288 The American Geologist. ^^J'' 19^4. 
even later, the ice crossed rather than followed the Cayuga 
and Seneca valleys; and (3) that the valley deepening could 
have been accomplished only when the ice was thin enough to 
be deflected in the form of valley lobes by the valleys that ex- 
tended transverse to its general motion. This limits the length 
of time for effective ice erosion, and also the depth, and hence 
efficiency, of the ice tool. Instead of having the entire period of 
glacial occupation, and the maximum depth of ice, for the deep- 
ening of these valleys, the erosive work, if glacial, must have 
been accomplished by relatively thin ice operating for only a 
part of the period of glaciation. 
Absence of Sufficient Glacial Drift : A point that has 
on several occasions been urged against the glacial erosion the- 
ory is that the materials eroded are not in evidence in corre- 
sponding deposits. This has never appealed to me as a vital 
point, though now that it is evidenfthat the ice erosion must have 
been accomplished mainly when the glacier margin was north 
of the divide of the Finger Lake valleys, and hence that the 
materials eroded from these basins must have been largely 
laid down within the confines of these valleys, it becomes a more 
effective argument against the ice erosion theory. The mo- 
rainic deposits in the Cayuga valley, for instance, are totally 
inadequate to refill the basin, or even to make more than a 
mere beginning ; and these deposits include a goodly percentage 
of materials from beyond the linuts of the Cayuga valley. 
But even with this limitation, the point is not a strong one 
against ice erosion. There is no way of detennining how much 
of the material eroded from the basins was carried beyond 
the divide as till, clay, and overwash sand and gravels. Nor 
are we in possession of facts which tell us how extensive 
were the deposits made when the glacier advanced, as it doubt- 
less did, in lobes along these valleys. Moreover, possible 
earlier ice advances may have left deposits, later swept away. 
I should consider the argument that there is lack of evidence 
of accumulated materials the weakest of all that have been 
advanced in opposition to the ice erosion theory for these val- 
leys. 
Summary of Facts Opposed: In summary it may be 
said that the evidences against the ice erosion theory are as 
follows: — (i) The fact that, while most of the tributaries are 
