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University of California Publications in Geology [ Vol. 9 



The Ultimate Stage. — The peneplain is, as the term implies, a 

 penultimate rather than an ultimate stage of geomorphic development 

 under persistence of uniform conditions. The final stage is a plain ; but 

 the term peneplain is conveniently used to cover both the penultimate 

 and the ultimate stages, since the latter is only rarely observed, or 

 inferred. Similarly the panfan is a penultimate stage in the reduction 

 of the relief of the desert under conditions of crustal stability. There 

 remains, therefore, the question of the ultimate stage and of the transi- 

 tion from the penultimate to the ultimate stage. 



A desert reduced to the panfan stage may still be subject to cloud- 

 bursts, unless the smoothing out of the relief react upon the climate 

 and minimize still farther the precipitation. If such a reaction be 

 assumed, then we must recognize in the consequent increase of aridity 

 a tendency which would make for preservation of the panfan stage. 

 If, however, the aridity be not intensified by the reduction of the 

 relief, then it seems probable that the forces which had hitherto been 

 engaged in building the embankment would become effective for its 

 modification. This modification would be manifest as a transfer of 

 finer material from higher to lower levels, which would build up the 

 playa at the expense of the fine and medium-sized material on the 

 middle slopes. The process would of course be in operation long before 

 the attainment of the panfan stage, particularly when cloud-bursts were 

 local to the fan slope and did not affect the subaerial front. But as 

 long as the latter continued to be a source of abundant detritus the 

 modifying tendency would be counteracted and masked by the normal 

 upbuilding process, except that it might appreciably contribute to a 

 slight upward concavity in the fan profile. With the disappearance 

 of the subaerial front the cloud-bursts falling upon the fan surface 

 would be more effective for the transfer of material from higher to 

 lower levels, the running water being no longer charged with a load 

 from above the fan. This transfer would first accentuate the upward 

 concavity of the fan profile by (1) leaving the upper part of the 

 embankment relatively little affected, owing to the large size of the 

 fragments and the greater porosity in that region; (2) degrading 

 the middle slopes where the material is finer and less porous ; and 

 (3) adding the transported material to the toe of the embankment 

 and to the playa. What the limits of this modifying process may be, 

 without the stimulus of crustal deformation, it is difficult to predict. 

 With increasing upward concavity the position of the coarse detritus 

 on the upper margin of the embankment would become less stable and, 



