30 H. L. FAIRCHILD— ICE EROSION THEORY A FALLACY 



called "hanging" valleys. Figure 1 is his figure, somewhat modified. 

 The words of his conclusion on this point are as follows : 



''It follows that the second feature ('hanging' valleys) of the typical glacial 

 canyons may naturally result from temporary occupation of water-cut canyons by 

 ice, and that it does not necessarily argue profound glacial erosion " (page 3(30). 



From this physical analysis he recognizes the possibility of some 

 basining erosion : 



" . . . Thus the excavations of depressions by direct ice-action has a defi- 

 nite though indeterminate limit, and can probably never exceed a moderate frac- 

 tion of the depth of the ice . . . " (page 3ti2). 



" . . . The irregularities of gradient peculiar to such canyons are not greatly 

 intensified, while glaciated rock basins are comparatively rare and of slight depth. 

 It equally follows that the occupation was only temporary and the sum of glacial 

 erosion relatively inconsiderable" (page 363). 



These conclusions of Doctor McGee adverse to ice erosion are spe- 

 cially significant because they are based on merely the mechanics of the 

 glacier as a plastic, moving body, taking into account weight, slope, 

 friction, and the potential energy available for mass motion, or the down- 

 stream impulse and weight at any point, constituting the intensity at 

 that point. It should be clearly understood that this analysis does not 

 take into account at all the important adverse factors due to the effect of 

 the accumulating subglacial drift burden. The problem is treated as 

 if the glacier had the power of constantly clearing its saw-cut. 



As a general conclusion it may safely be held that without glacial 

 widening of a valley there can be little deepening. This is a very im- 

 portant principle and can be effectively applied in special cases (see 

 page 40). 



From whatever side we attack the problem of glacial erosion the 

 physical and geologic study leads to the conclusion that all the erosion 

 factors are restricted or limited in their effectiveness. All the evidence 

 from observation and direct study of living glaciers and all the theoretic 

 conclusions derived by sound reasoning from known phenomena lead to 

 the confident judgment that stream glaciers commonly act on their beds 

 as flexible rasps. Rarely or locally they act as rigid planes. In either 

 case their erosive effect is either limited or exceedingly slow. They 

 scratch and polish their rocky beds, and to a limited extent they widen 

 their channels. 



If the glaciers were given unlimited time of millions of years they 

 might accomplish considerable cutting by the slow rasping process ; but 

 their work would lag far behind that of rivers. Any claim that they 

 can possibly cut faster than rivers is an untenable assumption. Their 



