Mudstones are detrital rocks made up of less than 1/16 mm diameter particles. If these fragments are coherent, then the rock is named siltstone if particles are above 1/256 mm size or claystone if below 1/256 mm. If these fragments are free, then the rock is called silt and clay. Mudstones containing 50% of calcium carbonate on average (35%-65% according to Vatan) are called loams. They’re usually bluish color. The color of mudstones in the area of Bages changes from the bluish gray or yellowish in sediments from marine origin, to the characteristic reddish in sediments from continental origin because they contain iron oxides.
The extraordinary hollow out of the morphology of Bages results from the weakness of mudstones in front of erosion. Mudstone materials predominate in farmed plains.
In 1st picture, huge potholes developed in bluish loams of marine Eocene in Marganell.
In 2ndpicture, there is one stratum of marine, yellowish colored mudstone displaying one stripe of very blue color.