i>LEISTOCENiE HISTORY OF THE GENESEE VALLEY 71 



most or all of central New York was included in this reversed 

 (obsequent) drainage, having northward flow down the inface of the 

 Allegheny table-land. The valleys of the Finger lakes represent the 

 work of such northward drainage/ but the most conspicuous and 

 the longest example is the Genesee river. 



During the long process of adjustment of the drainage there 

 were great changes not only in land altitude but also in the climatic 

 factors. Recently the Pleistocene ice sheets have plowed across the 

 region and in a capricious manner have left some valley stretches 

 filled with rock rubbish while other long stretches have been unfilled, 

 or even scoured and cleared. Not only has the preglacial topog- 

 raphy been thus obscured and the drainage diverted, but still greater 

 interference with the old drainage was produced by the damming 

 effect of the ice itself, in forcing the drainage into new lines, either 

 southward or in directions along the ice front.^ 



The Genesee, as above noted, is the longest stream in New York 

 which retains its preglacial northward course. It has persisted in 

 its northerly course in spite of obstructions and diversions, for a 

 little thought makes it evident that the damming and diverting effect 

 of the glacier must have been to destroy the original northward flow 

 rather than to produce such flow. 



Diversions of the river. Buried channels 



In the course of the Genesee river there are three stretches where 

 its valley shrinks to a narrow steepwalled ravine or canyon. One 

 is at Portage, a second near Mount Morris, and a third at Rochester. 

 The first two are parts of a single diversion of the river. Above 

 Portageville and below Mount Morris the valley is broad and flaring 

 and is evidently the result of weathering, storm wash and stream 

 transportation during millions of years. The canyons represent the 

 same kind of geologic processes as the open-valley stretches but 

 only a small fraction of the time. Quantitatively the canyons are 

 inharmonious with the rest of the valley, and have long been recog- 

 nized as the very recent or postglacial work of the river where it has 

 been forced from its old valley into a new path. 



Logically it must follow that for each of the new stretches of the 

 river course there must be a corresponding deserted stretch of the 

 ancient valley. As to the cause of the changes in the river's course 

 there is no disagreement among geologists, it being the interference 



1 See N, Y. State Mus. Bui. 45, p. 31-54; Geol. Soc. Am. Bui. 16: 55-56. 

 » For illustration of such glacial drainage see N. Y. State Mus. Bui. 106. 



