Engldc ici. l Drift.—Or os b y . 
207 
If“we could only know the normal ratio of vertical to hori¬ 
zontal or areal growth, we might, assuming that climatic os¬ 
cillations began at an early period of its history, reach a 
definite conclusion as to the probable extent of the ice-sheet 
when it first began to move or flow outward. We may, how¬ 
ever, attack the problem in another way. The equivalent of 
one foot of ice is, perhaps, not too large an estimate of the 
average annual excess of precipitation over ablation on the 
zone of maximum growth. Warm periods may arrest and 
reverse the areal growth while the central tracts continue to 
increase in thickness, though possibly at a diminished rate, 
thus accentuating the frontal slope. That is, while cold peri¬ 
ods must mean, on a plain country, relatively rapid horizontal 
growth and therefore a growth unfavorable to early movement, 
the ice-sheet becoming proportionally thinner; a moderately 
warm period, following a cold one, means continued growth in 
thickness with a great reduction of area, and hence a growth 
especially favorable to early movement, the ice-sheet becom¬ 
ing proportionally thicker. It seems probable, therefore, that 
a warm period following a cold period of two or three thou¬ 
sand years duration would induce movement in the sedentary 
ice-sheet, and that movement at this early period might atfect 
almost simultaneously the peripheral and central tracts. 
Assuming now that movement of the ice-sheet was inaugu¬ 
rated during a warm period, and that the southern margin of 
the ice has retreated far to the north through ablation, devel¬ 
oping a bold and aggressive front; it is obvious that the 
succeeding cold period must have caused a rapid extension 
of sedentar}^ ice southward from the front of the moving 
sheet; and the former would inevitably be progressively over¬ 
ridden and absorbed by the latter. It may be noted, also, 
that these phenomena would be repeated with each recurring- 
climatic cycle. This section may be summarized as follows: 
The Pleistocene ice-sheet was formed in situ , by snow accu¬ 
mulation, over the main part of the plain country within the 
glaciated area. The motion which this sedentary ice-sheet 
subsequently acquired probably originated in several ways: 
1. In the vicinity of the mountainous tracts, through its 
being progressively overridden and absorbed by piedmont 
glaciers. 
