1915] 



Lawson: The Epigcnc Profiles of the Desert 



29 



ments in general are irregularly prismatic in shape, which assumption 

 agrees with observation. 



For a given size of fragment the ease with which it may be moved 

 diminishes with the decrease of the angle of slope at the place where 

 it rests. Finally a minimum or limiting angle is reached below which 

 the fragment cannot be moved. This limiting angle thus becomes a 

 constant feature of that portion of the slope, say near the top of the 

 slope, throughout all its future degradation under the assumed climatic 

 conditions. For, if the angle of slope become lower, this and other 

 similar sized fragments would encumber it and protect the underlying 

 rock from further disintegration, and degradation would cease. But 

 the fragments shed from a rock slope vary greatly in size, and those 

 which determine the angle of slope are the spauls of maximum size, 

 or perhaps better, the maximum order of size. All loose detritus shed 

 from the rock slope of less order of size is readily carried down the 

 slope in times of cloud-burst. Those of the maximum order represent 

 the limit of size that can be transported. It may be presumed that 

 rocks similar as to lithology and structure shed fragments of about 

 the same maximum order of size, and, therefore, have slopes of about 

 the same angle for similar parts of mountain fronts ; whereas dissimilar 

 rocks shed fragments the maximum order of size of which may differ 

 greatly. 



But the angle of slope for the upper part of a mountain front in 

 homogeneous rock is not necessarily the angle at the lower part of the 

 same declivity. It is clear from the bare character of the desert fronts 

 that the sheets of water which wash down the detritus in times of 

 cloud-burst are underladen ; otherwise there would be a residue of 

 debris left behind and the slope would be undergoing alluviation and 

 not degradation. The fact that the flowing water is underladen 

 and that the volume of water increases in an arithmetical ratio as 

 it descends determines an acceleration of velocity toward the lower 

 part of the rock slope. This in turn determines that spauls of maxi- 

 mum order of size may be moved on a lower angle of slope than at 

 the top of the slope where the velocity is less. A lower angle is, 

 therefore, developed at the lower part of the rock slope. This in turn 

 tends to check the increase of velocity ; but, in view of the fact that 

 the slopes are bare and that the water which keeps them bare is 

 underladen, it is evident that the acceleration of the velocity while 

 lessened is not inhibited. Now in any desert range, as the rock slope 

 shortens by the rise of alluvium on its flank, the contrast between the 



