1915] 



Lawson : The Epigene Profiles of the Desert 



25 



upper than in the lower part. 1 The recession has been progressive 

 with the growth of the talus, and the portion of the battered scarp 

 which becomes buried is therefore a parabolic curve, the volume of 

 the talus being assumed equal to the volume of the rock in place, 

 while the portion which remains above the talus maintains a straight 

 slope. The development of the final combined facet from the initial 

 scarp is shown in the diagram, figure 1. 



Figure 1. — Under rainless conditions a mountain face OB would be reduced 

 to OPB u the lower curved portion of which OP would be burried in talus OSP. 

 The combined rock and talus slope 8PB, is the unchanging profile under the 

 assumed conditions. 



In the foregoing statement it is of course not implied that the 

 talus profile is found only in rainless climates; it may and does occur 

 under humid conditions, but here it is a temporary phase of the geo- 

 morphic evolution, whereas under rainless conditions it is a permanent 

 and characteristic feature of mountain fronts. 2 



ARID REGIONS 



Elements of the profile. — The scant rains of the arid regions are 

 sufficient to differentiate sharply the epigene forms there displayed 

 from those deduced above as characteristic of -the relief of a rainless 



i This is due, of course, chiefly to the fact that the amount of disintegration 

 is a function of the extent of surface exposed, and that this is greatest at the 

 top of the scarp where it intersects the reverse slope or a gentler slope in the 

 same direction, but if the scarp be high it may also be due partly to larger 

 contrasts of temperature in higher altitudes. 



-This was written before the appearance of Lane's abstract dealing with 

 the same profile in Bull. G. S. A., Vol. 26, No. 1, p. 75. 



