310 THE ANDES OF SOUTHERN PERU 



in the chapters immediately preceding. It is concluded that snow 

 moves upon 20° slopes if the snow is at least forty feet deep, 

 and that through its motion under more favorable conditions of 

 greater depth and gradient and the indirect effects of border 

 melting there is developed a hollow occupied by the snow. Actual 

 ice is not considered to be a necessary condition of either move- 

 ment or erosion. We may at once accept the conclusion that the 

 invariable association of the cirques and steepened profiles with 

 snowfields proves that snow is the predominant modifying agent. 



An argument for glacial erosion based on profiles and steep 

 cirque walls in a volcanic region has peculiar appropriateness in 

 view of the well-known symmetrical form of the typical volcano. 

 Instead of varied forms in a region of complex structure long 

 eroded before the appearance of the ice, we have here simple 

 forms which immediately after their development were occupied 

 by snow. Ever since their completion these cones have been 

 eroded by snow on one side and by ivater on the other. If snow 

 cannot move and if it protects the surface it covers, then this sur- 

 face should be uneroded. All such surfaces should stand higher 

 than the slopes on the opposite aspect eroded by water. But these 

 assumptions are contrary to fact. The slopes underneath the 

 snow are deeply recessed; so deeply eroded indeed, that they are 

 bordered by steep cliffs or cirque walls. The products of erosion 

 also are to some extent displayed about the border of the snow 

 cover. In strong contrast the snow-free slopes are so slightly 

 modified that little of their original symmetry is lost — only a few 

 low hills and shallow valleys have been formed. 



The measure of the excess of snow erosion over water erosion 

 is therefore the difference between a northern or water-formed 

 and a southern or snow-formed profile, Fig. 200. This difference 

 is also shown in Fig. 201 and from it and the restored initial pro- 

 files we conclude that the rate of water erosion is to that of niva- 

 tion as 1:3. This ratio has been derived from numerous obser- 

 vations on cones so recently formed that the interfluves without 

 question are still intact. 



Thus far only those volcanoes have been considered which 



