Résumé
In the Quaternary fluvial sequences of Northern France, deposits of tufa are mainly located in the border area between the chalky higher country and the plains of the lower country (Tertiary sandy and clayey substratum). Large superficial outcrops of fluvio-lacustrine tufa are observed in the south-western part of the coastal plain, in the Saint-Omer basin (Aa) and in the ancient Scarpe valley. They are dated of the Subatlantic Holocene, prior and contemporaneous to the Roman period.