Grottes du Foulon - a Jurassic cave

Located under the town of Chateaudun, 125km south-west of Paris or 40km south of Chartres, this limestone cave is unique in the world for its marine geodes of quartz and calcedony (microcrystalline type of quartz occurring in several different forms including onyx and agate). 

The whole area was once under the sea. The floor is of the Jurassic period with the rooms carved out by streams, though the water has long disappeared. It's not a very humid cave. There are no stalactites nor stalagmites but there are plenty of geodes formed from sea sponges that became covered in sediment and fossilised. There's a layer low down in the cave walls which we were told marks the disappearance of the dinosaurs. We saw branches of giant fossilised coral embedded in the ceiling and fossilised remains of a large marine animal skeleton that was discovered just a few years ago.

During a heat wave or a rainy day this is a cool place to be. The temperature inside is always 12 degrees C year-round. 300,000 years ago in paleolitihic times, humans thought this was a good place to be too. There's a display of primitive human tools from various eras, near the entrance. The cave consists of various pillars, galleries and a large vaulted ceiling known as the giant's cauldron as it was probably caused by a river spiralling through a natural chimney.

We took a guided visit which was conducted by the owner. He speaks very rapid French so I understood only 50-60% of the commentary. However, he did agree to let me take some photos at the end of the tour. Normally photos are forbidden. The coloured lights at the end of the tour display the cave's features quite nicely. You can buy minerals at the little shop or have a snack. Remember to wear a jacket as an hour inside the cave at 12 degrees C is chilly. Guided tours occur at 14h, 15h and 16h most days.
Chateaudun has a notable chateau to visit and the town is also interesting.

http://www.grottesdufoulon.sitew.com/#animation.A

https://www.youtube.com/watch?v=3gQKLkS8jBc