Chalk is a monomineral evaporative sedimentary rock (90%) formed by chalk crystals, hydrated calcium sulfate SO4Ca • 2H2O.

Age: Upper Triassic, Keuper (200 mi.).

Location: Surroundings of Adons, Senterada, Gerri de la Sal.

Uses: Stone for wall and ceiling coverings, cement additive, thermal insulation, mold making, etc.