632 



GUILCHER 



[chap. 24 



subsidence, shiftiiigs in sea-level, strength of waves and long-shore drift. The 

 Rhone delta does not divide into true sub-deltas, but large changes in discharge 

 in its various distrilnitaries have led to recent accretion at the eastern mouth 

 and erosion further to the west, where the submarine foreset beds have been 

 truncated and partially reworked into beaches. 



When the amount of sediment distributed over the delta sea-front by waves 

 is very small, as at the present mouths of the Mississippi, beach ridges may 

 completely fail to occur, but submarine bars appear and can emerge at the 

 mouths of the distributaries: they are known as theijs in the Rhone delta 

 (Duboul-Razavet, 195G) and cause frequent bifurcations at the passes of the 

 Mississippi (Welder, 1959). Combinations of natural levees along distributaries 



Fig. 11. Pass a Loutre, Mississippi delta, showing growing levees. (Photo. A. Guilcher.) 



and recurved courses of these channels under the influence of long-shore drift 

 which lead them to follow a course parallel to the coast are described as 

 "perilittoral levees" by Mrs. Duboul-Razavet and Pimienta. More purely marine 

 features reworking the coarse fraction of deltaic deposits are the cheniers or 

 successive beach ridges of south-western Louisiana (Byrne et al., 1959). They 

 form beautiful birdfoot complexes in the Rhone delta, where they have been 

 mapped by C. Kruit. Every kind of transition exists between digitated deltas 

 such as the Mississippi and stunted deltas such as the Guadalaviar mouth 

 near Valencia in Spain (see Gulliver's classification, 1898-1899). 



Climatic conditions diversify the processes in deltaic sedimejitation. One of 

 the most curious climatic types is the arctic or periglacial delta, principally 

 represented in Siberia (Lena, Yana, Indigirka, Kolyma), and existing also in 

 Canada (Mackenzie). The deposits are deeply frozen during the greater part of 



