The Idusi Formation forms the basal depositional unit of southwestern Tanzania. It is defined as the rock unit limited by the unconformable contact with underlying Precambrian metamorphics and the base of the overlying, fluviatile Mpera Sandstone Member of the Mchuchuma Formation. The Idusi Formation is subdivided into the Lisimba Member, comprising diamictites, lutites with dropstones, slump-mass deposits and laminates, and the overlying Lilangu Member, consisting of black, pyritic and kerogenous lutites. The type section is at Idusi Gorge, situated 6 km east of Lake Nyasa at 10°17'50''S. The thickness of the formation at the type section is 240 m, with the maxiumum observed thickness being 715 m. Plant fossils and palynological assemblages indicate an Asselian age, probably extending down into the late Carboniferous.The basal diamictites contain striated and facetted clasts, which identify them as glacial deposits. They mainly represent wash-out and slurried till deposits and also, less frequently, lodgment tillites. These are overlain by proximal and distal deposits of proglacial lakes, which were laid down during the retreat of the glaciers. Laminates of the upper Lisimba Member demonstrate progressively stronger seasonal control. Further amelioration of the climate, with substantially increased biological production both on land and in the water, is demonstarted by the black lutites of the Lilangu Member. They represent euxinic lake deposits formed during the final deglaciation phase. Deposition of the overlying, coal-bearing Mchuchuma Formation occurred under a temperate climate. The duration of the late Paleozoic glaciation is estimated at about 20 to 25 Myr. The last quarter of this time span was characterized by climatic amelioration. Global warming was accompanied by an eustatic rise in sea level and a marked decrease in atmospheric carbon dioxide.