150 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



ill several instances reached a level only 70 to 80 feet above Lake Erie 

 without striking the rock floor, but one boring near Mesopotamia is 

 thought to have reached rock at a level 70 feet above Lake Erie. 



As the gradation plain at the mouth of the Beaver stands nearly 300 

 feet above Lake Erie and is distant but 90 to 100 miles from some of these 

 borings in Grrand River Basin, it is evident that the northward slope is 

 rapid. In the 30 miles from the mouth to the head of the present Beaver 

 there appears to be a descent of fully 50 feet, and in the 50 miles to Sharon 

 of about 85 feet, thus giving an average slope of about 20 inches per mile. 

 This slope has possibly been made greater by Pleistocene changes in level, 

 though specific evidence is not at hand. 



The old gradation plain and the rock floor of the vallej^ excavated in 

 it are widely separated in altitude near the mouth of the Beaver, but 

 apparently approach each other rapidly in passing northward. The evidence 

 seems clear that the reversal of drainage took place before much of the 

 deep channeling occurred. 



In general the slope of the rock floor beneath the present streams is in 

 harmony with the present drainage, and in the main is the reverse of the 

 ancient system. There is, however, a part of the channel in which the 

 rock floor appears to be excavated to a level too low to correspond with 

 the floor at points lower down the stream. Such features were at first 

 interpreted to signify that the drainage was in the reverse direction from 

 the present line of discharge, but further examination has rendered it prob- 

 able that the excavation has been accomplished by a stream running in the 

 present line of discharge. On the lower course of the Mahoning the oil- 

 well records show the rock floor to be fully as low as at the mouth of the 

 Beaver and apparently about 90 feet lower than at the mouth of the Con- 

 oquenessing, midway of the Beaver Valley and 15 miles below the mouth 

 of the Mahoning. The piers of the railway bridge at the mouth of the 

 Conoquenessing are reported by R. .R. Hice, of Beaver, to stand upon the 

 rock floor, and are so distributed as to test the middle as well as the border 

 of the valley. Upon passing up the Mahoning, a few miles from the 

 deeply excavated part, the rock floor is found to have an altitude nearly 

 150 feet higher. There is* an abrupt descent just above Edenburg. Here 

 the buried floor seems to fall 165 feet in half a mile, as shown by four 

 wells reported by W. H. Raub, of Edenburg. The declivity may be even 



