294 THE ANDES OF SOUTHERN PERU 



with a thickness still greater, the resulting neve may actually be 

 forced up slight inclines whose declivity appears to approach 5° 

 as a limit. I have nowhere been able to find in truly nivated areas 

 reversed curves exceeding 5°, though it should be added that de- 

 pressions whose leeward slopes were reversed to 2° and 3° are 

 fairly common. If the curve were continued we should undoubt- 

 edly find it again turning to the left at the point where the thick- 

 ness of the snow results in the transformation of snow to ice. 

 From the sharp topographic break observed to occur in a narrow 

 belt between the neve and the ice, it is inferred that the erosive 

 power of the neve is to that of the ice as 2:4 or 1:5 for equal 

 areas; and that reversed slopes of a declivity of 10° to 15° may 

 be formed by glaciers is well known. Precisely what thickness of 

 snow or neve is necessary and what physical conditions effect its 

 transformation into ice are problems not included in the main 

 theme of this chapter. 



It is important that the proposed curve of snow motion under 

 minimum conditions be tested under a large variety of circum- 

 stances. It may possibly be found that each climatic region re- 

 quires its special modifications. In tropical mountains the sud- 

 den alternations of freezing and thawing may effect such a high 

 degree of compactness in the snow that lower minimum gradients 

 are required than in the case of mid-latitude mountains where 

 the perpetual snow of the high and cold situations is compacted 

 through its own weight. Observations of the character introduced 

 here are still unattainable, however. It is hoped that they will 

 rapidly increase as their significance becomes apparent; and that 

 they have high significance the striking nature of the curve of 

 motion seems clearly to establish. 



BERGSCHRUNDS AND CIRQUES 



The facts brought out by the curve of snow-motion (Fig. 195) 

 have an immediate bearing on the development of cirques, whose 

 precise mode of origin and development have long been in doubt. 

 Without reviewing the arguments upon which the various hy- 

 potheses rest, we shall begin at once with the strongest explana- 



