The geography in many ways mirrors that of the Mississippi Delta when nearing the coastal region of the nation. Having been subject to weathering and erosion for long periods, the characteristic landscape of this area is extensive level plains interrupted by occasional granite mountains. These features form a major landscape type of Nigeria and of West Africa as a whole. There are also smaller areas of younger granites found, for example, on the Jos Plateau.
Sedimentary strata dating from various periods overlay the older rocks in many areas. The sedimentary areas typically consist of flat-topped ridges and dissected plateaus and a characteristic landscape of extensive plains and no major rocky outcrops. This landscape is generally true of the basins of the Niger and Benue rivers as well as the depressions of the Chad and Sokoto basins in the far northeast and northwest of the country, respectively.
http://www.state.gov/r/pa/ei/bgn/2836.htm
