232 THE ANDES OF SOUTHERN PERU 



gradients in the direction of the oldest deposits, shows very 

 clearly a corresponding progression in the growth of the Andes 

 at intervals throughout the Tertiary. 



Thus we have aggradation in the Tertiary at the foot of the 

 growing Andes ; aggradation in the Pliocene or early Pleistocene 

 on the floor of a deep valley cut in earlier deposits; aggradation 

 in the glacial epoch; and aggradation now in progress. Basin 

 deposits within the borders of the Peruvian Andes are relatively 

 rare. The profound erosion implied by the development, first of 

 a mature topography across this great Cordillera, and second of 

 many deep canyons, calls for deposition on an equally great scale 

 on the mountain borders. The deposits of the western border are 

 a mile thick, but they are confined to a narrow zone between the 

 Coast Range and the Cordillera. Whatever material is swept be- 

 yond the immediate coast is deposited in deep ocean water, for 

 the bottom falls off rapidly. The deposits of the eastern border 

 of the Andes are carried far out over the Amazon lowland. Those 

 of earlier geologic periods were largely confined to the mountain 

 border, where they are now upturned to form the front range of 

 the Andes. The Tertiary deposits of the eastern border are less 

 restricted, though they appear to have gathered chiefly in a belt 

 from fifty to one hundred miles wide. 



The deposits of the western border were laid down by short 

 streams rising on a divide only 100 to 200 miles from the Pacific. 

 Furthermore, they drain the dry leeward slopes of the Andes. 

 The deposits of the wet eastern border were made by far larger 

 streams that carry the waste of nearly the whole Cordillera. 

 Their shoaling effect upon the Amazon depression must have been 

 a large factor in its steady growth from an inland sea to a river 

 lowland. 



