1915] 



Lawson: The Epigene Profiles of the Desert 



41 



an irregularly sinuous or serrate line. Nevertheless the general fact 

 remains that under uniform unchanging conditions we have built 

 up a sedimentary formation, which, by its tripartite stratiform sub- 

 division into coarse, medium and fine material would, and usually 

 does, suggest to the student of stratigraphy rather abrupt changes 

 of general conditions, whereas the only change that has transpired is 

 the gradual recession of the subaerial front, the source of supply of 

 the detritus. From a petrographic point of view these three divisions 

 of the embankment may be referred to in ascending order as fan- 

 glomerate, arkose and silt. It is noteworthy that all three increase 

 regularly in thickness away from the source of the material. All three 

 present two kinds of stratification. One of these is a formational 

 stratification determined by the upper and lower limits of the fan- 

 glomerate, arkose and silt respectively ; the other is the stratification 

 of actual deposition, which is parallel to slope of the surface of the 

 embankment, and is flatter than the formational stratification, being: 

 thus in contrast to ordinary false bedding, which is steeper than the 

 formational stratification. It is also noteworthy that a central or playa 

 deposit of silts well segregated from arkose can be developed, under 

 the assumed conditions, only at a comparatively late stage of the up- 

 building of the embankment. In earlier stages the silt of the central 

 region is mixed with arkose and at still earlier stages with both arkose 

 and fanglomerate. 



If our initial valley be wide-bottomed instead of V-shaped there 

 may be ample space for sorting, so that coarse or even medium detritus 

 may not reach the middle of the valley at any stage of the upbuilding 

 of the twinned embankment and lacustral deposition is possible. A 

 similar situation may arise if the valley be a graben. Hence the 

 central region of the twinned embankment may vary in the character 

 of its materials and, therefore, in structure according to the initial 

 profile of the valley at the inception of fan building. But for the 

 portion of the embankment that rests upon the suballuvial bench on 

 both sides of the valley the distribution of material and the resulting 

 structure are in all cases in a general way the same. 



The transverse profile of the embankments actually observable in 

 the desert is as even and uniform as is indicated in the discussion of 

 the ideal case, except that a slight upward concavity may in some 

 instances be detected. In the longitudinal profile, or that parallel to 

 the range, there is, however, a systematic irregularity, due to what 

 may be described as a transverse fluting of the embankment. 



