190 THE ANDES OF SOUTHERN PERU 



The rock masses upon which the mature slopes were formed 

 range from soft to hard, from stratified shales, slates, sandstones, 

 conglomerates, and limestones to volcanics and intrusive granites. 

 While these variations impose corresponding differences of form, 

 the graded quality of the slopes is rarely absent. In some places 

 the highly inclined strata are shown thinly veiled with surface 

 debris, yet so even as to appear artificially graded. The rock in 

 one place is hard granite, in another a moderately hard series of 

 lava flows, and again rather weak shales and sandstones. 



Proof of the rapid and great uplift of certain now lofty moun- 

 tain ranges in late geologic time is one of the largest contribu- 

 tions of physiography to geologic history. Its validity now rests 

 upon a large body of diversified evidence. In 1907 I crossed the 

 Cordillera Sillilica of Bolivia and northern Chile and came upon 

 clear evidences of recent and great uplift. The conclusions pre- 

 sented at that time were tested in the region studied in 1911, 500 

 miles farther north, with the result that it is now possible to state 

 more precisely the dates of origin of certain prominent topo- 

 graphic forms, and to reconstruct the conditions which existed 

 before the last great uplift in which the Central Andes were born. 

 The relation to this general problem of the forms under discus- 

 sion will now be considered. 



The gradients of the mature slopes, as we have already seen, 

 are distinctly moderate. In the Anta region, over an area several 

 hundred square miles in extent, they run from several degrees to 

 20° or 30°. Ten-degree slopes are perhaps most common. If the 

 now dissected slopes be reconstructed on the basis of many 

 clinometer readings, photographs, and topographic maps, the re- 

 sult is a series of profiles as in Fig. 127. If, further, the restored 

 slopes be coordinated over an extensive area the gradients of the 

 resulting valley floors will run from 3° to 10°. Finally, if these 

 valley floors be extended westward to the Pacific and eastward 

 to the Amazon basin, they will be found about 5,000 feet above 

 sea level and 4,000 feet above the eastern plains. (For explana- 

 tion of method and data employed, see the accompanying figures 

 127-128). It is, therefore, a justifiable conclusion that since the 



