219 
.Englacial Dr ift.—Crosby . 
slopes also. They seem to indicate an attempt on the part of 
the ice-sheet to carry or drag a large amount of drift up the 
stoss slopes. When the drift gains the crest, it is in part 
carried away into the body of the ice by the freer motion of 
the ice above this level, tending to leave the lee slopes bare, as 
already noted. But during the waning of the ice-sheet its 
movement over the rock hills and ledges finally became so 
feeble that it could no longer urge all^the^subglacial drift, the 
amount of which was probably being augmented by basal 
melting, up the stoss slopes, and it began to accumulate upon 
them. Each increment was so thoroughly compacted , nd 
pressed down by the ice, aided by the natural tenacity of the 
clay, that the effective stoss slope was gradually raised, until 
the accumulation of till finally over-topped the rocky obstruc¬ 
tion and became a drumlin. This view seems to be in entire 
accord with Prof. Chamberlin’s recent suggestion with regard 
to the origin of drumlins.* 
Perhaps the general conclusions to which my studies have- 
now led me may be best stated as follows: A large part, pro¬ 
bably the main part, of the preglacial surface detritus was 
englacial in the sedentary ice-sheet and remained so after the 
ice began to flow during the entire period of the growth, cul¬ 
mination, and early decline of the ice-sheet, or while the hard 
bed-rock surface was being actively abraded and striated. 
During this time, which embraced the greater part of the 
Glacial period, the preglacial detritus not originally incorpor¬ 
ated in the ice and the material worn from the ledges by the 
ice itself became englacial, in large parts at least, through 
overriding, shearing, and flexing movements of the ice, a hard 
surface of drift-shod ice in direct contact with the uneven 
bed-rock surface appearing to be essential to the rectilinear 
striation of the latter. During the later stages of the decMne- 
of the ice-sheet, basal melting set free considerable volumes 
of the hitherto englacial drift to form the ground moraine; 
and just as the frontal or terminal moraine, also composed of 
material set free by the ablation of the ice, records the cess¬ 
ation of the forward movement or invasion of the ice-sheet,„ 
so the basal or ground moraine records the gradual cessation 
of the glacial abrasion of the bed-rocks. The relative sud- 
*Journal of Geology, in, 480. 
