108 The American (reoUxjisl. August, is95.^ 
Ai-! an aid i'cr ck^arcr stateiiK'nt, the acconipanying figure, 
drawn on the same scale vertically as horizontally, may rep- 
resent a. section of the border of the departing ice-sheet along 
a distance of ten miles from south to north, where its origi- 
nal thickness, as probably for the vicinity of the Leaf hills, in 
northwestern Minnesota, was about a mile. Englacial drift, 
as 1 have shown in previous papers,* had been carried by the 
ice currents in some important amount into the basal quarter 
or third of the ice-sheet; and when the superficial melting or 
ablation reduced the ice border to a less thickness, this drift 
was gradually uncovered upon the ice surface. The rates of 
ascent of the frontal slope are taken in accordance with the 
upper limits of glacial action on mountains, and after careful 
consideration of the surface gradients of the Alpine glaciers 
and of the Greenland ice- sheet, as 400 feet in the first mile, 
200 feet in the second mile, and 150, 120, 100, 85, 75, 67, 60, 
and 55 feet in the third to the tenth miles, respectively, at- 
taining an altitude of 1,312 feet, or about a quarter of a mile. 
Thence we may suppose the ascent to average 50 feet per mile 
for the next nine miles, by which the altitude of a third of a 
mile, the probable \ipper limit of the englacial drift, would be 
reached. 
• OnitiMAL Maximum Thickness, Omc MiuE. 
{' . Approj-imate unpcr iimit of infr/aciaJ Drift, a third ol amile. ^"■ 
Fig. 1. Section of the border of the ice-sheet during its retreat. 
Scale, three miles to an inch. 
On areas where the ice-sheet built up large marginal mo- 
raines, and also wherever its drainage from ablation brought 
exceptional volumes of modified drift, or stratified gravel, 
sand and clay, directly supplied by the ice melting, we must 
believe that the amount of the englacial drift was greater than 
on other tracts having smaller moraines and little modified 
drift. Let us assume, therefore, for the definite illustrative 
case in which we are seeking to account for prominent mo- 
raine accumulations, that the total englacial drift, in the lower 
third or 1,760 feet of the ice-sheet, was equal to a thickness of 
15 feet. This may have been distributed, as shown in the ac- 
*Bulletin, G. S. A., vol. in, 1892, pp. 131-118: vol. v, 1894, pp. 71-86. 
Am. Geologist, vol. viii, pp. 376.385, Dec, 1891; vol. x, pp. 339-362. Dec.^ 
1892; vol. XII, pp. 36-13, July, 1893. 
