Vol. 6] Beid: The Geomorphogeny of the Sierra Nevada. 145 



ally found in the area under consideration. These are (1) an 

 original nearly or quite flat surface; (2) a homogeneous rock with 

 no disturbing features such as columnar structure, etc.; (3) the 

 absence of excessive precipitation of moisture with resultant 

 great stream erosion. In the Little Valley area faults displacing 

 the old plateau surface cut across the massive and homogeneous 

 granodiorite. The erosion of the steep steps due to stream action 

 is small compared to that of" the atmospheric agencies, so that the 

 scarps are not degraded in the same manner as slopes in a very 

 moist climate. If there then be assumed a fault dipping at a 

 steep angle, figure 3 represents the original profile. 

 zzzzzzzzzzzzzz zzza We will assume the simplest 



newly exposed surface, the material from which will roll or drop 

 down the steep cliff and form an apron with gentler slope at the 

 base. In the homogeneous rock we have assumed, the disinte- 

 gration for a considerable time will be quite even over the scarp 

 face and reduce it all equally. The profile then will be as in 

 figure 4. 



on the side and above. This results in greater disintegration at 

 the crest and a gradual rounding of the salient angle by a rolling 

 of the material below. The third stage in the evolution of the pro- 

 file is therefore as in figure 5. Here there are three distinct seg- 



condition of entirely fresh rock 

 below the broken flat surface. 

 Upon the formation of this 

 scarp the atmospheric agencies 

 will slowly disintegrate the 



Fig. 3. 



Fig. 4. 



There are two segments to 

 this curve. But, while the dis- 

 integration is progressing on the 

 face from the outside only, the 

 crest of the scarp is being at- 

 tacked by the atmosphere both 



Fig. 5. 



ments : top convex, center on 

 the original plane of the 

 fault, and bottom concave. 

 As the top becomes more 

 rounded and reduced and 

 the lowest slope aggraded by 



