CURRENT BEDDING OF FAN DEPOSITS 403 



of this kind, each consisting of inclined foresets and horizontal topsets, 

 have been found superposed. 



It would be difficult to conceive of a series of such deposits as having 

 been formed in a body of standing water. A single series might be inter- 

 preted as a small delta, but a succession of such series could not be 

 formed without periodic catastrophic subsidence after the completion of 

 each delta. Moreover, as such subsidence brings with it a transgression 

 of the waters, it is evident that as a result the zone of deposition would 

 be shifted landward, and that offshore deposits would follow on the old, 

 now submerged, delta. This type of cross-bedding is, however, eminently 

 characteristic of subaerial torrential stream deposits, but it is wholly 

 unknown so far from undoubted marine or lacustrine sediments. More- 

 over, since the deposits showing such cross-bedding are free from marine 

 or lacustrine organic remains, though often carrying those which can be 

 shown to be of land or river habitat, it seems that such a type of cross- 

 bedding may be taken as strongly indicative of a non-marine-torrential 

 river rather than a seashore deposit. 



The second type of current bedding is more properly described as a 

 cross-bedding, or better as a crisscross type of bedding, since the beds in- 

 cline in all directions. The angles of inclination vary greatly, and the 

 successively inclined series are not separated by horizontal beds, but by 

 erosion surfaces, which may also vary in amount and direction of inclina- 

 tion. This sort of bedding represents a process of cut and fill on a large 

 scale and it is most characteristic of eolian deposits at the present time. 

 Walther 4 has described it from the Libyan desert and Huntington 5 from 

 the deserts of Asia. One of its most characteristic features is the fact 

 that the oblique layers, where they come in contact with the underlying 

 erosion plane, show a marked tangency with this plane. 



In the modern eolian deposits of this type the inclined strata are rem- 

 nants of the anticlinal stratification of sand dunes. These dunes are 

 partly truncated, whereupon a second set of dunes is laid down on the 

 remains of the eroded first set. Repeating this several limes will, if the 

 layers become properly arranged, produce the structure in question, li is 

 essentially a progressive migration of sand dunes across the truncated 

 bases of the preceding series (figure 2). 



While a structure of this type can be formed on a small scale by mi- 

 grating ripple-marks, as shown by Gilbert, it is difficult to conceive how 



4 Denudation in dor Wtiste. AMiand. <l. Math. IMi.vs. Classe d. K. Sachs. Gesell. d. 

 naturw., vol. xvi, pp. 347-569. 1891. 

 B Bttll.<Geol. Soc. Am., vol. 18, pp. :'>7!> el set). 

 " Hull. Geol. Soc. Am., vol. 10, p. L89. 



