438 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



In the case of such formations, it is clear that other features than the 

 wind-worn character of the sand must be relied on to determine the 

 mode of origin of the final deposit, but on the basis of the structure 

 Sherzer considers that much of the Svlvania is a truly eolian deposit. 



Notwithstanding the examples just discussed, it is seen that under 

 modern conditions eolian action modifies terrestrial and fiuviatile de- 

 posits much more broadly than it does wave-worked sands. Eivers are 

 efficient carriers of sand; thick deposits may be made over subsiding 

 areas and much of each stratum is in turn broadly exposed to the air. 

 Where the sedimentation is slow the wind is given fullest opportunity 

 for work. On the other hand, sands which reach the sea are spread as 

 widely as wave action is effective on the bottom, and it is only on the 

 shore or on the upper portions of sands abandoned by a retreating strand 

 that wind action can come into play. The associations of eolian action 

 furnish, therefore, criteria of considerable value for separating forma- 

 tions chiefly marine from those chiefl}^ fluviatile and should occur much 

 more commonly in the latter. 



CLIMATES IMPLIED BY EOLIAX ACTION 



Desert climates and dominant dune structures. — There is necessarily 

 no sharp distinction between the combined wind and water structures of 

 arid climates and semi-arid climates, especially as surrounding uplands 

 may possess a semi-arid climate and furnish water to the true desert 

 below, or great rivers like the Xile may flow from well watered zones 

 through regions of arid climate. The following general distinctions 

 may, however, be drawn : In the true deserts wind is the dominating 

 activity. It not only shapes the sand but is the agent which abrades the 

 rocks and which sorts and transports both dust and sand. Where thick 

 sandstone formations show dominant dune structure and wind-worn 

 texture, as in the Jurassic white sandstones of Arizona, the inference is 

 strong that ancient deserts probably prevailed. 



Semi-arid climates and dominant combined structures. — It would ap- 

 pear that the degree to which wind action may modify the fluviatile or 

 marine sands in semi-arid or even in humid climates has not been appre- 

 ciated by some who have written of past conditions, nor that sufficient 

 distinction has been drawn between semi-arid and arid climates, widely 

 different in their terrestrial development and their relations to life. Ked 

 color, feldspathic sands, and the presence of some wind-worn material 

 have been taken as evidence of desert conditions in the Torridonian pre- 

 Cambrian and the Old Eed Devonian sandstones of Scotland, although 

 saline deposits are not present and the bulk of the material is such as 



