Mountain Ranges, Deserts and Landforms of Botswana

Geography

Drainage of the desert is by dry black valleys, seasonally inundated pans, and the large salt pans of the Makgadikgadi Pan in Botswana and Etosha Pan in Namibia. The only permanent river, the Okavango, flows into a delta in the northwest, forming marshes that are rich in wildlife. Ancient dry riverbeds—called Ouramba—traverse the central northern reaches of the Kalahari and provide standing pools of water during the rainy season.

A semi-desert, with huge tracts of excellent grazing after good rains, the Kalahari supports more animals and plants than a true desert, such as the Namib Desert to the west. There is little rainfall, and the summer temperature is very high.

Climate

The climate is sub-humid rather than semi-arid in the north and east, where the dry forests, savannahs, and salt lakes prevail. South and west, where the vegetation is predominantly xeric savanna or even a semi-desert, the climate is "Kalaharian" semi-arid. The Kalaharian climate is subtropical: average annual temperature greater than or equal to 18 °C (64 °F), at peaks reaching 40 °C (104 °F) and above, with mean monthly temperature of the coldest month strictly below 18 °C (64 °F), and is semi-arid with the dry season from April to September, the coldest six months of the year. It is the southern tropical equivalent of the Sahelian climate with the wet season during summer.

History

The Kalahari Desert was not always an arid environment. Geological and paleoclimatic evidence suggests that during parts of the late Pleistocene, the region experienced significantly wetter and cooler conditions. Fossil flora and fauna from Gcwihaba Cave in northwestern Botswana indicate that the area supported more abundant vegetation and permanent water sources between roughly 30,000 and 11,000 Before Present (BP), with the most humid period occurring after 17,500 BP. Sediment cores, pollen records, and fossilized animal remains from other parts of the Kalahari, including the Makgadikgadi Pans and Okavango Basin, corroborate these findings, showing evidence of large paleolakes and river systems that once covered much of what is now desert.

Geology

The Witwatersrand plateau consists of a 5000-to-7000-metre-thick (3.1–4.3 mi) layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2.97 billion years ago. The entire series of rocks, known as the "Witwatersrand Supergroup", consists of very hard erosion resistant quartzites, banded ironstones and some marine lava deposits, interspersed with softer, more easily eroded tillites, mudstones and conglomerates. The oldest rocks (laid down 2.97 billion years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2.71 billion years ago) are those that form the southern edge of the plateau.

Gold is found in the conglomerate strata of the younger members of the Supergroup, locally referred to as banket.

History

Although gold had been discovered in various locations in South Africa, such as Barberton and Pilgrim's Rest, as well as at several sites near the Witwatersrand, these were alluvial concentrates in contemporary rivers, or in quartz veins, in the form that gold had always been found elsewhere on earth. When George Harrison, probably accompanied by George Walker, found gold on the farm Langlaagte, 5 km (3 mi) west of what would become the city of Johannesburg, in an outcrop of conglomerate rocks, in February 1886, they assumed that this was alluvial gold in an old riverbed, that had been tilted as a result of earth movements.

Geology

As the ancestral Lake Makgadikgadi shrank, it left relic shorelines, which are most evident in the southwestern part of the basin. As the lake shrank numerous smaller lakes formed with progressively smaller shorelines. The relic shorelines at elevations of 940 m (3,100 ft) and 920 m (3,018 ft) can be seen mostly easily on Gidikwe Ridge, west of the Boteti River.

The geologic processes behind the formation of the basin are not well understood. It is conjectured that there was a gentle down-warping of the crust, with accompanying mild tectonics and associated faulting; however, no significant plate boundary faults have been identified. The main axis of the developing graben runs northeast–southwest.

Kubu Island and Kukome Island are igneous rock "islands" in the salt flat of Sua pan.

Location and description

Lying southeast of the Okavango Delta and surrounded by the Kalahari Desert, Makgadikgadi is technically not a single pan, but many pans with sandy desert in between, the largest being the Sua (Sowa), Ntwetwe and Nxai Pans. The largest individual pan is about 4,900 km2 (1,900 sq mi). A dry, salty, clay crust most of the year, the pans are seasonally covered with water and grass, and are then a refuge for birds and animals in this very arid part of the world. The climate is hot and dry, but with regular annual rains.

The main water source is the 330-kilometre-long (210 mi) Nata River, called Amanzanyama in Zimbabwe, where it rises at Sandown about 60 km (37 mi) from Bulawayo. A smaller amount of water is supplied by the Boteti River from the Okavango Delta.