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The volcanic Jebel Marra, located in west-central Sudan near the border of Chad, separates the Nile River and Lake Chad basins. The south- and southeast-flowing drainage from the mountains eventually empties into the Nile River; the west-flowing drainage empties into the Lake Chad basin. This mountainous area is a transition zone between the arid Libyan Desert to the north and the semiarid (savanna) landscape to the south. Several of the higher ridge lines of this range exceed 10 000 feet (3050 meters). The most obvious feature of the Jebel Marra is the Deriba caldera, which measures 3 miles (5 kilometers) in diameter and has one of its peaks rising almost 10 000 feet (3050 meters) above sea level. Geologists estimate that this volcano had its last major eruption in 2000 B.C. The volcanic plateau that formed outward for many miles from the Deriba caldera—especially to the southeast—shows deeply dissected volcanic rocks that present a formidable barrier to land travel through this region of Sudan.