A volcanically formed central plateau, isolated on three sides by low- lying desert dominates the Ethiopian landscape. The central plateau, often referred to as the Ethiopian highlands, has an average altitude of above 2,000m and includes 20 peaks of 4,000m or higher. The Ethiopia highlands are dramatically mountainous, no more than where hey are bisected by the Rift valley, which starts at the Red sea, then continues through the Denakil depression and through southern Ethiopia to Mozambique in Southern Africa. The part of the Rift valley, south of Addis Ababa, is notable for its string of eight lakes. The most extensive mountain ranges on the highlands are the Semien, which lie directly north of Gondar, and Bale, which lies in the southern highlands to the east of the Rift Valley. Mount Ras Dashen in the Semien is at 4,620m, the fourth highest peak in Africa. The highlands also form the source of four major river systems. The nest known of these is the Blue Nile or Abbay, which starts at Lake Tana in the northwest and supplies nine- tenths of the Nile’s water, which eventually reaches Egypt’s Nile valley.