38 'I'll' American Geologist. July, 1893 
t 
DefiniU Division between the Subglacial Till and the /<< 
carrying its enclosed Drift. — Observations of widely differenl 
glacial >t nation on adjoining parts of the same rock exposure, 
t he i wo sets of stria- being od surfaces of slightly different 
inclination which join each other with a bevelled edge or an- 
gle, convince me that a very definite plane divided the bottom 
of the moving ice-sheet with its enclosed drift, boulders, 
pebbles, and sand grains, which acted as graving tools, from 
underlying stationary drift accumulations, not less than from 
the immovable bed-rock. There was. at least in the places of 
these crucial observations, no pushing- or dragging forward of 
the drift beneath the ice. Several of these localities of adja- 
cent differently striated bevelled rock surfaces I have noted 
in southwestern Minnesota,* and others in Somerville, Mass. 
In all these places the contrasted directions of glaciation are 
apparently referable to deflections of a continuous ice-current, 
rather than to any withdrawal and subsequent new advance 
of the ice. During some considerable interval between the 
times of different courses of the glacial current, a very thin 
layer of stationary drift covering a part of the rock surface 
protected it from striation while the later ice erosion engraved 
its striae on the closely contiguous part of the same ledge. 
Here, and I think likewise generally, the onl} r transportation 
of drift took place within the moving ice-sheet, not by any 
sliding or rolling under it. 
Glacial Currents carry Drift <>»hj into the lower part "/tin 
Tee-sheet. — The conditions of the flowing ice which seem to 
me to have been efficacious to carry drift upward into it 
from tracts of plane or only moderately undulating contour, 
were the more rapid onflow of the ice-sheet in its upper and 
central parts and even in the portion near the ground but not 
in contact with it, than upon the bed of the ice-sheet where 
its movement was much retarded by friction. A very good 
analogy with the slowly rising currents which I believe to 
have existed in many portions of the base of the ice-sheet is 
afforded by the edges of alpine glaciers, where the crevasses 
extending diagonally upstream into the glacier testify that 
*Geology of Minn., vol. i, 1884, pp. 504-5, and 549-50. Compare with 
Prof. Chamberlin's memoir, "The Rock-Scorings of the iireat Ice Inva- 
sions," U. S. Geol. Survey, Seventh An. Rep., for 1885-'86, pp. 147-248, 
especially pp. 175, 17G, 200-207. 
