segment of the Central Alps along the Italian–Swiss border, bounded by the Simplon Pass and Pennine Alps (west-southwest), the Upper Rhône and Vorderrhein river valleys (north), Splügen Pass (Italian Passo dello Spluga) and the Rhaetian Alps (east-northeast), and the Italian lake district (south). At the western end of the range lies Mt. Leone (11,657 ft 【3,553 m】), the highest peak; and along the south slope are the deep Leventina and Mesolcina valleys. Important passes include Simplon (Simplon Pass), St. Gotthard (Saint Gotthard Pass), Lukmanier, and San Bernardino. The portion of the range east of St. Gotthard Pass is sometimes called the Adula Alps. The sources of the Rhône and Rhine rivers lie on the borders of this section of the Alps. Mountain climbing is popular in the region.