38 



University of California Publications in Geology [Vol. 9 



California west of the Colorado River near the Needles, as illustrated 

 in the photographs, 8 plates 1 and 2. Here broad alluvial embank- 

 ments, from six to ten miles across, rise by a gentle slope to the 

 summits of the ranges, where there are residual rock crests, or "nub- 

 bins," and stretches where the crest has entirely vanished as a pro- 

 tuberance above the general slope. Within a mile or more of the 

 summit one passes, without appreciable change of slope, from the 

 alluvial detritus of the embankment on to a bare rock surface extend- 

 to the nubbins of the crest. The upper limit of the alluvium is a 

 feather-edge. 



It thus appears that, in addition to the rock terraces of desert 

 ranges due, as explained by Paige, 9 to the stripping of suballuvial 

 benches of the alluvium which once mantled them, we have to reckon 

 with similar features, which may be evolved subaerially, and yet not 

 by any process of sheet flood erosion, but by the normal process of 

 recession of mountain fronts. 



The Alluvial Embankment. — The mode of growth of the alluvial 

 embankment determines certain features of structure which may be 

 of service as criteria for distinguishing it from other types of deposits 

 in the formations of past ages, when it appears as an element in 

 problems of stratigraphy. The discrimination desired may usually 

 be readily made in the coarser portions of the embankment by a mere 

 inspection of the character of the constituent fragments. The charac- 

 teristics of fanglomerate have been briefly described in a former 

 paper 10 and in good exposures there is little likelihood of it being 

 confounded with either a marine or a fluviatile conglomerate. The 

 finer deposits on the outer edge of the embankment and those of the 

 playa are, however, not so easily distinguished from delta deposits 

 either above or below sea-level, or from lacustrine deposits. It is 

 therefore desirable to point out in what respects the configuration and 

 internal structure of an embankment of the desert differs from other 

 sedimentary formations which are commonly met with in studies of 

 the earth's crust. 



Considered as a whole, the alluvial embankment of the desert is 

 elongated in a direction transverse to the movement of the material 

 which comes to it. It results from this that the grading of size of 



s These photographs and the descriptions of the conditions which they 

 illustrate were kindly supplied by Messrs. W. L. Moody and A. W. Lawson, 

 of the geological staff of the Southern Pacific Company. 



s Op. cit. 



io Univ. Calif. Pub!., Bull. Dept. Geol., vol. 7, no. 15, 1913. 



