764 K. F. MATHER FRONT RANGES OF THE ANDES 



The rejuvenated streams quickly lowered their beds in the softer strata, 

 and with the aid of their tributaries etched the landscape into its present 

 form. Thus the topography is that of extreme youth where the resistant 

 Mesozoic and Paleozoic formations are above baselevel, while the soft 

 profiles of the intervening areas have resulted from the swifter progress 

 made possible in the nonresistant Tertiary beds which occupy the inter- 

 montane lowland and the Chaco plain. 



