Grotte du Foulon

The guided walk through the Grotte du Foulon, at Châteaudun in the Eure-et-Loir, follows an 800-metre route designed as a geology lesson. In these caves covering about 10,000 square metres, many geodes line the vaults. These quartz geodes are the result of the crystallisation of silica; elsewhere the vaults and walls are made of flint.

Geologically, the chalk in which the once turbulent waters of the Loir carved this cave dates from 65 to 85 million years ago.

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