The imposing limestone bar of the Géruen mountain, its ruggedness originating from the  
Tithonian period (Late Jurassic period), contrasts with the fragility of the black marl or clay from the Oxfordian period.

The Tithonian limestone forms many of the peaks in the area, including the Rocher des Brigands. In France, this limestone, often with a very fine composition, has long been used in lithography (a process of printing). Large limestone blocks became detached from the Géruen ridge and seem to be "planted" in the underlying marl.

The Géruen mountain was entirely reforested at the end of the 19th century and the beginning of the 20th century so as to stop the devastating effects of land erosion. Although black pine trees dominate, ash and alder trees populate the damp valleys, while Scots pine and maple trees brighten up the sunny slopes.
The undergrowth is populated by a varied fauna. Wild game-birds are numerous (black grouse, hazel grouse ...).