I. Bowman — Physiography of the Central Andes. 207 



cover has been formed in 

 spite of the continual re- 

 moval of the smaller prod- 

 ucts by winds and a small 

 amount of rain wash. Fine 

 material, in general present 

 in great quantities when 

 rock is chemically decayed , 

 is here practically absent 

 and but partially fills the 

 interstices among .the slabs 

 and blocks. It is a cluttered 

 slope, minutely rough, but 

 in its distant aspect it is of 

 gentle declivity and great 

 smoothness. 



Somewhat r e g u 1 a r 1 y 

 spaced along the western 

 descent are the steep-walled 

 gorges of the westward flow- 

 ing streams. Their descent 

 is gentler than the 7° slope 

 of the plateau margin ; and 

 in many cases they have cut 

 profound canyons toward 

 their headwaters. The Que- 

 bracla Quisma of fig. 6 is 

 several hundred feet deep 

 where the trail ascends its 

 walls at 13,000 to 14,000 ft. 

 elevation, east of Pica, The 

 Chacarilla gorge, farther 

 south, is nearly a half mile 

 deep between the Victoria 

 mines and the oasis of Chaia. 

 The stream profiles, in spite 

 of the adjustments repre- 

 sented by these enormous 

 clefts, are still abnormal and 

 show a steepening of the 

 lower sections over the upper 

 below the point where they 

 pass the edge of the plateau, 

 thus clearly reflecting the 

 effects of recent elevation. 



From 12,500 ft. to 13,000 

 ft. the edge of the plateau is 

 occasionally marked by rock 

 ledges 20' to 10 ft. high. 



Fig. 



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 is 



15 



o 

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