and the mountains behind are extensively terraced to allow maximum land use. This coastal plain is part of the Arabian Peninsula coastal fog desert ecoregion. Both the holy city of Mecca and the city of Jeddah lie within the northern part of Tihamah. Najd East of the Hejaz and Asir lies the great plateau area of Najd. This region is mainly rocky plateau interspersed by small, sandy deserts and isolated mountain clumps. The best known of the mountain groups is the Jabal Shammar, northwest of Riyadh and just south of the An Nafud. This area is the home of the pastoral Shammar tribes, which under the leadership of the Al Rashid were the most implacable foes of the Al Saud in the late 19th and early 20th centuries. Their capital was the large oasis of Hail, now a flourishing urban center. Across the peninsula as a whole, the plateau slopes toward the east from an elevation of 1,360 meters in the west to 750 meters at its easternmost limit. A number of wadis cross the region in an eastward direction from the Red Sea escarpment toward the Persian Gulf. There is little pattern to these remains of ancient riverbeds; the most important of them are Wadi Hanifa, Wadi ar Rummah, Wadi as Surr, and Wadi ad-Dawasir. The heart of Najd is the area of the Jabal Tuwayq, an arc-shaped ridge with a steep west face that rises between 100 and 250 meters above the plateau. Many oases exist in this area, the most important of which are Buraydah, Unayzah, Riyadh, and Al Kharj. Outside the oasis areas, Najd is sparsely populated. Large salt marshes (sabkah) are scattered throughout the area. Northern Arabia The area north of the An Nafud is geographically part of the Syrian Desert. It is an upland plateau scored by numerous wadis, most tending northeastward toward Iraq. This area, known as Badiyat ash Sham, and covered with grass and scrub vegetation, is extensively used for pasture by nomadic and seminomadic herders. The most significant feature of the area is the Wadi as Sirhan, a large basin as much as 300 meters below the surrounding plateau, which is the vestige of an ancient inland sea. For thousands of years, some of the heavily traveled caravan routes between the Mediterranean and the central and southern peninsula have passed through the Wadi as Sirhan. The most important oases in the