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University of California Publications in Geology [Vol. 9 



example, we assume that our orogenic block is composed of fine-grained 

 incoherent rock and assume further the prevalence of powerful winds, 

 then the la. c t two of these processes might well be most important 

 agencies of degradation. But if we assume that our orogenic block is 

 composed of coherent, hard, elastic rock, then the products of mech- 

 anical disintegration woidd in general be of such a size as to be but 

 slightly affected by the wind, and the first two processes would be 

 almost the only agencies concerned in the degradation of the uplifted 

 mass. It is the latter case, the effect of mechanical disintegration and 

 gravitational transportation, that I desire here to discuss briefly as 

 an introduction to the consideration of more complex conditions in 

 the later part of the paper. For the purpose of simplifying the dis- 

 cussion I will further assume that the mountain is lithologically and 

 structurally homogeneous. 



Under the conditions assumed, our uplifted block may have initial 

 slopes which are (1) less than the slope of repose for loose material, 

 or, (2) greater than the slope of repose. The gentler slopes will remain 

 unaltered throughout the persistence of the rainless climate. The 

 surface will of course be at first subject to mechanical disintegration, 

 but in the absence of any transporting agency the products of dis- 

 integration will encumber the firm rock and protect it from further 

 attack. There can be no change of slope due to degradation. The 

 steeper slopes, on the other hand, cannot retain the fragments shed 

 by disintegration, and these lodge at the base of the slope in the form 

 of talus. The slope of the talus thus becomes the limiting slope of 

 all mountain facets evolved under such conditions. If we suppose a 

 steeper slope to come into existence, as for example by repeated 

 faulting, the disintegration of the surface and the direct action of 

 gravity will give rise to a talus at its base. As the latter grows it 

 maintains the constant slope of repose of loose material ; but its upper 

 edge encroaches upon higher and higher parts of the battered scarp. 

 While this encroachment is in progress the batter of the scarp ap- 

 proaches the slope of the talus ; and when the two coincide the combined 

 slope of the battered scarp and talus becomes the final, unchanging 

 mountain front. The talus ceases to grow and the rock slope above 

 ceases to diminish, the loose fragments on both lying at the limiting 

 angle at which gravity will move them. 



During the encroachment of the talus upon the battered scarp the 

 latter has been reduced in acclivity by more active recession in the 



