A mudstone layer crosses the lake origin limestone. The circle piece for scale purpose measures 6 cm diameter. Iron oxides discolor the limestone surface to ochre. Limestones of lake origin in Bages contain really small grains coming from the mud in lake beds. Most of the calcium carbonate comes from charophyta freshwater algae, exoskeleton of small ostracode crustaceae (from 0,4 to millimeters length) and snails shells. These rocks are dark gray after their content of organic matter. The lake origin limestones are were used in walls, mainly in the norh of the Bages district. Nowadays, lake origin limestones are extracted and ground for the manufacture of concrete and basic components of construction. [photo Florenci Vallès]