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University of California Publications in Geology [Vol. 9 



that they may have done so is clearly possible. This possibility is 

 not here introduced in a discussion of the origin of these valleys, but 

 to bring out the essential structure of the embankments which fill them, 

 whatever may have been their initial profile and mode of forma- 

 tion. Considering now the surface profile of the valley MFPF'M' 

 as typical of the present valley of the Great Basin, the materials 

 which form the surface of the embankment may be for convenience 

 classified into three grades, coarse, medium and fine. The point a 

 may be taken as the outer limit of the coarse material and b the outer 

 limit of the medium, and the fine would extend to the center of the 

 valley. Similarly at the successively preceding stages P 1} P 2 , P 3 , P 4 , 

 the points a x , a.,, a 3 , « 4 , would represent the outer limits of the coarse 

 material at the respective stages. At stages P s and P ti , however, the 

 coarse material would reach the center of the valley and would, there- 

 fore, not be sorted from the medium and fine detritus. Also at the 



Ml M 



Figure 4. — If the typical desert valley MFPF'M' were evolved from the 

 initial steep-sided valley NON', there would be developed the twinned embank- 

 ment FOF' resting on the cut benches FO and F'O and having two intersect- 

 ing systems of stratiform structure. 



stage P 1; b would be the outer limit of the medium-sized material and 

 the fine would go on to the center of the valley; but at stages P.,, P 3 , 

 P i} P 5 , P 6 , the medium-sized material would extend to the middle of 

 the valley and would therefore not be sorted from the fine. The same 

 would be true of the twin embankment on the other side of the valley. 

 Considering now the embankment in ideal cross-section, it follows 

 from what has been said that the coarse detritus is confined to the 

 bottom of the embankment and that its upper limit is the curve 

 drawn through the points a lf a.,, a 3 , a 4 , but that in the central region 

 the coarse is mixed with medium and fine material. Similarly the 

 coarse material is throughout the embankment, from the center of 

 the valley to a, overlaid by medium-sized material the upper limit of 

 which is the curve drawn through b, b x ; and above this in the central 

 or playa region is the fine detritus. Of course under actual conditions 

 the demarcations indicated by the points a, a 1} b, b 1} etc., are grada- 

 tional, and the locus of the mean position of these points in plan is 



