filarial Erosion. — Torr. 149 
Upon this basis our glaciated region is topographically young, 
or at most not far advanced into maturity. 
The length of time required to reach any one of these stages 
is a function of several variable factors, chiefly the amount of 
material to be removed, the rate of motion of the ice, and the 
thickness of the ice-sheet. The first of these factors is deter- 
mined by the pre-existing condition; the second is dependent 
upon the supply of snow and the underlying topography: and 
the third also varies with the ice supply, the topography of 
the land, and perhaps also with the rate of motion. Hence, in 
point of time, the rate of development is quite indefinite, but 
the ultimate result is uniform. 
The erosive action of a great ice-sheet is unevenly distrib- 
uted. Local erosion is most intense in the great valleys down 
which the ice moved with but little obstruction, since here, in 
addition to rapid motion, there is a greater thickness of ice. 
Perhaps, next to these places, hilltops are most powerful ly 
eroded, because here friction is less and consequently motion 
more rapid. Small valleys, transverse to the direction of ice 
flow, and to a less extent small valleys parallel to this direc- 
tion, brought about a condition of slack ice. Here detritus 
has very often been dragged and allowed to remain, while in 
some cases even loose fragments have not been removed from 
cliff faces.* In a more advanced stage of development these 
places might be made to give up their drift accumulations for 
erosive work. 
Just when the most intense general erosion exists in a 
great continental glacier is not determined, Mr. Upham, in 
the paper above quoted, considers the zone of most intense 
erosion to be not far from the periphery of the ice-sheet in a 
belt from fifty to two hundred miles within the ice boundary, 
but I should be more inclined to place it near the periphery 
of the center of ice distribution. Here the glacier remained 
longest, it was thicker here, and it seems also that the motion 
must have been at this place most rapid since the ice-sheel 
spread out from this center or feeding ground. In a valley 
glacier the reverse is true because the snow of a large area is 
concentrated in a relatively small valley: but in a continental 
*This may be seen in a number of places in central western Connect- 
icut.! 
