. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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, 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 extr
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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, 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 % 99% 98%. 10 mm 1 mm rrmi 100m lO^i- ^3 ^ 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
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