284 The American Geologist. ^^J^- i^o*- 
foot contour. It seems safe to entirely ignore the effect of 
their extension to the main valley axes, since it is certainly far 
less than the probable error in locating the exact level of the 
hanging valley bottoms. Since Cayuga lake bottom lies at 
least 50 feet, and Seneca valley bottom at least 640 feet be- 
low sea level, the amount of glacial erosion indicated is at least 
850 feet in the case of Cayuga and 1400- 1500 feet in the case of 
Seneca valley. This is far in excess of the estimate previously 
made, and in the case of Cayuga lake it takes no account of 
the filling in the lake bottom, which must be considerable. 
If the hanging valleys are taken as evidence there can be no 
escape from the conclusion that there has been this great amount 
of glacial erosion. 
A modification of the glacial erosion theory has been ad- 
vanced during the progress of the investigation of the prob- 
lem, and is still being considered. It is as follows : — During 
its first advance the ice deeply eroded the valleys ; during inter- 
glacial conditions the older gorges were cut ; with return of 
glaciation the valleys were deepened still further. During as 
many glaciations as this region experienced this process was 
continued. On this basis the older gorges are interglacial ; 
their cause is the lowering of their baselevel by the overdeep- 
ening of the valleys to which they were tributary. Since facts 
sufficient to establish or to overthrow this explanation are not 
yet at hand, it must stand at present merely as a working 
hypothesis. 
Facfs Opposed to the Ghicial Erosion TJicovy- \\'ith the 
progress of this investigation facts have appeared vrhich make 
the glacial erosion theory increasing!}- difficult to accept ; and 
I have found myself seriously doubting the explanation which 
formerly seemed to me established. 
Discordance of Tributaries : One opposing fact is the 
discordance of the tributary valleys. While the vast majority 
are hanging at a nearly uniform level, some lack this charac- 
teristic. The two best instances of discordance are Six i\Iile 
S£. 
Fig. 24. Profile of Six Mile creek, entering the Cayuga valley at Ithaca. 
(Scale same as Fig. 7.) 
