214 MAN AND THE GLACIAL PERIOD. 



the sea, which is about the height of the water-shed be- 

 tween the Licking and Kentucky Eivers. As the Ohio 

 River occupies a trough of erosion some hundreds of feet 

 in depth, and extending all the way from this point to the 

 mountains of western Pennsylvania, it would follow that 

 the ice which conveyed boulders across the Ohio River at 

 Cincinnati, and deposited them upon the highlands be- 

 tween the Licking and Kentucky Rivers, would so ob- 

 struct the channel of the Ohio as to pond the water back, 

 and hold it up to the level of the lowest pass into the 

 Ohio River farther down. Direct evidences of obstruc- 

 tion by glacial ice appear also for a distance of fifty or 

 sixty miles, extending both ways, from Cincinnati. 



The consequences connected with this state of things 

 are of the most interesting character. 



The bottom of the Ohio River at Cincinnati is 432 

 feet above the sea-level. A dam of 550 feet would raise 

 the water in its rear to a height of 982 feet above tide. 

 This would produce a long, narrow lake, of the width of 

 the eroded trough of the Ohio, submerging the site of 

 Pittsburg to a depth of 281 feet, and creating slack water 

 up the Monongahela nearly to Grafton, West Virginia, 

 and up the Alleghany as far as Oil City. All the tribu- 

 taries of the Ohio would likewise be filled to this level. 

 The length of this slack-water lake in the main valley, to 

 its termination up either the Alleghany or the Monon- 

 gahela, was not far from one thousand miles. The con- 

 ditions were also peculiar in this, that all the northern 

 tributaries rose within the southern margin of the ice- 

 front, which lay at varying distances to the north. Down 

 these there must have poured during the summer months 

 immense torrents of water to strand boulder-laden ice- 

 bergs on the summits of such high hills as were lower 

 than the level of the dam. 



Naturally enough, this hypothesis of a glacial dam at 

 Cincinnati aroused considerable discussion, and led to 



