UPPER OHIO DRAINAGE SYSTEM. 99 



the Clarion presents a very narrow valley, scarcely one-third the width of 

 the gradation plain of the Clarion and Lower Allegheny. This valley has 

 precipitous bluffs reaching a height of about 400 feet above the stream, or 

 nearl}' 250 feet above the gradation plain at the mouth of the Clarion. It 

 seems necessary to suppose either that a disproportionately small gradation 

 plain with high cliff borders lay in the narrow gorge, or that there has been 

 a reversal of drainage by which a small stream that had its source just 

 above the mouth of the Clarion and flowed northward was reversed and its 

 valley was recut to fit a new and larger stream. 



On the first supposition we naturally look to differences in the hard- 

 ness of strata for a possible explanation of the differences in the size of the 

 valley. If there has been but little change in the ancient Allegheny, we 

 must explain the fact that a stream not less than three times the size of the 

 Clarion excavated a valley only one-third to one-half as lai-ge. If the 

 subsequent addition of the Upper Allegheny be granted, we must account 

 for the fact that a drainage area about the size of the Clarion cut a valley 

 but one-third to one-half as large. 



The strata along the narrow portions of the Allegheny, from Franklin 

 to the mouth of the Clarion, as well as for some distance above Franklin, are 

 on the whole rather more resistant than those in which the gradation plain 

 of the lower course of the Clarion was carved. On the Allegheny there is a 

 considerable amount of the Lower Carboniferous conglomerate, in places 

 reaching a thickness of 75 feet, while on the Clarion there are the more 

 easily eroded Coal Measures sandstone and shale. The greater hardness 

 and resistance to erosion would naturally lessen the size of the valley, though 

 it scarcely seems adequate to produce so marked a difference. Upon turning 

 to tributaries of this part of the Allegheny, a more decisive line of evidence 

 in favor of a reversal of drainage is found. 



The tributaries of the Allegheny above the mouth of the Clarion have 

 channels that were not deepened to levels in harmony with a gradation 

 plain so low as that of the Clarion. While they have normal gradients on 

 their upper and middle courses, the streams descend by rapids and cascades 

 to the present Allegheny. This is done from a height of about 400 feet, 

 while the tributaries of the Clarion and of the Allegheny below the Clarion 

 descend in this way only 150 to 200 feet. This seems to indicate that 

 tributaries above the moiith of the Clarion formerly discharged into a stream 

 which had not reached so low a plain as that of the Clarion. 



