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part of the Mid-Sahara Rise of the central Sahara. Mostly in northwestern Chad, the mountains extend into northeastern Niger and southern Libya. The formation is about 300 miles (480 km) long and up to 175 miles (280 km) wide. The volcanic summit of Emi Koussi rises to 11,204 feet (3,415 m) above sea level and is the highest point in the Sahara. The mountains are formed of eruptive rocks that forced their way through a substratum of crystalline rocks covered by sandstone; the sandstone scarp rises steeply above the surrounding plains. Three great, dry watercourses that cut deeply into the mountains are indications of a formerly more humid climate. The Teda and Daza inhabitants, numbering about 8,000, are chiefly nomadic