The administrative regions (formerly called cercles) of Tiris Zemmour in the north, Adrar in the center, and northern Hodh ech Chargui in the east, which make up most of the Saharan Zone, are vast empty stretches of dunes alternating with granite outcroppings. After a rain, or in the presence of a well, these outcroppings may support vegetation. In the populated Adrar and Tagant plateaus, springs and wells provide water for pasturage and some agriculture. In the western portion of the Saharan Zone, extending toward Nouakchott, rows of sand dunes are aligned from northeast to southwest in ridges from two to twenty kilometres wide. Between these ridges are depressions filled with limestone and clayey sand capable of supporting vegetation after a rain. Dunes in the far north shift with the wind more than those in the south.