One can only be surprised and amazed by this beautiful river of stones. 6,5 million years ago, a lava flow coming from the south-east traveled more than 5 km in a shallow valley. It came out through the widely mouthed mouth of a conical-shaped volcano. This flow spread out and its cooling gave rise to numerous hexagonal prisms of a basaltic nature. Today, the relief has reversed because the Ance has dug its valley.
The basalt, more resistant to the action of erosion, thus constituted a plateau after having been the result of the cooling of a flow in a valley.
In the meantime, the edges of the flow were subjected to alternating freezing and thawing which detached the organs. Vast landslides and landslides on the clay slope did the rest.
You can discover this flow, located in the town of Saint-Julien-d'Ance, thanks to the chemin which allows you to climb to the top where there is an orientation table (course of about 3 km for a drop of about 140 meters).