GEOLOGICAL IMPORTANCE OF SEDIMENTATION 



343 



equal or exceed in thickness the fo reset beds. Where the area of the 

 delta becomes as great as was the case of the Carboniferous coal 

 swamps of the eastern United States, the resulting volume of land 

 deposits may far outweigh the volume of the foreset beds building 

 the delta front. 



At first thought it might be supposed that over a subsiding delta 

 region the only effect would be an encroachment of the sea, without 



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Fig. I. — Ideal section of delta built out into quiet water of constant level. 



Fig. 2. — Ideal section of delta built out into quiet water when subsidence just 

 balances deposition, resulting in a stationary shore-line. 



changing the slope of the delta surface, diminishing the subaerial por- 

 tion without increasing the rate of river deposition upon it. When a 

 dam is thrown across a stream, however, and the water-level artifi- 

 cially raised, the gradient is flattened, and the stream made more 

 sluggish for some distance upstream at levels higher than the upper 

 edge of the dam. Where similar elevations of the water-level occur 

 through natural causes, it is presumed, therefore, that the effect would 

 result in an enlargement of the zone of fresh-water morass and 

 increased river deposit, as well as a tendency to restrict the margin 

 of the delta. 



It is probable that many deltas reach a point beyond which they 

 advance but slowly. This occurs in those fronting large bodies of 



