206 The American Geologist. i-pril, 1896 
It seems a reasonable assumption that the period of the 
growth and culmination of the ice-sheet, equally with that of 
its waning and disappearance, must have witnessed marked 
climatic oscillations of long period. In fact, we would hardly 
be justified in supposing that the great crustal movement 
which gave us the Ice age was steadily progressive, without 
interruption or reversal, until its final culmination. During 
each period of climatic amelioration and increased ablation of 
the sedentary ice-sheet, its margin must have retreated to the 
northward; and since the ablation must have rapidly dimin- 
ished northward, snowfall exceeding ablation within a few 
miles of the margin, we have here another efficient cause of a 
high marginal gradient. That is. precipitation and ablation 
cooperate to accentuate the frontal slope of the ice-sheet. 
Now the chief factors in determining movement of the ice- 
sheet were, undoubtedly, its thickness and marginal gradient. 
We have few reliable data derived from observations on mod- 
ern ice-sheets to indicate the magnitude which these factors 
must attain to inaugurate movement in an ice-sheet formed 
in situ over such a dissected peneplain as is presented by the 
surface of a large part of Canada and the northern United 
States, many of the valleys having contrary or transverse di- 
rections. But the absence from a large part of the glaciated 
urea of mountains or dominant bights of land requires us to 
assume that over considerable tracts the sedentary ice-sheet 
did eventually begin to How without having experienced the 
overriding or shearing thrust of the piedmont ice-fields. The 
outward movement thus originating in the peripheral tracts 
during a period of excessive ablation must have extended 
backward, perhaps until it met the forward thrust of the 
piedmont glaciers. 
During a period of active growth of a sedentary ice-sheet, 
each annual snowfall advances its margin, and increases its 
thickness by amounts depending upon the distance from the 
margin. Although the zone of maximum precipitation does 
not necessarily coincide exactly with that of most rapid 
growth or maximum excess of precipitation over ablation, we 
may assume that they would not be widely separated, and 
that the locus of most rapid growth would, therefore, be. found 
at a moderate distance, say LOO to 200 miles from the margin. 
