Fig. 183—Two-cycle slopes and alluvial fill between Iluichihua and Chuquibambilla. The steep slopes on the inner valley border are in many places vertical and rock cliffs are everywhere abundant. Mature slopes have their greatest development here between 13,500 and 15,000 feet (4,110 to 4,570 m.). Steepest mature slopes run from 15° to 21°. Least steep are the almost level spur summits. The depths of the valley fill must be at least 300, and may possibly be 500 feet. The break between valley fill and steep slopes is most pronounced where the river runs along the valley wall or undercuts it; least pronounced where alluvial fans spread out from the head of some ravine. It is a bowldery, stony fill almost everywhere terraced and cultivated.
Similar conditions are well displayed at Huadquiña, where a fine series of terraces at the lower end of the Torontoy Canyon break the descent of the environing slopes; also in the Urubamba Valley below Rosalina, and again at the edge of the mountains at the Pongo de Mainique. It is exhibited most impressively in the Majes Valley, where the bordering slopes appear to be buried knee-deep in waste, and where from any reasonable downward extension of rock walls of the valley there would appear to be at least a half mile of it. It is doubtful and indeed improbable that the entire fill of the Majes Valley is glacial, for during the Pliocene or early Pleistocene there was a submergence which gave opportunity for the partial filling of the valley with non-glacial alluvium, upon which the glacial deposits were laid as upon a flat and extensive floor that gives an exaggerated impression of their depth. However, the head of the Majes Valley contains at least six hundred feet and probably as much as eight hundred feet of alluvium now in process of dissection, whose coarse texture and position indicates an origin under glacial conditions. The fact argues for the great thickness of the alluvial material of the lower valley, even granting a floor of Pliocene or early Pleistocene sediments. The best sections are to be found just below Chuquibamba and again about halfway between that city and Aplao, whereas the best display of the still even-floored parts of the valley are between Aplao and Cantas, where the braided river still deposits coarse gravels upon its wide flood plain.
CHAPTER XVI
GLACIAL FEATURES
THE SNOWLINE
South America is classical ground in the study of tropical snowlines. The African mountains that reach above the snowline in the equatorial belt—Ruwenzori, Kibo, and Kenia—have only been studied recently because they are remote from the sea and surrounded by bamboo jungle and heavy tropical forest. On the other hand, many of the tropical mountains of South America lie so near the west coast as to be visible from it and have been studied for over a hundred years. From the days of Humboldt (1800) and Boussingault (1825) down to the present, observations in the Andes have been made by an increasing number of scientific travelers. The result is a large body of data upon which comparative studies may now be profitably undertaken.
Like scattered geographic observations of many other kinds, the earlier studies on the snowline have increased in value with time, because the snowline is a function of climatic elements that are subject to periodic changes in intensity and cannot be understood by reference to a single observation. Since the discovery of physical proofs of climatic changes in short cycles, studies have been made to determine the direction and rate of change of the snowline the world over, with some very striking results.