Mass Movements in Hilly Areas (with Examples from Nigeria)

Abstract

In mountainous areas of great relief, mass movements occur regularly as the result of the normal cyclic landscape development process. However, mass movements also occur in areas of lower relief, i.e. in areas of less geomorphic activity. An extreme case of this type has been found in the development of cracks in a flat savannah area of Nigeria which has generally been ascribed to purely hydrological causes. Nevertheless, it can be shown that the development of the cracks occurs along the shear lines of the neotectonic stress field, so that geotectonic processes represent the ‘ultimate’ cause of the mass movements here as well. Conditions intermediate between those obtaining in mountainous and flat areas are found in hilly regions. In this instance, we have investigated a further particular area of Nigeria: the hills and river valleys near Ankpa in Benue State. Nevertheless, the instabilities in the ground may again have a much deeper primary cause. The primary unstable features appear as gullies; these are seen to form a definite orientation pattern; — which fact would point to a tectonic predesign. This contention can be substantiated by studying numerically the orientation pattern of the gullies/cracks and making a statistical analysis thereof. The pattern of the gullies is then compared with joint orientation measurements. It turns out that the pertinent orientation patterns agree within a few degrees with each other indicating that the gullies/cracks and the joints have been created by the same cause: viz. by the action of the stress field: the latter must be neotectonic, since the joints are found in recent laterite.Key wordsMass movementsneotectonicsstress fieldNigeria