GLACIAL FEATURES 305 



oped in the mature stages of topographic development in the gla- 

 cial cycle; (5) that the varying rates of snow, neve, and ice motion 

 at a valley head are the persistent features to which we must look 

 for topographic variations; (6) that the hypothesis here pro- 

 posed is applicable to all cases whether they involve the presence 

 of snow or neve or ice or any combination of these, and whether 

 bergschrunds are present or not; and (7) at the same time affords 

 a reasonable explanation for such variations in forms as the com- 

 pound cirque with its schrundline and terrace, the unbroken cirque 

 wall, the notched cirque, and the recessed, snow-covered mountain 

 slopes unaffected by ice. 



ASYMMETRICAL CREST LINES AND ABNORMAL VALLEY PROFILES IN THE 



CENTRAL ANDES 



To prove that under similar conditions glacial erosion may be 

 greater than subaerial denudation quantitative terms must be 

 sought. Only these will carry conviction to the minds of many 

 opponents of the theory that ice is a vigorous agent of erosion. 

 Gilbert first showed in the Sierra Nevada that headwater glaciers 

 eroded more rapidly than nongiacial agents under comparable 

 topographic and structural conditions. 11 Oddly enough none of 

 the supporters of opposing theories have replied to his argu- 

 ments ; instead they have sought evidence from other regions to 

 show that ice cannot erode rock to an important degree. In this 

 chapter evidence from the Central Andes, obtained in 1907 and 

 1911, will be given to show the correctness of Gilbert's proposition. 



The data will be more easily understood if Gilbert's argument 

 is first outlined. On the lower slopes of the glaciated Sierra 

 Nevada asymmetry of form resulted from the presence of ice on 

 one side of each ridge and its absence on the other (Fig. 200). 

 The glaciers of these lower ridges were the feeblest in the entire 

 region and were formed on slopes of small extent ; they were also 

 short-lived, since they could have existed only when glacial con- 

 ditions had reached a maximum. Let the broken line in the upper 



"Op. cit., p. 300; see pp. 579-588 and Fig. 8. 



