The Alps are divided by rivers and other topographic features into more than 40 subunits for which local names are commonly used. Well-known groups in the W Alps (from the Riviera to the Great St. Bernard Pass) include the Maritime, Ligurian, Cottian, and Graian alps, the Mont Blanc group, and Valle d'Aosta. The highest western peaks are Mont Blanc, Mont Pelvoux, Monte Viso, and the Gran Paradiso; the chief routes across this section are via the Mont Cénis Tunnel and the Great and Little St. Bernard passes.