300 TEE ICE AGE IN NORTH AMERICA. 



lar layers on the other side. The width of the gap cut by 

 the stream averages about a mile, with enlargements wher- 

 ever a tributary comes in from either side. The tributaries, 

 also, occupy corresponding narrow valleys of erosion, extend- 

 ing even to their very sources in the mountains. The nature 

 of the cause producing these narrow troughs, and its long- 

 continued operation in every one of these tributaries of the 

 Ohio, are not difficult to see. They have all been formed by 

 running water. The Tennessee, the Cumberland, the Ken- 

 tucky, the Wabash, the Miami, the Licking, the Scioto, the 

 Big Sandy, the Kanawha, the Hocking, the Muskingum, the 

 Big Beaver, the Monongahela, and the Alleghany, together 

 with their tributaries, all show the vast amount of water- 

 erosion along these lines of preglacial drainage. 



But even these do not, in their present condition, reveal 

 the whole extent of the effect of the constant and long-con- 

 tinued erosive forces of preglacial times. The ancient bed 

 of the Ohio River was certainly one hundred and fifty feet 

 deeper than that over which it now flows, it having been 

 filled with glacial debris to its present level. According to 

 Professor Newberry — 



At the junction of the Anderson with the Ohio, in Indiana, 

 a well was sunk ninety-four feet below the level of the Ohio 

 before rock was found. In the valley of Mill Creek, in the 

 suburbs of Cincinnati, gravel and sand were penetrated to the 

 depth of one hundred and twenty feet below the stream before 

 reaching rock. On the margin of the Ohio, at Cincinnati, 

 gravel and sand have been found to extend to a depth of over 

 one hundred feet below low-water mark, and the bottom of 

 the trough has not been reached. The falls of the Ohio, formed 

 by a rocky barrier across the stream, though at first sight 

 seeming to disprove the theory of a deep continuous channel, 

 really affords no argument against it ; for here, as in many 

 other instances, the present river does not follow accurately 

 the line of the old channel, but runs along one side of it. At 

 the Louisville falls, the Ohio flows over a rocky point which 

 projects from the north side into the old valley, while the deep 



