298 THE ANDES OF SOUTHERN PERU 



is clear that ice and not snow was the state at the bottom of the 

 mass in glacial times. Further, between the surface of the snow 

 and the surface of the bottom layer of the ice there must have 

 been every gradation between loose snow and firm ice, with the 

 result that a thickness much less than 900 feet must be assumed. 

 Precisely what thickness would be found at the foot of the cirque 

 wall is unknown. But granting a thickness of 400 feet of ice an 

 additional 300 feet for neve and snow would raise the total to 700 

 feet. 



The application of the facts in the above paragraph is clearly 

 seen when we refer to Fig. 197. The curve of snow motion of Fig. 

 195 is applied to an unglaciated mountain valley. Taking a 

 normal snow surface and filling the valley head it is seen that 

 the excess of snow depth over the amount required to give motion 

 is a measure at various points in the valley head and at different 

 gradients of the erosive force of the snow. It is strikingly con- 

 centrated on the 15°-20° gradient which is precisely where the so- 

 called process of basal sapping is most marked. If long continued 

 the process will lead to the developing of a typical cirque for it is 

 a process that is self-stimulating. The more the valley is changed 

 in form the more it tends to change still further in form because 

 of deepening snowfields until cliffed pinnacles and matterhorns 

 result. 



By further reference to the figure it is clear that a schrund 

 350 feet deep could not exist on a cirque wall with a declivity of 

 even 20° without being closed by flow, unless we grant more rapid 

 flow below the crevasse. In the case of a glacier flowing over a 

 nearly flat bed away from the cirque it is difficult to conceive of a 

 rate of flow greater than that of snow and neve on the steep lower 

 portion of the cirque wall, when movement on that gradient begins 

 with snow but 20 feet thick. 



In contrast to this is the view that the schrund line should lie 

 well up the cirque wall where the snow is comparatively thin and 

 where there is an approach to the lower limits of movement. 

 The schrund would appear to open where the bottom material 

 changes its form, i.e., where it first has its motion accelerated by 



