The Azawagh refers to the dry structural basin, which once carried a northern tributary of the Niger river, the Azawagh river, known as Dallol Bosso further south. The river, which ran some 1,600 kilometres (990 mi) in prehistoric times, dried up after the Neolithic Subpluvial and created a basin of some 420,000 square kilometres (160,000 sq mi). Its valley, which geologists call the Iullemmeden Basin, is bordered by the Hoggar Mountains and their foothills in the north, the Aïr Mountains in the east, and the Adrar des Ifoghas in the west.[4] The bedrock of the region is Cretaceous/Paleocene limestone and clay, which was cut by erosion and covered by aeolian sand in the Upper Pleistocene.[5]