386 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



the subaerial plain above from the subaqueous below. Vigorous wave 

 action develops a pronounced shore face and separates more sharply the 

 two parts of the topset surface, but in protected situations it may cease 

 to be a distinct feature of the delta. The shore face is geologically a line 

 of greatest importance, yet it is seen to be in places a somewhat in- 

 definite demarcation, the beds deposited on the two sides of the strand- 

 line showing approaches toward each other. This is especially true 

 of lagoons within the subaerial plain and of islands thrown up by the 

 waves on the surface of the subaqueous plain. On small deltas of coarse 

 material built into lakes, with weak wa^■e power, the shore face is prac- 

 tically coincident with the upper edge of the fo reset slope and the dis- 

 tinctions between the two parts of the topset plain are not emphasized. 

 It is such small deltas, however, which are commonly used as text-book 

 illustrations of delta structure. But in the large deltas — for example, 

 in that of the Mississippi — broad areas are covered Avitli shallow water, 

 and consist structurally of topset beds as closely allied with the land 

 surface as with the deep-water portions of the delta. From the stand- 

 point of living things, however, the shore face dividing the topset bedn 

 into two parts is the zone of fundamental importance, the strand which 

 separates the two worlds of life — the regions of continental and marine 

 sedimentation. 



The littoral zone is that belt of shore alternately covered and laid bare 

 by tides, or where these are insignificant in range, by the effects of power- 

 ful onshore or offshore winds. Special faunas and floras dwell in this 

 narrow belt adapted to alternate exposure to air and salt water. For that 

 reason the definition of the littoral should be restricted to the vertical 

 limits affected by the maximum range of the water level as recurring 

 once or twice per month and not extended as an indefinite term to in- 

 clude what may be near the shore, yet belongs wholly either to the sea or 

 land. The steepness of ihe outer shore face causes Ihc liiioral to be there 

 a narrow belt subject to wave action. It attains a greater breadth in the 

 lagoons and salt marshes which lie behind the beach, and reaches a maxi- 

 mum development under the flat and swampy condition attending delta 

 growth. Here there may be all gradations into fresh-water swamps of 

 the subaerial plain, but, as shown in previous studies, the breadth of the 

 littoral zone does not increase proportionately with the tidal range and 

 occupies normally but a small part of the entire delta surface.^ 



Subaerial plain. — The inner parts of the delta are covered alternately 

 by air and river water. The carrying agents comprise channel currents 



- Barren : Relative geological importance of continental, littoral, and marine sedimen- 

 tation. .Journal of Oeology, vol. xiv. lOOG. pp. 443-440. 



