Montserrat is formed of conglomerate that was originally deposited in a fan delta at the margin of the Ebro Basin, part of the foreland basin to the Pyrenees, during the middle Eocene. The conglomerate was derived from the uplifting Catalan Coastal Ranges to the southeast of the basin and the clast type is dominated by Triassic sedimentary rocks. The fan had an original size of between 100-150 square kilometres. The resistance of the Montserrat conglomerate to erosion compared to the rest of the fill of the Ebro Basin explains its current mountainous nature.