Eng la c i a l Dr ift .-—■ Crosby . 
221 
The history of an ice-sheet embraces, as regards the basal 
'temperature, two distinct and contrasted periods: (1) the 
period of growth and maximum development, when the tem¬ 
perature of the lower part of the ice is permanently below 
freezing; (2) the period of decline, when the basal tempera¬ 
ture is above freezing. The first is the period of active ab¬ 
rasion and scoring of the bed-rock, all detritus being frozen 
into the ice as fast as formed. Furthermore, the water result¬ 
ing from superficial summer melting of the ice, descending 
through crevasses to the basal portion of the sheet and re¬ 
freezing there, not only adds the newly formed detritus to the 
base of the moving sheet, but also, perhaps, favors an actual 
downward growth of the ice-sheet, whereby detritus which 
has previously become englacial is raised to greater hights in 
the ice, the growth of the ice-sheet being chiefly upward by 
snowfall in winter and downward by basal freezing in sum¬ 
mer. During the second period the ice-sheet wastes by basal 
as well as superficial melting, the englacial drift becomes 
subglacial (ground moraine) and glacial erosion gradually 
-ceases. 
The prevailing opinion among geologists, as recently colla¬ 
ted by Culver,* is evidently strongly against the efficiency of 
glacial erosion; and this trend of opinion is certainly justi¬ 
fied, so far as it is based upon observations on living glaciers. 
Observations on the velocity and abrasive power of modern 
ice-sheets have never been made. But the relatively high 
efficiency of the Pleistocene ice-sheet in this respect is clearly 
proved by the undisputed facts that, over practically the en¬ 
tire glaciated area north of the terminal moraine, all the pre¬ 
glacial sedentary soil and partially decayed rock was worn 
away, involving on a large aggregate area extensive erosion 
of the hard unaltered rocks; and that the prevailing color of 
the ground moraine, below the sharpty defined limit of post¬ 
glacial oxidation, is that of crushed rock and not of residuary 
or other preglacial detritus. The latter fact, which seems to 
have been but little regarded, is probably of equal significance 
with the first; and the general conclusion based upon both 
these is further sustained by observations upon the propor¬ 
tions of distinctively glacial detritus in the ground moraine.f 
* Trans. Wis. Acad. Sci.. etc., x, 339-366. 
fProc. Boston Society of Natural History, xxv, 131-138. 
