The only true desert in Africa south of the equator, the Namib lies largely within Namibia, on the south-eastern shore of the Atlantic Ocean. In the south it extends into South Africa, across the Orange River, while in the north it crosses the Kunene River into Angola. Three-quarters of it falls within Namibia, where it covers about a fifth of the country, an area four times the size of Switzerland.
With a coastline of 1 570km, the Namib occupies the western margin of the country in its entirety, from the ocean to the Great Escarpment. It is generally between 80 and 150km wide, but its width increases to some 200km in the far south, where tracts of desert overlap with part of the Orange River trough. The eastern limits of the desert coincide with the 100mm rainfall line at an altitude of about 1 000m above sea level.
Only eight smallish towns and villages are to be found in the entire Namib. Walvis Bay, Swakopmund, L岩tz, Oranjemund and Henties Bay lie on the coast, while Rosh Pinah, Aus and Arandis are in the inner desert. The coastal plain where the Namib now lies formed 70-65 million years ago as deep erosion cut into the escarpment and drove debris towards the ocean. Arid on the whole for the past 15 million years, the Namib is a desert on top of a desert, with petrified dunes buried beneath active sands. As such it is widely held to be the oldest desert on earth.
Moreover even the earlier Namib had a forerunner, a sand sea that sprawled across the Gondwana supercontinent 200-170 million years ago, when windblown sands covered most of the future southern Africa.
The present-day Namib originated five million years ago, after the icy Benguela Current had become fully established offshore, but its physical boundaries -- the Atlantic Ocean and the Great Escarpment -- are very much older. They began to form 132-128 million years ago when Gondwana disintegrated amid cataclysmic shifts in the crust of the earth.
For most of the year the Namib is devoid of surface waters, apart from some springs and seeps in otherwise dry riverbeds. The rivers at its northern and southern extremities are the only perennial ones. Both of them rise in other countries, the Kunene in Angola and the Orange in Lesotho, the latter on the opposite side of southern Africa.
Ephemeral rivers traverse the Namib from east to west in the wide open spaces between the Kunene and Orange. They rise in highlands in the interior, but seldom carry enough water to penetrate far into the desert, let alone flow into the Atlantic at the other end. Rivercourses in the northern half of the desert reach the ocean, but in the south come to a dead end in clay pans like Sossusvlei where dune fields block their path.
One of them, the Kuiseb, bisects the Namib. It forms the divide between the dune fields in the Great Sand Sea to the south and the gravel plains to the north. Not that either dunes or plains are confined to one or another part of the desert. Dune fields are also found beyond the gravel plains, on the Skeleton Coast in the far north, while gravel and sand plains lie to the south of the sand sea.
Nevertheless the sand sea and gravel plains contiguous with the Kuiseb rivercourse are the largest features of their kind in the Namib. Together they stretch across six degrees of latitude and account for more than half the length of the desert within Namibia. The sand sea on its own covers some 34 000km², an area nearly as big as Israel, Kuwait and Lebanon combined.
The highest mountain in Namibia, the Brandberg massif, is located in the inner Namib. Although other inselbergs stand isolated below the escarpment, like steep islands on the plains, mountains are few and far between in the desert proper.
Between the Kuiseb and the Orange the southern escarpment rises from the desert floor almost like a solid wall. Ephemeral rivers breach it in places and a few, like the Kuiseb, gouge out canyons as they descend into the desert.
On the coast the desert is generally open to the ocean, its narrow beaches joined to plains except where tall dunes rise abruptly from the high-water line, as they do in the Great Sand Sea and parts of the Skeleton Coast. The seashore is predominantly sandy.
On the whole it is a fairly straight coastline with few headlands to afford a bit of shelter from rough seas. Only two of the bays, Walvis and L岩tz, bite into the coast. Others once did, but silted up. One that was an anchorage for ships in the days of sail, Sandwich Harbour, survives as a shallow lagoon between the beach and desert dunes.
Confined to a short stretch of coast between Hottentot's Bay and a rock arch known as the Bogenfels, rocky shores with headlands and cliffs are found to the north and south of L岩tz Bay. The only islands in Namibian waters, none of them very big, lie off this part of the Namib coast.
THE GREAT SAND SEA
Over a span of 70 million years water and wind carried sediments across the coastal plain to the sea. The major sources of such deposits were the escarpment, the plateau behind it and the faraway interior of southern Africa.
The rivers that once rushed off the escarpment are nowadays of little account, except for the Orange or !Gariep, the great river that brought sand and diamonds to the Namib from the subcontinental interior. It is still doing so to this day and accounts in large measure for the presence of the Great Sand Sea.
Over aeons the palaeo-Orange deposited sediment in the Atlantic, where ocean currents swept it northwards and dumped it on storm beaches. Finally onshore winds drove the sand inland, to form dunes of all shapes and sizes, as well as sand sheets on the plains.
The Great Sand Sea extends northward for 400km from L岩tz Bay to Walvis Bay and as far as 140km inland from the coast.
Basically the sand consists of quartz grains mixed in a ratio of 9:1 with heavy minerals like garnet, ilmenite and magnetite as well as a little mica. The colours darken from pale buff in the west to deep red in the east as the ironoxide content of the sand increases.
The strength and direction of desert winds and the amount of sand available determine the shape and size of dunes. Apart from dunes anchored to vegetation, known as shrub-coppice dunes, all dunes are mobile to some extent, the crescent-shaped barchans most of all. Barchans in fact are popularly called "wandering dunes". They grow to a height of about 30m where strong winds blow mainly from one direction on parts of the coast with relatively little sand.
Widespread near the Orange River, around Walvis Bay and on the Skeleton Coast, all barchans in the Namib -- with south-westerlies behind them -- point and travel towards the north-east. They bury anything and everything in their path, tarry only as long as the wind allows and eventually move on, with communities of insects and reptiles on board.
A short distance inland from the barchans, transverse dunes are two and a half times as high, their long axis similarly across the path of the wind.
In the central and greater part of the sand sea, linear or longitudinal dunes lie parallel to one another and to the southerly winds, formations which in places are over 100km long and 120m high. They are arranged in nearly straight rows with valleys or so-called "dune streets" between them.
Their slipfaces change position with the seasons. Whereas they face north-east for much of the year when southerly winds prevail, they are turned around to face south-west in winter when the easterlies blow.
Mountainous piles of sand known as star dunes dominate the inland margin of the sand sea where high winds blow from all directions. They are reputed to be among the highest dunes in the world, as high as 220 m or even higher when they rest on a raised surface. Star dunes are named for their shape as seen from above, a lot of sharp ridges winding outwards and downwards from a central crest.
Shrub-coppice dunes, usually only a metre or two high, are common near the sea. They form around a bush or clump of grass when windblown sand lodges among the stems and leaves to form a hump-like mound. As the mound grows the roots lengthen to keep the plant above the surface.
A cool desert in Africa..........