432 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



the dune or the ripple-mark made by the oscillations of waves. As waves 

 and currents ma_y operate together, there are, however, all gradations 

 between ripple-mark and current-mark. In the papers which treat of 

 the theory of ripple-mark no distinction has commonly been made be- 

 tween the effects of waves and currents, both producing back eddies along 

 the bottom. It seems, however, to the writer from repeated observations 

 that where made clearly by a single cause the two structures can be sepa- 

 rated, waves producing a symmetrical system of ridges; currents, on the 

 other hand, resulting in ridges which are unsymmetrical in both plan 

 and section. 



The cross-bedded structures of fluviatile sands are the result of the 

 cutting out and filling in of channels and the downstream migration of 

 bars ; the slopes of the cross-bedding are commonly steep, from 15 to 30 

 degrees. Although showing considerable variation, they tend to slope in 

 one direction. The character of tlie cross-bedded strata of alluvial fans 

 has been described by Hobbs,^' and several illustrations of cross-bedding 

 ascribed to current action are given by A. Geikie.^^ Such cross-bedded 

 strata are especially discontinuous and indicate broken currents and 

 shifting channels. The effects are presumably much more striking than 

 the cross-bedding produced where waves are a powerful factor. On the 

 other hand, the thickness of a single cross-bedded stratum of fluviatile 

 origin is normally limited to a few feet, and in that respect is distinct 

 from the cross-bedding which results from the migration of dunes. 



Contrasts of marine and fluviatile action. — The modes of action of seas 

 and rivers on sand have been discussed at some length, as much to serve 

 as a guide to further observation and to prevent premature generaliza- 

 tion as to develop what is at present known. Under this heading will be 

 drawn briefly a comparison between the two. 



In marine deposits the coarser material is carried and deposited as the 

 result of great storms ; the finer interbedded material is the mark of les- 

 sened activity. In river action, on the contrary, the finer grained deposits 

 of the floodplain are made as the result of the waters of great floods or the 

 winds of dry seasons; the channel sands represent the silting up of 

 diminished streams in the stages of lessened activity. The sands of the 

 natural levees are spread out in sheets, however, at times of high water. 

 The sea is dominated by wave action ; the river and its sand-bearing 

 floods are dominated by current action. The waves tend to spread sand 

 in even sheets with evenly ripple-marked surfaces and a minute cross- 



^ Guadix formation of Granada, Spain. Bull. Geol. Soc. America, vol, 17, 1906, p. 

 291. 



28 Text-book of Geology, 1908, pp. 636-038, 



