268 THE ANDES OF SOUTHERN PERU 



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 rec- 

 ords 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 ren- 

 dered 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 insig- 

 nificant 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. 



The Pleistocene deposits fall into three well-defined groups: 

 (1) glacial accumulations at the valley heads, (2) alluvial deposits 



