I. Bowman — Physiography of the Central Andes. 397 



bine with rapid uplift and corresponding deep dissection to 

 make the feature a general one in the eastern Andes region. 



The strikingly general occurrence of hanging valleys in this 

 region constitutes them a type, and hanging valleys cannot 

 therefore be said to be peculiar to glaciated regions. But it 

 is the form of the valleys and the geologic structure that 

 with the hanging quality must determine the explanation. 

 ~No one could mistake the Y-shaped valley at X, tig. 27, for a 

 glaciated valley. ~Nor can one find here any equivalent for 

 the special features of glaciated regions even if the other more 

 obvious marks of glaciation were removed by post-glacial 

 stream erosion. 



Finally the eastern Andes are, after all, peculiar in the 

 sense that few regions have the particular combinations of 

 rapid and great uplift, deep dissection and strong and sudden 

 alternations of hard and soft rock there exhibited. The regions 

 in which these conditioning factors are absent do not have hang- 

 ing valleys except where glaciation has influenced the develop- 

 ment of slopes. 'No one should misinterpret these two kinds of 

 valleys merely because they have this in common, that they 

 are hanging with respect to the master stream. In the assem- 

 blage of detailed characters no other likeness between the two 

 types is discernible. 



The foregoing interpretation of the topography of the east- 

 ern Andes, the great eastern plateau of Bolivia, has peculiarly 

 interesting support in the drainage relations that are character- 

 istic of the entire region from Caupolican Bolivia to the south- 

 eastern border of the Republic at Tarija, the latter region as 

 described by Hoek.* The drainage is established upon the 

 surface in curious disregard of the structure. Forty miles 

 southwest of Tarija, the San Juan and Honda rivers, fig. 28, 

 flow northwest across the folded Silurian and Cretaceous sand- 

 stones and Silurian schists in courses that are utterly regardless 

 of the structure. Even the small Rupasco tributary of the 

 San Juan, after following a northward course in a synclinal 

 valley, turns west against the dip of the more resistant 

 schists and crosses one limb of the next anticlinal before joining 

 the master stream. The Tarija river itself is represented upon 

 Steinmann's mapf as crossing iour ridges of rock, varying 

 from Silurian schists to Cretaceous sandstones, in a distance of 

 twenty miles. In fact the most striking physiographic feature 

 of this map is the persistent way in which the drainage cuts 

 across ridge after riclge of rock of all degrees of hardness, dip, 



* Ante. 



f Erlauterung zur Routenkarte der Expedition Steinmann, Hoek, v. Bistrain 

 in der Anden von Boliven, 1903-1904 ( Petr. Geogr. Mitteilungen, Heft 1, 

 1906). 



