1915] 



Lawson: The Epigene Profiles of the Desert 



33 



the slope. This quantity is determined by the distance between the 

 ultimate crest of the degraded buried range and the median line of 

 the ultimate aggraded valley on either side. If these distances are 

 unequal, then the playas in the valleys stand at different altitudes, 

 the longer slope reaching out to the lower valley. It follows from 

 this that difference of altitude of the playas on the two sides of a 

 symmetrically sloped range introduces no element of instability into 

 the profile. The playas represent the bottoms of distinct enclosed 

 basins, but when the general condition above outlined has been attained 

 there is no tendency for the lower basin to capture the higher, so long 

 as the climatic conditions remain constant. Such capture may be 

 effected in part by the migration of the crest of an initially asymmetric 

 or heterogeneous range as an incident of the general process, but once 

 the rock crest vanishes the tendency to migration ceases and capture 

 is no longer possible. The surface thus evolved is, in its ideal com- 

 pletion, wholly one of aggradation, a vast alluvial fan surface to which 

 for convenience in discussion I propose to give the name panfan. 



The Panfan Stage of the Geomorphic Cycle of the Desert. — The 

 panfan may be regarded an end stage in the process of geomorphic 

 development in the same sense that the peneplain is an end stage of 

 the general process of degradation in a humid climate. The peneplain 

 closes the cycle of degradation and is a cut surface ; the panfan 

 closes a cycle of degradation and aggradation, is evolved by both 

 cutting and filling, and is a built surface. Like the peneplain, the 

 panfan is rarely observed in its ideal completion. The time required 

 for its completion is so long that diastrophic or climatic changes 

 usually interrupt the process, and even where it escapes those inter- 

 ruptions the final result is subject to destruction by the same changes. 

 We may not expect, therefore, to find the panfan a prevalent feature 

 of the relief of deserts which have been subject to such diastrophic 

 and climatic vicissitudes as have from time to time overtaken the Great 

 Basin or portions of it. Exceptionally, however, it does occur, as in 

 parts of southern California ; and in various stages of incompletion 

 is one of the commonest features of the desert. 



The recognition of the panfan as a phenomenon to be classified is, 

 however, not so important as is the perception of the process 5 whereby 



s This process has been fully recognized by Sidney Paige in his paper 

 "Rock-cut Surfaces in the Desert Ranges, " Journal of Geology, vol. 20, 1912, 

 and he has the honor of first summarily stating it. The conclusions set forth 

 in the present paper may be considered as an amplification of his observations, 

 although they were arrived at independently, and are, therefore, a corrobora- 

 tion and not merely a restatement of his views. 



