of the Cuzco Valley, Peru. 19 



of 18,000 to 21,000 feet, and into the plateau surface are cut 

 a number of canyons rivaling the Grand Canyon of the Colo- 

 rado in depth and ruggedness. In fact so deeply trenched are 

 the highlands, so narrow the spacing between the stupendous 

 gorges, and so commanding the volcanoes and erosion remnants 

 that it is difficult for one in the midst of these features to 

 view the region as a plateau. The sight of the forest is lost 

 in the contemplation of the trees. 



When the materials composing the plateau are examined 

 they are found to consist of continental and marine sediments 

 penetrated by igneous intrusions. The sediments have been 

 subjected to widespread disturbance; flat-lying rock is rare. 

 Folds of intricate design are everywhere encountered, and 

 faults are characteristic features. In this expanse of highly 

 disorganized strata the plateau surface has been developed 

 with slight regard to structure. In other words, the Peruvian 

 plateau is an uplifted erosion surface dating from late Mesozoic 

 time. For the purpose of description it will be called the Inca 

 peneplain. As in the case of Bolivia, pointed out by Professor 

 Bowman, 9 this conception of the Andes is widely at variance 

 with previous interpretations of South American physiography 

 and is quite out of accord with popular descriptions. In the 

 view here taken the bewildering display of lofty peaks and 

 profound canyons, which appear to be arranged in capricious 

 fashion, find their appropriate setting as features of an organ- 

 ized landscape systematically developed. Coropuna, El Misti, 

 and their companions are superposed on the western margin 

 of the Inca peneplain; Ausangate and Salcantay are gigantic 

 examples of erosion remnants formed of resistant rocks etched 

 by vigorous streams on the northeastern slopes of the Peruvian 

 plateau. This interpretation of the physiographic develop- 

 ment of southern Peru assigns to the Cuzco Valley its proper 

 relation as a trough of small dimensions sunk below the level 

 of the Inca peneplain. 



Land Forms in Cuzco Valley. 



Factors Conditioning Erosion. 



The Cuzco Valley, located between 13° 27' and 13° 40' 

 south latitude, is well within the Tropics. The heat incident 

 to low latitudes is to a large degree offset, however, by the 

 high elevation, and the influence of geographic position is 

 reflected chiefly by alternating wet and dry seasons. Scatter- 



8 This Journal, xxviii, pp. 197-217, 1909. 



