Erosion, which is caused principally by the sudden melting and retreat of ice aided by tectonic changes, has scooped out a deep longitudinal depression, best in evidence where in contact with folded Cretaceous rocks, which are lifted up by the Cenozoic granite. It generally separates the plateau from the first lofty hills, whose ridges are generally called the pre-Cordillera. To the west of these, a similar longitudinal depression extends all along the foot of the snowy Andean Cordillera.