Fig. 131—Terraced valley slopes at Huaynacotas, Cotahuasi Valley, at 11,500 feet (3,500 m.). Solimana is in the background. On the floor of the Cotahuasi Canyon fruit trees grow. At Huaynacotas corn and potatoes are the chief products. The section is composed almost entirely of lava. There are over a hundred major flows aggregating 5,000 to 7,000 feet thick.
The volume of the lavas is enormous. They are a mile and a half thick, nearly a hundred miles wide, and of indefinite extent north and south. Their addition to the Andes, therefore, has greatly broadened the zone of lofty mountains. Their passes are from 2,000 to 3,000 feet higher than the passes of the eastern Andes. They have a much smaller number of valleys sufficiently deep to enjoy a mild climate. Their soil is far more porous and dry. Their vegetation is more scanty. They more than double the difficulties of transportation. And, finally, their all but unpopulated loftier expanses are a great vacant barrier between farms in the warm valleys of eastern Peru and the ports on the west coast.
The upbuilding process was not, of course, continuous. There were at times intervals of quiet, and some of them were long enough to enable streams to become established. Buried valleys may be observed in a number of places on the canyon walls, where subsequently lava flows displaced the streams and initiated new drainage systems. In these quiet intervals the weathering agents attacked the rock surfaces and formed soil. There were at least three or four such prolonged periods of weathering and erosion wherein a land surface was exposed for many thousands of years, stream systems organized, and a cultivable soil formed. No evidence has been found, however, that man was there to cultivate the soil.
The older valleys cut in the quiet period are mere pygmies beside the giant canyons of today. The present is the time of dominant erosion. The forces of vulcanism are at last relatively quiet. Recent flows have occurred, but they are limited in extent and in effects. They alter only the minor details of topography and drainage. Were it not for the oases set in the now deep-cut canyon floors, the lava plateau of the Maritime Cordillera would probably be the greatest single tract of unoccupied volcanic country in the world.
The lava plateau has been dissected to a variable degree. Its high eastern margin is almost in its original condition. Its western margin is only a hundred miles from the sea, so that the streams have steep gradients. In addition, it is lofty enough to have a moderate rainfall. It is, therefore, deeply and generally dissected. Within the borders of the plateau the degree of dissection depends chiefly upon position with respect to the large streams. These were in turn located in an accidental manner. The repeated upbuilding of the surface by the extensive outflow of liquid rock obliterated all traces of the earlier drainage. In the Cotahuasi Canyon the existing stream, working down through a mile of lavas, at last uncovered and cut straight across a mountain spur 2,000 feet high. Its course is at right angles to that pursued by the stream that once drained the spur. It is noteworthy that the Cotahuasi and adjacent streams take northerly courses and join Atlantic rivers. The older drainage was directly west to the Pacific. Thus, vulcanism not only broadened the Andes and increased their height, but also moved the continental divide still nearer the west coast.
The glacial features of the western or Maritime Cordillera are of small extent, partly because vulcanism has added a considerable amount of material in post-glacial time, partly because the climate is so exceedingly dry that the snowline lies near the top of the country. The slopes of the volcanic cones are for the most part deeply recessed on the southern or shady sides. Above 17,500 feet (5,330 m.) the process of snow and ice excavation still continues, but the tracts that exceed this elevation are confined to the loftiest peaks or their immediate neighborhood. There is a distinct difference between the glacial forms of the eastern or moister and the western or dryer flanks of this Cordillera. Only peaks like Coropuna and Solimana near the western border now bear or ever bore snowfields and glaciers. By contrast the eastern aspect is heavily glaciated. On La Cumbre Quadrangle, there is a huge glacial trough at 16,000 feet (4,876 m.), and this extends with ramifications up into the snowfields that formerly included the highest country. Prolonged glacial erosion produced a full set of topographic forms characteristic of the work of Alpine glaciers. Thus, each of the main mountain chains that make up the Andean system has, like the system as a whole, a relatively more-dry and a relatively less-dry aspect. The snowline is, therefore, canted from west to east on each chain as well as on the system. However, this effect is combined with a solar effect in an unequal way. In the driest places the solar factor is the more efficient and the snowline is there canted from north to south.
CHAPTER XIII
THE EASTERN ANDES: THE CORDILLERA VILCAPAMPA
THE culminating range of the eastern Andes is the so-called Cordillera Vilcapampa. Its numerous, sharp, snow-covered peaks are visible in every summit view from the central portion of the Andean system almost to the western border of the Amazon basin. Though the range forms a water parting nearly five hundred miles long, it is crossed in several places by large streams that flow through deep canyons bordered by precipitous cliffs. The Urubamba between Torontoy and Colpani is the finest illustration. For height and ruggedness the Vilcapampa mountains are among the most noteworthy in Peru. Furthermore, they display glacial features on a scale unequaled elsewhere in South America north of the ice fields of Patagonia.