SAHARA, the great desert of North Africa. It forms the western part of a wide desert tract which extends from the Atlantic ocean through Egypt, across the Red sea and through Arabia to Mesopotamia. Its physical boundaries are in some directions clearly defined but in others vague, conventional and disputed. On the west the desert extends in many places to the Atlantic coast ; on the north to the foot of the Atlas mountains and to the Mediterranean sea east of these ranges. The eastern part, the Libyan desert, is flanked by the Nile, but the Nubian desert beyond differs very little from it ; whilst on the south the Sahara merges into the Sudan and the basin of the Niger and only in a few places does a natural boundary exist between them. The area of the desert is estimated at 31 million square miles, about the area of Europe minus Scandinavia. Its greatest length along lat. 20° N. is 3,200 m. and its breadth from north to south varies from Boo to 1,400 miles.
In such a vast region as the Sahara desert a wide range of topographical forms must occur. Some small tracts lie below sea level, but, on the other hand, approximately half the area stands above the 1,000 ft. contour line, the highest altitude (8,800 ft.) being reached in the volcanic cone of Tusidde (Tibesti massif). The horizontally bedded Cretaceous rocks form characteristic plateaux and escarpments which are dissected by an ancient river system and form a sharp contrast with the volcanic cones.
The Sahara Proper.—This is the region which extends south ward from the Atlas mountains as far as the highland tract of the central massif and down the western side of the latter to the basin of the Niger. For the most part it is a succession of low plateaux of Cretaceous rocks covered largely by dunes, locally called erg or igidi, although, in many parts of the west and south, Palaeozoic and Archaean rocks crop out. Along the Atlantic coast, in Mauritania, is a wide belt of dune country with recent deposits of salt and gypsum ; inland the dunes give place to tracts of peb bles, the eastern continuation of which is the low plateau coun try of Adrar and El Juf again covered with dunes, which extend as far as the central massif. To the north are the districts of Rio
de Oro, with a peneplained mass of Archaean and metamorphic rocks in the Wessat region; the plateaux of Dra'a and Tafilelt of Cretaceous and newer rocks, at the foot of the Anti Atlas and drained by the Wad Dra'a; the desert of Igidi, a large part of which is covered by dunes ; and the plateau of Tademait formed of flat Cretaceous rocks and formerly the watershed of this region. This plateau is continued eastward into the Hammada el Homra whilst to the north is the Great Erg, into which a spur of the hammada extends northward dividing it into the western and the eastern Erg.
The Central Tuareg Massif.—This comprises the Ahaggar plateau of Archaean and Palaeozoic rocks which occupies a central position in the desert and on which are several peaks of volcanic origin which do not rise above 8,000 ft. but are periodically snow capped. The mass is flanked on the west, north-west and north east by the lower plateaux of Ahenet, Muydir and the Tazili of the Asjer (5,000 ft.) respectively. The latter is continued south eastward by a low range, the Tummo, into the Tibesti mass of Archaean rocks in which occurs the highest peak in the desert, Mt. Tusidde (8,800 ft.), an extinct volcano, and still farther south-eastward into the lower range of Ennedi.
The Libyan Desert.—This is triangular in shape, extending from the Mediterranean sea to the Sudan. In the north-west is the hammada country of the Fezzan (q.v.) in which are the mountains of Jebel es Soda (Black mts.) which are continued south-eastward towards Kufara by the Huruj es Soda. On the coast in the extreme north are the Jebel el Akhdar ranges, but the rest of the desert is a wide expanse of almost level country, over 500,00o sq.m. in extent, which consists of dunes and sandy wastes of unascertained limits, and across which there is only one known route, running from north to south through the oases of Kufara.