1915] 



Lawson: The Epigene Profiles of the Desert 



31 



of the range than the preceeding layer and so diminishes the height 

 and breadth of the rock slope, the recession of which is at all stages 

 limited by the upper edge of the embankment. The rock surface 

 below this upper edge becomes fixed in position by burial. 



Truncation. — It follows from this that the buried slope is a shelf 

 or bench cut into the mountain, the inclination of which is somewhere 

 between that of the still exposed rock slope and that of the alluvial 

 embankment. This relation is shown in the diagram, figure 2. The 

 upper limit of the buried bench, the upper edge of the alluvial 

 embankment and the lower limit of the subaerial front are coincident 

 at all stages of the general process. "When the edge of the alluvium 

 reaches the crest the subaerial front has disappeared. What is true 



Figure 2. — If the surface of the alluvial embankment OS rises to P<S 1; the 

 rise is coincident with, and due to, the recession of the mountain front from 

 OB to P5,. The upper edge of the embankment migrates from to P, and 

 the rock-cut bench OP is evolved, but is buried as fast as it is cut. 



of one side of the mountain ridge is true of the other, so that the rocky 

 crest disappears by the meeting of the feather edges of the alluvial 

 embankments, or rather by their coalescence with the unmoved pro- 

 ducts of the disintegration of the crest rock. Thus, if the mountain 

 be lithologically and structurally homogeneous, it is reduced by trunca- 

 tion to a low rock ridge with symmetrical slopes wholly buried by 

 alluvium, which on either side is wedge-shaped in cross-section, the edge 

 of the wedge being at the crest and the butt out in the valley. The 

 upper surface of this alluvium is a gentle slope of great stability for 

 the prevailing climatic conditions. 



But homogeneity is rarely realized in mountain masses and the 

 heterogeneity varies the rate of the process in different parts and 

 diversifies the ideal symmetry of the result. Owing to differential 

 disintegration and recession, one part of the rock crest may disappear 

 while another portion is salient, and the migration of the lowering 

 rock crests may proceed unevenly. Similarly, resistant portions of 



