Hills and Ridges in Southwestern Nigeria

The Basement Complex rocks of southwestern Nigeria host a variety of residual hills. A distinction can be made among hills based on their parent materials and mode of formation. The hills include mesas, inselbergs, tors, regolith hills (i.e., erosional residuals in regolith), and regolith/forest-covered hills developed in amphibolite and diorite. Most common are inselbergs, which based on size have been classified into whalebacks, turtlebacks, bornhardts, and castle kopjes. They are either symmetrical or perfectly domical-bornhardts, elongated and symmetrical, or elongated and asymmetrical. They occur in clusters and with higher density in granitic plutons, but are scattered in various gneisses. These physical features have become a veritable touristic resource in the area. Good examples include the Idanre hills and the location of Abeokuta around the famous Olumo rock (which is actually a tor) in 1830.