IntroductIon The delta areas are part of terrestrial environments that-apart from volcanic phenomena-have undergone the most important transformations in recent millennia, and in particular since the post glacial sea attained its current level. This event occurred between 6000 and 5000 years B.P, and for the few millennia that followed, the sea level has varied only slightly, whereas the coastal area has undergone considerable changes. The most obvious cause of these changes in deltas is the

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... ume of sediment fluxes redistributed by fluvio-marine dynamics , as illustrated by the morphologic and stratigraphic classification of the deltas (Galloway, 1975). The possible variations of the sea level and local or regional lithospheric up-welling or subsidence phenomena have also played a role. In a simplified context where one of the factors can be considered as a constant, the comparison of the respective effects of the other two must be facilitated. Consequently, Abstract. The post-glacial transgression up to the present sea level, which occurred about 5500 years B.P., deeply modified the coastal zone morphology, especially in the river mouth region, where, in a few millennia, the geographical pattern changed from an open bay to a delta environment. The Salum islands case illustrates this evolution. Sediments deposits and anthropic shell-middens, built in a time interval starting 6000 years B.P. up to now, based on radiocarbon dating, provide a chronological framework of the delta evolution. At first, the open post-transgression bay was subject to filling-up. A major second stage, occurring not later than 2550 years B.P., is characterized by the formation of beach barriers. The third stage is the completion of the filling-up behind the beach barriers. The whole construction is under the distinct influences first, of the Sahelian and the Salum River sediment fluxes in the Northern part of the delta and then, the Gambia River sediment input in the Southern part. Detailed analyses of the sediment testify to this double origin of the various morphosedimentary exhibited units: tidal flats and beach barriers. Grain-size analysis, SEM examination and heavy mineral assemblages revealed sand as the dominant component in the Northern part. It originated from atmospheric and fluvial conditions favouring the coarse component input. To the South, a clay dominant influx, controlled by the Gambia River discharge, is responsible for back-barrier mud-flats, accumulated in relation to the reduction or the absence of the tide-dominated fluvial processes in the Salum River. Hence, the Diomboss arm does not form one of the delta distributaries between the Northern and the Southern part as previously considered but a residual space of the former mid-Holocene bay, separating the real Salum River delta to the North and the pre-Gambian delta to the South. All morphological units described in the delta are established at a level located within the present tidal range. There is no need to refer to sea level variations to explain the geomorphologic pattern. The sedimentological input and forcing seem to be the major agents of sedimentary unit distribution.