where sandiness is off-set by high clay and silt contents, and where leaching is limited by high water tables.
Furthermore, in relation to the Niger Delta in particular, not all deposits are the nutrient poor products (quartz, kaolinite clay and iron compounds) from the erosion of an old landscape. Via the Benue River, some very rich deposits originate from the young, volcanic Adamawa massif.
5.3.4 TROPICAL FOREST SOILS MAY BE SUBJECT TO EROSION
It is often assumed that there is no soil erosion beneath tropical rainforest. However, under certain conditions where there is very little plant ground cover, soil protection is dependant on leaf litter and other fallen vegetation. Very heavy rains can disturb this litter, allowing secondary raindrops to severely damage the soil surface and cause localised erosion. Potentially more damaging than primary raindrops, secondary raindrops are those that collect on vegetation and then fall to the ground. They are usually larger, and fall with a higher velocity.
Effects of this secondary rain-drop sheet erosion can be seen in the relatively natural rainforest of the Oban Hills in Southeast Nigeria, where roots that would not normally be above level ground are exposed on slopes. This is natural erosion and part of the geological cycle.
5.4 DISTRIBUTION OF THE NATURAL TROPICAL RAINFOREST BIOME
The natural tropical rainforest biomes generally correspond to those areas of the tropics with annual rainfalls exceeding 1200mm. This includes, in South America, the Amazon Basin, the Columbian and Ecuadorian Andes and the East coast of Brazil between Salvador and Rio de Janeiro; the whole of Central America including the Southern Caribbean; the Western Ghats of India; Southeast Asia (with the notable exception of the Vietnamese Highlands); the Indonesian/Melanesian and Philippine archipelagos; Hawaii; and the Northeast coast of Australia.
In Africa there are two main areas of the tropical rainforest biome (see Map 2). The Zaire Basin extends North-west to the Dahomey Gap; a second area stretches from the other side of the gap, as far west as Freetown. Other, smaller areas in Africa include areas of high local rainfall such as the East coast of Madagascar (see Map 1); riverine forest, as along the Zambezi and Ruvumu rivers, both in Mozambique; mangrove forest in small protected areas of the East African coast from Northern Kenya to Southern Mozambique; islands of montane forest in such places as the Ruwenzori Mountains, Mount Uhuru (Kilimanjaro) and Mount Kenya; and gallery forest where narrow river valleys maintain high humidity, such as in the Shire Hills of Malawi and the Adamawa massif in Cameroon/Nigeria.
However, with the exception of the Amazon and Zaire basins, a few very isolated areas (such as the highlands of the West half of New Guinea and Southern Guyana), and some national parks, the tropical rainforests in all the areas described are either gone, severely depleted or under extreme threat of destruction. It is only the size of the Amazon and Zaire basins, the largest areas of tropical rainforest on Earth, that has saved them so far.
Nigeria's main tropical rainforest biome runs in a belt from the coast to between 150 and 200 kms. inland. (Ibadan and Onitsha lie just within the belt, Enugu just outside, and Benin City in the centre.) It contains three ecozones in roughly parallel bands. Working inland from the coast, these are first the Brackish and then the Freshwater
