PREGLACIAL DRAINAGE. 307 



elusion. At Little Falls, a spur of archaean rocks projects 

 from the Adirondack Mountains and effectually separates 

 the preglacial drainage of the western part of the state from 

 that of the eastern. Professor Newberry had supposed that 

 there was room for a deep preglacial channel to lead around 

 this obstruction, but fuller examination shows that this 

 could not have been the case. The upper part of the St. 

 Lawrence is also crossed by continuous strata of archaean 

 rocks, showing that there was no preglacial channel permitting 

 drainage in that direction. 



To a superficial observer it would seem that the way was 

 equally closed into the Mississippi Valley. For it would 

 appear in the highest degree unlikely that the drainage of 

 Lake Ontario, whose bottom is now 507 feet below tide 

 level, could ever have found a way across the intervening 

 highlands which separate it from the Mississippi Valley, the 

 lowest place of which (at Chicago) is more than 600 feet above 

 tide level. But our conception of the depth of the glacial 

 deposits over this intervening area, and of the preglacial 

 gorges that have been filled by them has been enormously 

 enlarged by every fresh investigation of the region. 



The course of preglacial drainage in the upper basin of 

 the Allegheny River is worthy of more particular mention. 

 Mr. Carll, of the Pennsylvania Geological Survey, has conclu- 

 sively shown that previous to the Glacial period the drainage 

 of the valley of the upper Allegheny north of the neighbor- 

 hood of Tidioute, in Warren county, instead of passing south- 

 ward, as now, was collected into one great stream flowing 

 northward through the region of Cassadaga Lake to enter the 

 Lake Erie basin at Dunkirk, N. Y. The proof of this is that 

 between Tidioute and Warren the present Allegheny is shal- 

 low, and flows over a rocky basin; but from Warren north- 

 ward, along the valley of the Conewango, the bottom of the 

 old trough lies at a considerably lower level, and slopes to 

 the north. Borings show that in thirteen miles the slope of 



