The history of salt is closely linked to that of water and rock.
To know the origin of salt we must go back several billion years. The original atmosphere of the Earth was transformed and became a mixture of water vapour, carbon dioxide and nitrogen. Then the Earth cooled. The water vapour condensed and abundant rains appeared. On the surface, the water flowed and washed the rock, carrying away with it a part of their mineral salts (some of which contained chloride and sodium, the components of salt). This water, “enriched” with minerals, formed the oceans.
Salt does not exist only in the oceans. It is also found in underground deposits; we call this “rock salt”. It is formed through the evaporation of sea water.
At Salins-les-Bains, as elsewhere in the Jura, it is this type of salt that is found. It was formed around 215 million years ago, during the Upper Triassic. The sea then covered the entire east of France and Salins-les-Bains was found in the centre of a shallow lagoon. Under the action of the sun and the winds, the water evaporated. Cut off from the sea, the lagoon quickly became saturated in mineral salts which precipitated, meaning they settled to the bottom of the lagoon. Limestone was the first to be deposited, then gypsum and finally the salt, when 90 to 95% of the water had evaporated. Solidifying, the salt became a compact rock of the evaporite family (in reference to its means of formation via evaporation).
Thus formed, rock salt was covered by other layers of sediment, especially limestone and marl. Geological movements finished off the process of subsoil formation, displacing the rock, sometimes towards the surface, sometimes further underground. When the layer of salt is not too deep, rain water can seep into it and absorb some salt. Saltwater springs then appear on the surface.
In Salins-les-Bains, the strata of rock salt is found about 250 metres below the surface. Water infiltrations into the upper rock came forth in several saltwater springs, captured in three wells known as the wells of Amont, of Gré and of Muire.