342 ORTMANN— THE ALLEGHENIAN DIVIDE. [April 18, 



A. Formation of Mountains by Upheaval and Erosion. 



Lateral pression in a general direction from northwest to south- 

 east, in Permian and Postpermian times, formed the ancient and 

 original Alleghenian system, which consisted of a number of more 

 or less parallel folds (anticlines and synclines) running in a north- 

 east-southwest direction. These folds were pressed up against an 

 old block of Archaic rocks lying to the east of them, the Old Appa- 

 lachian belt of Davis (1907), now Piedmont plateau. They were 

 piled up highest in the eastern part, close to the old Archaic rocks, 

 but also in the southern parts the elevation was originally higher 

 than in the northern, and in this section not only folds, but also 

 faults, were formed. 



As soon as this mountain system began to develop, erosion set in. 

 The original drainage features conformed to the original structure; 

 the highest elevation being well to the east, the divide was situated 

 here, close to the old Archaic land, and the old rivers had to follow 

 the structure of the mountains, running first between the parallel 

 ridges in consequent, synclinal valleys, and finding their outlets at 

 certain points in a westerly (northwesterly) direction, toward the 

 interior basin. On the other side, toward the Atlantic Ocean, there 

 were shorter streams, originating also on the highest elevation, run- 

 ning east and southeast, and reaching the sea after having traversed 

 the belt of Archaic rocks. 



The longitudinal streams on the western side of the divide began 

 to carve out their valleys. But in addition, on top of the anticlines, 

 anticlinal valleys began to develop, running parallel to the synclinal 

 valleys, and very soon an important differentiation in the power of 

 erosion of these streams became evident, which is due to the geo- 

 logical structure and succession of rocks of the mountains. The 

 beds which compose them are all archaic and palaeozoic ; but while 

 the uppermost (Carboniferous) consist largely of hard sandstones, 

 in the lower beds (Devonian and older) softer shales and limestones 

 prevail. While the oldest rivers were running uniformly over sand- 

 stones, the anticlinal rivers, and chiefly those running on the highest 

 elevations, had the best chance to cut first through the sandstones 

 and reach the softer beds below. After this, these streams working 



