Rising to 221 metres, the rock appears suspended. It overlooks the green Saint Georges de Rouelley valley, in the Manche département and the Rouellé woods, in the Orne.
Created by millions of years of geological upheaval and erosion by the elements, the Fosse Arthour is today a water gap, a narrow, 70-metre deep gorge, cut through a rocky ridge and littered with huge blocks that have collapsed from the sides.
Made of Armorican sandstone, one of the hardest rocks from the primary era, these ridges have withstood the erosion better than the surrounding schists that form the bocage landscape.
This explains the striking contrast between this steep, rugged terrain, and the gentle relief off the surrounding hills. The views from the top of the water gap are stunning.
Footpaths open all year round: trails for pedestrians (GR22), horses and mountain bikes, climbing routes.
Access: St Georges de Rouelley car park, location "la fosse Arthour"
Guided tours: Contact the Normandie-Maine Regional Natural Park or the Mortain Tourist Office.