From a geological point of view, the subsoil of the islet of Brégançon, at the top of which rises the eponymous fort, consists of phyllades and quartzites from the Precambrian era. It is typical of the whole western part of the Massif des Maures, in the extension of which the islet is located. The schists, more or less hard and clayey, disintegrate easily on contact with air and form a layer of silico-clay soil that covers the rock that is therefore rarely visible exposed.