カメルーン国内陸部森林・サバンナ地帯の地形と表層物質の特性

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The main purpose of this paper is to describe and discuss the landforms and superficial deposits characteristic of humid tropical environment, with special attention to the spatial variations corresponding to the zonal arrangement of climate and vegetation, based on the 1975/76 season field research across the forest and savanna zones of inland Cameroon from 2°N to 9°30N. Some basic data and views used for the reconstruction of palaeoenvironment in the present-day forest zone are presented.<BR>In the inland plateau underlain by the Precambrian crystalline rocks and mantled with dense humid forest, the landscape consists principally of small hills 40-100 m high predominated by the convexity. Hillslopes of such hills are almost evenly covered by the colluvial deposits consisting of red to yellow ferrallitic soils and gravelly layers. The colluvial deposits contain prehistoric implements of African Middle Stone Age and younger. The wide-distribution of indurated cuirasses is also recognized in the forest zone of southern Cameroon. Under the forest environment, however, these cuirasses rarely outcrop to form ledges or cornices. Dome-shaped weathering front has been occasionally found beneath the colluvial deposits and weathering profile, but in the forest zone it never controls the slope form of the present landsurface.<BR>In contrast to the forested areas, in the savanna zone of the Adamaoua Plateau the landsurfaces are extensively capped with ferruginous or bauxitic cuirasses and such prominent land-forms associated with indurated cuirasses as mesa, buttes, ledges and cornices are developed in the dissected part of the plateau. In this savanna zone gullying is the most common present-day process operating upon the landsurfaces, and this process has been more or less accelerated by intensive grazing and repeated burning. Termites are most active in this zone and the large-scale mounds with a diameter of 20-30 m and a height of 3-4 m are found elsewhere.<BR>In the drier Sudanese savanna zone of the Bénoué Basin vegetation is much more xerophyitic than expected from annual rainfall (1, 400-1, 000 mm), and the landscape is characterized by the undulating plain with towering granitic inselbergs and buttes of Cretaceous sandstones which are fringed by pediments. The superficial deposits both of gently undulating landsurface and pediments are generally stony, and cyclic deposits are found elsewhere. The youngest ones may be the product of the last 500 years as they contain potsherds. The older deposits may be the product of the past drier environments which prevailed during the last glacial age. In this drier savanna zone, the extensively stone-paved landsurface may indicate that rain-wash is the most prevailing process under the present environment.<BR>As mentioned above south-north variations of landforms and superficial deposits can be observed undoubtedly. However, these observations are nothing but a first approximation. As the geology, ages, relief and some other elements of landsurface differ from one area to the other, it is difficult to make a complete comparison in a strict sence. These conditions may produce the intrazonal character of both landforms and superficial deposits. For instance, inselbergs associated with bare rock surface are distributed not only in the savanna and steppe zones but in the forested areas. Coarse-grained granitic rocks tend to form for groups and boulders both in the forest and savanna zones, though they are more common in the latter. The occurrence of these features is largely controlled by the nature of lithology of underlying rocks. Stone-lines or stony deposits on and near the present landsurface are most common in the Sudanese savanna zone, but they are also found in the southern humid areas as evidenced on the partly stripped landsurface on the south of the Sanaga River.
社団法人東京地学協会の論文

カメルーン国内陸部森林・サバンナ地帯の地形と表層物質の特性

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