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Posted to the web: 6/4/2002 12:56:35 PM
 
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Location and Spatial Extent:
Nigeria is located within the western coast of Africa, slightly north of the Equator. The Atlantic Ocean washes its entire southern part while the fast encroaching arid zone south of the Sahara Desert borders its northern part. It lies approximately between latitudes 4°N and 14°N and between longitudes 3°E and 15°E, encompassing a vast geographical area of contrasting landforms, climatic conditions and vegetation belts.

The surface area of the country is approximately 923,800 sq. km and, with over 100m people, Nigeria is the most populous African nation. It is bordered by the Republic of Cameroun to the east, Niger to the north and Benin Republic to the west. The southern boundary is formed by the 800 km Atlantic coastline, which includes the eastern sector of the Gulf of Guinea.

Physiographic Regions:

Nigeria exhibits a great variety of relief features encompassing uplands of 600-1,300m on the Jos Plateau, the north-central and the eastern highlands; and, lowlands of less than 20m in the coastal areas extending as far inland as 60 km from the shoreline. Covering an average distance of some 1,120 km from south to north, Nigeria displays physiographic regions of varying characters in relief, nature and spatial distribution. These attributes of the physiographic regions correspond to the occurrence of the three classes of rocks, namely, the igneous, metamorphic and sedimentary rocks.

These rocks account for the areas of uplift composed of basement complex formations and areas of lowland in the basins of sedimentation composed of deposits of tertiary rocks. The nature of the geological structure gives the two broad relief regions of high plateaux in the central and northern parts and lowlands in the southern parts of the country.

The Niger-Benue all-season drainage system, further cuts the uplands into three:

(i) the north-central plateau,

(ii) the eastern and north-eastern high- lands, and

(iii) the western uplands.

Consequently, due to fluvial action of the Niger-Benue system over the tertiary outcrops, lowland areas developed along:

(i) the Niger-Benue trough;

(ii) the Sokoto plains;

(iii) the Chad basin; and the depositional coastal area of the Niger Delta.

There are a few isolated highlands of volcanic rocks on the Jos and Biu plateaus, as well as on the Mandara highlands to the far east, along the Nigeria-Cameroun border, which are underlain by the basement complex of massive granitic formation. These isolated highlands, drained by swift-flowing streams and rivers, show strong resistance to denudation. They thus display very bold relief of dissected plateau with elevations of up to 1,300 m above sea level (a.s.l). Immense surface water resources, derived from swift-flowing streams and rivers which drain the highlands, flow into the Chad basin at the extreme north-eastern part of the country. On the bases of geological structure, origin and nature of rocks, and of the drainage system, Nigeria can be divided broadly into ten physiographic regions. These are:

Lowlands:

(i) The Niger Delta
(ii) The coastal lowlands
(iii) The Chad basin
(iv) The undulating plains of the interior low- lands
(v) The scarplands of south-central Nigeria
(vi) The Niger-Benue trough
(vii) The Sokoto plains


High Plateaus:
The western uplands
The north-central plateau.
The eastern and north-eastern high- lands


(i) The Niger Delta: This is the southern- most physiographic region of Nigeria protruding into the Gulf of Guinea and extending from the Benin river in the west to the Bonny river in the east. It extends over 450km from west to east, thus constituting about 60 per cent of the 800km Nigerian coastline. The Niger Delta is, therefore, an integral part of the southern lowlands and, with a catchment area of over one million sq.km, describes one of the largest delta systems in the world. The northern limit is at Aboh, where the Niger-Benue river system bifurcates into the Rivers Nun and Forcados, initiating the Niger Delta formation. This northward extension of the Niger delta is known to create tidal influences up to 50 km from the shoreline.

The Niger Delta is a low-lying depositional environment, truncated by an intricate network of streams, creeks and lagoons over extensive sand bars as they flow into the Gulf of Guinea. The Niger Delta, at its southernmost portionwhere it empties into the Atlantic Ocean, has developed a braided system of 21 estuarine fluvial (freshwater) systems.


The Coastal Lowlands: The Niger Delta is bounded on both the east and west sides by a strip of low-lying coastal plain known as the coastal lowlands, hardly more than 30m a. s. 1. This area consists of a maze of lagoons, creeks
and river estuaries bordering the entire shoreline washed by the Gulf of Guinea. The coastal lowlands to the west depict a remarkably intricate network of lagoon inlets, meandering creeks and extensive lagoons, with the Lagos Lagoon representing the largest lagoon system in West Africa. This area is also characterised by well developed river estuaries, such as those of the rivers Ogun, Shasha and Benin which flow into the vast network of creeks and lagoons.

The coastal lowland east of the Niger Delta is a uniform low-lying area termi- nating into creeks and swampy terrain at the shoreline. The prominent Cross River basin, of numerous tributaries, enters the Gulf of Guinea through the Cross River estuary. The reduced tidal influence, due to distance upstream of the estuary, makes possible the development of vast areas of freshwater swamp forest along the inland stretches, comprising the wetlands of the Eket and Calabar areas. This explains the existence of extensive freshwater swamp forest dominated by the raffia palm, and the rich oil palm bush in the area. The unique, monotonous configuration of low-lying swamp terrain at the shoreline gives rise to the strand coastline formation of the east, while the prograding sand bars of the west result in the formation of the barrier ridge-lagoon complex.

(iii) The Undulating Plains of the Interior Lowland: This is a physiographic region north of the coastal lowlands and the Niger Delta, with an average eleva- tion of about 100m a.s.l. They are underlain by young sedimentary forma- tions with a well-drained surface. By far better drained than the other two regions above, its sedimentary deposits describe the Cretaceous and Tertiary rocks of shales, coal, sandstones and clays, with abundance of limestone. The interior lowlands cover upper Lagos, southern Ogun, major parts of Ondo, Edo, Delta States to the west, and most parts of Anambra, Imo, Abia and Akwa I born States to the east.

(iv) The Scarplands of South-central Nigeria: These form a unique physio- graphic region truncated by the Niger- Benue trough as it runs southward. The scarplands of Nigeria exhibit plateau- like features with dip slopes terminating steeply into valleys of drainage basins. The most prominent is the Udi-Nsukka escarpment which terminates, by a steep slope, into the Cross River Basin. As the watershed of the numerous rivers flowing into the Cross River Basin, the scarpland to the east of the Niger is notorious for incidences of gulling and severe soil erosion. To the southwest of it, and close to the Niger valley near Onitsha, are the Awka-Orlu uplands which terminate with an east-facing escarpment into the Mamu valley.

The steep escarpment, formed near the lower Niger valley where the upland of the Esan area of Edo State terminates, forms the western extension of the Udi-Nsukka scarpland. The Asaba upland forms the western extension of the Awka-Oriu scarpland. The nature of landforms under the prevailing geomor- phic processes in this physiographic region has led to far-reaching land degradation problems, particularly of gulling and soil erosion.

(v) The Niger-Benue Trough: This is the physiographic region located north of the scarplands of south-central Nigeria Physical And Human Characteristics and south of the north-central plateau. The Niger-Benue trough runs from the southern boundary of the Sokoto plains in the north-west to the north-east near Yola, having passed through Lokoja from where it extends south to the Onitsha gap, north of the Niger Delta. The surface lies below 300m a.s.l. and it is underlain by sedimentary rocks. Denudational actions by the Niger-Benue river system liberate enormous alluvial sediments to form extensive flood plains along the trough. The area is thus favoured for agriculture, water for irrigation farming and a major source of fish. The immense surface and ground water resources left virtually untouched along the Benue basin of the trough call for careful exploitation and utilisation for maximum benefits.

The Sokoto Plains: This region occu- pies the north-western part of Nigeria. It has a generally uniform flat landscape at an average height of 150m a.s.l. A sedimentary region of sandstone, it is dissected by the Sokoto river and its tribu- taries, with large quantities of sediment deposition to form broad floodplains. Fluvial action on the plain surface has left few isolated flat-topped hills standing about 30m above the uniform plain. With improved irrigation schemes, the Sokoto-Rima basin is now intensively cultivated in the fadamas. The Sokoto plain is also highly influenced by wind action, as the depressions and valleys have been covered by drifting sands to further attain the uniformity in their relief.

The Chad Basin: The Chad Basin, like the Sokoto plains, is developed over young sedimentary Tertiary rocks; hence its near-flat plain over the landscape. It is located in the extreme north-eastern part of the country, where it forms a depression into which rivers drain. Its most important all-season river is the Shari, which flows from the Cameroun Republic. The other tributar- ies, including the Hadejia, Komadugu, Yobe and Yedseram, which flow through Nigeria, virtually dry up in the dry season. Afeature of this uniform landscape is the seasonal flooding of the immedi- ate lowlands and valley flood plains during the wet season. It also provides an immense potential for irrigation agriculture. Another feature is the occurrence of artesian reservoirs, which serve as important sources of water supply.

(viii) The Western Uplands: This physiographic region is underlain by granitic igneous and metamorphic rocks of the basement complex, which account for its bold relief as massive ridges and isolated dune-shaped inselberg. The general elevation is between 400-600m a.s.l. with extensively dissected landscape in which the higher parts form the drainage divides of streams flowing north into the Niger valley and south to the Atlantic Ocean. The elongated granite ridges and inselberg are of considerable heights above the general level. The Idanre Hills rise to about 900m while numerous peaks rising above 600m can be seen from the Oyo area, through the llesha/Ekiti areas, to the Kabba district area.

(ix) The North-central Plateau: This region is located north of the Niger-Benue trough, stretching northward and covering about one-fifth of Nigeria's total land area. This plateau occurs over the largest basement complex formation in the country, which has been greatly exposed due to denudational action of mature perennial rivers (such as the Kaduna River) flowing through broad valleys. The morphology of the area depicts a peneplain formation over an erosional surface. The average eleva- tion above sea level is about 700 m describing relatively high plains, which are, however, broken by peaks of between 850m and over 1000m in the Zaria area, and over 1300m in the Jos area. They include the inselberg and exposed volcanic plugs which are com- mon features in this region.

The Jos Plateau is a volcanic formation, which today is an erosional highland standing at about 1,300m a.s.l. and covering an area of almost 8,000 sq.km. It is thus, by far, the most conspicuous morphological feature formed on old basement complex rocks influenced by volcanic activities, with thick basalt surfaces from lava flows. The geological history and phases of volcanic activity of both older and younger granite, as well as older and younger basalt on the Jos Plateau account for the varying granitic and basaltic surfaces and imposing landforms and the tremendous tin deposits as well as numerous waterfalls on the plateau.

(x) The Eastern and North-eastern Highlands: This region includes the areas east of the north-central plateau and the highlands of the Nigeria- Cameroun border on the far eastern part. The north-eastern highlands at the Nigeria-Cameroun border are located east of the Benue valley as mountain ranges towards the north into the Mandara mountains and south into the Adamawa highlands. The entire border, covering almost 1,200 km, constitutes a fragile physiographic region containing crater lakes of volcanic origin.

Some of these crater lakes composed of thermal springs and gas are now under threat of collapse, necessitating Nigeria- Cameroun trans-border co-operation. The region is heavily dissected by swift-flowing rivers taking their sources from the area and flowing into the Benue and Cross rivers. The Mandara mountain range stands up to between 1,200 and 1,500 m, while the Adamawa highlands including the Alantika and Shebshi Mountain ranges rise to between 1,800 and 2,400m. . Further south are the Obudu Plateau and Oban hills which reach about 1,200m.

This physiographic region stretches from north to south and displays a striking departure from the largely monotonous Landscape in many other parts of the country. This is exemplified in the contrasting relief features from the very high grass-covered mountain ranges of the far north-east, through the deeply incised river profiles east of the Benue river trough, to the bold plateau surfaces further south in the Obudu area of south-eastern Nigeria

 

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