The Puy de Sancy (1,886 metres) is a stratovolcano, i.e. a volcano made up of stratified layers of lava or lava and slag. It is part of the Monts Dore massif, which includes four stratovolcanoes. The Puy de Sancy was formed on the southern slopes of the Monts Dore between 1 and 0.25 million years ago. Geologists estimate its original height at about 2,500 metres. Since the end of volcanic activity in the area, erosion has shaped the rugged relief of steep ridges and wild valleys. There are also rounded landforms, plateaus and even a crater lake – Lake Pavin – which was formed only 7,000 years ago.