6o JOHN LYON RICH AND EDWIN A. FILMER 



definitely claimed to have found such deposits. Tarr^ mentions 

 two or three exposures of possible older drift near Ithaca, but 

 believes that the evidence is inconclusive, and Gilbert^ recognizes 

 the possibility of two till sheets and the certainty of "an epoch of 

 local till erosion by a glacier," but is led to no definite conclusions 

 further than this. 



There is, however, another criterion of multiple glaciation for 

 the application of which the Finger Lake valleys furnish ideal 

 conditions. This criterion is the amount of work performed during 

 interglacial intervals by streams whose normal profiles have been 

 thrown out of harmony by glacial erosion or deposition; streams 

 which must, therefore, resort to gorge-cutting in their endeavor to 

 bring their beds back to normal grade. 



A glacier moving along a valley tends to overdeepen it and to 

 leave tributary valleys hanging. As soon as the ice withdraws, the 

 streams in the hanging tributary valleys begin the work of bringing 

 their beds down to the normal profile of equilibrium. In doing this 

 they cut gorges in the lower ends of the hanging valleys. Given 

 sufficient time after an ice invasion, the tributary streams would 

 enter the main valley at grade through narrow, gorgelike valleys 

 cut in the bottoms of the hanging valleys. These gorges, narrow 

 at first, would, after their streams have reached grade, continue to 

 widen in a normal manner under the influence of the ordinary 

 weathering agencies combined with lateral swinging of the stream. 



After a single glacial epoch the streams in all the hanging val- 

 leys would be flowing in such gorges and would, in time, bring their 

 beds down to a grade accordant with that of the main stream. 

 Suppose, now, a second glacial epoch should intervene. The 

 chances are that the main valley would be deepened still further 

 by ice erosion while the gorges in the tributary valleys would 

 become more or less clogged with drift. On the withdrawal of this 

 second ice sheet the gorge bottoms of the tributary valleys would 

 be left hanging above the bottom of the recently deepened main 

 valley. 



' Watkins Glen-Catatonk Folio, No. 169, U.S. Geol. Survey. 

 =" G. K. Gilbert, "Boulder Pavement at Wilson, N.Y.," Jour. Geol., VI (1898), 

 771-75- 



