JSnglacial Drift. — Crosby. ' 233 
and is very largely represented in the modified drift. But 
except to a very limited extent it is entirely unidentifiable 
as to its source and the distance of its transportation. The 
vast deposits of modified drift in southeastern New England, 
and the great average thickness of till in Ohio and other in- 
terior states, not to mention distinctly morainal accumula- 
tions, indicate, however, when compared with the scanty 
deposits of drift over many northern areas, in New England 
and Canada, that a large volume of the older, finer, and less 
readily identifiable part of the drift is relatively far-travelled. 
In its earliest stages the ice-sheet, we ma} T reasonably sup- 
pose, wore away and absorbed a considerable thickness of 
rotten rock underlying the residuary soil; and during its 
maximum stage, as already noted, the hard rocks suffered 
glacial abrasion on the lee slopes as well as on the stoss slopes. 
The conditions must then have been very unfavorable to the 
rending of the ledges and the detachment of fragments and 
boulders: but this came later, with the decline of the ice- 
sheet, when the flowage pressure so far predominated over the 
vertical pressure that the ice pulled away from, instead of 
following down, the lee slopes. 
Approaching the subject in this way, I can see no escape 
from the conclusion that the rock fragments and boulders 
must date chiefly from the later stages of the ice-sheet. 
Hence they must have been, in general, the last material to be 
absorbed by the ice-sheet and the first to be deposited by 
basal melting. Under favorable conditions of flexing or 
shearing a small part of this material attained a high level in 
the ice and enjoyed a long glacial transport ; but the fact that 
most of it is still near the parent ledges will, I judge, be 
found quite consistent with the englacial theory, if due allow- 
ance be made for the relatively short time that it was enclosed 
in the ice, and for its basal position and the low velocity of 
the basal layers of the ice. Although the total forward move- 
ment of the ice, as indicated by far-travelled erratics, appears 
to have been as much as five or six hundred miles, and even 
in some parts of the glaciated areas perhaps a thousand miles, 
a basal slipping of one-twentieth of that distance or less 
would probably be regarded as sufficient to account for the 
erosion of the bed-rock surface and the normal distribution 
