Drainage Features of the Upper Ohio Basin. 255 



the Clarion as compared with the Clarion-Lower Allegheny 

 valley. Evidence is also found in an elevated watershed 

 which crosses the Allegheny immediately above the mouth of 

 the Clarion. Further evidence is found in the rock shelves or 

 old rock floors of the Allegheny and French creek valleys. 



Taking up these lines of evidence in the order named, atten- 

 tion may be called to the presence of a broad base-plane or 

 gradation-plane* a mile or more in average width which fol- 

 lows the lower Allegheny, standing about 200 feet above the 

 present stream. f This broad gradation-plane follows up the 

 Clarion but does not extend up the Allegheny above the 

 mouth of the Clarion, the Allegheny having a narrow valley 

 with precipitous bluffs reaching a height of nearly 400 feet 

 above the stream. It seems necessary to suppose either that a 

 disproportionately small gradation-plane with high cliff bor- 

 ders lay in this narrow gorge (having a breadth only one-third 

 to one-half that of the Clarion and Allegheny gradation-plane), 

 or that there has been a reversal of drainage by which a small 

 stream that formerly flowed northwestward through this gorge 

 to join the French creek outlet was reversed and its valley 

 recnt to fit the new and larger stream. We naturally look to 

 differences in the hardness of strata for a possible explanation 

 of the difference in the size of the valleys. On the supposi- 

 tion that there has been but little enlargement of the pregla- 

 cial Allegheny through additions due to glacial agency we 

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

 as large as the Clarion excavated a valley only one-third to 

 one-half as great. If the ice caused the addition of the two 

 upper sections of the Allegheny, and not of the section under 

 discussion, we must account for the fact that a drainage area 

 about the size of that of the Clarion cut a valley but one-third 

 to one-half as great. 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 are those in which the gradation- 

 plane of the lower course of the Clarion was carved, there 

 being in the former a considerable thickness of the subcarbon- 



* For use of terms see Physiography in the University, W. M. Davis, Jour, of 

 Geol., vol. ii, No. 1, p. 77. A degradation plane or profile (fluvial), results from 

 erosion in excess of deposition, and is the common type. An aggradation plane 

 or profile (Salisbury) results from deposition in excess of erosion, the plane being- 

 built up. A graded or gradation plane or profile (Gilbert), results from a balance 

 between erosion and deposition, the work of the stream being expended in 

 widening its valley. This is equivalent to a plane or profile of equilibrium 

 (Davis), and to a rather strained application of base-level or base-plane (Powell). 

 It is advocated by Gilbert and Davis because it avoids this strained use, is meas- 

 urably convenient, and falls in with the preceding terms to form a symmetrical 

 series. 



f Bull. U. S. Geol. Survey, No. 58, 1890, pp. 22-24. 



