Located Between the 20th and 28th degree of North latitude, the Saharan provinces extend over a length of 950 km, while its width varies between 300 and 500 km.
They cover an area of 252,120 square km and form a set of plains and plateaus offering great topography uniformity, thereby opposing the Eastern Sahara bristling with mountains. The structure of this topography, very worn because it used to be emerged, is relatively simple. There are three main sets of north-south general orientation.
• Reguibat dorsal, in the Eastern region, from hinterland Laguira until the South-east of the province of Smara. It is a plain made of Precambrian lands on which rock remains are accumulated. It is slightly dominated to the North by primary and cretaceous plateaus covered by tertiary sediments and to the West by Gada plateaus. Accidental storm waters have formed closed basins or sebkhat (e.g. ldjil Sebkhat).
• The Gada, dominating the coastal plain, extends to the North until the Draâ valley and to the South until the 23rd parallel. Composed of limestones plateaus, topography and rain cause very weak and occasional flow of wadis (rivers); whereas inside the side valley, Saguiat El Hamra flows almost every year sometimes from end to end. Towards the South, the flow becomes increasingly rare.
The coastal plain is made of tertiary and quaternary deposits; some isolated dunes and small closed basins (Sebkhat Tah) break the monotony of the landscape.
The coast, which is generally straight, features a series of small sandstone cliffs and lagoons that are isolated by barrier beaches covered with dunes aligned in the direction of trade winds. The coastline is exposed to swell, except Dakhla and Laayoune, but the tide is of low amplitude.