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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.

A series of Pleistocene deposits extending over 155 m below sea level was drilled at Aux Coudres Island, in the middle St. Lawrence Estuary, Quebec. The series, which basis is unknown, is divided into two sedimentary units: the lower Baie-Saint-Paul Glacial Complex facies (−155 to −125 m), which is correlated with the Illinoian (isotopic stage 6), and a stratified sequence referred to as the île aux Coudres Formation. The latter is subdivided into four zones: a very compact lower clay (−125 to −102 m), rhythmites with Paleozoic schist debris (−102 to −71 m), and prodeltaic silts and deltaic sandy silts with brackish water benthic foraminifera (−71 to −2 m). The spore and pollen content indicates a schrub tundra followed by an afforestation sequence of a boreal forest that changes to an Abies forest and then to an interglacial mixed forest with Betula, Jugions, Carpinus or Ostrya, Carya, and, at the top, Betula, Tsuga, Quercus, and Ulmus. The accumulation of the sediments of the Île aux Coudres Formation required approximately 3500 years, beginning with a deep marine environment (about 300 m) followed by shallowing waters during the subsequent glacioisostatic rebound phase of the regression. The sedimentation is assigned to a main postglacial marine invasion, referred to as the Guettard Sea, which occurred prior to two regional glacial episodes and was partly contemporaneous with Bell Sea invasion in the Hudson Bay lowlands. A major postglacial sedimentary influx in the Atlantic Ocean, during the Illinoian-Sangamonian transition and at the beginning of the Sangamonian (transition 6–5 and early substage 5e) is inferred from this marine event.