Résumé
The Seine and the Somme are the two main rivers flowing from northwestern France into the Channel. During the Pleistocene cold stages both rivers were tributaries of the River Manche which was exporting sediments into the central deeps of the Channel. The River Seine has a very well developed terrace system recording incision that began at around 1 Ma. The same age is proposed for the beginning of the main incision in the Somme Valley on the basis of morphostratigraphy, pedostratigraphy, palaeontology, palaeomagnetism and ESR datings. The uplift rate deduced from analysis of the Seine and Somme terrace systems is of 55 to 60 m/Ma since the end of the Lower Pleistocene. The response of the two rivers to climatic variations, uplift and sea-level changes is complex and variable in the different parts of the river courses. For example, the evolution of the lower Seine system is influenced by uplift and climate changes but dominated by sea-level changes. In the middle Seine the system is beyond the impact of sea-level variations and shows a very detailed response to climatic variations during the Middle and Upper Pleistocene in a context of uplift. The Somme Valley response appears to be more homogeneous, especially in the middle valley, where the terrace system shows a regular pattern in which incision occurs at the beginning of each glacial period against a general background of uplift. Nevertheless, the lower Somme Valley and the Palaeo-Somme in the Channel area indicate some strong differences compared with the middle valley: influence of sea-level variations and probably differences in rates of tectonic uplift between the Channel and the present continent. The differences in the responses of the two river valleys during the Pleistocene are related to differences in the size of the fluvial basins, to the local tectonic characteristics, to the geometry of the platform connected to the lower parts of the valleys and to the hydrodynamic characteristics of each river. Finally, it is shown from these examples that the multidisciplinary study of Pleistocene rivers is a very efficient tool for the investigation of neotectonic activity.