 something to do with the present size of this lake. In its western 

 end the lake is shallow; if the eastern end, at the mouth of the 

 Niagara River, were depressed 10 feet, the lake counties of the 

 state, west of Huron River, would have their areas increased. 

 Land tilting in postglacial times has tended to drown the western 

 end of this basin. There was one period, since the ice retreated, 

 when the lake was only a small fraction of its present size; indeed, 

 according to some investigators, the basin held no lake, only a 

 river. The obvious effect of the postglacial tilting, in increasing 

 the area of a lake in the Erie basin, in nowise detracts from the 

 part taken by ice erosion in making the basin. To be assured 

 that glacial erosion operated here we have only to recall the evi- 

 dences of it on the islands about Sandusky; the limestone resisted 

 the corrasion of ice more than did the neighboring shales. 



To what extent glacial erosion has been a factor in the develop- 

 ment of the basins of the other Great Lakes cannot be proved. 

 The amount of erosion, if the glacier made their present depths, 

 is varied. Lake Superior is the deepest, but ice erosion is invoked 

 less in accounting for its basin than in the case of the other lakes. 

 Lake Michigan occupies a rock basin; its maximum depth, 870 

 feet, at first thought, would indicate a great deal of erosion. When, 

 however, we consider its length and the width of the basin, as it 

 is brought out in a cross section, with the same horizontal and 

 vertical scales, this depth does not appear so great. Lake Huron, 

 having a maximum depth of 702 feet is also in a rock basin. 



