PHYSIOGRAPHIC AND GEOLOGIC DEVELOPMENT 263 



over the desert in the early morning, that in the most favorable 

 places moistens even the dune surfaces and that has least penetra- 

 tion on the steep semi-protected dune fronts. Sand later blown 

 up the dune front or rolled down from the dune crest is en- 

 couraged to remain near the cornice on an abnormally steep slope 

 by the attraction which the slightly moister sand has for the dry 

 grains blown against it. Since dunes travel and since their front 

 layers, formed on steep slopes, are cut off to the level of the sur- 

 face in the rear of the dune, it follows that the steepest dips in 

 exposed sections are almost always less than those in existing 

 dunes. Exceptions to the rule will be noted in filled hollows not 

 re-excavated until deeply covered by wind-blown material. These, 

 re-exposed at the end of a long period of wind accumulation, may 

 exhibit even the maximum dips of the dune cornices. Such will 

 be conspicuously the case in sections in aggraded desert deposits. 

 On the border of the Majes Valley, from 400 to 500 feet of wind- 

 accumulated deposits may be observed, representing a long period 

 of successive dune burials. 



The peculiar blending of the contact lines of dune laminae, re- 

 lated to the tangency commonly noted in dune accumulations, is 

 apparently due to the fact that the wind does not require a graded 

 surface to work on, but blows uphill as well as down. It is pres- 

 ent on both the back-slope and the front-slope deposits. Its finest 

 expression appears to be in districts where the dune material was 

 accumulated by a violent wind whose effects the less powerful 

 winds could not destroy. 



It is to the ability of the wind to transport material against, 

 as well as with, gravity, that we owe the third distinct quality of 

 dune material, the succession of flowing lines, in contrast to the 

 succession of now flat-lying now steeply inclined beds character- 

 istic of cross-bedded material deposited by water. One dune trav- 

 els across the face of the country only to be succeeded by another. 5 

 Even if wind aggradation is in progress, the plain-like surface in 

 the rear of a dune may be excavated to the level of steeply inclined 



5 The best photograph of this condition which I have yet seen is in W. Sievers, Siid- 

 und Mittelanierika, second ed., 1914, Plate 15, p. 358. 



