THE INTERGLACIAL GORGE PROBLEM I 35 



tion of the United States. In western Pennsylvania, however, the presence 

 of Pleistocene river gravels on rock terraces several hundred feet above the 

 bottom of the present gorge of the upper Allegheny River indicates that the 

 last stage of the active erosion did not begin there until after the first ice 

 invasion, though the uplift and the inauguration of the erosion in the lower 

 reaches of the river may have been somewhat earlier. The uplift recorded 

 by the rock terraces immediately adjacent to the Susquehanna in the eastern 

 portion of the state is of questionable date, but would appear to be of late 

 Tertiary or early Pleistocene age. 



In the Elkland-Tioga region there appears to be a slight notching in the 

 bottom of the old valley of Pine Creek and some of its tributaries, but it is 

 believed that this was not produced until after the southward deflection of 

 the lower portion of the creek through the gorge south of Ansonia. This 

 diversion, as will be described more fully in the discussion of the earliest 

 glacial stage, was probably due in great measure to the accumulation and 

 overflow of waters ponded in front of the advancing ice-sheet, and the con- 

 sequent reduction of the divides and the' cutting of a new channel in which 

 the stream persisted even after the ice had disappeared. The notching of 

 the bottom of Pine Valley and its branches was a result of the diversion 

 through the new and lower channel, and affords no evidence of uplift. 



The Elkland-Tioga region is not far from the Finger Lake 

 region. It is, however, on the south side of the divide, and 

 includes some of the streams which are tributary to the head- 

 waters of the Susquehanna River. If rejuvenation had effected 

 the Laurentian drainage, this would tend to increase rather than 

 diminish the advantage of the northward-flowing streams over 

 those flowing southward. 



The hills, which rise steeply at the southern end of the lake 

 valley, become lower and more gently sloping as you pass north- 

 ward. The valley also widens rapidly toward the north. While 

 a mature stream valley ought to become wider and the walls 

 more rounded toward the mouth of the stream, the change here 

 is so rapid as to suggest that there must be some other explana- 

 tion to account for a part of the difference. One cause which 

 has probably contributed to this end is the northward differential 

 depression which occurred at the close of the glacial period. 

 If this depression amounted to no more that two feet per mile, 

 it would have made a difference of one hundred feet in the 

 relative height of the land at the ends of Cayuga Valley. It 

 has been suggested that the difference in the topography at the 



