148 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



upon the gradient of the Redbank only in its lower 20 miles. Such an 

 abnormal deepening is accounted for by the sudden enlargement of the 

 drainage area to several times its former size in consequence of the diver- 

 sions of drainage previously discussed. It is improbable that a simple 

 change in the altitude, or in the general slope of the region, would produce a 

 result of precisely this nature. The main stream, of course, usually leads 

 in rejuvenated excavation, but not in such a disproportionate degree as this 

 nor in precisely this method. 



In the Clarion Valley the present stream has a fall of about 500 feet 

 in the lower 75 miles below Ridgway and the old fluvial plain about 340 

 feet Redbank River falls about 400 feet in the lower 40 miles below 

 Brookville, but the old fluvial plain falls only about 280 feet. In the 62 

 miles from Falls Creek station to the mouth of Redbank River, the stream 

 has a fall of 550 feet and the old fluvial plain a fall of 430 feet. The 

 Conemaugh falls about 425 feet, and the old fluvial plain about 300 feet in 

 the 64 miles below Johnstown. Of these old fluvial plauis the one on the 

 Clarion shows the lowest fall, about 4.5 feet per mile. If these data are 

 compared with those given in the table on the fluvial plains of the Lower 

 Allegheny, it will be found that both in its present and in its old stream beds 

 the Lower Allegheny has a much lower gradient than its main tributaries. 



BEAVER RIVER. 



Beaver River is formed by the junction of the Shenango and Mahdning 

 rivers, and has a drainage area of about 3,000 square miles, of which, per- 

 haps, 1,800 square miles are -in Pennsylvania and the remainder in Ohio. 

 Attention has already been called to the evidence that the Beaver consti- 

 tuted tlie former line of discharge for the Upper Ohio and its main 

 tributaries as far to the northeast as the Clarion River. The restoration of 

 the old system of drainage given on fig. 1 (p. 89) serves to show its natural- 

 ness compared with the present system. The Conoquenessing, an eastern 

 tributary, has a northwestward trend to its junction witli the Beaver, and 

 formerly continued in this direction toward the Lake Erie Basin, but now 

 it turns abruptly southward to enter tlie Ohio. The former outlet of Shp- 

 pery Rock Creek was in a course north of west through the valley now 

 drained by Big Run entering the Shenango at Newcastle. This finds a 

 natural continuation northwestward along the old line to Sharon, Pa. The 

 upper course of Mahoning River is northward from its source in Columbiana 



