The Ri/vers and Yalleys of Pennsylvania. 229 



on only the weakest rocks ; but after this little stream had grown 

 to a good-sized river, further rising of the land, probably in the 

 time of the Jurassic elevation, allowed the river to sink its 

 channel to a greater depth, and in doing so, it encountered the 

 hard Medina anticline of Jack's mountain ; here it has since 

 persisted, because, as we may suppose, there has been no stream 

 able to divert the course of so large a river from its crossing of a 

 single hard anticlinal. 



The doubt that one must feel as to the possibility of the pro- 

 cesses just outlined arises, if I may gauge it by my own feeling, 

 rather from incredulity than from direct objections. It seems 

 incredible that the waste of the valley slopes should allow the 

 backward growth of N at such a rate as to enable it to capture 

 the heads of C, Tn, F, and so on, before they had cut their beds 

 down close enough to the baselevel of the time to be safe from 

 capture. But it is difficult to urge explict objections against the 

 process or to show its quantitative insufficiency. It must be re- 

 membered that when these adjustments were going on, the region 

 was one of great altitude, its rocks then had the same strong 

 contrasts of strength and weakness that are so apparent in the 

 present relief of the surface and the streams concerned were of 

 moderate size ; less than now, for at the time, the Tyrone, 

 Frankstown and Bedford head branches of the Juniata had not 

 acquired drainage west of the great Nittany-Bedford anticlinal 

 axis, but were supplied only by the rainfall on its eastern slope 

 (see section 39) — and all these conditions conspired to favor the 

 adjustment. Finally, while apparently extraordinary and difficult 

 of demonstration, the explanation if applicable at all certainly 

 gives rational correlation to a number of peculiar and special 

 stream courses in the upper Juniata district that are meaningless 

 under any other theory that has come to my notice. It is chiefly 

 for this reason that I am inclined to accept the explanation. 



31. Reversal of larger rivers to southeast courses. — Our large 

 rivers at present flow to the southeast, not to the northwest. 

 It is difficult to find any precise date for this reversal of flow 

 from the initial hypothetical direction, but it may be suggested 

 that it occurred about the time of the Triassic depression of the 

 Newark belt. We have been persuaded that much time elapsed 

 between the Permian folding and the Newark deposition, even 

 under the most liberal allowance for pre-Permian erosion in the 

 Newark belt ; hence when the depression began, the rivers must 



