Not far from the Col des Sagnes mountain-pass, Serre Blanc and Eau Amère bear witness to the presence of gypsum in their own particular way.

Gypsum is a sedimentary rock which belongs to the evaporite family (rocks composed mostly of minerals produced by evaporation of saline solutions). Gypsum is formed by the concentration of sea water and its subsequent evaporation.The presence of gypsum testifies to a warm and dry climate when it was deposited about 250 million years ago.

A soft rock, readily absorbing water, gypsum played an active part in the tectonic movements that built the present-day terrain and landscape by facilitating the displacement of the rock-layers that rested on it. It is these movements and deposits that have accumulated large quantities of gypsum here.

From another point of view, gypsum is a rock that has been used in construction for thousands of years: after a moderate heating process (120-130°C) it produces plaster, the uses of which have not ceased to be innovated since Classical Antiquity Times.

Nearby, in Clamensane and Caire, two plaster kilns and mills still exist and bear witness to the traditional use of this natural material. The one in Clamensane is still operational.