392 I. Bowman — Physiography of the Central Andes. 



gradients ; and all spurs terminate in scarped fronts partially 

 modified in outline by the huge and widely extended alluvial 

 fans formed at the mouth of each deep-cut ravine. The fiat- 

 topped appearance of the spurs is very striking indeed and 

 compels attention in every view from the pass of Puca-Puca 

 to the Tunari of Cochabamba and about the whole Cliza 

 basin. The mature profile is easily recognizable, though it is 

 fast being destroyed by vigorous stream dissection. The 

 margin of the basin exhibits a progressively greater amount of 

 dissection with approach to the outlet, where the maximum 

 incision of the outlet stream is perhaps 150 ft. 



Fig. 24. 



Fig. 24. Slope relationship in the Urubarnba valley near Cuzco, Pern. 



The valley of Urubarnba, fig. 2i, near Cnzco, has features 

 very closely resembling those of the Cliza and Cochabamba 

 basins, save for their formation on a much larger scale. The 

 interstream areas have mountainous instead of spur propor- 

 tions ; the frontal scarps terminating the mature slopes are 

 nearly two thousand feet high ; the ravines are gorges or can- 

 yons ; and the alluvial fans at the canyon mouths are some- 

 times several miles wide. 



A physiographic interpretation of the forms of the eastern 

 Andes must include attention to the valley and basin filling 

 which is everywhere so prominent. A period of deep dissec- 

 tion distinctly below the level of the present drainage was 

 followed by a period of alluviation, of partial valley filling. In 



