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



perature) could favor erosion, the Upper Kern glacier should have had 

 great potency. 



An important fact to be noted in connection with the fact of slight 

 glacial erosion in the Sierras is the existence of " hanging " valleys. 



"... The apex of these cones is generally several hundred feet above the 

 canyon floor. These cones are the product of streams which enter the canyon far 

 above its bottom and which, in many cases, flow in glaciated trenches. These 

 trenches are fine examples of hanging valleys." (Page 329; italics not in the original.) 



Lawson divides the Upper Kern valley into two parts : 



" an upper part, in which the Kern has been engaged in vertical corrasion since the 

 retreat of the ice, and a lower part in which it has been engaged in aggradation." 



He thinks that the aggrading portion 



11 represents an overdeepeningof the canyon floor hy glacial scour, a process which 

 has been exemplified in many glacial troughs, such as in some of the lochs of 

 Scotland and fiords of Norway. . . . Both the degraded and aggraded portions 

 of the glaciated canyon are U-shaped in profile, but the most perfect U-shaped 

 profile is in the former, and is true of the rocky floor and sides of the canyon. 

 The U-shaped effect of the aggraded portion is largely due to the talus slopes at 

 the base of the canyon walls. If the rocky bottom of the canyon beneath the 

 meadow floor be also U-shaped, as is very probable, then the depth of the aggra- 

 dational accumulation may not be more than a hundred feet." (Page 349.) 



The only doubtful matter would seem to be that of the conjectural 

 basin. The fact of less perfect U form of the aggrading section, implying 

 less total erosion, taken in connection with the small volume of morainal 

 debris, and specially the absence in the latter of clayey material or product 

 of abrasion, throws serious doubt on the suggestion of an erosional basin 

 beneath the meadows. However, it is not an important matter, as a 

 basin of 100 feet, or even greater depth, is inconsequential in the valley 

 stretch of perhaps 8 or 9 miles. 



We find in the Sierras all the important features which have been 

 appealed to for proving enormous ice erosion, particularly U-shaped 

 canyons and hanging valleys; but the geological experts are practically 

 unanimous that the Sierran valleys are preglacial, and at the most have 

 only been more or less modified by glaciers. But now, if these topo- 

 graphic features can exist in the Sierras without great deepening of trunk 

 valleys, then it seems illogical and unreasonable to require vast deepen- 

 ing in Norway or Alaska to account for the same features. 



The only equivocal matter which we find in the Sierran study relates 

 to the shallow rock-basins, but they can be explained without requiring 

 any scouring of live rock ; and in any case they are insufficient founda- 

 tion for an assumption of thousands of feet of deepening. 



