leaching the lands they were drilled to irrigate. Successive agricultural projects, many of which were designed primarily to encourage Bedouin settlement, have increased water resource exploitation. In the early 1990s, large-scale agricultural projects have relied primarily on such underground aquifers, which provided more than 80% of the water for agricultural requirements. In fiscal year (FY) 1987, about 90% of the total water demand in the kingdom was consumed by agriculture. Topography and natural regions The Arabian Peninsula is an ancient massif composed of stable crystalline rock whose geologic structure developed concurrently with the Alps. Geologic movements caused the entire mass to tilt eastward and the western and southern edges to tilt upward. In the valley created by the fault, called the Great Rift, the Red Sea was formed. The Great Rift runs from the Mediterranean along both sides of the Red Sea south through Ethiopia and the lake country of East Africa, gradually disappearing in the area of Mozambique, Zambia, and Zimbabwe. Scientists analyzing photographs taken by United States astronauts on the joint United States-Soviet space mission in July 1975 detected a vast fan-shaped complex of cracks and fault lines extending north and east from the Golan Heights. These fault lines are believed to be the northern and final portion of the Great Rift and are presumed to be the result of the slow rotation of the Arabian Peninsula counterclockwise in a way that will, in approximately ten million years, close off the Persian Gulf and make it a lake. On the peninsula, the eastern line of the Great Rift fault is visible in the steep and, in places, high escarpment that parallels the Red Sea between the Gulf of Aqaba and the Gulf of Aden. The eastern slope of this escarpment is relatively gentle, dropping to the exposed shield of the ancient landmass that existed before the faulting occurred. A second lower escarpment, the Jabal Tuwayq, runs north to south through the area of Riyadh. In the south, a coastal plain, the Tihamah, rises gradually from the sea to the mountains. Hejaz extends southward to the borders of mountainous Yemen. The central plateau, Najd, extends east to the Jabal Tuwayq and slightly beyond. A long, narrow strip of desert known as Ad Dahna separates Najd from eastern Arabia, which slopes eastward to the sandy coast along the Persian Gulf. North of Najd a larger desert, An Nafud, isolates the