188 THE ANDES OF SOUTHERN PERU 



imaginary) valley floor which extends without important change 

 of expression to the border of the Cordillera. No extensive cliffs 

 occur on the restored surface, and none now occur on large tracts 

 of the still undissected upland. Since the mature slopes represent 

 a long period of weathering and erosion, their surfaces were cov- 

 ered with a deep layer of soil. Where glaciation at the higher 

 levels and vigorous erosion along the canyons have taken place, 

 the former soil cover has been removed; elsewhere it is an impor- 

 tant feature. Its presence lends a marked softness and beauty to 

 these lofty though subdued landscapes. 



The graded mountain slopes were not all developed (1) at the 

 same elevation, nor (2) upon rock of the same resistance to de- 

 nudation, nor (3) at the same distance from the major streams, 

 nor (4) upon rock of the same structure. It follows that they will 

 not all display precisely the same form. Upon the softer rocks at 

 the lowest levels near the largest streams the surface was worn 

 down to extremely moderate slopes with a local relief of not more 

 than several hundred feet. Conversely, there are quite unreduced 

 portions whose irregularities have mountainous proportions, and 

 between these extremes are almost all possible variations. Though 

 the term mature in a broad way expresses the stage of develop- 

 ment which the land had reached, post mature should be applied 

 to those portions which suffered the maximum reduction and now 

 exhibit the softest profiles. At no place along the 73rd meridian 

 was denudation carried to the point of even local peneplanation. 

 All of the major and some of the minor divides bear residual ele- 

 vations and even approximately plane surfaces do not exist. 



Among the most important features of the mature slopes are 

 (1) their great areal extents — they are exhibited throughout the 

 whole Central Andes, (2) their persistent development upon rocks 

 of whatever structure or degree of hardness, and (3) their pres- 

 ent great elevation in spite of moderate grades indicative of their 

 development at a much lower altitude. Mature slopes of equiva- 

 lent form are developed in widely separated localities in the Cen- 

 tral Andes : in every valley about Cochabamba, Bolivia, at 10,000 

 feet (3,050 m.) ; at Crucero Alto in southern Peru at 14,600 feet 



