292 THE ANDES OF SOUTHERN PERU 



action — the accumulation of waste, the strong crevassing, the 

 stream-like character of the discharging snow, and the pro- 

 nounced topographic depression in which it lay — that much natter 

 gradients would serve, possibly not more than 15°, for a snow 

 mass 150 feet wide, 30 to 40 feet thick, and serving as the out- 

 let for a set of tributary slopes about a square mile in area 

 and with declivities ranging from small precipices to slopes of 30°. 



We may say, therefore, that the factors affecting the rate of 

 motion are (1) thickness, (2) degree of compactness, (3) diurnal 

 temperature changes, and (4) gradient. Among these, diurnal 

 temperature changes operate indirectly by making the snow more 

 compact and also by inducing motion directly. At higher eleva- 

 tions above the snowline, temperature changes play a decreas- 

 ingly important part. The thickness required varies inversely as 

 the gradient, and upon a 20° slope is 20 feet for wet and compact 

 snow subjected to alternate freezing and thawing. For dry snow 

 masses above the zone of effective diurnal temperature changes, 

 an increasing gradient is required. With a gradient of 40°, less 

 than 50 feet of snow will move en masse if moderately compacted 

 under its own weight; if further compacted by impact of falling 

 masses from above, the required thickness may diminish to 40 

 feet and the required declivity to 15°. The gradient may decrease 

 to 0° or actually be reversed and motion still continue provided 

 the compacting snow approach true neve or even glacier ice as a 

 limit. 



From the sharp topographic break between the truly glaciated 

 portions of the valley in regions subjected to temporary glacia- 

 tion, it is concluded that the eroding power of the moving mass 

 is suddenly increased at the point where neve is finally trans- 

 formed into true ice. This transformation must be assumed to 

 take place suddenly to account for so sudden a change of function 

 as the topographic break requires. Below the point at which the 

 transformation occurs the motion takes place under a new set of 

 conditions whose laws have already been formulated by students 

 of glaciology. 



The foregoing readings of gradient and depth of snow are 



