Capcir is a region of highlands enclosed by Capcir massifs to the west, one of the summits of which – Pic Péric (2,810m) – is the highest point; Madres massif to the east, the summit of which culminates at 2,469m, opening on to the departments of Aude and Ariège to the north, and the areas of Conflent and Cerdagne via the pass of Quillane to the south.
The name of 'Capcir' (opening to the north) is derived from Caput Circii (literally meaning 'head of the Cers' or 'towards the Cers').
Glaciers had moved back from the Capcir about 10,000 years AD.
Fontrabiouse Cave had developed in the primary limestone of the Devonian. Therefore a very ancient limestone.
The limestone bed is called the 'Syncline from Mérens to Villefranche' by geologists. In fact, the syncline goes from Mérens (Ariège) to Villefranche-de-Conflent (Pyrénées-Orientales). In other words, it is 45km long and several hundreds of meters thick.
We must remind that a syncline is a downward, U-shaped fold. Water flows in the centre towards the lowest part.
In the area of Fontrabiouse, the limestone bed is sloping to the east, and the underground stream appears again in the open air in the village of Fontrabiouse (Font Rabiosa meaning 'Raging Spring').
Speleological explorations conducted for more than 40 years by the 'Entente Spéléologique du Roussillon' have demonstrated that the geology of Fontrabiouse Cave was not as simple as one could think. In fact, since the Pyrenees are a growing mountain, Carlit peak is progressively rising and this phenomenon always causes damage. Speleologists have observed a number of rifts: some of them are of no importance and can be easily crossed, but others are important and prove to be real barriers. Three large rifts with a north-south orientation cut the limestone bed over a length of 4km from the downstream part including the cave to the upstream part situated towards the refuge of La Jassette in the Val de Galbe.
The explorations carried out in the downstream part permitted to discover 12km galleries on 5 storeys.
The explorations carried out in the upstream part via the 'Trou Qui Aspire' (TQA) permitted to discover a chasm 310m deep and a river 1.5km long.
1.5km of galleries have not been discovered yet between the two sections of galleries. If several storeys exist, we expect that 7km of galleries can still be discovered.