LOWERED DIVIDES 237 



courses, which so often lead across the sites of ancient divides, are due 

 to the fact that glacial deposits have so graded the valleys as to make 

 streams of two systems unite in a single course. For instance, in both 

 Post Creek and Pony Hollow valleys, moraines form the present divides, 

 and outwash gravel plains supply a grade down which the streams flow 

 across ancient divide sites. The same is true of Cayuta creek. The 

 course of the Chemung west of Elmira is also determined by outwash 

 gravels, which spread fan-shaped from the neighborhood of Horseheads, 

 both toward Elmira and Big Flats, making the part of the valley near 

 Horseheads higher than those parts farther away from the moraines, 

 during the building of which the outwash gravels were supplied. 



By these morainic and outwash gravel deposits the present divides 

 have to a large degree been determined, and, as a result of the changes 

 thus brought about, the drainage of this divide region has been pro- 

 foundly altered; but without a previous lowering of the divides these 

 decided changes in drainage would not have been possible. 



Interpretation 



statement of hypotheses 



Three different hypotheses suggest themselves as possible explanations 

 of the drainage peculiarities described above: (1) Ice erosion; (2) ero- 

 sion by ice-born streams ; and (3) headwater erosion, lowering divides 

 and capturing opposing headwaters. The ice-erosion hypothesis must 

 include not only the last ice advance, but also earlier ice advances, of 

 which, however, there is no direct evidence so far discovered in this region. 

 The same is true of the hypothesis of ice born streams ; and, in addition, 

 the possibility of water erosion during both the advance and retreat of 

 each ice sheet must be considered. These hypotheses will be considered 

 separately. 



ICE- EROSION HYPOTHESIS 



A number of facts are opposed to this hypothesis. In the first place, 

 as has been stated elsewhere* there is in this region definite evidence 

 of weak ice erosion during the last ice advance. Residually decayed rock 

 abounds in the southern half of the quadrangle, and some is present even 

 on the margin of Cayuga lake below the edge of the steepened slope. 

 Moreover, gorges cut in the steepened slope have not been erased nor 

 distinctly modified in form by ice erosion. If it is true that the last ice 

 advance in a main valley like the Cayuga trough did not perform marked 



*R. S. Tarr: American Geologist, vol. xxxiii, 1901, p. 286; Journal of Geology, vol. xiii, 1905, 

 p. 160. 



XXXII— Bull. Geol. Soc. Am., Vol. 16, 1904 



