390 I. Bowman — Physiography of the Central Andes. 



in which the very heart of the plateau and the residual moun- 

 tains is being attacked by the torrential streams. 



A comparison with the conditions in the Maritime Andes 

 is important at this point to estimate the correspondence of 

 development there, to reenforce the earlier statement that the 

 first mountains of the region were all but obliterated, and to 

 show that broad regional uplift, not orogenic movement, is 

 responsible for the present loftiness of the central Andine 

 tableland. Fig. 12 represents the view looking southwest 

 near the pass at Crucero Alto on the railway line from Lake 

 Titicaca to Arequipa in southern Peru. The camera is at 

 about 14,400 ft. The description applied to the preceding view 

 could be applied word for word to this view. All elements of 

 form and structure in the two are comparable, except that the 

 greater altitude of the camera in the latter photograph brings 

 the level of the now dissected peneplain into the view and 

 renders both cycles appreciable at a single glance. 



A striking fact is the occurrence of the mature slopes right 

 up to many of the divides near the western border of the 

 eastern plateau. This is admirably shown on the coach-road 

 from Oruro to Cochabamba, where the waste-cloaked valley 

 heads are thoroughly organized with respect to the drainage. 

 It is only some distance down valley that the dissection 

 due to the last uplift is topographically expressed. 



The disappearance of the slopes of maturity on approach to 

 the eastern edge of the Andes is not less marked than the 

 similar disappearance of the even-crested upland in this direc- 

 tion. Both alike are broken down by the terrific dissection 

 of the mountain torrents that in many places descend 12,000 

 and 13,000 ft. in less than 75 miles, or with an average 

 gradient of over 150 ft. per mile. The mature slopes in their 

 final expression eastward appear as skeleton shoulders upon 

 the valley sides, a typical occurrence being on the trail a few 

 hours' ride north of Inca Corral. 



The most interesting expression of the slopes of maturity is 

 not, however, their perfect development in regions in which 

 by virtue of favorable position they are not preserved. It is 

 their persistent occurrence in localities now undergoing vigor- 

 ous dissection that gives strongest support to the explanation 

 based upon three cycles of erosion. Unfortunately, the natural 

 limitations of a camera did not make it possible to secure a 

 photograph in which all the elements of form characteristic of 

 the three cycles were expressed in a single view. Too great a 

 vertical range exists in the position of the three planes of 

 erosion to make their common expression possible from a single 

 position. The two localities selected are among a list of at 

 least a dozen and choice among the list is difficult because of 





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