heart of the peninsula from the steppes of northern Arabia. South of Najd lies one of the largest sand deserts in the world, the Rub al Khali. A recent global remote sensing analysis suggested that there were 706km² of tidal flats in Saudi Arabia, making it the 37th ranked country in terms of tidal flat area. Hejaz The western coastal escarpment can be considered two mountain ranges separated by a gap in the vicinity of Mecca in Tihamah. The northern range in the Hejaz seldom exceeds 2,100 meters, and the elevation gradually decreases toward the south to about 600 meters. The rugged mountain wall drops abruptly to the sea with only a few intermittent coastal plains. There are virtually no natural harbors along the Red Sea. The western slopes have been stripped of soil by the erosion of infrequent but turbulent rainfalls that have fertilized the plains to the west. The eastern slopes are less steep and are marked by dry river beds (wadis) that trace the courses of ancient rivers and continue to lead the rare rainfalls down to the plains. Scattered oases, drawing water from springs and wells in the vicinity of the wadis, permit some settled agriculture. Of these oases, the largest and most important is Medina. South of Hejaz, the mountains exceed 2,400 meters in several places with some peaks nearing 3,000 meters. The eastern slope of the mountain range in Asir is gentle, melding into a plateau region that drops gradually into the Rub al Khali. Although rainfall is infrequent in this area, a number of fertile wadis, of which the most important are the Wadi Bishah and the Wadi Tathlith, make oasis agriculture possible on a relatively large scale. A number of extensive lava fields (harrat) scar the surfaces of the plateaus east of the mountain ranges in the Hijaz and give evidence of fairly recent volcanic activity. The largest of these beds is Khaybar, north of Medina; another is Al Harrah, part of the large volcanic field Harrat Ash Shamah. Famous cities of Hejaz include the holy city of Medina and the city of Taif. Tihamah The rugged western face of the escarpment drops steeply to the coastal plain, the Tihamah lowlands, whose width averages only sixty-five kilometers. Along the seacoast is a salty tidal plain of limited agricultural value, backed by potentially rich alluvial plains. The relatively well-watered and fertile upper slopes