306 THE ANDES OF SOUTHERN PERU 



part of the figure represent the preglacial surface and the solid 

 line beneath it the present surface. It will not matter what value 

 we give the space between the two lines on the left to express non- 

 glacial erosion, since had there been no glaciers it would be the 

 same on both sides of the ridge. The feeble glacier occupying the 

 right-hand slope was able in a very brief period to erode a de- 

 pression far deeper than the normal agents of denudation were 

 able to erode in a much longer period, i.e., during all of intergla- 

 cial and postglacial time. Gilbert concludes: "The visible ice- 

 made hollows, therefore, represent the local excess of glacial over 

 nonglacial conditions." 



In the Central Andes are many volcanic peaks and ridges 

 formed since the last glacial epoch and upon them a remarkable 



SonSW NorNE s N 



Fig. 200 — Diagrammatic cross-section Fig. 201 — Postglacial volcano recessed 



of a ridge glaciated on one side only; on shady southern side by the process of 

 with hypothetical profile (broken line) nivation. Absolute elevation 18,000 feet 

 of preglacial surface. (5,490 m.), latitude 14° S., Maritime 



Cordillera, Peru. 



asymmetry has been developed. Looking southward one may see 

 a smoothly curved, snow-free, northward-facing slope rising to a 

 crest line which appears as regular as the slope leading to it. 

 Looking northward one may see by contrast (Fig. 194) sharp 

 ridges, whose lower crests are serrate, separated by deeply re- 

 cessed, snow-filled mountain hollows. Below this highly dissected 

 zone the slopes are smooth. The smooth slope represents the 

 work of water ; the irregular slopes are the work of snow and ice. 

 The relation of the north and south slopes is diagramm atically 

 shown in Fig. 201. 



To demonstrate the erosive effects of snow and ice it must be 

 shown: (1) that the initial slopes of the volcanoes are of post- 

 glacial age; (2) that the asymmetry is not structural; (3) that the 

 snow-free slopes have not had special protection, as through a 

 more abundant plant cover, more favorable soil texture, or other- 

 wise. 



