| Summary: | National audience Since the beginnings of geology in the XVIIIth century, the massif of Fontainebleau has been a melting pot of new ideas. It has several remarkable geological features: extensive linear quartzite ridges, spectacular quartzite lenses within uncemented white sands, outstanding Calcite de Fontainebleau sand calcite crystals, and also a rare biodiversity due to a juxtaposition of contrasted landscapes.This guide presents a geological history revised in the light of the evolution of geological knowledge. It shows that the Fontainebleau quartzites are not related to the deposition of sands some 35 million years ago, but formed in geological recent landscapes less than 500 000 years ago, after the original sands were exposed and incised by rivers. The characteristic facies of the sands are due to subsurface alteration in Quaternary periglacial environments. The quartzites formed behind springs at the edges of valleys, where groundwater cooled when it came into contact with frozen ground. The sand calcites precipitated when cold surface waters infiltrated the landscape, warming as they seeped to depth and came into contact with groundwaters. Sands were leached and bleached by the infiltration of acidic organic-rich waters that originated in organic-rich soils in the tundra taiga environments. The brittle fractures that spectacularly cut quartzite pans occur only in outcrop resulted most probably from successive shrink-swell thermal effects.The guide shows how new data can change old ideas and initiate new concepts that gradually generate a quite new geological model. The remarkable aspects of the geology of the Fontainebleau massif are explained by its evolution during the Quaternary glacial periods. This model gives coherency to observations that initially seemed to be quite independent of each other.The model now proposed for the Fontainebleau massif applies to the other Tertiary sand formations of the Paris Basin, all of which are characterised by white sands with quartzite lenses and sand calcite ...
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