Cornieules (or rauhwackes) are essentially dedolomitized, veined or brecciated carbonate rocks or marls with a characteristic porous to cellular appearance. In hand specimen, they can be divided into two types, those with a veined structure, and those showing a brecciated structure. In the Fréjus region and in most other areas both inside and outside the Alps, cornieules are restricted in occurrence to evaporitic sequences. They are frequently associated with gypsum or anhydrite, which suggests a genetic link between cornieule and evaporites. This was confirmed when a full petrographic investigation was carried out, first on a Triassic evaporite sequence and then on cornieules. Field and petrographic evidence has revealed that cornieules are formed from a starting material of gypsum and dolomite that had been fractured, veined and brecciated many times. The causes of fracturing were probably tectonic in the initial phases, and non-tectonic later. Due to the major post F4 metamorphism, the gypsum was replaced by calcite and the calcite was subsequently dolomitized. Dolomitization was accompanied by a 12–13% increase in porosity. When the rocks were brought near to the earth’s surface, chemically active sulphate-rich percolating solutions invaded the rocks, brought about dedolomitization and formed the cavernous structure of cornieule.

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