300 



THE ANDES OF SOUTHERN PERU 



familiar with. Johnson's apparently complete proof of their 

 genetic relation to the cirques. But it was less surprising to dis- 

 cover the position of the few observed — high up on the cirque 

 walls and always near the upper limit of the snowfields. 



A third fact from regions once glaciated but now snow-free 

 also combined with the two preceding facts in weakening the whole- 

 sale application of Johnson's hypothesis. In many headwater 

 basins the cirque whose wall at a distance seemed a unit was really 

 broken into two unequal portions; a lower, much grooved and 

 rounded portion and an upper unglaciated, steep-walled portion. 

 This condition was most puzzling in view of the accepted explana- 

 tion of cirque formation, and it was not until the two first-named 

 facts and the applications of the curves of snow motion were 

 noted that the meaning of the break on the cirque became clear. 



Referring to Fig. 198 we see at 

 once that the break occurs at y 

 and means that under favorable 

 topographic and geologic condi- 

 tions sapping at y takes place 

 faster than at x and that the re- 

 treat of y-z is faster than x-y. 

 It will be clear that when these 

 conditions are reversed or sapping at x and at y are equal a 

 single wall mil result. On reference to the literature I find that 

 Gilbert recently noted this feature and called it the schrundline* 

 He believes that it marks the base of the bergschrund at a late 

 stage in the excavation of the cirque basin. He notes further that 

 the lower less-steep slope is glacially scoured and that it forms 

 "a sort of shoulder or terrace." 



If all the structural and topographic conditions were known in 

 a great variety of gathering basins we should undoubtedly find 

 in them, and not in special forms of ice erosion, an explanation 

 of the various forms assumed by cirques. The limitations in- 

 herent in a high-altitude field and a limited snow cover prevented 



Fig. 198 — The development of cirques. 

 See text, p. 299, and Fig. 199. 



G. K. Gilbert, Systematic Asymmetry of Crest Lines in the High Sierra of 

 California. Jour. Geol., Vol. 12, 1904, p. 582. 



