Most of Mauritania is made up of low-lying desert, which comprises part of the Sahara. Along the Senegal River (which forms the border with Senegal and is Mauritania's only perennial river) in the southwest is the semiarid Sahel with some fertile alluvial soil. A wide sandstone plateau (rising to c.1,500 ft/460 m) runs through the center of the country from north to south. In the southeast is the Hodh, a large basin in the desert.
The majority of the population is of Berber, Arab, Tuareg, and Fulani descent, and many still live a nomadic or seminomadic existence. Those of Berber, Arab, and mixed Berber-Arab background are sometimes called Moors, Maurs, or Maures. The remainder of the population mostly belong to the Tukolor, Soninke, Bambara, and Wolof ethnic groups and live as sedentary agriculturalists near the Senegal River. Recurrent droughts in the late 20th cent. forced many nomads from the countryside into the urban area of Nouakchott.
Virtually all the inhabitants of the country are Muslim, and many belong to the Qadiriyya brotherhood. The great majority of Mauritanians use Hasaniya Arabic, which, along with Wolof, is an official language. Other indigenous languages such as Pular and Soninke are also widely spoken. The country has a complex social caste system, with light-skinned Moors usually in positions of power and black Africans often at the bottom of the social ladder. In 1981, Mauritania became the world's last nation to officially ban slavery. Nonetheless, the United Nations and other groups report that slavery persists, with thousands of Haratines, the Arabicized Africans known as black Moors, held in involuntary servitude. In 2007 legislation was enacted that, for the first time, provided for criminal penalties for keeping slaves.