Enquête game "Le mystère de la pierre gravée"

During his latest digs, your history teacher, who is also a great specialist in palafittic sites (a rather complicated word to describe prehistoric sites built on stilts), discovered an engraved stone. You can see it at the tourist office. It's up to you to decipher it!
Valisette for hire from the Clairvaux-les-Lacs office