The Sahara is the most obvious of the natural barriers that divide Africa. Although speckled with oases and crossed by well-established tracks (1), this great expanse of sand desert and arid steppe separates the Arab north from the rest of the continent so effectively that ' Africa south of the Sahara', or 'sub-Saharan Africa', is a familiar conventional term. The Nile (8) flows across one end of the Sahara but its value as a link between north and south is reduced by its cataracts, and by the way its headwaters are entangled in swamps and in the precipitous Ethiopian mountains.
Just south of the desert, the broad belt of open savannah or pasture land known as the Sudan -- not to be confused with the Sudan republic (10) -- runs nearly the full width of the continent, embracing the upper Niger and the Chad basin. Relative ease of movement in this belt has in the past permitted the creation of extensive African states, and the spreading out of peoples like the Fulani over wide areas (C, E).
South of the Sudan, dense tropical rain-forest blankets parts of the Guinea Coast and most of the Congo basin. The Congo, the lower Niger and other rivers provide a few routes through it (P).
Southern and eastern Africa is dominated by a great mass of high plateaux and mountains, much of it open grassland with only a limited amount of tree cover. While movement is easier here than in the rain-forest, there are few natural routes except the scattered waterways provided by the eastern lakes (P). Africa's major rivers are all blocked to navigation by falls or rapids, particularly where they descend from the plateaux to the coastal lowlands. There are, moreover, few natural harbours along its coasts, and no intrusive arms of the sea of the kind that give almost all of Europe easy access to maritime trade. In general, the interior is more isolated from the outside world than that of any other continent.