134 GEORGE C. MATS ON 



by glacial erosion." There is also good reason for believing that 

 the divide has been lowered by stream erosion. The ice in its 

 advance would close the outlet of the lake valley, causing a lake 

 to be formed between the ice front and the divide. The drain- 

 age of this lake across the divide would continue until the ice 

 had advanced to the divide. In receding the ice would again 

 cause the formation of a lake in the valley, which would exist 

 from the time the divide was uncovered until the ice retreated 

 far enough to uncover a lower outlet. The drainage across the 

 divide would naturally tend to lower it. The amount of erosion 

 would vary with the length of the time. The presence in Cayuga 

 Valley of the well-developed terminal moraine of the Wisconsin 

 glacial epoch points to the existence of an ice-dam in the valley 

 for a long time. This fact points to the probability of the divide 

 having been considerably lowered by stream erosion. The effect 

 of the drainage across the divide would be influenced by the rela- 

 tive altitude of the northward- and southward-flowing streams. 

 If the- southward-flowing streams had cut considerably below 

 the level of those flowing northward, the water would fall into 

 the deeper valleys, and the divide might be destroyed in a very 

 short time by the recession of this waterfall. If, as seems more 

 probable, the northward-flowing streams had reached the lower 

 level, the removal of the divide would be much slower. On this 

 point Professor T. L. Watson has said: 



It can hardly be doubted that the Laurentian tributaries were the stronger 

 streams, therefore encroaching upon the territory of the other system, and 

 thereby causing the southward migration of the divide.^ 



We should also bear in mind that there is a possibility of a 

 differential uplift having rejuvenated the Laurentian streams just 

 before the glacial period. Mr. M. L. Fuller^ failed to find evi- 

 dence of this uplift in the area covered by the " Elkland-Tioga 

 Folio": 



It has been frequently urged among geologists that the advent of the 

 earliest Pleistocene ice-sheet was preceded by a general uplift of the north- 

 ern half of the continent, affecting the surface throughout the northern por- 



^"Some Higher Levels in tlie Post Glacial Development of the Finger Lakes," 

 Report, N. Y. State Museum, Vol. I (1897), p. R. 68. 



"M. L. Fuller, "Elkland-Tioga Folio," U. S. Geological Survey, p. 7. 



