Holocene stratigraphy and paleoecology of three sites in the inland border zone of the northern France (Calais) coastal plain show alternation of marine and fresh-water environment Fresh water was supplied by small rivers running from the loam and chalk hills bordering the coastal plain. During the Atlantic period, in the Aa valley (connected with the coastal plain proper via a narrow gap) fully marine conditions prevailed along the valley axis. At the borders and in the tributary valleys a rather open marsh occurred. In the Aa valley as well as in a small valley nearby but more openly connected with the coastal plain, fluvial deposition and, seaward of this, peat formation (now more swamp) increased after late-Atlantic marine regression. In both valleys, independency is presumed between marine-induced water-level fluctuations and fluctuations in fluvial deposition during the Subboreal period. Deposition of lake marl at about Roman time (or later) would have been made possible through local marine abrasion of peat near to the chalk hinterland.