Crosby. 1 
134 
[May 21 
rather than minimize the importance of glacial erosion, they do not 
lend any real support to the view that the recent ice-sheet pro- 
foundly modified the topography of the glaciated area. It is still 
safe to say that such topographic changes as can he proved should 
he attributed far more to the distribution than to the formation of 
the drift. We may pass now to a brief consideration of the bear- 
ing of some of the facts brought out by the analyses upon the 
problems of tho transportation of the drift and the movement of 
the ice-sheet. 
It is clear enough that the till, when truly subglacial and not 
euglacial, was not moved forward bodily or en masse. But the 
glaciated character of nearly all the coarser material, as well as its 
arrangement and the general structure of the till, indicates that it 
must have experienced differential horizontal movements or flowing 
in which, normally, every particle or fragment slipped or was 
squeezed forward with reference to those immediately below it, the 
velocity diminishing downward through the friction of the under- 
lying ledges. In other words, nearly every mass, large or small, 
soft enough to be shaped or modified by the movement exhibits a 
typical glaciated form ; and this proves not only that the glaciation 
was not limited to masses which were firmly caught between the 
ice and the solid ledges, but also that it was in every case essen- 
tially a slipping and not a rolling movement. We look in vain for 
evidence that the bowlders in the till were rolled over and over 
during their transportation. The great bowlder in Madison*, N. II., 
probably the largest in New England, is a case in point. Although 
it has been carried nearly two miles from its parent ledge, it is 
still right side up and with its original orientation but slightly 
changed. Of course the melting of the ice and settling down of 
the till would naturally result in the overturning of many bowlders 
which had escaped rotation during their transportation. In short, 
the principle governing the movement of a river and of the ice- 
sheet itself applies also in the case of the ground moraine or till. 
These differential horizontal movements mean that the till acted 
as a lubricant for the ice-sheet; and the clayey element, especially, 
cooperating in many cases with the pent up subglacial waters, must 
have greatly facilitated the onward progress of the ice. We find 
here an important contrast between a continental ice-sheet and an 
ordinary valley glacier. The latter, in a few centuries, sweeps its 
narrow and tortuous path free of the original cla3 r ey soil of the 
