During the Secondary Era (or Mesozoic period), thick geological marl formations (a mixture of clay and limestone) were deposited several times in our region.  The first level, at our feet, dates back to the Lias period, a period of the Secondary Era dating from 185 to 170 million years ago.

It corresponds to a period of the deepening of the Alpine Sea, which hosted vast quantities of silty-quartz sediments in an oxygen-poor environment, which is the origin of the black colour of these rocks.

The enrichment of the sediment in limestone gives a harder rock which better resists erosion creating the Feston Ridge.

At La Robine-sur-Galabre, the fossil remains of an ichthyosaur from the Lias period is preserved and presented on site, being made securely accessible by the Geological Nature Reserve of Haute-Provence .

At least two other occurrences of black marl deposition are known in the region: in the upper Jurassic period and in the mid-Cretaceous period. It is from this last period that the ichthyosaurus of Chanolles (in the Municipal District of Prads Haute-Bléone) dates, where the site has been secured and managed for public-visits by the Geological Nature Reserve of Haute-Provence.

These different geological formations, more or less completely exposed, reveal local fossils (ammonites, bivalves, vertebrates) which are protected by the Geological Nature Reserve of Haute-Provence.