The block diagram represents all of these features, though of necessity somewhat more closely associated than they occur in nature. Reference to the photographs, Figs. 121-124, will make it clear that the diagram is somewhat ideal: on the other hand the photographs together include all the features which the diagram displays. In descending from any of the higher passes to the valley floor one passes in succession down a steep, well-like cirque at a glaciated valley head, across a rocky terminal moraine, then down a stair-like trail cut into the steep scarps which everywhere mark the descent to the main valley floors, over one after another of the confluent alluvial fans that together constitute a large part of the valley fill, and finally down the steep sides of the inner valley to the boulder-strewn bed of the ungraded river.
We shall now turn to each group of features for description and explanation, selecting for first consideration the forms of widest development and greatest significance—the high-level mature slopes lying between the lofty mountains which rise above them and the deep, steep-walled valleys sunk far below them. These are the great pasture lands of the Cordillera; their higher portions constitute the typical puna of the Indian shepherds. In many sections it is possible to pasture the vagrant flocks almost anywhere upon the graded slopes, confident that the ichu, a tufted forage grass, will not fail and that scattered brooks and springs will supply the necessary water. At nightfall the flocks are driven down between the sheltering walls of a canyon or in the lee of a cliff near the base of a mountain, or, failing to reach either of these camps, the shepherd confines his charge within the stone walls of an isolated corral.
In those places where the graded soil-covered slopes lie within the zone of agriculture—below 14,000 feet—they are cultivated, and if the soil be deep and fertile they are very intensively cultivated. Between Anta and Urubamba, a day’s march north of Cuzco, the hill slopes are covered with wheat and barley fields which extend right up to the summits (Fig. 134). In contrast are the uncultivated soil-less slopes of the mountains and the bare valley walls of the deeply intrenched streams. The distribution of the fields thus brings out strongly the principal topographic relations. Where the softer slopes are at too high a level, the climatic conditions are extreme and man is confined to the valley floors and lower slopes where a laborious system of terracing is the first requirement of agriculture.
The appearance of the country after the mature slopes had been formed is brought out in [122] . The camera is placed on the floor of a still undissected, mature valley which shows in the foreground of the photograph. In the middle distance is a valley whose great depth and steepness are purposely hidden; beyond the valley are the smoothly graded, catenary curves, and interlocking spurs of the mature upland. In imagination one sees the valleys filled and the valley slopes confluent on the former (now imaginary) valley floor which extends without important change of expression to the border of the Cordillera. No extensive cliffs occur on the restored surface, and none now occur on large tracts of the still undissected upland. Since the mature slopes represent a long period of weathering and erosion, their surfaces were covered with a deep layer of soil. Where glaciation at the higher levels and vigorous erosion along the canyons have taken place, the former soil cover has been removed; elsewhere it is an important feature. Its presence lends a marked softness and beauty to these lofty though subdued landscapes.
The graded mountain slopes were not all developed (1) at the same elevation, nor (2) upon rock of the same resistance to denudation, nor (3) at the same distance from the major streams, nor (4) upon rock of the same structure. It follows that they will not all display precisely the same form. Upon the softer rocks at the lowest levels near the largest streams the surface was worn down to extremely moderate slopes with a local relief of not more than several hundred feet. Conversely, there are quite unreduced portions whose irregularities have mountainous proportions, and between these extremes are almost all possible variations. Though the term mature in a broad way expresses the stage of development which the land had reached, post mature should be applied to those portions which suffered the maximum reduction and now exhibit the softest profiles. At no place along the 73rd meridian was denudation carried to the point of even local peneplanation. All of the major and some of the minor divides bear residual elevations and even approximately plane surfaces do not exist.
Among the most important features of the mature slopes are (1) their great areal extent—they are exhibited throughout the whole Central Andes, (2) their persistent development upon rocks of whatever structure or degree of hardness, and (3) their present great elevation in spite of moderate grades indicative of their development at a much lower altitude. Mature slopes of equivalent form are developed in widely separated localities in the Central Andes: in every valley about Cochabamba, Bolivia, at 10,000 feet (3,050 m.); at Crucero Alto in southern Peru at 14,600 feet (4,450 m.); several hundred miles farther north at Anta near Cuzco, 11,000 feet to 12,000 feet (3,600 to 3,940 m.), and [129] shows typical conditions in the Vilcabamba Valley along the route of the Yale Peruvian Expedition of 1911. The characteristic slopes so clearly represented in these four photographs are the most persistent topographic elements in the physiography of the Central Andes.