ABSTRACT
The Ruwenzori Mountains lie on the equator in the centre of Africa. Several peaks exceed 4500m and carry many small glaciers amounting to 4.5 sq.km. There are also moraines and other evidence of three major Pleistocene glaciations, the maximum extents of which are represented by the Lake Mahoma (260 sq.km), Rwimi Basin (300 sq.km?) and Katabarua (500 sq.km?) Stages, the first being dated to shortly before 14,700 BP and the two latter estimated to be 100,000 BP or more. Lesser moraines indicate younger retreat stages and minor readvances such as the Omurubaho Stage (10,000 BP ?) and the Lac Gris Stage (100–700 BP). Historical records show a considerable retreat of the glaciers during this century. The greater extent of the Katabarua Glaciation was due to its being a plateau ice-cap but, following drainage incision, the ice tongues of the subsequent valley glaciations reached far lower altitudes down to 1900m.
The equilibrium line altitudes (ELAs) of the present and former glaciers have been estimated by the Area-Height-Accuraulation method. Because of the large number of glaciers at each stage, it has been possible to estimate the statistical precision of these estimates and to map iso-ELA lines which represent notional ELA surfaces. The vertical distance between these surfaces is a good measure of the difference in climate. All the surfaces are characterised by similar downward slopes to the East-south-east. The present ELA is at about 4600m but at the time of the major glacial stages it was depressed by 600–700m. This crudely indicates a reduction in mean temperature of about 4°C but the climate may have been still colder but rather drier. The present annual precipitation is 2000–2500mm.
The mountain is not volcanic but an uptilted block on the edge of the Western Rift. However most of the uplift seems to have been completed before these glaciations so that the ELA changes represent climatic change not tectonics. The present peaks seem to be the products of fluvial and glacial erosion since uplift started, and there were considerable changes in the drainage system after the Katabarua Glaciation.
The sequence of glacial stages on the Ruwenzori correlates well with those on the other East African mountains, using such criteria as size, extent, weathering and erosion, ELAs and a limited number of C14 and K/Ar dates.
