EASTERN ANDES: CORDILLERA VILCAPAMPA 211 



In examining this group of features it is important to rec- 

 ognize the essential fact that though the number of moraines 

 varies from valley to valley, the differences in character between 

 the moraines at low and at high elevations in a single valley are 

 constant. It is also clear that everywhere the ice retreated and 

 advanced periodically, no matter with what topographic features 

 it was associated, whether those of maturity or of youth in the 

 glacial cycle. We, therefore, conclude that topographic changes 

 had no significant part to play in the glacial variations in the 

 Cordillera Vilcapampa. 



The country west of the Cordillera Vilcapampa had been re- 

 duced to early topographic maturity before the Ice Age, and then 

 uplifted with only moderate erosion of the masses of the inter- 

 fiuves. That on the east had passed through the same sequence 

 of events, but erosion had been carried much farther. The reason 

 for this is found in a strong climatic contrast. • The eastern is 

 the windward aspect and receives much more rain than the west- 

 ern. Therefore, it has more streams and more rapid dissection. 

 The result was that the eastern slopes were cut to pieces rapidly 

 after the last great regional uplift ; the broad interfluves were nar- 

 rowed to ridges. The region eastward from the crest of the 

 Cordillera to the Pongo de Mainique looks very much like the 

 western half of the Cascade Mountains in Oregon — the summit 

 tracts of moderate declivity are almost all consumed. 



The effect of these climatic and topographic contrasts is mani- 

 fested in strong contrasts in the position and character of the gla- 

 cial forms on the opposite slopes of the range. At Pampaconas 

 on the east the lowermost terminal moraine is at least a thousand 

 feet below timber line. Between Vilcabamba pueblo and Puquiura 

 the terminal moraine lies at 11,200 feet (3,414 m.). By contrast 

 the largest Pleistocene glacier on the western slope, nearly twelve 

 miles long, and the largest along the traverse, ended several miles 

 below Choquetira at 11,500 feet (3,504 m.) elevation, or just at the 

 timber line. Thus, the steeper descents of the eastern side of the 

 range appear to have carried short glaciers to levels far lower 

 than those attained by the glaciers of the western slope. 



