In the secondary geological era, the finest particles - that is to say fine sands, silts, and clays - formed by the mineralogical degradation of the granite rocks of the Massif Central were carried to the sea. Driven by a maritime current oriented south/east, they formed an immense sandbank parallel to the coast then located at the level of the Provençal Drôme near Montélimar. This deposit, which currently extends over more than 62 mi (100 km) long, a few tens of miles wide and around 196 ft (60 m) thick from the south of the Ardèche department towards the Var department, gave rise to the ocher formations of the area of Apt visible in Rustrel.