BEAVER RIVER DRAINAGE SYSTEM. 151 



more precipitous, as the wells are not situated so as to limit more closely 

 the space occupied in the descent. The variations in the valley floor 

 appear to be such as might result from the recession of a fall or cascade, 

 and this suggestion harmonizes with the more clearly indicated history 

 of the region, which appears to be as follows: 



Before the ice invasion forced the waters of the upper portion of 

 the Grand River Basin across the divide, which apparently stood near the 

 State line above Edenburg, there was only a small tributary leading down 

 to the valley occupied by the old north-flowing Monongahela system. 

 But when the waters of the upper Grrand River Basin were forced over the 

 divide in large volume and descended the steep slope of the little valley, 

 deep scouring at the mouth would naturally result and the formation of 

 cascades or falls would readily follow. These would work upstream as 

 the erosion progressed. They appear to have reached a point just above 

 Edenburg when a later incursion of the ice stopped the process and filled 

 the deep valley with debris. The formation of a pool was favored by the 

 softness of the rock in this portion of the valley and the hardness of the 

 strata encountered near the mouth of the Conoquenessing. 



The present system of drainage displays considerable variation in the 

 slope of its stream beds, some portions being very sluggish, while other por- 

 tions present rapids and even low cascades. The Shenango falls but 2 to 

 2 J feet per mile in the upper 22 miles above Jamestown, Pa., the source 

 of the river being in a swampy lowland at an altitude only 1,025 feet above 

 tide. In the next 30 miles, from Jamestown to Sharon, there is a fall of 

 4 to 5 feet jDer mile. From Sharon to the mouth of the Shenango, a dis- 

 tance of 24 miles, the average fall is 2^ feet per mile. The Mahoning has 

 a fall of about 3 feet per mile in the 35 miles from Warren, Ohio, to its 

 mouth, but the northward-flowing headwater portion is more rapid. In 

 the 23 miles from its head to Beaver Falls the descent of the Beaver is 

 only about 2 feet per mile, but in the lower 5 miles it makes a descent of 

 52 feet, or more than 10 feet per mile. 



