I. Bowman — Physiography of the Central Andes. 395 



valleys. We are obliged to conclude that the epeirogenic 

 movement which deformed the peneplain was accompanied by 

 block-faulting which formed the interior basin and that no 

 significant amount of further faulting has occurred to mar the 

 nice relation of slopes and grades which are the product of the 

 second cycle of erosion. The second uplift, whereby the third 

 and present cycle of erosion was inaugurated, is topographically 

 unexpressed in the interior basin. Its self-contained character, 

 in common with the great interior basins of all desert regions, 

 removes it from the immediate effects of uplift. Its floor is a 

 local baselevel of erosion. Therefore, while expressing the 

 forms of maturity, it does not represent those of recent dissec- 

 tion. In fact, we may say that the second cycle of erosion is 

 here still in progress, the slopes are becoming gentler and flat- 

 ter, and in the absence of local tectonic movements the cycle 

 may progress much nearer completion. The formation of a 

 locally baseleveled area is, however, imperilled by the vigorous 

 attack of the neighboring streams outside the basin, — the La 

 Paz, Rio Grande, and Pilcomayo. These now head near the 

 border of the basin and will eventually tap it if existing condi- 

 tions of relief and rainfall are indefinitely prolonged. 



In the non-glaciated parts, on the eastern border of the 

 Andes, the slopes are not generally graded in the drainage 

 systems tributary to the Atlantic. The last uplift was appar- 

 ently sudden and undoubtedly pronounced and the eastern edge 

 of the eastern Andes is a well-defined fault scarp 1,000-4,000 

 feet in height. The plains at the foot of the scarp are less than 

 a thousand feet above seal ev el. High plateaus and residual 

 mountains are near low plains and have led to enormous and 

 rapid dissection. The jaggedness, insecurity, and unorganized 

 character of the valley and "mountain" forms of this eastern 

 section are its most conspicuous features below the limit of 

 glacial action (8,500 ft. more or less). The valleys generally 

 have a sharp V profile. The exceptions are explained by struc- 

 tural conditions, in every case examined in detail. Dissection 

 has not yet advanced to a maximum in the western half of the 

 eastern plateau, hence in this half the graded and relatively 

 flat slopes of the previous (second) cycle dominate ; in the 

 eastern section, as a consequence of the great dissection caused 

 by rapid and great uplift and well-watered slopes, well-defined 

 hanging lateral valleys occur abundantly below the limit of 

 glacial action. This is one of the most interesting of the 

 physiographic results of our work in the eastern Andes. We 

 had not expected to see the condition at all, to say nothing of 

 seeing it so generally developed. It was particularly interest- 

 ing because one rides in a single day from a splendidly 

 glaciated region with all the "discordant" features diagram- 



