240 R. S. TARR — DRAINAGE FEATURES OF CENTRAL NEW YORK 



glacial lakes were capable of such profound erosion as that necessary to 

 account for the many deep, long cuts in the divide region of southern 

 New York. 



From the facts, as indicated in the above outline, it is evident that the 

 peculiar valley conditions of this area are not the result of the action of 

 streams supplied by the melting of the Wisconsin ice sheet, and that in 

 several instances, at least, the conditions were brought about long before 

 the advent of this ice sheet. The hypothesis that these conditions were 

 brought into existence by possible earlier ice advances is weakened by 

 several facts aside from the fact that studies in this region have not yet 

 revealed any evidence of the presence of former ice sheets. It must, 

 nevertheless, remain as a working hypothesis until either some other 

 explanation is established or this one eliminated. 



HEADWA TER-EROSION H YPO THESIS 



On this hypothesis the assumption is that streams gnawing at their 

 headwaters have in many cases so lowered the divides as to permit com- 

 plete diversion of streams across these divides when the valleys have 

 been graded up by glacial deposits. By this hypothesis it is further 

 assumed that the gnawing back at the headwaters often caused an en- 

 croachment of one stream system on the opposing system and the con- 

 sequent capture of the headwaters of rival streams in favorable cases. 

 At the present stage of the study it does not seem wise to enter into the 

 consideration of why this headwater erosion may have been in progress 

 nor exactly how it was operating. Further study over a wider area may 

 make this consideration desirable at a later time. 



The difficulties in the way of accepting either ice erosion or ice-born 

 stream erosion as an explanation of the peculiar valley conditions of this 

 area may be considered as favorable to the headwater-erosion hypothesis. 

 Moreover, many of the facts opposing the rival hypotheses directly sup- 

 port headwater erosion. The doubly flaring condition of the valleys is 

 a type of form that such headwater erosion would be expected to pro- 

 duce. 



If two streams heading in the narrow part of the Chemung valley 

 west of Elmira were lowering their divides, just such a condition as exists 

 west of Elmira would be produced. This process of divide lowering 

 seems to have been in progress behind the small hills 2 miles southwest 

 of Elmira and behind those northeast and northwest of Horseheads. In 

 fact, in many places there are lowered divides in all stages of lowering, 

 from simple notching of the divides to such complete reduction as to 

 permit burial beneath glacial deposits. 



