74 JOHN LYON RICH AND EDWIN A. FILMER 



which led us to beheve that the Wisconsin ice had played but a 

 small part in the widening and modification of the 600-foot gorge. 

 It seems to be still an open question whether the ice of the latest 

 Wisconsin epoch ever extended beyond the belt of strong terminal 

 moraines just south of Ithaca/ If it did not, we should expect only 

 feeble erosive action from the Wisconsin ice in the Ithaca region. 

 It is interesting to note, in this connection, that Tarr, in his report 

 on the Watkins Glen-Catatonk FoHo, emphasizes the slight erosion 

 by Wisconsin ice. 



7. Postglacial interval. — -This has been marked, in the case of 

 Six Mile Creek vaUey, by the partial re-excavation of the older 

 gorges and by the cutting of postglacial gorges in places where the 

 postglacial stream found itself outside its former course. The post- 

 glacial seems to have been the shortest of the intervals of deglacia- 

 tion. The gorges are relatively small and show much less stream 

 erosion than the others. In spite of the fact that climatic differ- 

 ences may have influenced the rate of gorge-cutting, it seems 

 legitimate to judge, roughly, the length of an interglacial interval 

 by the size of its gorges. 



ALTERNATIVE HYPOTHESES 



Before the interpretation outUned above can be considered 

 estabhshed, certain questions call for consideration. The first of 

 these is whether the observed relation of the 200-foot gorge to the 

 600-foot gorge^that is, cut in its bottom — 'necessitates for its expla- 

 nation the intervention of a glacial epoch; the second is whether 

 the 600-foot gorge may not be a product of preglacial rejuvenation. 



In answer to the first of these questions it may be said that the 

 whole problem resolves itself into a search for an adequate reason 

 why the stream, after cutting the 600-foot gorge to a low gradient— 

 evidently to its local base level^and widening it considerably, 

 should suddenly have renewed its downcutting sufficiently to have 

 formed the 200-foot gorge in the bottom of the older one. Several 

 possible explanations present themselves : {a) there may have been 

 a lake in the main valley whose level was suddenly lowered; {h) the 



I Chamberlin, in his report on the "Terminal Moraines of the Second Glacial 

 Epoch" in the Third Annual Report of the U.S. Geol. Survey, states, as a possibility, 

 that the moraine in question represents the limit of Wisconsin ice movement. It 

 seems to the writer that this possibility has never been satisfactorily set aside. 



