This area is famous for the early Oligocene Fontainebleau Sandstone formation, a very pure and clean white sandstone that is host to two types of collectible specimens (see Thiry et al, 2015 for a good summary):

1 - Quartzite "pan" concretions cemented by quartz (not calcite) that can show bizarre, botryoidal or "custard-like" shapes known locally as "gogottes". These formed in the Quarternary when groundwater contacted permafrost within river channels incised into the sandstone and then cooled. The quartz cement precipitated as both crystallographically-aligned euhedral overgrowths on quartz grains and as isopachous (aka fortification) layers around the grains.
2 - Sand-included calcite crystals (and ~1 cm spherule concretions) that typically occur as clusters of rhombohedrons. They are also Quarternary and formed concomitantly with the quartzite "pans" as cold, carbon dioxide-rich groundwater infiltrated from above and warmed when it contacted the water table.

35 million years ago, the sea occupied the place of the current forest.

The plateau is composed of sediments of some fifty meters thickness.

This white sand is one of the purest in the world and is used for glassware (Murano in Venice), optical fibres, etc.

This sand has formed the rocky bars that characterize the landscape of the forest.

The silica sand is so white that the sun reflects off of it like a mirror, blinding your eyes and burning your skin, therefore, on sunny days, wearing sunglasses and protective clothing is a must.

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References