The Kamasian, or Second, Pluvial of the middle Pleistocene Epoch corresponds to the Mindel in Europe. A dry but not a desert climate is implied by the Kamasian-Kanjeran Interpluvial levels at Olduvai Gorge. The Kanjeran, or Third, Pluvial occurred during the middle Pleistocene and corresponds to the Riss Pluvial in Europe.
An arid phase, which greatly reduced forest land, is revealed in the Kanjeran-Gamblian Interpluvial levels, lasting from about 60,000 to 55,000 years ago. That period corresponds to an important tectonic phase marked by uplift and subsidence in North Africa and activity along all the faults, in particular those in eastern Africa. It was at that time that eastern Africa assumed its present topographic character.
During the Gamblian, or Fourth, Pluvial, which occurred from approximately 30,000 to 15,000 years ago, three distinct humid phases are separated by drier intervals. During those phases the dimensions of Lake Chad and those of the glaciers of Mount Kenya and of Kilimanjaro diminished rapidly. The postpluvial phase that followed the period, equivalent to the postglacial phase of the Northern Hemisphere, was marked by a succession of alternating dry and humid stages and by the desertification of both the Sahara and the Kalahari, a process that began about 3,000 bce.
Mount Kenya, central Kenya.© Jiri Kasal/Fotolia
Alfred Kröner
Land
Relief
The physiography of Africa is essentially a reflection of the geologic history and geology that is described in the previous section. The continent, composed largely of a vast rigid block of ancient rocks, has geologically young mountains at its extremities in the highlands of the Atlas Mountains in the northwest and the Cape ranges in the south. Between these mountainous areas is a series of plateau surfaces, with huge areas that are level or slightly undulating, above which stand occasional harder and more resistant rock masses. Surrounding these surfaces is a zone of plateau slopes below which are narrow coastal belts widening along the Mediterranean coast, the coastlands of Tanzania and Mozambique, a narrow belt between the Niger and Cunene (Kunene) rivers, and an area northward of the Gambia and Sénégal rivers.
Physical regions of AfricaEncyclopædia Britannica, Inc.
Kilimanjaro (19,340 feet [5,895 metres]) is the highest point on the continent; the lowest is Lake Assal (515 feet [157 metres] below sea level) in Djibouti. In proportion to its size, Africa has fewer high mountains and fewer lowland plains than any other continent. The limited areas above 8,000 feet are either volcanic peaks or resistant massifs. All the land below 500 feet occurs within 500 miles of the coast, except for two small basins in the Sahara.
Summit of Kilimanjaro, northeastern Tanzania.© Shawn McCullars
The higher areas of the south and east are in marked contrast to the considerably lower elevation of the western and northern parts of the continent. South of a line drawn from near the mouth of the Congo River to the Gulf of Aden, most of the land lies 1,000 feet or more above sea level, and much of it exceeds 3,000 and even 4,000 feet. North of the line there is relatively little land above 3,000 feet, most of the area being between 500 and 1,000 feet above sea level; there are also broad coastal lowlands, except in the region of the Atlas Mountains and, in the east, beyond the Nile.
MoroccoThe rugged Atlas Mountains surround a valley in Morocco.Victor Englebert/Photo Researchers
Afromontane moorland of tussocky grasses, giant groundsel, and lobelias on the slopes of Mount Kenya.Caroline Weaver/Ardea London
Margherita Peak in the Ruwenzori Mountains, Uganda© Lauré Communications/Paul Joynson-HicksThe highest extensive areas are to be found in Ethiopia, parts of which exceed 15,000 feet. Southward the East African Plateau is highest in Kenya, where it is often 8,000 feet or more above sea level; there are occasional volcanic peaks that are much higher, such as Kilimanjaro, Mount Kenya (17,058 feet), Meru (14,978 feet), and Elgon (14,178 feet). The Ruwenzori (Rwenzori) Range—sometimes called the Mountains of the Moon—which reaches its highest elevation at Margherita Peak (16,795 feet) on the borders of the Democratic Republic of the Congo and Uganda, is not volcanic in origin. From East Africa the plateau extends southward, often with a well-defined though not continuous escarpment particularly noticeable in the Drakensberg of Southern Africa, where Ntlenyana, or Ntshonyana, is 11,424 feet and Mont-aux-Sources 10,823 feet high. There the plateau edges are especially marked, because the rock formations are hard and horizontal, whereas in Ethiopia they are conspicuous because of faulting. Where the rocks are softer and less resistant, the escarpment is not so pronounced and so forms less of a barrier to climatic influences and to human movement.
The Atlas Mountains.Encyclopædia Britannica, Inc.
Algeria: geographyThe Ahaggar Plateau rises from the barren landscape of the Sahara in southern Algeria.Geoff Renner/Robert Harding Picture LibraryTo the north and west of the plateau area of the southern parts of the continent there is a general descent to the lower areas of the basins of the Congo, Niger, and Nile rivers. The only large areas that extend above 3,000 feet are in the folded ranges of the Atlas Mountains and in the central Sahara, where resistant granites form the massifs of Ahaggar and Tibesti. The interior uplands of western Africa and of Cameroon consist of ancient crystalline rocks, reaching considerable heights only in the Fouta Djallon plateau in Guinea, in the Guinea Highlands, which also extend over the borders of Sierra Leone and Liberia, in the Jos Plateau in Nigeria, in the Adamawa region of Nigeria and Cameroon, and in the Cameroon Highlands. There are extensive low-lying areas near the coast and in the basins of the Sénégal, Gambia, Volta, and Niger–Benue rivers. The high areas of Darfur in Sudan (more than 10,000 feet) and of Mount Cameroon (13,435 feet) are volcanic in origin and are evidence of the same tensions that have resulted in rifting and volcanism in East Africa.
The East African Rift System constitutes the most striking and distinctive relief feature of the continent. Associated with its formation was the volcanic activity responsible for most of the higher peaks of East Africa, including Kilimanjaro. Seismic and volcanic disturbances are still recorded in the western portions of the rift valley system. In the Virunga Mountains, northeast of Lake Kivu, there are periodic outbursts (about every 10 or 12 years) that have created a series of lava flows. One of these volcanoes dammed the rift valley and converted a large area, formerly drained by a tributary of the Nile, into Lake Kivu.
Mountains and lakes of East Africa.Encyclopædia Britannica, Inc.
Fishing boat on Lake Nyasa (also called Lake Malawi).Neil Cooper—Panos PicturesThe rift valley extends for about 4,000 miles, its course being clearly marked by many of the lakes of East Africa as well as by the adjacent volcanic peaks. From the Gulf of Aqaba it can be traced southward along the Red Sea and into the Ethiopian Plateau to Lakes Rudolf, Naivasha, and Magadi in Kenya. Farther south, through Tanzania, the line of the rift is not quite so obvious. The walls that constitute the eastern rim have been more easily eroded, while the lakes of this area are generally smaller and not in line, and some of them are only waterless salt beds. The largest of these lakes are Natron and Manyara, with Eyasi in a side branch of the main rift. The edges are obvious enough to the south in Malawi, where a huge crusted block collapsed along the parallel faults that constitute the steeply rising slopes of Lake Nyasa (Malawi). The lake is 360 miles long but never more than 50 miles wide; it has a maximum depth of 2,310 feet. The rift then follows the line of the Shire Valley to reach the Indian Ocean near Beira, Mozambique.