Les travaux récents menés sur le Pléistocène de la France du Nord-Ouest permettent de compléter et de réviser les tableaux stratigraphiques publiés dans les années 80. Par rapport à ces premières synthèses, les nouveaux apports sont essentiellement représentés par les éléments suivants : - Définition de nouvelles formations marines et fluviatiles pour le Pliocène et le Pléistocène inférieur dans le Bassin de Carentan (Normandie) ; - Mise en évidence de nouvelles formations marines pléistocènes et holocènes en Normandie ; - Définition des systèmes fluviatiles pléistocènes de la Seine moyenne et de la moyenne Somme ; - Mise en évidence de séquences de référence pour l'Eémien et le Début Weichsélien au sein de la Formation fluviatile de la Lys (Nord), et pour le Flandrien dans la plaine maritime du Nord ; - Mise en évidence d'une longue séquence de référence pour le Pléistocène moyen de versant dans le bassin de la Somme (Picardie) ; - Proposition de nouveaux stratotypes et d'une nouvelle synthèse pédostratigraphique pour le dernier cycle Interglaciaire-glaciaire en milieu loessique sur la base de nouvelles données de terrain, d'analyses sédimentologiques, de mesures de susceptibilité magnétique et de datations TL et IRSL. - Parallèlement, des recherches sur l'Argile à Silex ont permis de préciser le mode de formation et l'attribution chronostratigraphique de ce faciès d'altération présent dans l'essentiel du Nord-Ouest de la France. Enfin, la prise en compte de ces éléments nouveaux débouche sur une révision de la carte des formations superficielles de la France du Nord-Ouest publiée en 1987 par F. Joly et dont une nouvelle version est proposée dans le cadre de cet article synthétique.

No abstract

Research on permafrost in France has concentrated on field evidence of fossil permafrost and the mapping of its possible distribution. Few attempts have been made to estimate permafrost thickness. Studies devoted to this subject in North America and Siberia have shown that permafrost may be very thick (several hundred metres), may be rapidly laid down (in < 2000 years in northwest Canada) and causes complex water flow patterns between the base of the permafrost and the soil surface. Using the "Gelsol" code developed by the Laboratoire central des ponts et chaussées (Central Laboratory of the French Department of Public Works) for geotechnical purposes, this paper presents the first results of modeling permafrost development at depth. These preliminary tests show the influence of parameters such as mean annual ground temperature, heat flow, thermal conductivity related to lithology, and water content of the rocks involved on permafrost depth during the last glacial cycle (Weichselian). The results from simple cases indicate that the permafrost was from ten of metres to over 300 m thick. Although these are only calculated values, they are much higher than the few figures found in the literature, and must betaken into consideration when searching for fossilized traces of deep permafrost.

From 1989 to 1991, a rescue-dig took place at Poses, «Le Vivier- Le Clos-Saint-Quentin». The area studied is situated on the flood-plain, which explains in this place important alluvial deposits, and consequently exceptionnal conservation of neolithic and chalcolithic seulement levels. On four hectares, the excavation has induced the study of seven settlements and also secondary areas (particularly combustion structures of polynesian type and accumulation of blocks of sandstone). The two oldest settlements belong to the «post-rubané» cultures (Villeneuve-Saint-Germain, Cerny) and the five others to the chalcolithic cultures. The most important resuit concerns, on one hand, the first neolithisation of the country and, on the other hand, the development of a chrono-cultural framework for the Late Neolithic - Early Bronze Age transition. The excavations have shown the existence of original late neolithic features (assemblages 3 and 4), have confirmed the large impact of Bell Beakers espansion in a late time in the Lower Valley of the Seine River and their evolution (assemblages 5 and 6), and finally have allowed to discover an unknown chalcolithic feature (assemblage 7). This operation enables us also to approach the theme of the development of marginal lands in areas liable to flooding.

Regolith formations are surficial fragmental and unconsolidated rocks whose genesis and properties result from successive periods of surface processes. They comprise:-autochthonous formations resulting directly from successive periods of surface weathering in the upper 200 metres of the Earth's crust;-allochthonous formations in which the unconsolidated deposits result from weathering and transport and have not yet been affected by burial diagenesis. Regolith studies are currently at the crossroads of new scientific challenges and new environmental needs. Not only do regolith formations support a full spectrum of biological activity, including human activities, they also provide a source of extractable materials and form the substratum of soils. As such, they should be considered as a capital to be managed and protected. Historically, geological maps have focused mainly on substratum geology. Although the inclusion of regolith formations in the 1:50,000-scale geological map series of France since the 1980s has considerably increased our knowledge of France's regolith, this is still is patchy and generally insufficient for applied uses despite increasing societal pressures demanding up-to-date regolith maps. To produce an updated map at the scale of France is an ambitious project that must necessarily begin with an inventory of existing geological cartographic data relating to regolith formations. A digital two-layered regolith map of France, representing both the allochthonous and autochthonous formations, was produced in 2009 at a scale of 1:1,000,000. Each layer is accompanied by a confidence index map, and a brief descriptive note of the mapped formations (in French) is also available. A critical analysis of the two map layers reveals not only the very uneven nature of our current knowledge in terms of data quantity and quality, but also its deficiency as regards the expectations of the public authorities. Today, therefore, we require surface studies combining geology, geophysics and geochemistry in order to enhance our understanding of the natural processes governing the distribution and evolution of subsoil formations. This approach is critical for developing appropriate guidance in the use and management of natural and groundwater resources. The 2009 map can be viewed and interrogated at http://infoterre.brgm.fr, and a "pdf " of the map and corresponding project report can be downloaded from