> The Pyrenees
In the Pyrenees, limestone terrains exist in two forms, formations of primary limestones (Cambrian, Silurian and Devonian) in the central crystalline area, and secondary chalks (Jurassic and Cretaceous). The latter form a continue strip along the area of the north Pyrenees from the Atlantic to the Mediterranean. They are present either in narrow bands associated with the folds of the central area, or in more or less isolated formations. Karst systems are frequent, and some caves attain colossal dimensions (Pierre-Saint-Martin, Henne-Morte). Often found in high and medium mountain areas, these caves are difficult to access and only a small number have been opened up for tourist visits In the eastern part, (Pyrénées-Orientales and Aude) the very ancient limestones of the primary (Cambrian and Devonian) form secondary ranges among the crystalline rocks. In bancs generally tilted or heavily folded, they are often metamorphosed into marble (the Lachambre System, Canalettes, and Fontrabiouse).
In the middle part (Ariège, Haute-Garonne, Haute Pyrénées), more recent chalks from the Jurassic and the Cretaceous form massifs aligned in secondary ranges along the North Pyrenees in front of the axial crystalline area: Plantaurel, Petites Pyrénées. These well karstified bands of limestone are the site of some remarkable watercourses (the underground rivers of Labouiche and the Mas d’Azil in Ariège), or very extensive systems (the Arbas Massif in Haute-Garonne). In the far west of the chain (Pyrénées-Atlantiques) the crystalline formations of the Pyrenean spine have practically disappeared, giving way to great ensembles of cretaceous chalks. Here, the karsts are similar to those in the central area of the Pyrenees. Whereas in the high mountains major caverns (Pierre Saint Martin) are common but restricted to specialists, on the other hand the Basque Country has some caves of great quality both for their concretions and for the archaeological heritage they contain.
> The Causses, Cévennes and Languedoc
This vast sector stretches across the Grands Causses and the Cévennes, along the southern border of the Massif Central. The Grands Causses are high table-lands deeply divided by the valleys of the Lot, the Tarn, the Jonte and the Dourbie, all flowing into the Aquitaine basin. Their gorges separate (from north to south) the Causse de la Sauveterre, the Causse Méjean, the Causse Noir and the Causse du Larzac.
Towards the south and south-east, two other limestone areas fall away in steps to the plains of Languedoc; they are lower, drained by the rivers of the Mediterranean basin: the Hérault, the Gard, the Ardèche and the Gardons. In the north of this region is the band of Cévennes limestones and in the south the ensemble formed of the Petits Causses du Hérault, the Garrigues and the Vivarais. Geologically these areas consist of secondary chalks. Chalk landscapes are predominant, but show great diversity from soft marls to more solid limestones.
The tops of the Grands Causses, at some 1000 metres, are made of solid limestone and dolomites from the middle Jurassic whereas the Cévennes limestones at only 800 metres are composed of rocks from the upper Jurassic. On the Petits Causses and the Garrigues, which rarely exceed 600 metres, younger rocks as recent as the cretaceous break the surface. These deposits were subject long ago to intense karstification giving impressive tower-like outcrops such as Montpellier-le-Vieux, the Cirque de Mourèze and the Bois de Païolive. Beneath, there are extremely diversified underground systems. But even though the region is riddled with thousands of caves, galleries, potholes and rivers, only a score have been opened up for visitors.
On the other hand some of them are among the most beautiful in France or even the world.
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