Pre-Wisconsin Drift in Finger Lake Region 
13 
Topography favors both ice-erosion and ice-stream aggradation. 
These conspicuous valleys, digital-like in arrangement, because of 
their general north-south trend, molded the basal ice of the deploy- 
ing sheet into forms that expedited erosion. Furthermore, the 
fact that these valleys sloped toward the overriding wedges of 
ice facilitated the acquiring of a load which in turn augmented the 
erosive power of the ice up to the time when the amount of this 
load became so great that the basal ice lost in velocity; it then did 
little degradational work. In consequence of this differential 
erosion we find that approximately the southern thirds of these 
valleys are zones of ice-aggradation. Therefore, Professor Tarr’s 
Qoo-foot-contour upper limit of most active erosion^® defines a 
plain which dips into the Allegheny Plateau. The present attitude 
of this plain of erosion embodies some post-glacial deformation 
due to warping; but, neglecting the effects of this warping,-^® it is 
not likely that the plane would define a surface even parallel to 
its original attitude. Concerning the relation which this part of 
our continent bore to sea-level while the Wisconsin ice-sheet was 
active, we have insufficient data to warrant any but very general 
conclusions. 
It is evident, then, that so far as the north-south valleys are con- 
cerned, exposures of the old drift are more apt to be found in a 
belt skirting the zone of heavy drift in the southern parts of the 
valleys; northward from this hypothetical belt erosion may have 
been very active, tending to remove the earlier deposits; southward, 
aggraded glacial rubbish has probably covered these deposits. 
Few of the quite mature transverse valleys belonging to an inter- 
rupted but well-developed drainage cycle, above alluded to, have 
been described. The more nearly transverse to ice-movement 
such valleys lie, the less ice-erosion they are subject to. Subse- 
quent invasions of ice presumably have not removed much of the 
residual rock waste that escaped the earliest glaciation; nor would 
Popular Science Monthly, Vol. Ixviii (1906), p. 389. 
G. K. Gilbert, U. S. Geological 'SmwQYj 1 8th Annual Report 
pp. 603-606; H. L. Fairchild, Bulletin of the Geological Society of America, Vol. X 
(1899), pp. 66-68. 
R. S. Tarr, American Geologist, Vol. xxxiii (1904), pp. 271-291; F. Carney, 
Journal of Geography, Vol. ii (1903), pp. I15-124. 
