The heavily eroded volcanic bar of the Coiron borders the Bas-Vivarais to the north. From the Col de l'Escrinet to the Rhône, its black basalts interrupt the line of the hillsides and create the powerful contrast of the dykes (eruptive rocks forming a wall) of Rochemaure. The upper part looks like a vast bare plateau (average altitude 800 m) rising from the Rhône towards the north-west.