PHYSICAL SETTINGPost Comment PHYSICAL SETTING Nigeria
Geology: The state is underlain by metamor rocks of the basement complex, which outcrop over many parts. Rocks of the basement complex found here are schists, associated with quartzite ridges of the type found in llesa area. The metamorphic rocks are largely undifferentiated, but two specific rock groups may still be identified.
The first group consists of the migmatite complex, including banded magmatic and auguen gneisses and peg matites with outcrops in llesa and lfe Areas. Metasediments consisting of schiests and quartzites, calsilicates, meta conglomerates, amphibiolites and metamorphic iron beds make up the second group. They are found in two and lkire areas. Other parts of the state are underlain by undifferentiated metamorphic rocks.
Relief and Drainage: The land surface is gen erally undulating and descends from an altitude of over 450m in ljesa area to 150m and below in the southern parts of the state. Two main relief regions may be identified, the first is the inselberg land scape which is part of the Yoruba highlands, while the second is the coastal plain.
The region of inselberg landscape covers more than half of the state. The northern part is charac terised by numerous domed hills and occasional flat topped ridges, the more prominent hills in this region, are found at llesa, lgbajo, Okemesi, Elu and Oba. At Erin ljesa, there is a sharp drop in the ele vation, and this has given rise to a waterfall which has become one of the tourist attractions of the state.
Many rivers, including the Osun River from which the state derives its name, have their source in the northern part of the state. The Osun River is perennial and its volume fluctuates with seasons. The river flows through a narrow valley throughout its course across basement complex rocks. Two dams, at Ede and Ire, provide water for the inhabitants of the state.
Soil: The soils belong to the highly ferruginous tropical red soils associated with basement com plex rocks. As a result of the dense humid forest cover in the area, the soils are generally deep and of two types, namely, deep clayey soils formed on low smooth hill crests and upper slopes; and the more sandy hill wash soils on the lower slopes.
The well drained clay soils of the hill crest and slopes are very important, because they provide the best soils for cocoa and coffee cultivation in the state. The lighter loams are more suitable for cultivating the local food crops, such as yam, cassava and maize. Soil degradation and soil erosion are generally not serious in the state, but considerable hill wash is recorded along the slopes of the hills.
Vegetation: The state is covered by secondary forest and in the northern part, the derived Savannah mosaic predominates. Originally, virtually all parts of the state had a natural lowland tropical rain forest vegetation; but this has since given way to secondary forest regrowths. Among the rea sons for this are fuelwood production, road con struction, clay and sand quarrying and traditional farming practices.
Human interference, by way of cocoa plantation, has also replaced the forest. Hence, the natural tree species have given way to oil palm (elacis guinniensis), gmelina and dense thickets. Mature forests still exist in the Owu forest reserve at the southern part of the State. Part of this high forest has recently been cleared to make way for forest plantations of tectona grandis and gmelina arborea.
Wildlife: As in other parts of the country, hunt ing is an important traditional occupation in the state. Because of this, the game population of the state has dwindled considerably. None of the thirty three game reserves in the country is located in the state, and although there is a forest reserve, it was not established to protect game. The fauna species found in the state, include grass cutter, antelope and bus pig (warthog).
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