THE METAMORPHISM OF GLACIAL DEPOSITS 483 
ice areas and bands of drift constructed by former ice.t~ A con- 
servative estimate of the depth of Wisconsin ice over the Erie basin is 
at least 2,000 feet. This figure is based on two considerations: 
the present difference in level between Lake Erie and altitudes south 
that were covered by ice is about 800 feet. The ice reached south 
of the Erie basin approximately 200 miles; if its surface sloped even 
six feet per mile, this would represent a depth of 1,200 feet which, 
plus the 800 feet due to the difference in altitude, makes approximately 
2,000 feet. The basal pressure per square foot for clear ice of this 
thickness would be 115,500 pounds. 
In New York state, there is a greater difference in altitude, even 
when we neglect the overdeepened portions of the major Finger 
Lake valleys. ‘The range in altitude alone would give 1,500 feet of 
ice; this, in connection with the surface slope of the ice, would give 
a depth of approximately 2,500 feet, which represents a basal pres- 
sure per square foot of over 144,000 pounds. Both these computa- 
tions, it is noted, are for clear ice. Knowing that the ice-sheet 
must have carried constantly some drift, these figures undermeasure, 
perhaps, the real pressure. That the subjacent deposits would be 
compressed by the weight of this ice is undebatable. 
Adams has shown that a condition of rock-flowage was induced 
in marble by a pressure of about 18,000 pounds per square inch.? 
The pressure on the sediments, as discussed above, is in either case 
more than 9,ooo pounds per square inch. 
Another possible factor associated with the question of pressure is 
the development of heat. Even the laggard motion of an ice-sheet 
represents energy which through basal interference is converted into 
heat. ‘This heat may have no other manifestation than the wastage 
of ice near the friction zone. Whether a dead load upon compres- 
sible matter evolves heat in the absence of appreciable movements 
along planes developed in this matter is a question on which the writer 
is not informed. 
3. It is thought, furthermore, that pressures are evolved by chem- 
ical changes going on in this drift. Such pressure is an accompaniment 
of hydration when the hydrated mineral is confined as must be the 
1 Chamberlin and Salisbury, Geology, Vol. III (1906), pp. 356-58. 
2 F. D. Adams, Bulletin Geological Society of America, Vol. XII (1901), p. 457. 
