290 THE ANDES OF SOUTHERN PERU 



ness, due to increase in amount of the accumulated snow; that 

 bowlders are actually transported by snow is also shown by their 

 presence on the lower margins of nivated tracts. 



Our argument may be made clearer by reference to the ob- 

 served action of snow in a particular valley. Snow is shed from 

 the higher, steeper slopes to the lower slopes and eventually ac- 

 cumulates to a marked degree on the bottoms of the depressions, 

 whence it is avalanched down valley over a series of irregular 

 steps on the valley floor. An avalanche takes place through the 

 breaking of a section of snow just as an iceberg breaks off the 

 end of a tide-water glacier. Evidently there must be pressure 

 from behind which crowds the snow forward and precipitates it 

 to a lower level. 



As a snow mass falls it not only becomes more consolidated, 

 beginning at the plane of impact, but also gives a shock to the 

 mass upon which it falls that either starts it in motion or acceler- 

 ates its rate of motion. The action must therefore be accom- 

 panied by a drag upon the floor and if the rock be close-jointed 

 and the blocks, defined by the joint planes, small enough, they will 

 be transported. Since snow is not so compact as ice and permits 

 included blocks easily to adjust themselves to new resistances, we 

 should expect the detached blocks included in the snow to change 

 their position constantly and to form irregular scratches, but not 

 parallel striae of the sort confidently attributed to stone-shod ice. 



It is to the plasticity of snow that we may look for an ex- 

 planation of the smooth-contoured appearance of the landscape in 

 the foreground of Fig. 135. The smoothly curved lines are best 

 developed where the entire surface was covered with snow, as in 

 mid-elevations in the larger snowfields. At higher elevations, 

 where the relief is sharper, the snow is shed from the steeper 

 declivities and collected in the minor basins and valley heads, 

 where its action tends to smooth a floor of limited area, while 

 snow-free surfaces retain all their original irregularities of form 

 or are actually sharpened. 



The degree of effectiveness of snow and neve action may be 

 estimated from the reversed slopes now marked by ponds or small 



