FLUVIAL DYNAMICS AND LANDSCAPE EVOLUTION FROM THE LATE-GLACIAL TO THE HOLOCENE PERIOD IN THE RENNES BASIN (ILLE-ET-VILAINE, FRANCE)

The construction of the "République" metro station, in the heart of the historic center of Rennes,revealed three meters of very well preserved deposits of the ancient Vilaine river. The lack of dataconcerning the fluvial dynamics of the Vilaine available today motivated this paleoenvironmentalstudy.Given the technical constraints of the urban context, two sets of transects were madeperpendicular to the actual riverbed. All transects were recorded, but only the three mostrepresentative were described in detail and sampled for grain size, pollen anddendrochronological analyses. All features that could be accurately dated were sampled toprovide a chronological framework for as many sedimentary levels as possible.The fluvial dynamics of the Vilaine river show an initial phase that was quite active until theSubboreal, followed by the abandonment of some side channels, as the main course movedsouthward to the center of the major bed. The changing dynamics of the streams and the numerouschannel intersections provide a fragmented sedimentary pattern that is difficult to date.Before the first traces of worked wood (4550-4325 cal BC), human activity had little effect.Anthropogenic opening of the natural forest seems evident at the end of Atlantic period. Duringthe Subboreal, the site was regularly flooded. During the digging of the docks in the 19th century,the Roman and older layers were destroyed. The sedimentary sequence ends with an undisturbedorganic clay sedimentation which, according to archaeological and pollen data, testifies to anintense human activity.This paleoenvironmental study has made it possible to specify the main stages of socialdevelopment and their effects on the dynamics of the Vilaine river. It also allowed us to comparethese results with those of other large rivers in northern France.