The broad regional uplift of the Peruvian Andes in late Tertiary and in Pleistocene times carried their summits above the level of perpetual snow. It is still an open question whether or not uplift was sufficiently great in the early Pleistocene to be influenced by the first glaciations of that period. As yet, there are evidences of only two glacial invasions, and both are considered late events on account of the freshness of their deposits and the related topographic forms. The coarse deposits—nearly 500 feet thick—that form the top of the desert section described above clearly indicate a wetter climate than prevailed during the deposition of the several hundred feet of wind-blown deposits beneath them. But if our interpretation be correct these deposits are of late Tertiary age, and their character and position are taken to indicate climatic changes in the Tertiary. They may have been the mild precursors of the greater climatic changes of glacial times. Certain it is that they are quite unlike the mass of the Tertiary deposits. On the other hand they are separated from the deposits of known glacial age by a time interval of great length—an epoch in which was cut a benched canyon nearly a mile deep and three miles wide. They must, therefore, have been formed when the Andes were thousands of feet lower and unable to nourish glaciers. It was only after the succeeding uplifts had raised the mountain crests well above the frost line that the records of oscillating climates were left in erratic deposits, troughed valleys, cliffed cirques and pinnacled divides.
The glacial forms are chiefly at the top of the country; the glacial deposits are chiefly in the deep valleys that were carved before the colder climate set in. The rock waste ground up by the ice was only a small part of that delivered to the streams in glacial times. Everywhere the wetter climate resulted in the partial stripping of the residual soil gathered upon the smooth mature slopes formed during the long Tertiary cycle of erosion. This moving sheet of waste as well as the rock fragments carried away from the glacier ends were strewn along the valley floors, forming a deep alluvial fill. Thereby the canyon floors were rendered habitable.
In the chapters on human geography we have already called attention to the importance of the U-shaped valleys carved by the glaciers. Their floors are broad and relatively smooth. Their walls restrain the live stock. They are sheltered though lofty. But all the human benefits conferred by ice action are insignificant beside those due to the general shedding of waste from the cold upper surfaces to the warm levels of the valley floors. The alluvium-filled valleys are the seats of dense populations. In the lowest of them tropical and sub-tropical products are raised, like sugar-cane and cotton, in a soil that once lay on the smooth upper slopes of mountain spurs or that was ground fine on the bed of an Alpine glacier.
Fig. 179—Snow fields on the summit of the Cordillera Vilcapampa near Ollantaytambo. A huge glacier once lay in the steep canyon in the background and descended to the notched terminal moraine at the canyon mouth. In places the glacier was over a thousand feet thick. From the terminal moraine an enormous alluvial fan extends forward to the camera and to the opposite wall of the Urubamba Valley. It is confluent with other fans of the same origin. See [Fig. 180]. In the foreground are flowers, shrubs, and cacti. A few miles below Urubamba at 11,500 feet.