394 THE ICE AGE IN NORTH AMERICA. 



which had been flowing in that direction was so obstructed 

 that it was turned over the col which existed a little below 

 Cincinnati, and the permanent channel was opened which 

 is now followed by the Ohio River. During this period of 

 erosion there was a temporary lake formed above Cincinnati, 

 but of comparatively short duration, for the immense flow 

 of water from the whole upper basin of the Ohio would work 

 with great rapidity to cut down the col below the city. 



After this there followed in immediate succession, the 

 Cincinnati ice-dam proper, of which an account has been 

 given. For a distance of fifty miles the ice advanced into 

 Kentucky to heights of land fully 500 feet above the present 

 level of the river, and must have obstructed the drainage up 

 to that level. This was doubtless of comparatively short 

 duration, but while it continued it would set the water in 

 the whole valley above to that height, which would submerge 

 Pittsburg to a depth of 300 feet, and reach far up both the 

 Monongahela and the Allegheny rivers. The shortness of 

 its duration, however, makes it difficult to distinguish its 

 deposits from those which had been made during the con- 

 tinuance of the earlier ice-dams. While admitting there- 

 fore, that the ''Cincinnati ice-dam" was at first somewhat 

 overworked in accounting for the deposits in the Upper Ohio 

 Valley, it is by no means ruled out of the problem. The Cin- 

 cinnati ice-dam still remains a fact with which all students 

 of the region are compelled to reckon. 



As already intimated, however, the existence of the Cin- 

 cinnati dam would necessitate an overflow either into the 

 Kentucky River or around the margin of the ice, and such a 

 channel, if it ever existed, should be easily discovered. 

 But though I have diligently searched for it, it has so far 

 eluded observation. This has inclined me to account for 

 the Kentucky deposits as results of annual ice gorges, dur- 

 ing the climax of the period, in the narrow channel below the 

 city. But even so the phenomenon is none the less impres- 

 sive, and would involve the flooding of the upper Ohio and 

 its tributaries to the extent stated. 



