Exploration archéologique de 170 hectares de plaine maritime (Bourbourg, Saint-Georges-sur-l'Aa, Craywick, Nord de la France); restitution de la fermeture d'un estuaire au Moyen Âge et mise en évidence de mares endiguées

As part of the expansion of the port of Dunkirk, more than 170 hectares of the maritime plain were explored on the future "Zone de Grande Industrie" (heavy industry area) in the former estuary of the Denna (or Déna, or L'Enna) river, sub-catchment of the small coastal Aa river. The usual preventive archaeological diagnosis by trial trenches was accompanied by archival research, a geophysical survey and geomorphological surveys (cross-sections and 3m deep test pits). The integration of the different results leads to an understanding of the sector better than each discipline would have done separately. The first meters of deposits consist of tidal sands intersected by tidal channels. The upper decimeters are possibly siltier. The apparent conductivity survey allows spatializing the lithostratigraphic data. Their comparison, as well as information from the plot network, highlight the western edge of the estuary, contemporary with human occupations. In the same way, a comparison of archaeological remains collected during the survey with the geophysical/stratigraphical data allows to understand the evolution of that landscape, along with the expansion and contraction of dwellings, since the first remains around the 10th-13th century on the western edge of the estuary. From that, dwellings move to the northeast to a long time active channel of the Denna whose its axis corresponds to the current drainage system. The electrical conductivity survey also revealed anomalies that turned out to be broad and deep anthropogenic features (up to about 40m in diameter and more than 5m deep, i.e. 2.5m below mean water level). Although abundant, these structures can easily be unnoticed by trial trenches. Three structures were tested. Their filling is variable but has a common point to be sealed on the bottom and to have walls made with clumps of clay. These features are interpreted as diked ponds, some examples remain on the coast of the North Sea (Friesland, Zeeland and Germany). They were used to collect fresh water while the surrounding shape embankment was used to protect it from the highest tides.