384 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



Strong tides scour the lower channels, keeping them large and open. 

 But the inrushing tide has greater carrying power than the ebb unless 

 the latter is assisted by a strong river current. Consequently, although 

 there is great channel scour, sedimentation is all tlie more rapid in the 

 two zones of slack water, that of the salt marshes on the one hand and 

 the adjacent shallow sea bottom on the other, and the greater strengtii of 

 the flood tide tends to keep even the marine sediment near the land. 

 The carrying of material in suspension until deep Avater is reached pre- 

 vents to that degree the building of deltas. But the silt deposited over 

 the salt marshes at full tide tends on the other hand to build rapidly 

 upward the floodplain to the level of high tide, so that tides serve in this 

 respect to extend the subaerial plain. If the river empties into a shallow 

 sea the sediment which is dropped from suspension where the tidal cur- 

 rent weakens will commonly be within the reach of the waves and the 

 deposit, like that from the river currents proper, diminishes the shore 

 action of the waves, thus doing indirectly its part toward extending the 

 delta. The strength of the tidal action in itself does not therefore con- 

 trol ultimately the presence or absence of deltas, though the contrary 

 view is commonly expressed in text books, based on observations on the 

 scouring power of tides in estuaries. The Indus delta, for example, is 

 built in the presence of a tidal range of 10 feet and that of the Ganges 

 against a tide of 16 feet. 



Waves are the more real destroyers of the delta front. They plane 

 back the shore, and by keeping the finer material in suspension it is 

 swept out to deeper water. If it were swept radially out from the land, 

 and if the limiting depth of wave action were soon reached, the effect 

 would be chiefly to build outward the subaqueous platform of the delta 

 at the expense of the subaerial plain, thus modifying the form but not 

 destroying the individuality of the delta. But the lateral component of 

 wave action works the material along shore for indefinite distances. Cur- 

 rents parallel to the shore aid in the dispersion and, cooperating with 

 strong wave action over a shallow sea — as, for example, in the Korth 

 Sea — the sediment may be widely distributed both along shore and away 

 from the land. Thus wave action aided by lateral currents, either of 

 tidal or non-tidal origin, tends to destroy the delta by redistribution of 

 its material. The waves work most strongly against the shore, and by 

 piling up beaches and by excavating the foreshore serve to differentiate 

 more sharply the subaerial and submarine portions of the Helta. 



From this brief statement of the parts played by the several factors in 

 delta construction and destruction it is seen that the two directly oppos- 

 ing processes are, first, those of floodplain aggradation, building the land 



