The Rivers and Valleys of Pennsylvania. 241 



on the lower courses of the larger rivers, which having already- 

 cut their channels down close to baselevel and opened their 

 valleys wide on the softer rocks, were then " estuaried," or at 

 least so far checked as to build wide flood-plains over their lower 

 stretches. Indeed, the flood-plains may have been begun at an 

 earlier date, and have been confirmed and extended in the later 

 time of depression. Is it possible that in the latest stage of this 

 process, the almost baselevelled remnants of Blue mountain and 

 the Pocono ridges could have been buried under the flood-plain 

 in the neighborhood of the river ? 



If this be admitted, it is then natural for the river to depart 

 from the line of its buried channel and cross the buried ridges on 

 which it might settle down as a superimposed river in the next 

 cycle of elevation. It is diflicult to decide such general questions 

 as these ; and it may be diflicult for the reader to gain much 

 confidence in the eflicacy of the processes suggested ; but there 

 are certain features in the side streams of the Susquehanna that 

 lend some color of probability to the explanation as offered. 



Admit, for the moment, that the aged Susquehanna, in the later 

 part of the Jura-Cretaceous cycle, did change its channel some- 

 what by cutting to one side, or by planation, as it is called. 

 Admit, also, that in the natural progress of its growth it had 

 built a broad flood plain over the Siluro-Devonian lowlands, and 

 that the depth of this deposit was increased by the formation of 

 an estuarine delta upon it when the country sank at the time of 

 the mid-Cretaceous transgression of the sea. It is manifest that 

 one of the consequences of all this might be the peculiar course 

 of the river that is to be explained, namely, its superimposition 

 on the two Pocono synclinal ridges in the next cycle of its 

 history, after the Tertiary elevation had given it opportunity to 

 re-discover them. It remains to inquire what other consequences 

 should follow from the same conditions, and from these to devise 

 tests of the hypothesis. 



36. Evidence of superitnposition in the Stisquehanna tributa- 

 ries. — One of the peculiarities of flood-plained rivers is that the 

 lateral streams shift their points of union with the main stream 

 farther and farther down the valley, as Lombardini has shown in 

 the case of the Po. If the Susquehanna were heavily flood- 

 plained at the close of the Jura-Cretaceous cycle, some of its 

 tributaries should manifest signs of this kind of deflection from 

 their structural courses along the strike of the rocks. Side 



