A layer of limestone in Els Tres Salts, in the river Llobregat (Sant Fruitós de Bages). Limestones are calcareous rocks made of, basically, calcium carbonate. The most frequent limestones in Bages are arranged in really resistant to the erosion layers. They come from whether marine or lake origin. Nowadays limestones are mined in quarries as a material for construction while, formerly, they were used to obtain lime. Marine limestones of Bages contain a lot of microfossils because their were originated in past coral reefs. [photo Florenci Vallès]