The Geology of Sierra Leone

Abstract

I. Introduction. The territory of Sierra Leone in British West Africa comprises a Colony and a Protectorate, covering a total area comparable with that of Ireland. The Colony consists mainly of a mountainous peninsula about 25 miles long and 9 miles in mean width. The Protectorate can be divided physiographically into a low-lying coastal plain, and an interior upland region composed of a series of broken plateaux [11]. The mountains of the Colony, rising abruptly from the seaward margin of the coastal plain, consist of a great noritic complex [9]. The main outcrop of the older schists and gneisses, as well as that of the Rokell River Series, falls entirely within the limits of the coastal plain [12]. The upland plateaux are composed of a group of great granite-masses, mostly younger than the schists and gneisses of the coastal plain [12]. In the north-western corner of the Protectorate, near Saionia, the southern margin of the great plateau of horizontal sandstone that extends over a large part of French Guinea runs for a few miles along the Anglo-French boundary [8]. The main outcrop of the older schists and gneisses extends along the coast of the Protectorate, from French Guinea to Liberia. It is extensively overlain by clays, sands, and gravels of Pleistocene and recent age; consequently, except along the beds of the larger rivers and on the flanks of low hills scattered over the plain, the ancient basement-rocks are very poorly exposed. In most respects these older schists and gneisses do