634 



GUILCHER 



[chap. 24 



of quartzose sand. Waves can rework the fluviatile sand into beaches or bars, 

 when the river discharge is low during the dry season. 



Another deltaic environment is found at the mouth of tropical rivers flowing 

 into the sea after a long and low course across arid or semi-arid countries, e.g. 

 the Senegal River (Tricart, 1955, 1956). At the time of high river discharge 

 (August to November), numerous basins adjoining the lower valley are flooded 

 and fed with silt and clay. Then the discharge becomes extremely low or even 

 practically nil for several months, so that, owing to the exceedingly low gradient 

 of the Senegal River, sea-water enters deeply into the distributaries with the 

 tide, and is driven into the basins when the floods of the next rainy season 

 arrive. Other sources of salt for basins lying close to the sea are spray from the 



10 mm 1mm 100^ 10/^ 



1/x O.V 



99.6 % 



99% 

 98% 



10 mm 1 mm 



rrm i 



100m lO^i- 

 ^3 



0.1^ 

 4 



0.4 % 



Fig. 13. Enveloping cumulative curves of grain-size distribution in four environments in 

 the Xorth Mahavavy delta, Madagascar. (From data in Berthois and Guilcher, 1956.) 



Legend — 1: coarse sediments in channels (7 samples); 2: basins (2 samples); 

 3: coastal mangroves (3 samples); 4: beaches on delta sea-front (6 samples). 



heavy surf beating upon the outer beach and percolation of sea-water through 

 the coastal dunes at high tide. As a consequence of this, the water in the basins is 

 saline. Since a large proportion of the basins are dry at the end of the dry season 

 or even before, the salt crystallizes and salt mixed with clay, silt and sand is 

 transported by the wind and deposited in more sheltered areas. Owing to the 

 climatic environment, the inter-levee basins are thus sebkhas, or saline bahadas, 

 and the role of salt in deltaic sedimentation is much more imi)ortant than it 

 would be under a humid climate. Rather similar processes have been reported 

 from the Laguna Madre flats, coastal south Texas, by Fisk (in Russell, 1959), 

 Price (1958) and others. 



