DRA INA GE MODI PICA Tl ONS 675 



The constantly increasing magnitude of the river basins in 

 the Appalachian Valley from New River to the Susquehanna, at 

 once suggests the idea that the northern streams have been more 

 favored in their development than the southern streams, and as 

 a result have grown largely at the expense of the latter. Roan- 

 oke River has but a few square miles of its basin within the 

 rockv walls of the Blue Ridge ; James River has pushed farther 

 toward the northwest, but is still confined well within the limits 

 of the zone of folded rocks ; Potomac River controls much more 

 of the area in question and is practically limited in its north- 

 western side by the Alleghany front ; Susquehanna River has 

 not only gained control of a large portion of the valley, but has 

 also extended its headwaters far back into the Alleghany pla- 

 teau of western Pennsylvania and southern New York. It is at 

 once obvious that these streams are working under different con- 

 ditions ; and judging from their arrangement, it is probable that 

 much, if not all, of this difference is due to difference in amount 

 of interior uplift, and also to the location of the axis of the 

 movement. 



The water parting between the Atlantic streams and those 

 belonging to the Ohio River drainage basin forms a remarkably 

 regular line which crosses the valley obliquely from the Blue 

 Ridge south of Roanoke, Virginia, to McKean county, Pennsyl- 

 vania. From the criteria already established this would appear 

 to mark the position of an axis of uplift which has been mainly 

 instrumental in shaping the drainage basins on either side. 



From the James to the Susquehanna the basins are decidedly 

 unsymmetrical ; the divides have migrated toward the southwest, 

 until the divide between the Susquehanna and the Potomac 

 approaches close to the left bank of the latter stream, and the 

 divide between the Potomac and the James allows to the former 

 about twice as great an area as it does to the latter. This migra- 

 tion is certainly due to a tilting of the surface toward the north- 

 east, which favored the development of the Susquehanna at the 

 expense of the Potomac, the Potomac at the expense of the 

 James, and the James at the expense of the Roanoke River. 



