Fig. 127—Topographic profiles across typical valleys of southern Peru. They are drawn to scale and the equality of gradient of the gentler upper slopes is so close that almost any curve would serve as a composite of the whole. These curves form the basis of the diagram, Fig. 128, whereby the amount of elevation of the Andes in late geologic time may be determined. The approximate locations of the profiles are as follows: 1, Antabamba; 2, Chuquibambilla; 3, upland south of Antabamba; 4, Apurimac Canyon above Pasaje; 5, Abancay; 6, Arma (Cordillera Vilcapampa); 7, divide above Huancarama; 8, Huascatay; 9, Huasentay, farther downstream; 10, Rio Pampas. The upper valley in 8 is still undissected; 7 is practically the same; 8a is at the level which 8 must reach before its side slopes are as gentle as at the end of the preceding interrupted cycle.
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 débris, 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 mountain ranges in late geologic time is one of the largest contributions 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 presented 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 topographic 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 discussion 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 result is a series of profiles as in [127] . If, further, the restored slopes be coördinated 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 explanation of method and data employed, see the accompanying figures 127-128). It is, therefore, a justifiable conclusion that since the formation of the slopes the Andes have been uplifted at least a mile, or, to put it in another way, the Andes at the time of formation of the mature slopes were at least a mile lower than they are at present.