The South Pyrenean Zone consists of a Mesozoic-Eocene sedimentary sequence which has detached from the Axial Zone within evaporitic horizons of the Middle or Upper Triassic and consequently was transported southward. The basement of this sequence does not outcrop. The southward motion was channelised by two major conjugated faults, in the west by the more or less north-south-trending folds and thrusts near the Cinca river (Mediano and Boltaña anticlines), and in the east by the northeast-southwest-trending en echelon wrench faults at the Segre river. At the latter, the thrust system forms a break-back (hindward-thrusting) imbricate emergent fan which developed during the latest Eocene and early Oligocene. Due to constriction, the sedimentary cover was forced into several internal overthrusts, examples being the nappe of the Monte Perdido and the nappe of the Cotiella in the northwest. More centrally placed is the Bóixols Thrust Sheet which continues farther east in the Pedraforca Thrust Sheet (upper unit). The Bóixols Thrust Sheet is hindward-thrusting but also overrides the Montsec Thrust Sheet to the south. Its sediments reach 5000 m in thickness and are mostly Lower Cretaceous in age. The Montsec Thrust Sheet correlates with the lower unit of the Pedraforca Thrust Sheet. It consists of a 2000 m thick layer of Upper Cretaceous limestone followed by Lower and Middle Eocene syntectonic conglomerate, sandstone and shale.