434 J. BARBELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



show neither structure nor fossils. False bedding oblique to the even 

 regular bedding is occasionally observed, but the homogeneous material 

 conceals its frequent presence. Eipple and current marks, however, are 

 rare or absent. Similar formations may possibly also be of marine ori- 

 gin, but it seems probable from the normal presence of wave action that 

 ripple-marking should occur more commonly in the latter. It would 

 seem unsafe, however, to assume in the present state of knowledge either 

 a fluviatile or marine origin for a formation on the basis of such struc- 

 tures alone. 



Effects of wind. — Cross-bedding and ripple-mark of most noteworthy 

 development occur as the result of wind action on river or beach sands. 

 In semi-arid or arid climates during the dry season the shrunken streams 

 lay bare large areas of loose sands which are swept to leeward for indefi- 

 nite distances. The delta of the Indus furnishes an example of such a 

 fluviatile deposit greatly modified by wind action. The dunes of such 

 regions advance by the deposition of successive layers of sand on the 

 leeward face. With each change of wind some shifting of the dune takes 

 place. The marching of the dune involves the continuous destruction 

 and construction of the bedding, but in regions of aggradation the basal 

 parts of the dunes remain and become permanently buried. Huntington 

 has called attention to the fact that cross-bedding of eolian origin attains 

 a much larger scale than cross-bedding by water currents. He shows 

 also that eolian cross-bedding is furthermore deposited on curved sur- 

 faces which approach tangency to a horizontal plane at the bottom.-^ 

 Aqueous cross-bedding, on the other hand, is commonly developed as 

 plane slopes at a distinct angle to both the horizontal planes which limit 

 the structure. These distinctions have been forcibly urged as evidence 

 that the Triassic, and especially the white Jurassic sandstones of north- 

 western Arizona, are ancient desert sands.^^ If gravels occur in connec- 

 tion with such wind-blown sands certain of the i:)ebbles may be expected 

 to show the smoothed facets and sharp edges which are developed by 

 wind action, giving rise where carried to perfection to tlie form of peb- 

 bles known as dreikanter. Eipple-mark on marine sands is best devel- 

 oped on nearly horizontal surfaces, since the action of the waves is closely 

 limited by depth. Winds, however, are not so restricted in action and 

 develop ripple-mark on the long sloping sides of dunes. Furthermore, 



2" Some characteristics of the glacial period in non-glaciated regions. Bull. Geol. Soc. 

 America, vol. 18, 1007, plates 80-8S. pp. 351-888. 



='"' Ellsworth Huntington and .T. W. (ioldthwait : The Hurricane fault in the Toquer- 

 ville district, Utah. Bull. Mus, Comp. Zool., Harv. Col., vol. xlii, 1004, pp. 214-210. 



