GEOLOGICAL IMPORTANCE OF SEDIMENTATION 437 



upon the tidal rise than upon the greatness of the river and its delta 

 which faces the tidal wave. This may be due to the great quantity 

 of sediment furnished to the sea by the rivers, swept back bv the 

 tide and dropped at its flood, or to the subsidence which frequently 

 characterizes such areas and which tends to drown the seaward end 

 of the delta. Probably both causes contribute. This is illustrated 

 by noting that on the Mississippi delta this width of marsh is 27 

 miles, while in Mobile Bay near by it is from nothing up to 2 miles. 

 At Boston the marshes average less than a mile in width, while the 

 Indus delta, with the same tidal range, shows them at least 17 miles 

 wide. The delta of the Ganges in the presence of a tide of 16 feet is 

 flooded by the tides for a distance of 30 miles, while in the Bay of 

 Fundy and the Basin of the Minas the salt marshes average less than 

 a mile in width. 



The conclusion is geologically of some importance. The broad 

 development of a littoral zone is largely independent of tidal influ- 

 ence, since, where the tides are small, oscillations of level through 

 storms may develop it to practically the same width. The topo- 

 graphic character of the littoral zone, as indicated by dominance of 

 mud-flats, size of tidal channels, etc., is, however, dependent upon 

 the tidal range as one factor. Where the land is shelving, as along 

 the eastern and southern coasts of the United States, the shore may 

 be much broken with estuaries, but the land and the water are sepa- 

 rated by a littoral, which, including both its upper and lower por- 

 tions, does not average more than a mile in width. The littoral is 

 therefore of insignificant area compared to the breadth of the coast 

 plains on the one hand, and of the shallow sea on the other. 



The littoral becomes broadest where a great quantity of sedi- 

 ment is poured into the sea by a great river and a contest ensues 

 between the sea and the land. But in such places the river, if suffi- 

 ciently powerful, builds out a land surface delta in the face of the 

 storms and tides, while the undertow of the latter builds out another 

 extensive platform, submerged in gradually deepening water, and 

 ending where a depth is attained of from 50 to 100 fathoms. Even 

 beyond this limit important quantities of the finer sediment are swept, 

 forming occasionally bottom deposits from 200 to 800 miles from 

 land. 



