Enghicial Drift. — Crosby. 
217 
‘dragged down the lee slopes and away from the elevations 
without losing its subglacial position. This must have in¬ 
volved the striation and polishing of the lee slopes, and the 
^conditions were unfavorable for the detachment of blocks of 
rock—the rending and quarrying operations of the ice-sheet. 
Later, when the vertical pressure was less and the velocity of 
flow greater, the ice hugged the lee slopes less closely and the 
■conditions became favorable for the detachment of blocks by 
the ice moving under the combined vertical and flow r age pres¬ 
sure across the crests of the elevations. If the ice actually 
pulled away from the lee slopes to an appreciable extent, or 
even tended to do so, the local relief of pressure may, perhaps, 
have led to the freezing on these slopes of subglacial wrnter. 
This sedentary ice, penetrating the joint-cracks of the rocks 
and by its expansive power starting the joint-blocks from 
their positions, and later, by its continued growth, becoming 
continuous with the moving ice, may have assisted in pluck¬ 
ing away blocks and fragments of rock from the lower as well 
ns the upper portions of the lee slopes, thus tending to main¬ 
tain the high angle of declivity so.characteristic of lee slopes. 
The broken and precipitous character of typical lee slopes 
is, to my min'd, rather inconsistent with the passage dowm 
them, during the detachment and removal of the blocks, of 
much ground moraine; and this conclusion is in harmony 
with the facts that they are not now, as a rule, banked high 
with till, and that w T e often find a surface train of angular 
blocks leading awav from them. That the blocks thus borne 
awmy from a lee slope were, in many cases, carried in the ice 
instead of being dragged along beneath it is proved by the 
occurrence of entirely angular and unglaciated forms, and the 
fact that, as in the case of the great Madison boulder in New' 
Hampshire, the original orientation of the blocks is sometimes 
unchanged.* 
In the preceding paragraph I have but followed in the foot¬ 
steps of Prof. Chamberlin, for he has shownf very fully and 
■clearly that the ice flowing over and around prominent ledges 
.and rocky hills will naturally carry awrny in true englacial 
fashion many angular blocks and more or less of other forms 
*Appalachia, vi, 66. 
t Journal of Geology, i, 47-60, and 255-267. 
