*9*3.] ORTMANN— THE ALLEGHENIAN DIVIDE. 345 



Virginia, the mountain-type of erosion encroaches upon the plateau, 

 and, for instance, the valley of the upper Tygart and Greenbrier 

 valley are largely anticlinal valleys of the mountain-type (see Fon- 

 taine, 1876, p. 9), so that the eastern edge of the Allegheny Plateau 

 is pushed back westward. In the region between James and New 

 River and beyond (toward the southwest), conditions become more 

 complex by the development of faults, and here the eastern edge of 

 the plateau (Cumberland Plateau) is formed by a tremendous fault, 

 which brings the Carboniferous down to about the same level with 

 the Cambrian. (See maps and profiles in Rogers, 1884; also geo- 

 logical map by Willis, 1912; as to the faulting, see Lesley, 1865; 

 Stevenson, 1887; Powell, 1896, p. 79.) 



B. Stream Capture. 



There is yet another factor which contributed to make the struc- 

 ture of the Alleghenies more complex. We have seen that the orig- 

 inal divide of the waters probably was well to the east, not far from 

 the old Piedmont land. It is clear that from this divide the way to 

 sea-level (the Atlantic Ocean) was short and direct, while westward 

 it was long and devious. This produced a much steeper grade of 

 the eastern streams, and consequently the eroding power of the latter 

 must have been much greater than that of the western streams. The 

 eastern rivers had thus the first chance to saw through the divides 

 westward. This resulted in the general law that the Atlantic 

 streams have the tendency to cut into and to encroach upon the 

 region which originally drained westward. This general law is not 

 without exceptions, but such are rare. 



Also the Atlantic streams have been subject to stream capture be- 

 tween themselves; Campbell (1896, p. 675) points out the unsym- 

 metrical development of their basins, with the divides shifting toward 

 the southwest; the Susquehanna developed at the expense of the 

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

 that of the Roanoke. Similar conditions probably existed on the 

 western side. 



is correct only in so far as this escarpment represents the eastern boundary 

 of the Allegheny Plateau, but it does not correspond to the same structural 

 line as the Allegheny Front in Pennsylvania. 



