132 GLACIAL FOEMATIONS OF ERIE AND OHIO BASINS. 



while on the old line of northward di-ainage for the Tionesta and on the 

 portion of the Conewango north from Russellbiirg, Pa., the rock floor shows 

 a perceptible northward slant. This singular feature may perhaps be due 

 to a recession of the Thompson col through valley excavation, as suggested 

 by Carll,^ and the formation of a pseudo-col north of Warren at a point 

 where excavation and recession were interrupted by a later filling with 

 glacial gravel. 



The valley leading westward from the lower course of Brokenstraw 

 Creek into the Oil Creek Basin has been insufficiently tested by borings to 

 furnish satisfactory evidence concerning the slope of its rock floor or the 

 altitude of the floor compared with that of the Lower Conewango. A 

 decision between these two routes can scarcely be rendered until a better 

 knowledge of the western line is obtained. 



OLD MIDDLE ALLEGHENY DRAINAGE SYSTEM. 



This basin includes the lower portion of the Tionesta (below Barnes- 

 ville post-office), the Allegheny from the old col at Thompson's to near the 

 mouth of the Clarion, and the tributaries of this part of the Allegheny, 

 except the headwaters of Oil and French creeks, as indicated in fig. 6. 

 Several lines of evidence unite in indicating that this district formerly 

 discharged northwestward along or near the lower course of French Creek 

 nearly to Meadville and thence past Conneaut Lake to Conneaut Creek and 

 the Lake Erie Basin. Evidence in favor of the reversal in the part below 

 the mouth of French Creek is found in the narrowness of the Allegheny 

 Valley above the mouth of the Clarion, as compared with the Clarion- 

 Lower Allegheny Valley. Evidence is also found in an elevated tract which 

 is crossed by the Allegheny immediately above the mouth of the Clarion, 

 and in the rock shelves or old rock floors of the Allegheny and French 

 Creek valleys. 



Attention has already been called (p. 129) to the broad gradation plain, a 

 mile or more in average width, which follows the Lower Allegheny at a level 

 about 200 feet above the present stream, and to the fact that this gradation 

 plain follows up the Clarion, but does not extend up the Allegheny above 

 the mouth of the Clarion, for that part of the Allegheny has a narrow valley 

 with precipitous bluffs reaching a height of nearly 400 feet above the stream. 



'Second Geol. Survey Pennsylvania, Eept. I*, 1883, p. 311. 



