344 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



the Great Allegheny Valley, then follows, east of the mountains, a 

 much older section of the country ; (3) the Piedmont Plateau, a pene- 

 plain, the remnant of the Old Appalachian land; and finally toward 

 the ocean comes an additional physiographic division, (4) the Coastal 

 Plain, lying between the Piedmont Plateau and the sea, of various 

 width, which consists of marine deposits of much younger geological 

 age (Cretaceous and Tertiary) (see McGee, 1888, Powell, 1896, 

 Davis, 1907). 



In the southern Appalachians this division is somewhat modified. 

 The boundary between 2 and 3 is more developed (Blue Ridge) and 

 is called the Appalachian Mountains, while no. 2 has more of a 

 valley character and is called Appalachian Valley. No. 1 is called 

 Cumberland Plateau (see Hayes, 1899, PI. 1). 



The boundary between the Coastal Plain and the Piedmont Pla- 

 teau is well marked by an escarpment forming a falls line for the 

 streams traversing the Piedmont Plateau. The Allegheny Moun- 

 tains, and chiefly the Allegheny Valley, are marked off from the 

 Piedmont Plateau by the flank of an anticline, consisting largely of 

 archaic rocks, known in Virginia as Blue Ridge, and continued into 

 Pennsylvania as South Mountain. But farther north this ridge be- 

 comes obscure, and Piedmont Plateau and Allegheny Valley are 

 more or less indistinct. In southern Virginia the Blue Ridge widens 

 out and becomes a more important member of the system, finally 

 reaching in North Carolina the highest elevation (see above). The 

 Great Allegheny Valley is very distinct northwards, in Pennsylvania, 

 Maryland and northern Virginia, forming a broad and flat limestone 

 valley, and is sharply differentiated from the more western moun- 

 tains and valleys. Farther south it merges more or less with the 

 mountain region, which consists of several broad and flat limestone 

 valleys, separated by longitudinal ridges formed by monoclinal 

 harder rocks. 



The boundary between the Allegheny Mountains and the Alle- 

 gheny Plateau is well marked in Pennsylvania and Maryland by the 

 western flank of an anticline, known as Allegheny Front. Farther 

 south this may be traced to a certain distance, 10 but then, in West 



10 Willis, 1896, p. 186 (also Abbe, 1899, p. 70), use the name Allegheny 

 Front much farther South, for the escarpment west of Bluestone River: this 



