Lake Bosumtwi, Ashanti

Abstract

THE following paper was written during six months' reconnaissance and prospecting work in the immediate vicinity of Lake Bosumtwi, which lies approximately 20 miles south-east of Kumasi, the capital of Ashanti, at an altitude of approximately 350 feet above sea-level. The shape is roughly circular with an average djameter of 5 miles at the lake surface and 6I2 miles at the caldera rim which surrounds the lake depression. The inner slopes of the latter, rising up to I2oo feet above lake-level, are very steep, whereas the outer slopes are gentle and at times scarcely recognizable when approaching the lake from the neighbouring country. As the name derives from obosom meaning God and twe the name of the lake spirit, the spelling Bosomtwe appears to be more correct than the usual Bosomtwi.I The lake is an object of native superstition. Any effort to solve the problem of origin of this mysterious lake is faced by exceptional obstacles. The lake is still within the dense forest belt of Ashanti, accessible only along native paths and hunters' tracks, which are quite numerous, and occasionally by a stream-bed. If one wishes to examine beyond this, one is forced to cut a way through the thick bush, and even then the soil is covered with decaying vegetation and undergrowth. The slopes of the depression are so steep that after a heavy rainfall it is neither possible to climb down nor up, and unfortunately the lake depression is known to be a locality of exceptionally frequent and heavy rainfall. The geology and character of the problem are extremely complicated. With the exception of practically recent clays and very scanty, relatively young agglomerates, the only rocks in the vicinity are of Birrimian (precambrian) age. But these comprise a whole petrological collection: sedimentary, metamorphic and igneous rocks; of the latter intrusive, hypabyssal and extrusive, acid, intermediate and basic, effusive and explosive types. Of the breccias some are possibly of tectonic origin. But what of all this is actually connected with the origin of the lake? The whole earth's history lies between. Rocks of Birrimian age can look as fresh as if they were recent, whereas nearly all types of quite young rocks may under the highly tropical conditions of West Africa change soon into lateritic or bauxitic alteration products. Previous work on the subject is scanty. In spite of very little concrete evidence, three theories have been put forward: the meteoric theory; origin of the basin through subsidence; volcanic origin. The meteorite theory originates from Malcolm Maclaren (i), and since the date of his publication (I93I) meteoric craters have become known from all parts of the world (2). At one time two points seemed strongly in favour of this theory: the apparent absence of any young volcanic material or other geological features explaining this sudden deep depression, and the fact that this lake is an isolated occurrence incomparable with any other geological