The picture displays the origin of the convex shapes of the monoliths in Montserrat type relief. It was obtained at the bottom of the peak Cortins (Mura), near the Montcau. The red band is made of several thin layers of sandstone and clay which are much easier to erode than the thick and compact layers of conglomerates that wrap it. The erosion dig out the red band. The process may result in a rock shelter. [photo Florenci Vallès]