LARGER RELATIONS OF DELTAS 385 



upward and outward, giving convexity and unity to the delta; and, 

 second, marine planation, carrying the sea inward and downward and 

 tending to maintain the straightness of tlie shoreline. River and tidal 

 currents greatly modify the outline of the delta and the place of growth, 

 but do not control the existence of the subaerial and subaqueous plains. 

 Delta-building is thus due to a dominance of fluvial over marine action. 



The hottomset beds. — A delta consists typically of several parts, as 

 shown in figures 3 and 4, pages 396 and 399, and are more or less dis- 

 tinctly unlike in nature. The outermost deposits consist of materials 

 which have settled slowly from suspension in water, making an extended 

 mantle of gradually diminishing thickness. These are the bpttomset 

 beds, and do not differ in any easily recognizable way from those deposits 

 of similar depth where waves and currents have prevented the formation 

 of a delta. Stratigraphically they are, therefore, not to be discriminated, 

 and the demonstration of deltaic conditions must rest on other lines of 

 evidence. 



The foreset beds. — N'earer shore is the steeper delta face or foreset slope, 

 made by the accumulation o^ that coarser material swept outward by 

 currents and waves until the depth of wave base is reached, the carrying 

 power disappears, the material finally settles, and being built out at the 

 top tends to develop the front at a relatively steep angle. Where the 

 sediment is dominantly coarse and but little of it carried in suspension, 

 the steepness of the foreset slope approaches the angle of repose. But 

 where, as is the case with large rivers, the detritus is mostly fine in tex- 

 ture the foreset beds are built largely by material settling from suspen- 

 sion. Both in slope and texture there is less distinction from the other 

 parts of the delta, the foreset beds grading especially into the hottomset 

 beds, the greater steepness being due to the greater rapidity of settling 

 near the limit of wave action. 



The topset beds — Subaqueous plain. — The inner part of the delta con- 

 sists of the topset beds, whose upper surface slopes gently seaward at 

 such a grade as is necessary to convey that detritus which is swept along 

 the bottom to the edge of the foreset slope. The topset surface consists 

 of two distinct portions, the subaqueous and subaerial plains, separated 

 by a narrower transition belt — the shore face and littoral zone. The 

 subaqueous plane is the outer part permanently beneath the water level. 

 Across it material is transported by wave action and currents of the open 

 water. It is characterized by being the home of marine organisms, 

 affected by waves rather than currents, and never exposed to the air. 



The shore face and littoral zone. — The shore face is the relatively 

 narrow slope developed by the breaking waves, a slope which separates 



