The Chastreix-Sancy National Nature Reserve offers an exceptional diversity with its very mountainous alpine landscapes and its exceptional geology. Val de Courre and Val d’Enfer with their crests and steep reliefs show significant differences in elevation, while the Fontaine Salée valley is wide open and dotted with boulders, vestiges of latest glaciations. As for the volcanic edifices – trachyandésite domes, basalt flows, dykes, breccias – they are numerous and well-marked.