1915] Lawson: The Epigene Profiles of the Desert 



39 



its constituent material is transverse to its general elongation, the 

 tendency being for the coarser material to remain on one side and 

 the fine to be carried to the other, at any and every stage of its growth. 

 Owing to the fact that the embankment transgresses the wasting 

 mountain mass, the fan growing broader as it rises, the fine material 

 of a late stage of its growth may be, and usually is, directly above 

 the coarse material of an earlier stage, with a gradation between. 

 The reverse condition of coarse above fine never occurs except Avhere 

 the embankment has been dislocated by a large fault, the scarp of 

 which would supply a local source of coarse material. 



But from a formational or stratigraphic point of view the em- 

 bankment which flanks a wasting desert mountain ridge cannot be 

 considered alone. Nearly all such embankments have a twin or sym- 

 metrical relation to another embankment on the other side of the 

 valley, and the fill of the valley must be considered as a whole. From 

 this point of view the formation is a twinned embankment with 

 similar structure on both sides of its axis of symmetry. In the later 

 stages of its growth the central part, the playa, is composed of fine 

 silts and sands flanked on either side by arkose, the latter in turn 

 grading towards the sides into coarse fanglomerate. But as to the 

 distribution of the different grades of material in the early stages 

 of the growth of the twinned embankment in particular eases we can 

 speak only in a speculative way, because we rarely have information 

 as to the configuration of the country when it first came under the 

 sway of aridity. We may, however, discuss to advantage an assumed 

 case for the purpose of clarifying our ideas as to the essential features 

 of the internal structure of the embankment, recognizing that the 

 results deduced will vary greatly with other assumptions as to the 

 initial condition. 



We may suppose : ( 1 ) That a deep V-shaped valley evolved by 

 erosion under humid conditions is suddenly brought under conditions 

 of aridity; (2) that by crustal warping or by alluviation the valley 

 becomes an enclosed basin; and that (3) the rocks are homogeneous. 

 In the course of time, by the recession of the valley sides from ON 

 and ON' to FM and F'M' in figure 4, the suballuvial benches OF and 

 OF' will be cut and the surface of the twinned embankment will rise 

 to FPF'. At this stage the profile of the valley and the adjacent 

 mountains will be that of the ordinary desert valley in the Great 

 Basin. I do not suggest that the broad alluviated valleys of the Great 

 Basin thus originated as valleys of erosion in a humid climate; mil 



