The Rivers and Valleys of Pennsylvania. 247 



While the quality of these processes appears satisfactory, I am 

 not satisfied as to the sufficiency of their quantity. If diversion 

 was successfully practiced at the crossing of the Tuscarora anti- 

 cline, why not also at the crossing of Jack's mountain anticline, 

 on which the river still perseveres. It is difficult here to decide 

 how much confidence may be placed in the explanation, because 

 of its giving reason for the location of certain streams, and how 

 much doubt must be cast upon it, because it seems impossible and 

 is not of universal application. 



39. Migration of the Atlantic- Ohio divide. — There are certain 

 shifted courses which cannot be definitely referred to any particu- 

 lar cycle, and which may therefore be mentioned now. Among 

 the greatest are those by which the divide between the Atlantic 

 and the Ohio streams has been changed from its initial position on 

 the great constructional Nittany highland and Bedford range. 

 There was probably no significant change until after Newark 

 depression, for the branches of the Anthracite river could not 

 have begun to push the divide westward till after the eastward 

 flow of the river was determined ; until then, there does not seem 

 to have been any marked advantage possessed by the eastward 

 streams over the westward. But with the eastward escape of 

 the Anthracite, it probably found a shorter course to the sea and 

 one that led it over alternately soft and hard rocks, instead of 

 the longer course followed by the Ohio streams over continuous 

 sandstones. The advantage given by the greater extent of soft 

 beds is indicated by the great breadth of the existing valleys in 

 the central district compared with the less breadth of those in the 

 plateau to the west. Consider the effect of this advantage at the 

 time of the Jurassic elevation. As the streams on the eastern 

 slope of the Nittany divide had the shortest and steepest courses 

 to the sea, they deepened their valleys faster than those on the 

 west and acquired drainage area from them ; hence we find 

 reason for the drainage of the entire Nittany and Bedford district 

 by the Atlantic streams at present. Various branches of what are 

 now the Alleghany and Monongahela originally rose on the 

 western slope of the dividing range. These probably reached 

 much farther east in pre-Permian time, but had their headwaters 

 turned another way by the growth of the great anticlinal divide ; 

 but the smaller anticlines of Laurel ridge and Negro mountain 

 farther west do not seem to have been strong enough to form a 

 divide, for the rivers still traverse them. Now as the headwaters 



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