202 THE ANDES OF SOUTHERN PERU 



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 dissec- 

 tion 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 moun- 

 tain spur 2,000 feet high. Its course is at right angles to that 

 pursued by the stream that once drained the spur. It is note- 

 worthy 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 considera- 

 ble amount of material in post-glacial time, partly because the cli- 

 mate 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 con- 

 tinues, but the tracts that exceed this elevation are confined to the 

 loftiest peaks or their immediate neighborhood. There is a dis- 

 tinct 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 rami- 

 fications up into the snowfields that formerly included the highest 

 country. Prolonged glacial erosion produced a full set of topo- 

 graphic forms characteristic of the work of Alpine glaciers. Thus, 

 each of the main mountain chains that make up the Andean sys- 

 tem has, like the system as a whole, a relatively more-dry and a 



