MODIFICATION OF DELTA 617 



on the delta, such as dust brought from a distance and wind- 

 arranged material from the surface of the delta itself. Additional 

 river deposits in the form of natural levees may be built by the 

 overflowing river, which leaves much of its material near the bank, 

 which is thus raised above the surface of the delta on either side. 

 The floor of the channel may also be raised by aggradation, and 

 the river thus flows at a level much higher than that of the sur- 

 rounding country. Where the levees are artificially raised the 

 river bed may come to occupy a position far above the normal 

 level of the delta or flood plain. Thus the channel of the Po has 

 been elevated in this manner to such an extent that it is said to 

 rise above the tops of the houses in the town of Ferrara (Le Conte- 

 ^6:26). Where the sea temporarily floods part of the delta, de- 

 posits of salt may occur, as in the Rann of Cutch and other low- 

 lying delta districts. The Nile delta is likewise characterized by 

 saline deposits along the coast, due to evaporation of the sea water 

 which at intervals floods the surface. Over these salt beds sand 

 dunes are seen to wander. Deposits of vegetal material in swamps 

 further characterize many delta surfaces. Such deposits may be 

 marine marsh deposits or fresh water swamp deposits, or both. 

 Salt and gypsum deposits are not uncommon on the lower part of 

 the deltas in arid climates. 



Relation of Delta Building to Crustal Movements. From the 

 known relations of rivers to delta building it would appear that 

 pieriods of strong river activities are also, as a rule, periods of 

 pronounced delta formation, while, conversely, periods of dimin- 

 ished river work will be, on the whole, periods of slow and com- 

 paratively limited delta building. In general, youthful ungraded 

 conditions of a land with corresponding high relief favor delta 

 building, while mature or graded conditions characteristic of regions 

 of low relief are correspondingly unfavorable to delta building. 

 There are, however, modifying circumstances which may, to a cer- 

 tain extent, reverse these results, but as a general working proposi- 

 tion they may be confidently accepted. Furthermore, conditions of 

 youth and high relief with active river work are productive of an 

 abundance of coarse waste, which will result in the outward and 

 upward building of deltas with strongly contrasted slopes in fore-set 

 and top-set beds, while the finer waste supplied in lesser quantities 

 by a region of relative maturity will tend to build outward deltas 

 of gently inclined layers and, on the whole, of uniformity 

 of slopes or confluence of top-set, fore-set, and bottom-set beds. 

 In youthful regions of much waste supply the subaerial part of 

 the delta will become a prominent feature, while the reverse will 



