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University of California Publications in Geology [Vol. 9 



the mass become shoulders and these are usually isolated in the rising 

 flood of alluvium, as it creeps up the slope, and appear as island-like, 

 conical rock hills protuberant above the slope. This diversity in 

 plan and in profile is usually notable in the incomplete stages of the 

 general process. At completion all the rock crest has disappeared, 

 but the line of meeting of the opposing embankments may remain 

 sinuous. Many of the island-like conical hills become eventually 

 buried ; but others that rise above the ultimate limit of the alluvial 

 slope remain, of course, protuberant, till as local centers of recession 

 and alluvial distribution they waste slowly away, and become mere 

 patches of the bedrock obscured by the residual unmoved products 

 of disintegration, nearly, but not quite flush with the general slope. 



Under exceptional conditions the resultant profile may be asym- 

 metric. For example, if the initial profile of the mountain be steep 

 on one side and so gentle on the other that alluvial debris cannot be 

 transported over it, then the reduction of the mass will proceed on 

 the steep side only. When the rock surface on this side has been 

 wholly buried by alluvium and the process of reduction has come to 

 an end, the profile will be that of the fan slope ; but on the other 

 side it will be that of the initial slope, although the surface may be 

 encumbered by the sedentary products of disintegration. 



The Resulting Surface. — The result described has been attained 

 in southern California; and various stages of the process may be 

 observed throughout the Great Basin. It is a stage of geomorphic 

 development at which the processes of degradation and aggradation, 

 in so far as they are due to the agency of water, both almost cease. 

 There are no exposed rock slopes to be degraded and the embank- 

 ments can receive no farther increment. The slopes of the latter are 

 close to the minimum angle for transportation and therefore are not 

 themselves susceptible of vigorous attack. With the disappearance 

 of the rock crest the process of degradation changes from one of 

 relative rapidity to one of extreme slowness. The wind doubtless tends 

 to modify the result, but in the Great Basin this tendency is extremely 

 ineffective ; and no important modifications have been observed to 

 affect any stage of the general process or the result, except that locally 

 sand derived from the playas may drift over the alluvial slopes in the 

 form of dunes, which may thus be partially incorporated into the 

 growing embankments. 



The symmetry of profile which is attained in the normal process 

 of concurrent degradation and aggradation in the desert is of course 

 a symmetry of angles of slope. It does not extend to the length of 



