Engineial Drift. — Crosby . 
205 
have tended in some measure to check or arrest the motion of 
the ice which had flowed outward from the mountains. Ow¬ 
ing, however, to the forward motion of the piedmont glaciers, 
as well as to their termination on tracts where a short time 
before ablation had been in excess of snowfall, they must 
have terminated somewhat abruptly or with high marginal 
gradients; and the conditions were, therefore, extremely fa¬ 
vorable for their overriding the new and still stationary ice¬ 
fields by which they were invested, in the manner indicated 
by the experiments of Favre, Bailey Willis, and others, on the 
compression and folding of sedimentary deposits of unequal 
thickness or rigidity.* The overridden tract or zone of ice 
must slowly acquire the motion of the overriding sheet, and 
thus in its turn come to override other tracts. In fact, it 
seems to me very probable that this process of overriding and 
absorption would continue almost indefinitely, extending, pos¬ 
sibly, over a large part of the glaciated area. The only alter¬ 
native views are that the piedmont glaciers became station¬ 
ary, or that they were able by simple thrust to induce motion 
in the embryo and still sedentary ice-sheet across a breadth 
of hundreds of miles. The theory of overriding lies between 
these extremes. 
Assuming a uniform annual snowfall over the area of the 
sedentary ice-sheet, it is obvious that since its area is gradually 
extended southward by the progressive climatic refrigeration, 
while the annual ablation as gradually diminishes northward, 
its thickness must increase backward from the margin. To 
the surface gradient thus resulting must be added the south¬ 
ward gradient of the surface of the land, which was probably 
augmented by differential continental uplift. 
If, as Upham holds ,\ and, as certainly seems most probable, 
the precipitation of snow over the growing ice-sheet was not 
uniform, but greatest for the first 100 to 200 miles inward 
from the margin, the surface gradient must have culminated 
on these peripheral tracts, diminishing gradually backward. 
This condition would, obviously, favor an early beginning of 
outward movement or flow in the marginal zone, and tend in 
an equal degree to retard motion in the central area. 
*See also Upham’s paper on “Drumlins and Marginal Moraines of 
Ice-sheets,” Bull. Geol. Soc. of America, vii, 22. 
fBull. Geol. Soc. of America, vi, 344. 
