I INTERGLACIAL GORGES 67 



will appear from the following description taken in connection with 

 the map (Fig. 2). There is an older gorge (hereafter referred to 

 from its general width as the 600-foot gorge), 500-600 feet wide, of 

 low gradient, in the bottom of which a second much narrower 

 gorge, which we will call the 200-foot gorge, has been cut to a base- 

 level some 100 feet lower than the other. Both of these gorges 

 antedate the last ice invasion, being still filled by glacial deposits 

 except where they have been cleared out by the postglacial stream. 

 A third, much younger, gorge (postglacial) is found in places 

 where drift and lake delta deposits have so completely filled the 

 earlier gorges that in re-excavating its valley the stream took a new 



Fig. 4. — Sketch showing bottom and sides of gorge as it appears from upstream 



course which, in this case, did not correspond with the former 

 one. In cutting down in this new course the stream, encounter- 

 ing rock, developed the postglacial gorge. 



The bottom and walls of the oldest, or 600-foot, gorge are exposed 

 in numerous places for a distance of over a mile. The first evidence 

 is seen just below the Ithaca waterworks pumping plant, where a 

 rock bench, representing the old gorge bottom and east wall, shows 

 in section where the postglacial stream has been undercutting its 

 banks. The characteristics of this section as seen from upstream 

 are indicated in the accompanying sketch (Fig. 4) . A little farther 

 upstream, just below the road, is an interesting section best illus- 

 trated on the map (Fig. 2). A rock island (A of Fig. 2) is found 

 between the buried 200-foot gorge on the south and the recent post- 

 glacial gorge. The top of the rock in this island Hes approximately 



