Aubrac is composed of a vast granite plateau topped in the centre by volcanic basalt rocks. The volcanism, like that of Hawaii, has spread the liquid lava to form a soft flowing landscape. Some rocks, peaks, mesas are in contrast with the plateau (point that culminates at Signal de Mailhebiau 1469m). At the “boraldes” (streams) we find gneiss and mica schist and even limestone in some places.
The great “causses” are composed of limestone and dolomite which dates from the jurassic period and formed at the bottom of the seas by the accumulation of calcareous marine animals.
Karstic erosion (limestone dissolved by water) creates underground caves (Aven Armand - Dargilan - Bramabiau), while the free flowing waters dig out impressive gorges and canyons (gorges de la Jonte et gorges du Tarn). Dolomite erodes in its own way by sculpting out rocks (corniche de la Jonte, Nîmes le vieux, Montpellier le vieux).
The “cevenole” part and the Aigoual massive are principally composed of granite, dated at around three hundred and thirty million years, and of schist which is older. The topography of the area is marked by the roughness of the slopes (especially on the schist), and the (1567m) mount Aigoual granite, in a phase of uplift, has pushed back the surrounding rocks.
This contact has created a transformation (generating seams) and numerous defunct mines are testimony to this geologic wealth.
More to the south, after Le Vigan, the “Cirque de Navacelles” has been formed by a meander of the river “Vis” which has cut out a deep canyon in the limestone.
The “Seranne” mountain, between the Cirque de Navacelles and St Guilhem, is composed of limestone rocks formed on a coral reef 145 million years ago. On the southern buttress the “Cirque de l’Infernet” (or “edge of the world”) opens up a deep pass through the mountains of St Guilhem.