AFRICA - GEOGRAPHY Africa is largely a plateau, broken and elevated in the south and lower towards the north, with fold mountains adhering to the northern flank, and it thus contrasts very strikingly with Euro Asia in which fold mountains of successive series in diverse stages of denudation occupy a large proportion of the surface. The mean elevation of Africa is about 2,000ft., that of Asia 3,117ft. Asia has much quite low as well as very high land. Africa, as a plateau, has little at either extreme and in particular the land over 1 o,000 ft. in Africa is merely small groups of peaks.
The high southern part of the plateau reaching north almost to lat. 1o° S. has sharp slopes around its edges on the" west, south and east, with a narrow coastal plain below, broadening in the north-west and especially the north-east in Angola and Portuguese east Africa respectively. Towards the south this plateau has a high rim, over 6,000ft. above sea near Windhoek in the west and in Basutoland on the south-east. The southern edge is stepped, with the Nieuwveld range above the Great Karroo, the Little Karroo as the step next below and the coastal plain next below that. Within the high rim are considerable areas of centrally placed lower land, the South Kalahari in the south, the depression draining into Lake Ngami (now practically dry) and the Barotse land depression, but all keep above the 1,5ooft. contour and each is a local depression in a table-land most of which is at more than 3,000ft. above the sea. Generally speaking the plateau-edge dips down, with a small coastal ledge, into fairly deep water, but towards the Cape of Good Hope there is a fair area of shallow water off-shore.
The steepness of the edge of the plateau is important as a de terminant of the character of the rivers, long sections of which flow in deep slots cut back into the plateau from its edge. The Orange river is fed from the rains that fall on the Drakenberg range and flows west through lands of drought ; the Limpopo and Zambezi, the two largest on the east flow out to the broad sandy alluvial flats of Portuguese East Africa and the slot-like character of a large part of the Zambezi-channel is generally known from the occurrence of the famous Victoria Falls. In the south-west the Zambezi system interlaces with that of the Taukhe or Tioghe and receives surplus water from it at times. The rest of the Taukhe water loses itself in the swamps and saltpans of the f or mer Lake Ngami basin. The lower parts of the three great rivers have contributed to the lowering of considerable areas of the plateau below the 3,000ft. level. The Zambezi river, about 2,000111. long, drains an area of half a million square miles, the Orange river, about 1,3oom. long and with a drainage area of about i7o,000sq.m. but more than that amount'Dust be added if we define the basin orographically and so include in it larger water less tracts. Towards the north this southern plateau tends to run rather higher and there is a continuous belt of highland across the continent from the coastal plain on the west to Lake Nyasa on the east, in which region the typical form of the southern plateau is interfered with by important rifts to be considered next. Al lowing for complexities the southern plateau of Africa is thus essentially an elevated surface with still higher edges for the most part and with, also, considerable depressions, usually desertic in character, near the median north to south axis of the plateau. North of this southern plateau, on the west, is the basin depres sion, largely floored by younger rocks and now occupied by the Congo and its numerous feeders. The basin is almost enclosed by land over 1,5ooft. in height, for the main river escapes through a deep cut between the hills out to sea; there is general agreement that the basin is a depression in the original plateau surface that has been an inland sea. To the east of the basin are series of physi cal features that together constitute one of the major physical problems of the earth's surface. They are collectively designated the Rift valley system and have been the subject of much dis cussion, for which reference may be made especially to the files of the Geographical Journal (London, especially 1916-21) and to Krenkel Geologie Afrikas (1925). In this region in later tertiary and early Pleistocene times great movements occurred and earth quakes are still a feature. Of ten a block has sunk between parallel fractures and thence has come the name of Rift valley. A western rift may be traced from Lake Nyasa via Tanganyika and Lake Albert to Gondokoro. Many of the fractures run either north west to south-east (the so-called "Erythraean" direction) or, in the north, north-east to south-west (the "Somali" direction). A middle rift line goes north-north-east from Nyasa via Lake Nai vasha to Lake Rudolf, with branchings and changes en route; though its general direction is north to south, its component frac tures are generally north-west to south-east and north-east to south-west, with some north to south ones in the north. The easterly rift line is less important and goes on the whole north-east wards from about the middle of Lake Nyasa. In the floors of the rifts lie the long narrow lakes, Nyasa 1,645ft. above sea-level but with a maximum depth of 2,58oft. so part of its floor is over 9ooft. below sea-level, Tanganyika, also very deep and normally 2,624ft. above sea-level but this level is so variable that the outflow from Tanganyika is quite intermittent, and Rudolf at only i,i8oft. Some other lakes are small; that of Naivasha is over 6,iooft.
Many portions of these lakes have sharp walls and are obviously filling depressed zones. The western rift is the youngest and sharpest ; the most important movements may have been as late as the Pleistocene period. The rifts all show volcanic activity which is most marked along the eastern sides of the middle one, where occur the great peaks of Kenya (17,007f t.) and Kiliman jaro, on which Kibo peak (19,32i ft.) has long been held to be the culmination of Africa but Nilsson (1927) claims to have found points on Kilimanjaro 13ft. and i7ft. higher, respectively, than the highest previously known. Ruwenzori, on the eastern side of the western rift is 16,794ft. high and has a block structure ap parently with faulted sides ; interestingly enough it is in about the same latitude as the other great peaks mentioned and they are all on a belt of land, higher than most of the flanking territory, that stretches from the volcanic Cameroons (v. inf.) to the east coast. This cross-line scheme is noticeable in great basins (Niger, Chad and Bahr-el-Ghazal) to the north-west and is evidently of some importance in the story of the deformations that Africa has under gone.
It seems clear that at Lake Albert there was faulting on a large scale both before and after the country was arched up into a wrinkled dome while, farther east, the middle rift may be con nected with faulting near the median axis of a long arch. Faulting of domed and arched structures, supplemented by vulcanicity on a large scale, both occurring so recently as to leave features ex tremely sharp, are thus the characteristics of the country. Be tween the arched areas of Lake Albert and the middle rift in a downfold lies Lake Victoria of area approximately 26,000sq.m. and more or less rectangular but fractures determine only a small part of its outline and its shores are not high-cliffed. Unlike the rift lakes, it is less than so fathoms deep. It is at 3,720f t. above sea-level.
It is well known that the zone of fractures extends northwards marking the east and west edges of the Abyssinian highland, the series of cliff lines bordering the Gulf of Aden and the Red Sea, and the rift lines along the Gulf of Aqaba to the Dead Sea and the Jordan valley. All through this zone one finds systems of parallel fractures and a considerable number are north-west to south-east, others are north-east to south-west, others apparently north to south.
The relations of land and water, both inland waters and the ocean, have naturally varied a lot with the great changes of land levels, but apparently since present levels were reached, and there fore probably well on in the Pleistocene period, lakes of immense size existed in the rift valleys of East Africa. Shore lines 600ft. above present water-levels have been traced over long distances and there is not much doubt that during the major glacial phases of the Pleistocene ice age in Europe the mountains of East Africa were glaciated much farther down than at present.
The northern extension of the rift system to Lake Rudolf and beyond it north-westwards to the lowlands of the White Nile divides east Africa from Abyssinia and Somaliland. The latter is a plateau sloping south but broken sharply on the north, where a steep slope marks the coast of the Gulf of Aden, and less sharply on the south-east to the Indian Ocean as well as on the south west towards the Juba river basin which may be described as a widening of the coastal plain. Socotra is essentially a broken off projection of Somaliland. Abyssinia on the other hand is a re markable mountain land bounded east and west by great fractures of remarkable sharpness giving steep cliffs down to the Red Sea and to the White Nile lowland. The rift system of the Red Sea and that of the Gulf of Aden both enter into relationship with an Abyssinian rift which runs from the sea to Lake Rudolf and di vides Abyssinia from Somaliland. Abyssinia differs entirely from any folded mountain range ; it is a high and deeply dissected block full of evidences of volcanic activity and provided with powerful rivers, feeders of the White Nile (Sobat, etc.) the Blue Nile and the Atbara, indicating its high (largely summer monsoonal) rain fall. It lies for the most part more than 5,000ft. above sea-level and reaches up to nearly 15,1ooft. at Ras Dashan in the north. Lake Tana (Tsana) in the highlands is actually 5,69oft. above sea-level.
The Nile may be said, in a way to begin as the Kagera, the chief feeder of Lake Victoria. It leaves that lake on the north and after passing through the lake swamp of Kioga it goes to the north-eastern end of Lake Albert. Issuing again from the northern end of that lake, it emerges in due course from the rift system into a great flat area less than I,so0ft. above sea-level and framed by higher land save in the north-east and giving so little fall on the river that it becomes obstructed by floating vegetation. It is noteworthy that this upper Nile basin, the Lake Chad basin and the main Niger basin, all more or less enclosed, are nearly in the same latitude, with the still more enclosed basin of the Congo to the south. Basins of this kind are thus a marked feature of the geography of the old block of Africa. After the Nile (Bahr-el Jebel) has received the Sobat from the east it flows north through increasingly barren country into a slot cut in the great desert. The transition from the fertile slot up the, of ten steep, slope to the desert plateau is a very sharp one and the river has no tribu taries in its desert section. It thence follows that, though the Nile is 4,03.7m. long from the Kagera source to the sea, its basin area is only 1,107,227sq.m., whereas the Congo, rather under 3,000m. long but very rich in tributaries, has a basin area of about 1,425,00o square miles.
North of the Congo basin one may go from the Gulf of Guinea across to the extreme east without touching a point less than 1,5ooft. above sea. This rise or swell of the land surface has, on its north-west border, the high line (north-east to south-west) of the Cameroons, apparently an old mountain structure rejuvenated by volcanic outpourings. On this line stands the great peak of the Cameroons (13,37oft.) at the corner of the Gulf of Guinea, and it is continued into the sea giving the island of Fernando Po (Spanish) and those of S. Thome and Principe (Portuguese). On Fernando Po, Clarence Peak is over 9,000ft. high.
North of the Gulf of Guinea the land soon rises above the 600ft. level and towards the west there is a fairly marked slope up from the sea to over 1,5ooft. above its level, the slope faces south-west and above it towards the north is a patch above 3,000 f t. in height while one small area near the northeast border of Liberia reaches nearly I o,000ft. This region, with a heavy, mon soonal, summer rainfall is the birthplace of the feeders of the Niger. Thence that river flows north-east demonstrating the gen eral slope of the land down to the Sudan and Sahara, then east and south-east and south to reach the gulf in the lowland between the eastern end of the West African coastal edge and the high land of the Cameroons. Its length is about 2,600m. and basin area about 800,000 square miles.
Between the main Niger basin and that of the Bahr-el-Ghazal and White Nile lies a basin of internal drainage with the shallow, shrinking Lake Chad in its southern half and a depressed area with some swamps in the north-east. Lake Chad is about 85oft. above sea-level, and receives drainage from the south-east and west. The other depressed area is, in parts, less than 600ft. above sea but the drainage to it is of very small account. North of the Niger and Chad basins is the immense arid belt of the Sahara. It reaches the Atlantic along a low practically waterless coast i,000m. long from the mouth of the Senegal to the streams of the Atlas. The western part of the Sahara is generally low and has no heights reaching r,sooft. but in the northern centre is the Ahaggar pla teau, connected with the eastern end of which are north-west to south-east lines of higher land one going north-west to the Gulf of Qabes and one south-east crowned by the Tibesti highlands. Eastward, heights diminish again but there is practically a contin uous cliff edge along the Nile slot on both sides and the continua tion of the desert plateau on the east stands for the most part more than i,5ooft. above sea with a considerable area over the 3,000ft. line.
The desert has large sandy areas especially in the west with regions of stony desert (hammada) in the northern centre and a good deal of stony desert to the east. A number of oases break the monotony of the desert, some being north of Lake Chad ; some of the land around the Tibesti highland (reaching 8,000ft.) also gets more moisture and has a population aggregated in certain centres. Wadis from the Ahaggar plateau and elsewhere in the desert are interesting evidences of a former greater rainfall and from a study of animal remains as well as of finds of human implements it seems that the Sahara was more or less a grassland during some phases at any rate of the early ages of man. The probability is that periods of greater rainfall in the Sudan more or less coincided with periods of Pleistocene glaciation in Europe. The desert area is estimated at 3,500,000 square miles. The Nile slot cuts the desert without modifying it appreciably, and opens out north to the famous delta west of which the coast is not high save in the Barca or Cyrenaica plateau which reaches i,5ooft. not far from the sea.
The Atlas range, the north-western part of the continent, is orographically a part of the mountain arcs that frame the basins of the western Mediterranean. It reaches a height of 14,000ft. at Bu Uriul and Ari Aiaschi and runs, generally, east-north east to west-south-west. The eastern half has the Tell Atlas on the north and the Sahara Atlas on the south side with the plateau of the Shotts, several lakes without outlets, between them. The western region has the Mid Atlas continuing the first and the Great Atlas continuing the second ; the Anti Atlas is parallel to and south of the Great Atlas. To the north of the Mid Atlas the separate coastal mountain range of Er Rif curves north-west to the Strait of Gibraltar beyond which the curved high line is continued in the Sierra Nevada of southern Spain.
South of the eastern ends of these highlands is a more or less east to west line of low land containing the Shott el Jerid (about 52ft. above sea-level) and the Shott Melrir (nearly iooft. below sea-level) both without outlet. Africa is remarkable for the ex tent of land, 3,75o,000m. in basins of inland drainage.
Africa is singularly poor in islands. Madagascar 229,82osq.m., is one of the large islands of the world (smaller than New Guinea and Borneo) ; it is separated from Africa by the rather deep Mozambique Channel and in structure it resembles Africa. Bio logically it has links both with Africa and with the East Indian archipelago. Socotra, east-north-east of Cape Guardafui, a contin uation of the Somaliland plateau is the only other African island of any size. The Canary and Cape Verde archipelagoes of the north-west and the Comoro archipelago of the east are volcanic (but pebbles of continental, or plutonic, material have been sent to Europe from Anjovan, one of the lesser Comoro Islands) ; the Seychelles are a continental remnant. There are coral islets be tween the Comoros and the Seychelles. Mauritius, Reunion and Rodriguez, the two last entirely, the first mainly volcanic, lie in the Indian Ocean east of Madagascar; Ascension, St. Helena and Tristan da Cunha in the south Atlantic far to the west. (X.)