Archeological research around the Picton gulf, now Marais Poitevin, in the west centre of France, ifs regularly faced with the problem of shorelines whose variations are difficult to ascertain owing to the many parameters involved : hydrological, geological, human, etc. The Picton gulf, open to the Atlantic over a long period of time, was subjected to both the highest tides and to a sustantial filling from river sediments, such as those contributed by the Sèvre Niortaise. On both sides of this vast synclinorium, in Charente-Maritime, Vendée or Deux-Sèvres, the landscape has been substantially modified by geological phenomena. Ancient and recent studies supply shoreline approximations for the various periods. Aerial photography, via the archaeological sites observed, helps to confirm or modify certain opinions. Over the last twodecades Maurice Marsac photographed the most important sites, neolothic, gallo-roman and mediaeval, relevant to the present subject. With the help of theses records we aim at pinpointing the shoreline with maximum possible precision, on the basis of substantial, well-dated sites, and at drawing a general, tentative map. Over the various periods Maurice Marsac' s aerial photographs revealed a good number of sites which now are in dry valleys, some of them still flooded in winter. Successive summer and winter prospecting enables data to be compared, and this special natural milieu to be better understood. The study of ancient geography appears therefore to be an essential background for reseach, concerning for instance man and his environment, regional economy at various times, means of communication.