There are few prominent surface features in Mali, which is crossed by two river systems–the Niger and the Senegal. In the southwest are low mountains deeply notched by valleys formed by the coursing of water. A second upland, in the circle formed by the Niger River, is virtually a plateau and contains Hombori Tondo, 1,155 m (3,789 ft), the highest point in Mali. In the northeast is Adrar des Iforas, an extension of Algeria's Ahaggar Mountains. The republic is divided into three natural zones: the Sudanese, an area of cultivation covering some 200,000 sq km (77,200 sq mi) in the south and in the inland delta (a pre-Tertiary lake bed into which the upper Niger once flowed); the Sahelian; and the Saharan.