210 



THE ANDES OF SOUTHERN PERU 





of very loose material, are well preserved. They indicate clearly 

 that the time since their formation has witnessed no wholesale 

 topographic change. If (1) no important topographic changes 

 have taken place, and (2) a vigorous glacier lay for a long period 

 back of a given moraine, and (3) suddenly retreated several 

 ■miles and again became stable, we are left without confidence 

 in the application of the topographic hypothesis to the glacial 



features of the Vilcapampa 

 region. Glacial retreat may 

 be suddenly begun in the 

 case of a late stage of topo- 

 graphic development, but it 

 should be an orderly retreat 

 marked by a large number 

 of small moraines, or at 

 least a plentiful strewing of 

 the valley floor with debris. 



The number of moraines 

 in the various glaciated val- 

 leys of the Cordillera Vil- 

 capampa differ, owing to 

 differences in elevation and 

 to the variable size of the 

 catchment basins. All val- 

 leys, however, display the 

 same sudden change from moraine to moraine and the same 

 characteristics of gradient. In all of them the lowermost 

 moraine is always more deeply eroded than the higher 

 moraines, in all of them glacial erosion was sufficiently pro- 

 longed greatly to modify the valley walls, scour out lake basins, 

 or broad flat valley floors, develop cirques, aretes, and pinnacled 

 ridges in limited number. In some, glaciation was carried to the 

 point where only skeleton divides remained, in most places broad 

 massive ridges or mountain knots persist. In spite of all these 

 differences successive moraines were formed, separated by long 

 stretches either thinly covered with till or exposing bare rock. 



O I6887.W\; 



15870 

 JIG300 ,. 



Mt.Pumasillo 



Scale of Miles 



O 1 



Fig. 139 — Glacial features on the eastern 

 slopes of the Cordillera Vilcapampa. 



