The lac Noir has soil made of black peat, which darkens its reflection. Some glacial lakes become bogs after a slow evolution. The transformation happens following successive colonization of plants such as sphagnum moss or cottongrass. Their decomposition forms peat, a substance formerly used for heating and still employed to distill whiskey in Ireland. Peat height is estimated to increase by 4 to 6 inches (10 to 15 cm) per century. The lac Noir was reconstituted over the bog thanks to a dam.