Abstract : The eastern part of the region of Valence and the basin of Crest constitute a part of the central Rhone Valley separating the Massif Central from the Alps . This synclinal depression which core is made of miocene rocks has a North-South orientation and is widely covered with quaternary alluvial deposits. The sedimentological study of the neogene detrital rocks permits, on the hand to emphasize the major roIe played by the alps drifts regarding the filling of the basin and , on the other hand to reconstitute the paleogeography of the region. In the basin of Crest, the transgression begins at the Burdigalian; during the Langhian the sea gradually stretches aIl over the Valence Plain. At the upper Tortonian , the regime becomes continental then after an intense messinian erosion a new marine pliocene ingression invades the paleo-valleys of the Rhone and its affluents . The hydrogeological study reavealed the first global approach of caracterisation of the molassic aquifer . The accurate kwnowledge or the geological context leads to the proposaI of a simple sheme of the aquifer reservoir . A draft of the piezometry has been plotted . The estimation of the hydrodynamic parameters to differenciate the superficial water table flowing lnto the quaternary alluvial deposits from the deep water table flowing into the molassic sands. The vertical heterogneity as well as the horizontal one is obvious and the geochemical study confirms the existence of a stratification of the miocene aquifer.The nappe is mainly supplied from its top by infiltrating of precipitations, other possible supply means are envisaged.