PREGLACIAL DRAINAGE. 303 



they had formerly occupied. Thus it is that the glaciated 

 region became again a region of waterfalls. Almost every 

 stream entering Lake Erie from the south exhibits waterfalls 

 produced in this manner. In Minnesota the falls of St. 

 Anthony, at Minneapolis, and of Minnehaha, a few miles be- 

 low, were thus produced, and, so, are post-glacial in their 

 origin, the ancient channels having been filled with glacial 

 debris. 



The falls of Niagara are due to the same cause. The pre- 

 glacial outlet to Lake Erie was dammed up and buried by 

 glacial debris, so that the water was compelled to seek 

 another channel. Before the Ice age there was no Niagara 

 River, and Lake Erie is, in fact, but a glacial mill-pond. 

 The falls of the Genesee at Rochester are also clearly due to 

 the same cause. The preglacial valley in the Genesee is now 

 deeply buried. " Between Mount Morris and Rochester the 

 river follows its preglacial valley, but flows for much of this 

 distance on an alluvial plain that closely resembles the filling 

 of an old lake ; above and below the limits named the river 

 has cut a new channel since glacial times, giving some of the 

 best natural sections in the State, and its old course is choked 

 with drift." * 



The falls of the Mohawk at Cohoes, also, doubtless indi- 

 cate the existence of a deeply buried channel somewhere in 

 the vicinity, connecting, as we shall see a little later, the 

 basin of Lake Ontario with the Hudson. 



The evidence that the preglacial outlet of Lake Erie was 

 much lower than its present outlet, lies in the fact that sev- 

 eral rivers now entering the lake from the south flow at a 

 level two hundred feet above that formerly occupied by them, 

 since that distance has been penetrated beneath their present 

 bottom before reaching rock. From various borings at Cleve- 

 land it appears that rock bottom at the mouth of the Cuy- 

 ahoga lies more than 500 feet below the level of the lake, 

 and is at a depth of at least 200 feet twenty miles in land. 



*See Professor W. M. Davis, in the ''Proceedings of the Boston 

 Society of Natural History," vol. xxi, p. 359. 



