Fluvial dynamics and landscape evolution from the late-glacial to the Holocene period in the Rennes basin (Ille-et-Vilaine, France)

Résumé 0

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 data concerning the fluvial dynamics of the Vilaine available today motivated this paleoenvironmental study. Given the technical constraints of the urban context, two sets of transects were made perpendicular to the actual riverbed. All transects were recorded, but only the three most representative were described in detail and sampled for grain size, pollen and dendrochronological analyses. All features that could be accurately dated were sampled to provide 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 the Subboreal, followed by the abandonment of some side channels, as the main course moved southward to the center of the major bed. The changing dynamics of the streams and the numerous channel 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. During the 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 undisturbed organic clay sedimentation which, according to archaeological and pollen data, testifies to an intense human activity. This paleoenvironmental study has made it possible to specify the main stages of social development and their effects on the dynamics of the Vilaine river. It also allowed us to compare these results with those of other large rivers in northern France.