Mauritania, a country in the western region of the continent of Africa, is generally flat, its 1,030,700 square kilometres forming vast, arid plains broken by occasional ridges and clifflike outcroppings. Mauritania is the world’s largest country lying entirely below an altitude of . It borders the North Atlantic Ocean, between Senegal and Western Sahara, Mali and Algeria. It is considered part of both the Sahel and the Maghreb. A series of scarps face southwest, 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 . 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 is a prominent feature of the north-central region. Kediet ej Jill, near the city of Zouîrât, has an elevation of and is the highest peak.