18 



H. E. Gregory — A Geologic Reconnaissance 



gorges in the plateau rim and in some places have trenched 

 the upland surface, they are within an arid zone and therefore 

 carry little water to the ocean. The initial topographic expres- 

 sion of the northeast rim of the Peruvian plateau, in a region 

 of heavy rainfall, has undergone extensive modification in 

 consequence of vigorous headwater erosion by tributaries of the 

 Ucayali and the Madre de Dios. In the latitude of Cuzco 

 the surface of the plateau is deeply dissected by the Paucar- 

 tambo, the Urubamba, and especially by the Apurimac, which 

 carries water to the Atlantic from points less than 100 miles 



Fig. 9. 



OAUSANGATE. N. VILCANOTA 



C 2 SALCANTAY Cli)Z.CO j 



AMAZON PLAINS R URUBAMBA ii R. APURIMAC ! OCOROPUNA PACIFIC OCEAN 



Fig. 9. Diagram of a portion of Southern Peru, including the Cuzco 

 Valley; based on topographic data from all available sources. The 

 block lies approximately between parallels 13 and 16 degrees south 

 latitude. Drawn by Gladys M. Wrigley. 



from the Pacific. These major streams crossing the plateau 

 in a northwesterly direction have swift flow interrupted by 

 rapids, but their descent is gentle compared with that of 

 streams on the rim of the plateau. Between its source and its 

 junction with the Urubamba, a distance of about 450 miles, 

 the fall of the Apurimac is approximately 1,400 feet; Rio 

 Majes, which receives its waters from the same area, descends 

 an equal amount in about 90 miles. 



As shown on the outline map (fig. 2) the heart of the plateau 

 is attacked by tributaries to the great interior basin. These 

 streams, however, carry no debris beyond the plateau borders, 

 and their strength is sapped by a semiarid climate and a high 

 base-level of erosion, the surface of Lake Titicaca, 12,500 

 feet above tide. 



Above a general surface at 13,000 to 15,000 feet the Peru- 

 vian plateau is studded by snow-capped peaks attaining heights 



