THE INTERGLACIAL GORGE PROBLEM 1 49 



While we cannot eliminate the possibility of a preglacial reju- 

 venation, we can limit the effect in Ten Mile Creek valley. The 

 bottom of the broad valley in Ten Mile Creek is about 720 feet 

 A. T., and there is a shelf in the drift-filled gorge at 620 feet A. T. 

 A differential uplift increasing at the rate of three feet per mile 

 (which would be a fair estimate) to the northeast might cause 

 trenching to 620 feet A. T. without producing a rejuvenation of 

 the streams of the " Elkland-Tioga Folio." To permit erosion 

 to the lowest known point in this gorge would call for an uplift 

 twice as great, while to reach the lowest known depth of any of 



Fig. 6. — A view of the middle ridge looking down stream. 



the gorges would require an uplift which would effect the streams 

 south of the divide. Therefore, it seems safe to conclude that 

 the main trenching of the broad valley was interglacial, and that 

 in the case of Ten Mile Creek the preglacial uplift did not cause 

 the stream to cut below 620 feet A. T. There is considerable 

 evidence that the broad valley of Six Mile Creek contains two 

 drift-filled gorges with their bottoms below 540 feet A. T. This 

 fact would seem to indicate at least two periods of deep trench- 

 ing after the first glacial invasion. 



The lengtli of the deglaciation intervals. — In considering the 

 length of the intervals of deglaciation, we are confronted by a 

 great many diflficulties. One of the most troublesome points is 

 our lack of knowledge of the depth of the various gorges. 

 Another difficulty, which is almost equally troublesome, is the 

 uncertainty as to their chronological order. From the fact that 



