212 



THE ANDES OF SOUTHERN PERU 



s;**//-;"/.?/. 



It seems at first strange that the largest glaciers were west 

 of the divide between the Urubamba and the Apurimac, that is, on 

 the relatively dry side of the range. The reason lies in a strik- 

 ing combination of topo- 

 graphic and climatic condi- 

 tions. Snow is a mobile 

 form of precipitation that is 

 shifted about by the wind 

 like a sand dune in the 

 desert. It is not required, 

 like water, to begin a down- 

 hill movement as soon as it 

 strikes the earth. Thus, it 

 is a noteworthy fact that 

 snow drifting across the 

 divides may ultimately cause 



the largest snowfields to lie 



where the least snow ac- 

 tually falls. This is illus- 

 trated in the Bighorns of 

 "Wyoming and others of our 

 western ranges. It is, how- 

 ever, not the wet snow near 

 the snowline, but chiefly the 

 dry snow of higher altitudes 

 that is affected. What is 

 now the dry or leeward side 

 of the Cordillera appears in glacial times to have actually re- 

 ceived more snow than the wet windward side. 



The topography conspired to increase this contrast. In place 

 of many streams, direct descents, a dispersion of snow in many 

 valleys, as on the east, the western slopes had indirect descents, 

 gentler valley profiles, and that higher degree of concentration of 

 drainage which naturally goes with topographic maturity. For 

 example, there is nothing in the east to compare with the big spur- 

 less valley near the pass above Arma. The side walls were so 



Fig. 140 — Glacial sculpture in the heart 

 of the Cordillera Vilcapampa. In places the 

 topography has so high a relief that the glaciers 

 seem almost to overhang the valleys. See Figs. 

 96 and 179 for photographs. 



