Geological section through the Sable de Fontainebleau in relation to the cold front. (a) Progressive deepening of the cold front over time. (1) No permafrost or limited discontinuous permafrost in the early glacial period leading to the formation of horizontal quartzite slabs at the groundwater table. (2) Thick permafrost during the glacial maximum. The groundwater is confined and under pressure, leading to the formation of irregular quartzite masses. (b) Schematic of the spatial disposition of the watertable and a cold front versus paleodune ridges incised by a perpendicular valley leading to preferential silicification of these paleostructures.