388 I. Bowman — Physiography of the Central Andes. 



Features of the Second and Third Erosion Cycles. 



The deformation whereby the peneplain once existing here 

 was uplifted was epeirogenic in nature, and affected the entire 

 central Andine region. But the first uplift did not bring the 

 country to the level at which we now find it. Intermediate 

 between the present level of erosion and the peneplain we find 

 another cycle expressed. In it the slopes were in general 

 developed to the point of maturity, and neither the complete- 

 ness of development of the peneplain on the one hand, nor the 

 remarkable activity of erosion in the eastern Andes on the 

 other, overshadows the field expression of these mature slopes, 



Fig. 22. 



Fig. 22. — Graded, waste-cloaked, nndissected slopes of maturity near 

 Colomi, Bolivia, northeast of Cochabamba. 



either in areal extent or in perfection of development. A 

 later and second uplift once more encouraged the dissecting 

 streams with the result that both the uplifted peneplain of the 

 first cycle and the mature slopes of the second cycle are fast 

 disappearing under vigorous stream attack. The most recent 

 episode in the region is glaciation, which has, in some places, 

 partially refilled the valleys with alluvium, and softened the 

 once sharper outlines of the valley forms. 



The most striking development of mature slopes observed 

 was at Colomi (fig. 22) in a mountain valley about half way 



