430 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



definitely waves even as great as those of the present open oceans. The 

 explanation should rather be sought in some nodal effect, or the drag- 

 ging, partial breaking, and recovery of waves of translation in very 

 shallow water, or perhaps some relations of waves to undertow currents. 

 ^Miatever the explanation may be the fact remains, however, that such 

 structures are presumably formed on the subaqueous plain, and their 

 migration across a bottom on which sand was accumulating could give 

 rise to a moderate degree of cross-bedding, but one presumably distinct 

 from the steeper and more irregular cross-bedding due to current action 

 where the latter becomes a dominating action. 



If cross-bedding in a sandstone or limestone, presumably marine, 

 occurs on a large scale it is possible that it may be due to wind action on 

 material abandoned by the sea or blown inland. Grabau has called atten- 

 tion to an instance on the shores of the Eed Sea where an eolian lime- 

 stone of foraminiferal tests is accumulating many miles inland,-® and 

 the eolian limestones of Bermuda blown inland from beach materials are 

 well known. Such cooperation of wave and wind action is favored in 

 shallow waters and makes more uncertain the direct interpretation of 

 ancient formations and calls for a closer study of modern sedimentation 

 under analogous conditions. 



Effects of currents. — Currents as carrying and depositing agents are 

 especially characteristic of fluviatile action. In estuaries scoured by 

 strong tides current action is also dominant, but they are in -fact en- 

 larged river channels alternately invaded by sea and river waters. In 

 connection with irregularities in the coastline, waves also produce cur- 

 rents, as the result of concentrated undertow or obliqueness to the coast- 

 line. Such effects in seas and lakes are, however, local and exceptional as 

 compared to the broad areas where wave action is dominant. The results 

 of marine currents may be seen on the coast charts which show the en- 

 trances to Delaware and Chesapeake bays and also off Cape Hatteras. 

 The conditions which give rise to wave-formed currents are connected 

 especially witli tlic inequalities of coastline resulting from a recent 

 crustal movement and are rather closely restricted to shallow water and 

 the vicinity of coasts. The waves and the currents which they generate 

 are, however, in continual opposition, the one tending to fill up, the 

 other to scour out. The leveling effects of strong wave action prevent 

 in consequence such sharply defined and undercut channels as are devel- 

 oped by rivers. The slope of their sides probably does not average more 

 than one in twenty-five, but the great volumes and consequent large 



2fl Oral commimication. Washington, D. C, December 20, lOH. 



