Drainage Features of the Upper Ohio Basin. 251 



The Tipper Tionesta-Conewango Baa%n. — Mr. Carll also 

 called attention to the evidence of reversal of drainage in the 

 Tionesta and Conewango basins, which are connected with the 

 present Allegheny at Warren below the old col near Kinzua. 

 There is evidence here of the same decisive nature as that 

 cited for the upper section of the Allegheny. The abandoned 

 northward outlet of the upper Tionesta leading to the present 

 Allegheny near Warren is very plainly outlined, and the 

 numerous oil wells of the region show that the rock floor of 

 the abandoned channel slopes northward. The old col where 

 reversal took place is readily located near Old Sheffield (Barns- 

 ville P. 0.), where the stream enters a narrow gorge scarcely 

 one-fifth the width of the abandoned channel, the latter being 

 60 to 120 rods wide while the new gorge is but 10 to 15 rods. 



That the upper Tionesta-Conewango basin did not discharge 

 southward along the present course of the Allegheny seems 

 evident from a constriction of the valley which sets in near 

 Thompson's station, about twelve miles below Warren, as 

 indicated by Mr. Carll who located the old col at that point. 

 The narrowness of the Allegheny at Thompson's as well as for 

 some miles below, is in striking contrast with the portion of 

 the Allegheny valley between Thompson's and Warren, and 

 also that of the main tributaries (the Brokenstraw and Cone- 

 wango), the width being scarcely one-third of a mile while the 

 width of the valleys named is nearly a mile. In both situa- 

 tions the bluffs are mainly of soft easily erodible strata, the 

 hard subcarboniferous conglomerate which farther down the 

 Allegheny is an important factor in the valley bluffs being 

 present here only in the highest parts of the bordering upland. 

 Evidence of reversal is also shown by the valleys of small side 

 streams. Below the supposed col these have not been so 

 broadly opened to the level of the Allegheny as they have 

 above and the streams descend near their mouths through nar- 

 row trenches fully 100 feet in depth. It would appear, there- 

 fore, that the col could not have stood less than 100 feet above 

 the present river bedat Thompson's, or 1220 feet A. T. This 

 view is confirmed by remnants of the old rock floor preserved 

 in an occasional rock shelf beneath early glacial gravels. At 

 Tidioute, some eight miles below the supposed col, the lowest 

 discovered limit of erosion prior to the deposition of these 

 gravels is about seventy feet above the present stream, whereas 

 at Warren, fifteen miles above the col, the erosion seems to 

 have reached a level even lower than that of the present 

 stream. 



It is probable that the line of discharge was through the Con- 

 ewango (reversed), though a deeply filled broad valley con- 

 necting Brokenstraw creek with Oil creek along the line of 



