578 FRANK CARNEY 
That the resulting forms due to the aggradational action of an ice- 
sheet overriding these two types of drift arrangement would not be 
identical seems reasonable. 
The drift as described under condition (2) would suffer much less 
from a second invasion. The deposits in the major valleys—i. e., the 
valleys transverse to the direction in which the ice is moving—would 
be somewhat protected from erosion; the weight of the overriding ice 
would tend to indurate this drift. But the drift in valleys tributary 
to these, since they trend more in unison with the moving ice, must 
suffer much more from erosion. When such accumulations are rather 
thick, it is probable that a drumlinoid form is the resultant of degrada- 
tion by a second invasion of ice, particularly in these tributary 
valleys. 
The most marked erosional effects, however, are observed in the 
old drift as distributed under condition (3). These valleys accord 
with the direction of ice-movement; if they open toward the approach- 
ing ice, greater obstruction is offered to its progress, hence greater 
erosion results; if they lead away from the feeding ice, the disturbance 
of the adjacent material may not be so marked. In the former case— 
i.e., the northward flaring valleys—the older drift, if not eroded, is 
apt to be deeply buried because of the intense aggradational work of 
the valley lobes which characterized the margin of the waning ice- 
sheet. In the latter case the ice-erosion is less effective; the 
augmented ice-front drainage has degraded, shifted, or covered 
with later outwash the earlier deposits. The application of this 
principle probably varies inversely with the size or width of the 
valleys. 
But the old drift in the minor valleys of condition (3) has suffered 
less from ice-erosion. The stage of development of these minor 
valleys, and their degree of transverseness to the moving ice, 
are important factors in controlling the extent of ice-erosion in 
them. 
Furthermore, under all these conditions we should find more old 
drift preserved in areas where during either pre- or inter-glacial time 
the drainage has suffered rejuvenation. The chances of such old 
drift being later revealed is greater in the transverse drainage lines of 
condition (3). 
