440 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



favored by the nature of the material. It is especially characteristic of 

 semi-arid climates, but extends beyond to a more limited degree into 

 both arid and sublmmid climates. It is commonly marked by such 

 minor wind activities as the rippling of sandy surfaces and the filling 

 of mud-cracked plains by sand. The latter is driven forward in a thin 

 stratum without the development of dune structure unless the sand is 

 very abundant and the winds are strong. The great quantity of animal 

 life existing at present in the semi-arid districts of Africa and the great 

 number and variety of footprints in the Triassic strata of Connecticut 

 associated with these evidences of semi-aridity show how widely such 

 climates differ in their life relations from true deserts, and a study of 

 the rainfall maps of the world shows how characteristic are such climates 

 of continental interiors and, in certain latitudes, even in the proximity 

 of the ocean. 



BREADTH OF EOLIAN ACTION AS A CRITERION OF FLUVIATILE DEPOSITS 



The preceding sections have shown that river sands- are much more 

 broadly exposed to the air than sands of subaqueous origin ; that sands 

 where so exposed are subjected to wind action under climates which now 

 widely prevail and support great quantities of life, and that certain com- 

 bined structures of marked character not uncommonly arise. The condi- 

 tions for the preservation of such evidences by burial are, furthermore, 

 much more favorable in the case of aggrading rivers than for the de- 

 posits which fringe the shore. The arguments are therefore of cumula- 

 tive force which show the value of partial wind action as a high but not 

 absolute criterion of fluviatile origin and suggest the importance of 

 careful search for such structures in ancient formations. 



SIGNIFICANCE OF CONGLOMERATES 



Gravels are transported by ice, by rivers, and by waves, giving rise to 

 conglomerates of glacial, fluviatile, and marine origin. Glacial gravels 

 may be eliminated from the discussion, leaving the distinctions between 

 marine and terrestrial conglomerates to be considered. 



W. D. Johnson pointed out that in the Tertiary deposits of the High 

 Plains the gravel courses where exposed to observation are greatly elon- 

 gated in the direction of the streams^^ — that is, in the direction leading 

 away from the source of supply. Mansfield has noted that shore gravels, 

 on the contrary, are extended in courses parallel to the margin of the 

 deposit.^^ Ancient conglomerate formations, however, are commonly 



"The high plains and their utilization. 21st Ann. Rept. U. S. Geol. Surv., part IV, 

 1001. p. 034. 



"The characteristics of various types of conglomerates. Journal of Geology, vol. xv, 

 1907, p. 554. 



