Relief and Main Physical Features

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Figure 8. Topography and Drainage

Much of Nigeria's surface consists of ancient crystalline rocks of the African Shield. Having been subject to weathering and erosion for long periods, the characteriÍÍÍÍstic 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 (see fig. 8).

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. The most dramatic of the sedimentary landscapes are in southeastern Nigeria, where thick sedimentary beds from the Abakaliki Uplift to the Anambra Basin have been tilted and eroded. This process has resulted in a rugged scarp land topography with east-facing cliffs at in the Udi Hills, north of Enugu, and in the area around Nanka and Agulu.

Although relatively little of the Nigerian landscape has been shaped by volcanic episodes, there are two main areas of volcanic rock. They are found on the Biu Plateau in the northeast, extending into some localized volcanic areas along the eastern border with Cameroon, and on the Jos Plateau in the northern center of the country.

The elevational pattern of most of Nigeria consists of a gradual rise from the coastal plains to the northern savanna regions, generally reaching an elevation of 600 to 700 meters. Higher altitudes, reaching more than 1,200 meters in elevation, are found only in isolated areas of the Jos Plateau and in parts of the eastern highlands along the Cameroon border. The coastal plain extends inland for about ten kilometers and rises to an elevation of forty to fifty meters above sea level at its northern boundary. The eastern and western sections of the coastal plain are separated by the Niger Delta, which extends over an area of about 10,000 square kilometers. Much of this is swampland, separated by numerous islands. The coastal plain region penetrates inland about seventy-five kilometers in the west but extends farther in the east. This region is gently undulating with elevation increasing northward and a mean elevation of about 150 meters above sea level. Much of the population of southern Nigeria is located in these eastern and western coastal plains and in some of the contiguous areas of the coast and the lower Niger Basin.

Separating the two segments of the coastal plain and extending to the northeast and northwest are the broad river basins of the Niger and Benue rivers. The upper reaches of these rivers form narrow valleys and contain falls and rapid 755s. Most t of the lower portions, however, are free from rapids and have extensive floodplains and braided stream channels. To the north of the Niger and Benue basins are the broad, stepped plateau and granite mountains that characterize much of northern Nigeria. Such mountains are also found in the southwest, in the region between the western coastal plains and the upper Niger Basin. The western wedge between Abeokuta and Ibadan and the Niger Basin reaches elevations of 600 meters or more, while the extensive northern savanna region, stretching from Kontagora to Gombe and east to the border, includes extensive areas with elevations of more than 1,200 meters or more at its center. The mountainous zone along the middle part of the eastern border, the Cameroon Highlands, includes the country's highest point (2,042 meters). In the far northeast and northwest, elevation falls again to below 300 meters in the Chad Basin in the far northeast and the Sokoto Basin in the northwest.

Data as of June 1991