21 - On the fringes of the Devès

A volcanic land

After a very long period of effusive volcanism, the Devès went through explosive eruptions of varying intensity. The most violent, generally linked to rising magma meeting a small amount of water, cut large craters into the surface of the plateau: the maars. The material thrown out in these explosions spread over wide areas, building up around the edges of the craters in gently sloping layers. Many of them filled with water immediately after the eruption ended. Today, only the Bouchet maar is still occupied by a lake, 28 metres deep. The other maars have filled in through the build-up of volcanic material or lake sedimentation. Near Costaros, the Péchay maar was the subject of attempts at agricultural drainage; it was nonetheless refilled with water because of the poor quality of the pastures, which were too wet.

Other, weaker explosions opened smaller vents and built scoria cones, locally called gardes. These volcanic cones are unevenly distributed: numerous and scattered around Cayres and Costaros, they gather along an important line of fracture from the Devès to La Durande, and become rarer on the Saint-Jean-Lachalm plateau. Under the effect of erosion, the summit crater of these volcanoes has disappeared and their shapes have softened. Through solifluction, the finest elements of the cones slid down the slopes and mixed with those of the plateau, giving the soil a fairly uniform reddish tint.