Kenya is notable for its topographical variety. The low-lying, fertile coastal region, fringed with coral reefs and islands, is backed by a gradually rising coastal plain, a dry region covered with savanna and thornbush. At an altitude of over 1,500 m (5,000 ft) and about 480 km (300 mi) inland, the plain gives way in the southwest to a high plateau, rising in parts to more than 3,050 m (10,000 ft), on which most of the population and the majority of economic activities are concentrated. The northern section of Kenya, forming three-fifths of the whole territory, is arid and of semidesert character, as is the bulk of the southeastern quarter. In the high plateau area, known as the Kenya Highlands, lie Mt. Kenya (5,199 m/17,057 ft), Mt. Elgon (4,310 m/14,140 ft), and the Aberdare Range (rising above 3,962 m/13,000 ft). The plateau is bisected from north to south by the Great Rift Valley, part of the geological fracture that can be traced from Syria through the Red Sea and East Africa to Mozambique. In the north of Kenya the valley is broad and shallow, embracing Lake Rudolf (Lake Turkana), which is about 207 km (155 mi) long; farther south the valley narrows and deepens and is walled by escarpments 600–900 m (2,000–3,000 ft) high. West of the Great Rift Valley, the plateau descends to the plains that border Lake Victoria. The principal rivers are the Tana and the Athi, both flowing southeastward to the Indian Ocean, and the Ewaso Ngiro, which flows in a northeasterly direction to the swamps of the Lorian Plain.