The Cévennes form the south eastern fragment of the Massif Central, separated from the related Montagnes Noires by the limestone Causses. The basement rocks of granites and schistes were uplifted by the Variscan orogeny forming a discontinuity, with the subsequent erosion infilling the lower voids for much of Permian and Triassic period (280-195 Ma), while changing sea levels added a thick limestone covering, with only the tops of the Cévennes protruding as islands in the Jurassic sea. This in turn was eroded, The Cévennes forms the watershed between the Atlantic and Mediterranean. In late Cretaceous and early Tertiary times further mountain building occurred. The Alpine orogeny lifted and deformed the Alps and the Pyrenees though the Massif Central acted as a rigid block, and the cover rocks remained mostly horizontal. Some have been folded through later faulting at the time of the opening of the western Mediterranean in Tertiary times. The principal rivers of the region have cut deeply into the limestone forming deep gorges: Gorges du Tarn, Gorges de la Jonte, Lot, Gorges de l'Ardèche, Cèze etc.