Mauritania is generally flat, with vast arid plains broken by occasional ridges and cliff-like outcroppings. A series of scarps face south-west, longitudinally bisecting these plains in the center of the country. The scarps also separate a series of sandstone plateaus, the highest of which is the Adrar Plateau, reaching an elevation of 500 meters (1,640 ft). Spring-fed oases lie at the foot of some of the scarps. Isolated peaks, often rich in minerals, rise above the plateaus; the smaller peaks are called guelbs and the larger ones kedias. The concentric Guelb er Richat (also known as the Richat Structure) is a prominent feature of the north-central region. Kediet ej Jill, near the city of Zouîrât, has an elevation of 1,000 meters (3,281 ft) and is the highest peak.
Approximately three quarters of Mauritania is desert or semidesert. As a result of extended, severe drought, the desert has been expanding since the mid-1960s. To the west, between the ocean and the plateaus, are alternating areas of clayey plains (regs) and sand dunes (ergs), some of which shift from place to place, gradually moved by high winds. The dunes generally increase in size and mobility toward the north.