Upham.] 
236 
[Feb. 18, 
and appear to have been mostly obstructed and closed by the 
transportation and deposition of modified drift. The waning 
ice-fields were then deeply incised by brooks and rivers pouring 
over them in the descent to their border and to the adjacent land 
lately uncovered by the glacial retreat. Hydrographic basins of 
the ice-sheet probably extended 50 to 200 miles or more from its 
margin, resembling those of a belt of country along a sea coast ; 
but the glacial rivers, and their large and small branches, had 
much steeper gradients than those of the present river systems 
on the land surface, and often or generally they flowed in deep 
ice-walled channels, more like canons than ordinary river valleys. 
The ablation of the ice-surface by sunshine and rains, and its 
deeper melting by water-courses, sculptured it into hills, ridges, 
and peaks, and these were doubtless most conspicuously isolated 
by deep intervening valleys close to the receding ice-margin. It 
is therefore easy to see how ice-masses would be left here and there 
as peninsulas or islands outside the waning glacial boundary, es- 
pecially where rivers debouched from the ice-sheet to the land ; 
and thus the basins of our lakes in modified drift were filled each 
with its hill of ice while sedimentation was going on around them. 
Evidence that a large amount of drift was contained in the 
lower part of the ice-sheet on certain areas near its border, where 
massive terminal moraines were being accumulated, is afforded by 
lakes Benton, Shaokatan, and Hendricks in southwestern Minne- 
sota, with deeply eroded glacial river-courses extending from them 
south westward through the high Coteau des Prairies ; 1 and on 
belts to which currents of the ice-sheet converged from large 
areas on each side, there was also much drift within the ice, as is 
shown by the osar or esker of Bird’s Hill, near Winnipeg. 2 On 
each of these tracts the amount of material held in the ice, which 
President Chamberlin denominates englacial drift , was equal to a 
uniform thickness of not less than 40 feet. Wherever abundant 
gravel, sand, and clay deposits were spread by the waters of the 
glacial melting, the volume of the englacial drift was great. Only 
part of it is comprised in these beds of modified drift ; and the 
portion which was not carried away by the glacial streams but fell 
Geological and Natural History Survey of Minnesota, Ninth annual report, for 
1880, pp. 322-6 ; Final report, vol. i, 1884, pp. 603-4. 
2,< Glacial Lake Agassiz in Manitoba,” Geol. and Nat. Hist, Survey of Canada, An-, 
nual report, vol. iv, for 1888-89, Part E, pp. 36-42, 
