98 GLACIAL FORMATIONS OF ERIE AND OHIO BASINS. 



level of the rock floor at these wells it appears probable that the old river 

 entered the Lake Erie Basin at a level at least as low as the present surface 

 of the lake, 573 feet. 



EXTENT OF THE OLD MONONGAHELA SYSTEM. 



The old Monongahela system appears to have embraced the entire 

 drainage of the Monongahela, the Allegheny as far up as Clarion River, 

 the portion of the Ohio above Bellaire, the greater part of the present drain- 

 age basin of the Beaver, and the lowland known as the Grand River Basin. 

 The greater part of this drainage system lies outside the glacial boundary, 

 but the trimk stream and its small tributaries northward.from the southern 

 end of the Beaver Valley are within the limits of glaciation, and the Alle- 

 gheny and Ohio valleys have been filled to some extent by glacial gravel 

 transported by streams beyond the ice margin. 



The portion of the Ohio above the mouth of the Beaver, the Mononga- 

 hela Valley, and the portion of the Allegheny below the mouth of Clarion 

 River have a system of gradation plains that slope in harmony with the 

 present lines of drainage toward the Beaver Valley. They fall less rapidly 

 than the present streams, as is to be expected of such a system of gradation 

 plains. Thus on the Monongahela, as shown by White,^ the fall of the 

 present stream is 190 feet in the 206 miles below Weston, West Virginia, 

 while the gradation plain descends only 110 feet in the same distance. 

 On the Allegheny the present stream falls 163 feet in 82 miles below the 

 mouth of Clarion River, while the gradation plain descends but 120 feet in 

 this interval. In the 26 miles from the head of the Ohio at the junction of 

 the Allegheny and Monongahela down to the Beaver the gradation plain 

 has a descent of about 30 feet. 



At this point it seems necessary only to consider the reasons for not 

 including in the old Monongahela system the part of the Allegheny di'ain- 

 age basin above the mouth of the Clarion River. These will be but 

 briefly discussed, since the Allegheny drainage is treated in some detail 

 farther on. 



THE OLD DIVIDE ON THE ALLEGHENY. 



The gradation plain which is found on the Lower Allegheny continues 

 up the Clarion, as well as up tributaries which enter below the mouth of the 

 Clarion. The portion of the Allegheny immediately above the mouth of 



lAm. Geologist, Vol. XVIII, 1896, pp. 368-379. 



