28 H. L. FAiRCHILD — ICE EROSION THEORY A FALLACY 



Shearing of the upper and more rapidly moving layers of the glacier 

 ice over the lower, basal, and laggard layers seems to be established by 

 observation. To whatever degree this factor is active it diminishes the 

 velocity factor at the bottom of the ice and antagonizes erosion. It must 

 be more effective where the bottom ice is obstructed. It argues specially 

 against the flow of ice at the bottom of basins, and implies that the ice 

 resting in a basin is likely to form a bridge over which the upper ice can 

 travel. This is an important point with reference to the assumed glacial 

 origin of lake basins. 



The mechanics of ice flow in basins has been well discussed by Culver 

 (pages 363, 364 of paper cited on page 17). His conclusions, adverse to 

 basining erosion, are certainly sound for the open, graded sections of 

 valleys. The most effective abrasion must be on the rims of the basins, 

 the tendency of which is to obliterate the basin rather than to make it, 

 except, perhaps, at the foot of a steep slope. 



The U shape of ice-worn channels is generally accepted as of diag- 

 nostic value. Like other physiographic features, it may be indefinite 

 and liable to different interpretations by different observers, and the 

 psychologic equation must not be forgotten. The U form belongs to 

 stream valleys in a stage of their development. But such stream valley 

 when scoured by ice is so much more striking, because of its uniformity 

 and smoothness (just as a drumlin is more noticeable than a hill of the 

 same general shape but of uneven surfaces), that it has been assumed as 

 peculiar to ice erosion. Probably in man}' cases the ice has done noth- 

 ing more than to clear the preglacial valley of its talus and other accu- 

 mulations and to give the conspicuous smoothed surfaces. The accord- 

 ance of the glaciated U-shaped sections of mountain valleys in their 

 general proportions and their grade with their unglaciated extensions 

 prove that the amount of ice erosion has not been great; but to what- 

 ever extent ice erosion has occurred the U form implies that it has in- 

 cluded widening of the valley. Some glacialists- have concluded from 

 field study that glaciers tend to widen their valleys much faster than to 

 deepen them, and the analysis of the mechanics of the glacial stream 

 flow leads to the same conclusion. No critical study, so far as the writer 

 is aware, has reached any different conclusion. 



It is not believed possible for a glacier to deepen its valley to any 

 appreciable extent without corresponding increase in width. The rea- 

 sons for the more rapid widening of the valley are not difficult to find. 

 It is not conceived as possible that " plucking " or quarrying or any 

 other mass-erosion can occur at the bottom of a glacier, under the great 

 pressure, after the saliences are planed off and the rock surface has been 

 smoothed. The only possible wear at the bottom is the slow process of 



