In this majestic setting, the river has cut a narrow gorge through folded limestone layers that were straightened vertically when the Alps were formed. This particular morphology, which corresponds to a fold cut perpendicular to its axis, is known as a clue. Clues are relatively common in the region. They form a kind of geographical and climatic barrier. In winter, rain to the south of the Barles clue is often replaced by snow to the north.