The drainage of the south-west of Darfur flows towards the River Mamun, a perennial stream, also an affluent of the Shari, which, of course, empties into the southern end of Lake Chad.
Water supply.Broadly it may be said that the country to the north and east of the Marra range resembles that of Kordofan in its character and usual dearth of water, whilst to the west, south-west, and south it is much better watered and more fertile. During the rains water is here everywhere plentiful, whilst at this season much of Southern Darfur becomes marshy and difficult to travel over. In the rainy season too the principal wadis, especially those in the south and south-west, are perfect torrents, and, although their beds are dry soon after the cessation of the rains, water is generally to be found in abundance held up by the clayey strata at a few feet below the surface. In Eastern Darfur the wells are of considerable depth and at great distances from each other, especially south of Dara and Taweisha, and the people are dependent to a great extent on water melons (batikh) and to a less extent on Tebeldi trees (Adansonia digitata) which are such a feature of Dar Hamar, the adjoining district of Kordofan.
The deepest wells are at Karnak, where water is only obtained at 250 feet. At Burush on the Fasher-Obeid road and on the road to Taweisha, and at Taweisha itself, the wells, which pass through strata of chalk and marl, average from 100 to 130 feet in depth.
At El Fasher the wells are of no great depth and at the end of the dry season water is obtainable at 35 feet.
The nearer one approaches the central group of mountains the depth at which water is found diminishes. At 3,200 feet above the sea it is found by excavating in the sandy beds of khors, but at 4,000 feet there is running water which becomes more abundant still further to the west of Jebel Marra.
Geology.The geological formation is very varied; in the west the mountains show a volcanic origin; in the north and south granite and sandstone are the prevailing rocks; in the east the soil is sandy and contains a quantity of iron, which is worked to a small extent.
In the east and north-east, granite predominates, with the exception of a strip between Foga and El Fasher, where red and white sandstone crops out.
In the north, Wadi Melit and the hills in its neighbourhood are of gneiss. To the north-east of this, granite again predominates at Saya, whilst still further north, Jebel Tagabo is of sandstone.
Jebel Meidob contains both sandstone and granite; this group has been much distorted by volcanic agency, and beds of lava are to be seen in all directions. To its south-west lies Bir El Melh,[124] an extinct crater, which to outward appearance is an insignificant hill, but has a depth of about 150 feet. Here is a small lake strongly impregnated with alkaline matter, while sweet water springs issue from the sandstone and granite declivities.
The Jebel Marra group is also of volcanic origin; lava and granite are to be found everywhere, but there is no sandstone; small peaks of pink granite crop up here and there between these mountains and El Fasher.