386 I. Bowman — Physiography of the Central Andes. 



lofty plateau, scarcely broken by residual ranges of only com- 

 paratively slight extent, is viewed at 14,000 ft. from the 

 summit of the pass east of Pretoria Station, on the coach-road 

 between Oruro and Cochabamba. 



Before considering further the physiography of the eastern 

 Andes, it is well to recall the main facts of the physiography 

 of the western Andes that are correlated with those of the 

 eastern Andes. It will be remembered that three cycles of 

 erosion are found to have occurred (see Part I). The first may 

 be called the great denudation, when a widespread baselevel 

 of erosion was developed and above which only occasional peaks 

 and ranges were able to survive because of superior hardness 

 or advantage of position. Hard and soft rocks, simple and 

 complex structures, high and low masses, save for the excep- 

 tions noted, were brought down to one common level but little 

 above the sea. This is the great dominating topographic fact of 

 the region, the organizing principle of the physiography. It 

 represents a time interval of great length. Marine sediments 

 younger than the Tertiary are nowhere found in the central 

 Andine region, except in a very narrow zone near the sea in 

 northern Chile (Eocene and doubtful Miocene). Elsewhere 

 only terrestrial deposits occur. Even the Eocene deposits 

 represent very limited invasions of the sea in the area now 

 known as the coastal plateau. We have here an old land area 

 long denuded and brought at last to a baseleveled condition. 

 What the age of this peneplain is it seems now impossible 

 to say. It appears from the paleontologic record to have 

 suffered its chief deformation in the Tertiary, and if this 

 inference be correct we have between South America 'and 

 North America a very striking parallelism of topographic 

 development. 



The uplifts to which the eastern and western Andes are 

 commonly accredited are thus seen to have little to do with 

 the present elements of mountain form there displayed. Initial 

 topographic irregularities responsive to structural conditions 

 were largely obliterated; and the orogenic movements could 

 only be said to be physiographically important as they occa- 

 sionally determined the foci of those residual heights which 

 survived the great denudation. It must, therefore, be emphati- 

 cally stated that the central Andes as we know them to-day are 

 the direct products of orogeny. The parallel ranges now 

 observed, say between Sucre and La Paz, are not, in general, 

 the axes of first disturbance. They are the products of revived 

 erosion acting subsequent to a period in which the initial oro- 

 graphic features were largely destroyed. 



