there are four main features of the physical geography. the whole eastern half of darfur is covered with plains and low hills of sandy soils, known as "goz", and sandstone hills. in many places the "goz" is waterless and can only be inhabited where there are water reservoirs or deep boreholes. to the north the "goz" is overtaken by the desert sands of the sahara. a second feature are the "wadis", seasonal watercourses ranging from small rivulets that flood only occasionally during the wet season to large "wadis" that flood for most of the rains and flow from western darfur hundreds of miles west to lake chad. many "wadis" have pans of alluvium with rich soil that are also difficult to cultivate. the west of darfur is dominated by the third feature, basement rock, sometimes covered with a thin layer of sandy soil. basement rock is too infertile to be farmed, but provides sporadic forest cover that can be grazed by animals. the fourth and final feature are the marrah mountains, that rise up to a peak at deriba crater where there is a small area of temperate climate, high rainfall and permanent springs of water.