St. Lucia is a volcanic island, the younger part of which is the mountainous southern half, and the older the hilly but more nearly level northern half. The highest mountain, Mt. Gimie, rises 950 m (3,117 ft) above sea level. Better known are the two peaks on the southern coast, Gros Piton (798 m/2,619 ft) and Petit Piton (750 m/2,461 ft), which together form one of the scenic highlights of the West Indies. The lowlands and valleys of the island have fertile soil and are irrigated by many streams. The island has beautiful beaches, some with black volcanic sand.