The sand dunes of Béorlots are accessible by a paved road and constitute a unique testimony to the geological phenomenon which occurred in the forest of Fontainebleau at the end of the tertiary era thirty-four million years ago. At this time took place the last marine invasion of the Paris Basin, including the forest of Fontainebleau. As the sea retreated, it left behind a thick mantle of silica-rich sand. Then in the Quaternary were formed the famous sandstone chaos of the massif, and these dunes due to wind phenomena.
Yet these dunes are in danger. They were overgrown with tall pines. They were uprooted in 2001. But the pines, which grow easily in the open, are gradually coming back.
It is estimated that these quaternary dune fields, which appear here and there in the national forest, cover between 15 to 25% of the Massif de Fontainebleau.