726 K. F. MATHER FROXT RANGES OF THE ANDES 



which are a complete contrast to the broad late-mature valleys occupied 

 in the intermontane lowlands. The more northerly of the two, the Eio 

 Grande, is eventually tributary to the Amazon, while the Rio Pilcomavo 

 contributes its waters to the Paraguay and River Platte. Other rivers, 

 such as the Saipuru, Charagua, Cuebo, and Vitiacua, rise within one of 

 the inner sierras and flow thence across the lowlands through broad, open 

 valleys until they reach the western side of the next ridge to the east. 

 Through it they invariably make their way in extremely youthful canyons. 

 Beyond the mountain front the majority of the smaller streams lose 

 themselves among the sand hills of the Chaco. 



Except in time of unusually low water, Rio Grande and the Pilcomavo 

 can not be forded, but may be passed in the ferries situated at the 

 crossings of the main trail between Santa Cruz and Yacuiva. The other 

 rivers may be forded almost anywhere along their courses, although 

 certain of the fords are dangerous or even impassable at times of high 

 water, during the rainy seasons. 5 



PHYSIOGRAPHIC HISTORY 



It is obvious from the foregoing description of the physiography and 

 drainage system of this region that its stream pattern was inherited 

 from a previous erosion cycle. The modern streams are neither conse- 

 quent on, nor adjusted to, the existing land surface and geologic struc- 

 ture. Two alternative theories should be considered : The master streams 

 may be antecedent: they may be following courses determined before the 

 episode of crustal deformation responsible for the folds and faults of 

 the sierras and cuestas, courses which they maintained regardless of the 

 crumpling of the mountain ranges directly athwart their paths. Or 

 the rivers may be superimposed. Subsequent to the crumpling of the 

 strata into approximately their present attitude, the entire region was 

 reduced to base level and the easterly courses of the master streams were 

 developed without regard to the structures beneath the peneplain, struc- 

 tures which were etched in bold relief subsequent to the rejuvenation of 

 the streams coincident with the regional uplift which inaugurated the 

 existing cycle of erosion. 



In the preceding descriptions of the sierras and cuestas in this region, 

 emphasis has been frequently placed upon the close accordance in summit 

 level within each mountain group. The comparatively plain surface 

 projected through these summits rises gently toward the west, as each 

 successive range of the Andes system is a few hundred feet higher than 



5 K. F. Mather, 1922 6. 



