The lac Blanc (White lake) extends in the hollow of a small cirque dating from the Quaternary. It is dominated, on the southwest side, by the foothills of the Polset mountain group. It has also seen its surface shrink considerably due to the advance of rock slides. The steep slopes that dominate the lake on the southern side are essentially made up of sandstones and clayey schists. At an altitude of 3,500 fr (1,055 m), the lake freezes in winter. It will not regain its transparency, due to its sandy bottoms, until spring.