I9I3-] ORTMANN— THE ALLEGHENIAN DIVIDE. 343 



in a less resistant material, had the advantage, and thus the anticlinal 

 valleys were more deeply excavated than the synclinal valleys. This 

 process advanced farthest in the eastern section of the mountains, 

 so that what was once the highest elevation became finally a deeply 

 excavated valley. 



This general process was repeatedly interrupted by the fact that 

 the whole region was reduced to base level. One of these periods 

 of base level conditions is most important to us, that of Cretaceous 

 times,, when most of the mountain region was a peneplain, little ele- 

 vated above the sea, but with certain hi41s (monadnocks) standing 

 above this level. In Postcretaceous times a reèlevation took place, 

 and the rivers began their work again, according to the same laws, 

 but with complications due to the base-level period. During the 

 latter, they had acquired courses across the strike of the mountains, 

 and these were inherited by the later rivers, and often they were 

 compelled to cut across hard rocks, thus forming so-called water 

 gaps, which have no apparent connection with the original geological 

 structure. 



The difference in the erosion has produced a physiographical 

 differentiation within the whole system. In the western parts, where 

 the Pre-Carboniferous soft rocks have not been reached, either 

 synclinal valleys are present, or the drainage system is independent 

 on the structure, irregular or dendritic. This section has been base- 

 leveled rather completely in the past, and thus it is of the character 

 of a plateau, and has been called the Alleghenian Plateau. The 

 eastern parts, which were originally much higher, have been much 

 cut into by the anticlinal streams, which have carved out broad lime- 

 stone valleys, with high ridges of harder rock between them, so that 

 this region has a more mountainous character, and is known as the 

 Allegheny Mountains proper. Within these mountains, farthest to 

 to the east, where there was once the highest elevation, an exception- 

 ally broad valley has been excavated, called the Great Allegheny 

 Valley. 



Thus we have, going from west to east across the mountains (see 

 Plate XII.) : (i) The Allegheny Plateau; (2) the Allegheny Moun- 

 tains, with numerous ridges and valleys, the most eastern valley being 



