Misplaced Pages

#65934

56-754: Searsia lancea commonly known as karee (archaically karree ), is an evergreen, frost hardy, drought resistant tree, which can reach up to 8 metres in height with a 5-metre spread. It is one of the most common trees on the Highveld and in the Bushveld in South Africa, but not found in the Lowveld . In North America, where it is naturalised, it is known as African sumac and willow rhus . S. lancea bears many names by locals in South Africa . By far

112-834: A continental divide , with the run-off to the north draining into the Indian Ocean via the Crocodile and Limpopo rivers, while the run-off to the south drains via the Vaal into the Orange River and ultimately into the Atlantic Ocean . Because of the extraordinary quantities of gold that have been extracted from the Witwatersrand rocks, the South African currency was named the rand in 1961 upon

168-662: A 56 km long east–west ridge over which several rivers running to the north form waterfalls, giving rise to the name Witwatersrand, which in Afrikaans means "Ridge of White Waters". There was no free oxygen in Earth's atmosphere until about 2 billion years ago, before the appearance and sufficient development of photosynthesizing cyanobacteria . The oxygen produced by these microorganisms rapidly reacted with, amongst others, Fe ions dissolved in water, precipitating insoluble red iron oxide ( hematite ). For various reasons,

224-471: A shallow sea, conveniently named the "Witwatersrand Sea". The resulting 2500–4500 m thick layer of sediments is termed the "West Rand Group" of Witwatersrand rocks. The second phase, which lasted for 200 million years, followed on from the first phase, with on-land deposits, resulting from the retreat of the Witwatersrand Sea, leaving a wide almost flat coastal plain over which rivers from

280-567: Is Botha's lark ( Spizocorys fringillaris ) and the two endemic mammals – the Free State pygmy mouse ( Mus orangiae ) and the rough-haired golden mole ( Chrysospalax villosa ). As well as the python, other reptiles include the Nile crocodile ( Crocodylus niloticus ), Nile monitor ( Varanus niloticus ), rock monitor ( Varanus albigularis ), and giant girdled lizard or sungazer ( Smaug giganteus ). Like many other areas of grassland all over

336-451: Is also one of the largest meteor impacts to have left its imprint on present-day Earth geology. A meteor 10–15 km across created a 300 km diameter crater, distorting all the rock strata within that circle. Johannesburg is just within the outer edge of this impact crater. In the immediate vicinity of the impact all the subterranean strata were uplifted and upturned, so that Witwatersrand rocks are exposed in an arc 25 km away from

392-516: Is clear that for the next 200 million years the flood plain was repeatedly inundated, sometimes eroded, and sediments re-deposited. The result was a 2500 m thick layer of rock that is termed the "Central Rand Group", which together with the "West Rand Group", forms the "Witwatersrand Supergroup". It is the younger Central Rand Group that contains most of the gold bearing conglomerates, locally referred to as banket , that are today of great economic importance. The "Central Rand Group" of deposits

448-864: Is generally devoid of mountains, consisting of rolling plains, especially in the Free State, sometimes interrupted by rocky ridges such as the Witwatersrand , the Magaliesberg , and Vredefort Dome . The Vaal River and its tributaries form the main water drainage system of the Highveld. Tributaries of the Orange River drain the most southerly regions of the Highveld. The Highveld rainy season occurs in summer, with substantial afternoon thunderstorms being typical occurrences in November, December, and January. Frost occurs in winter. Cities located on

504-405: Is sometimes carried out at depths of 4 kilometres (2.5 mi) below the surface. The Witwatersrand Basin is a largely underground geological formation which surfaces in the Witwatersrand. It holds the world's largest known gold reserves and has produced over 40 000 tonnes (1.3 × 10  ozt), which represents about 22% of all the gold accounted for above the surface. The basin straddles

560-779: Is to its north where the Magaliesberg separates the Highveld from the Bushveld. The continuation of the Great Escarpment to the south separates the Highveld from KwaZulu-Natal. The south-eastern portion of the Great Escarpment (that portion of the Great Escarpment most commonly referred to as the Drakensberg ) rises to over 3000 m and forms the boundary between KwaZulu-Natal and Lesotho. The latter mountainous region is, however, not generally referred to as Highveld, whose boundary at this point runs just inside

616-689: The Eastern Cape , Northern Cape , North West , Limpopo , and Mpumalanga Provinces of South Africa. The highest part of the Highveld, around 2,100 m (6,900 ft), is its northeastern well-defined boundary, where the plateau escarpment (the Mpumalanga Drakensberg ) separates it from the Mpumalanga Lowveld , (containing, amongst others, the Kruger National Park ). Another well defined boundary

SECTION 10

#1732779695066

672-645: The crust is derived from gold in the mantle which resulted from a meteorite bombardment some 3900 million years ago (i.e., at approximately the time that the Kaapvaal Craton formed). The gold bearing meteorite events occurred millions of years after the segregation of the Earth's core. The gold in the Witwatersrand Basin area was deposited in Archean river deltas having been washed down from surrounding gold-rich greenstone belts to

728-530: The grassland acts as a natural filter, providing sources of clean water. Naturally occurring vegetation in the Highveld consists of different types of well-established grassland depending on the varying amounts of rainfall across the area: subtropical and temperate grassland , with true savannah not dominating the ecosystem until more tropical latitudes. The major grass species are Hyparrhenia hirta and Sporobolus pyramidalis and among these are other grasses and herbs. Trees and shrubs never thrived due to

784-494: The "West Rand Group" of sediments caused the Witwatersrand sea to retreat. The area of the craton on top of which Johannesburg is now situated, became a vast riverine plain, which extended along the entire northern and western shoreline of the shrunken sea, in an arc extending from Evander in the east, through Johannesburg, Carletonville and then southwards to Klerksdorp and Welkom in the south-west. The rivers formed braided deltas with many interlacing, slow flowing channels where all

840-484: The "Witwatersrand Sea" to the south, where the earlier sediments that form the older rocks of the Witwatersrand Supergroup had been deposited. Some of these gold bearing fan deltas are now at depths of 4 km (2.5 mi) below the surface. Although many of the older mines, around Johannesburg, are now nearly exhausted, the Witwatersrand Basin still produces most of South Africa's gold and much of

896-413: 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

952-497: The Archean Eon, but island arcs did form. It was the coalescence of several of these island arcs that led to the formation of the Kaapvaal Craton , one of the first microcontinents to form on Earth about 3.9 billion years ago. Its size and position relative to Southern Africa today are indicated in the diagram on the left. About 3 billion years ago local cooling of the underlying asthenosphere caused subsidence of

1008-526: The Highveld include Johannesburg , Pretoria , Bloemfontein , Vereeniging , Welkom , Carletonville , and the cities of the West Rand and East Rand . The diamond-mining city of Kimberley lies on the border of the Highveld and the southeastern Kalahari. About half of the gold ever produced in the world has been mined on the Highveld since 1880. The largest deposits are located in the Witwatersrand, which centres on Johannesburg, with smaller deposits in

1064-842: The Lesotho-Free State border, about 2,000 m (6,600 ft). From its eastern boundary, the Highveld slopes gently downwards to be bounded by the Great Karoo to the south, the Kalahari desert to the west, the Bushveld to the north, the Mpumalanga Lowveld to the northeast, KwaZulu-Natal to the east, and the Lesotho Highlands, or Mountains , to the southeast. The Highveld covers an area of almost 400,000 km (150,000 sq mi), or roughly 30% of South Africa's land area. The Highveld terrain

1120-606: The Rand or, less commonly, the Reef ) is a 56-kilometre-long (35 mi), north-facing scarp in South Africa . It consists of a hard, erosion-resistant quartzite metamorphic rock, over which several north-flowing rivers form waterfalls, which account for the name Witwatersrand, meaning 'white water ridge' in Afrikaans . This east-west-running scarp can be traced with only one short gap, from Bedfordview (about 10 km [6.2 mi] west of O.R. Tambo International Airport ) in

1176-619: The Vredefort Dome, virtually all the other outcrops occur in an arc approximately 80–120 km from the centre of the impact crater, to the west, north-west, north and north-east. Thus, it is possible that if it had not been for the Vredefort asteroid strike 2 billion years ago, we would either have never discovered the rich gold deposits beneath the Southern African surface, or they would have been eroded away during

SECTION 20

#1732779695066

1232-463: 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

1288-524: The basin is deeply buried under younger rocks, but outcrops occur in Gauteng , the Free State , as well as in some of the surrounding Provinces. The outcrop in Gauteng forms the Witwatersrand ridge, from which the basin and its rocks derive their name. It was on the southern portion of this ridge that gold was first discovered on the farm Langlaagte in 1886, 5 km west of Johannesburg . Since this gold

1344-522: The declaration of the republic. Witwatersrand and the Rand are names for the conurbation that developed along the range , although the terms are falling into disuse and Witwatersrand was the "W" in PWV (Pretoria-Witwatersrand-Vereeniging), the initial name of Gauteng province. In this context, it has lent its name to institutions including the University of the Witwatersrand (Wits University) and

1400-501: The defunct Rand Afrikaans University (RAU, now part of the University of Johannesburg ), and to towns and regions such as the East Rand , West Rand and Randburg . The Witwatersrand plateau consists of a 5 000 -to- 7 000 -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

1456-430: The east, through Johannesburg and Roodepoort , to Krugersdorp in the west (see the diagram at left below). The scarp forms the northern edge of a 7-to-10-kilometre-wide (4–6 mi) plateau (or ridge) which rises about 200 m (660 ft) above the surrounding plains of the Highveld . A number of picturesque Johannesburg suburbs, including Observatory , Linksfield Ridge and Upper Houghton are located along

1512-464: The first glaciation episodes on earth. Within 60 million years, up to 4500 m of sediment had accumulated on the granite base, to become the "West Rand Group" of rocks that contribute over 60% of the total thickness of the Witwatersrand Supergroup. Uplifting of the north of the Kaapvaal Craton, in addition to orogenesis (mountain formation), towards the end of the deposition of

1568-468: The frequent fires that occurred in the dry season and the heavy grazing (once by wild animals and now by livestock ). The Highveld is home to a number of endangered animals, including straw-coloured fruit bats ; Africa's largest snake, the African rock python ( Python sebae ); mountain zebras ; and South Africa's national bird, the blue crane ( Anthropoides paradiseus ). The only endemic bird species

1624-467: The fruits were pounded, water added and left to ferment, producing an evidently refreshing beer. The tree is a good shade tree for gardens, parks and pavements. It favours areas rich in lime in the Karoo and Namibia. Highveld The Highveld constitutes almost the whole of the Free State , and Gauteng Provinces, and portions of the surrounding areas: the western rim of Lesotho, and portions of

1680-475: The geology of the Witwatersrand Basin and its exposure in the Johannesburg region, was a massive meteor impact 110 km to the south-west of Johannesburg 2.02 billion years ago. The impact was close to the present village of Vredefort, which has given its name to the geological remnant of this immense event: the Vredefort Dome. Not only are the remains of this impact among the oldest on Earth, but it

1736-675: The gold that is accounted for today. Not all the conglomerates contain gold, and of those that do (known as "reefs" by the miners), the gold is not uniformly distributed throughout the layer, but tends to occur in streaks, where the pebbles that make up the conglomerate are larger than elsewhere. Here the gold is associated with other minerals, especially iron pyrite and uraninite , as well as carbon rich materials such as kerogen , or bitumen , which occurs in small balls less than 1 mm (0.04 in) in size, called "flyspeck carbon", or as continuous layers about 10–20 mm (0.4–0.8 in) thick. The gold-bearing conglomerates occur chiefly in

Searsia lancea - Misplaced Pages Continue

1792-445: The heavy materials brought down from the mountains were deposited: large pebbles, and heavy minerals, such as gold , iron pyrite , and uraninite . The gold was in its free elemental form. Cyanobacteria grew in relative abundance in these mineral rich waters. The kerogen , or bitumen , that is found in association with the gold deposits almost certainly represents what remains of these Archean photosynthesizing micro-organisms. It

1848-677: The historic Witwatersrand Gold Rush . Harrison is believed to have sold his claim for less than £10 before leaving the area, and he was never heard from again. Harrison's original Zoekers' (in English: seekers', or prospectors') Claim No 19 was declared a national monument in 1944, and named Harrison's Park. The park is on the busy Main Reef Road, immediately west of Nasrec Road. In 1887 Cecil John Rhodes registered "The Gold Fields of South Africa" in London, South Africa's first mining house, with

1904-416: The impact centre. There are no gold deposits in these outcrops. The meteor impact, however, lowered the Witwatersrand basin inside the crater. This protected it from erosion later on; but, possibly more importantly, bringing it to the surface close to the crater rim, near Johannesburg. In fact, apart from the Witwatersrand outcrops (i.e. where these rocks are exposed at the surface) in the immediate vicinity of

1960-703: The most common name for this tree is karee , which derives from the Khoekhoegowab name !areb . This is mostly used by speakers of Afrikaans . Other Afrikaans names are Rosyntjiebos from rosyn ( raisin ) and bos ( bush ) . In the Sotho–Tswana languages , the names mosilabele in Southern Sotho , mokalabata in Northern Sotho and mosabele in Tswana are cognates. In Khelobedu ,

2016-696: The names is motshakhutshakhu and mushakaladza in Venda . Among Nguni languages the name umhlakotshane in Zulu and Xhosa is used and in Swati the name given is inhlangutshane The tree is dioecious . It has a graceful, weeping form and dark, fissured bark that contrasts well with its long, thinnish, hairless, dark-green, trifoliate leaves with smooth margins. It bears small yellow flowers followed on female trees by bunches of small yellow-green flattish fruits, which are relished by birds. In earlier times

2072-401: The north and west. Rhenium - osmium isotope studies indicate that the gold in those mineral deposits came from unusual 3 billion year old mantle -derived intrusions known as komatiite , present in the greenstone belts. Apart from the obvious hollowing out of the rocks below southern Johannesburg, causing unpredictable sinkholes, surface instabilities and earth tremors, the bringing to

2128-415: The north formed wide braided river deltas , into some of which rich deposits of gold were deposited. The resulting 2500 m thick layer of rock is termed the "Central Rand Group". The "West Rand Group" and "Central Rand Group" of rocks together form the "Witwatersrand Supergroup", the full horizontal extent of which is termed the Witwatersrand Basin. There were no continents during the early stages of

2184-463: The north-east to Theunissen in the south-west, and 150 km (93 mi) wide stretching from Steynsrus in the south-east to Coligny in the north-west, with a small subsidiary basin at Kinross . Gold occurs only along the northern and western margins of this basin, but not in a continuous band. The gold-bearing rocks are limited to six sites where Archean rivers from the north and west formed fan deltas, with many braided channels, before flowing into

2240-619: The northern Free State near Welkom and Virginia . The Highveld is also exceedingly rich in diamonds , coal , vanadium , and manganese . The pollution in this region is also very high principally due to coal thermal power stations producing nitrogen oxide and sulfur dioxide . Agriculture on the Highveld is generally dominated by extensive grain production and the grazing of beef cattle , with more intensive production of maize , wheat , sorghum , citrus fruits , groundnuts , sunflowers , and vegetables , occurring in irrigated areas and farmland closer to urban areas. The peat base of

2296-509: The old provinces of Transvaal and the Orange Free State , and consists of a 5000–7000 m thick layer of Archean , mainly sedimentary rocks laid down over a period of about 260 million years, starting about 3000 million years ago. The entire series of rocks, known as the "Witwatersrand Supergroup" consists of quartzites , banded ironstones , mudstones , tillites , conglomerates and some marine lava deposits. Most of

Searsia lancea - Misplaced Pages Continue

2352-685: The phenomenon is called, has become a major ecological problem, because it dissolves many of the heavy elements, such as the uranium , cadmium , lead , zinc , copper , arsenic and mercury found in the mine dumps, facilitating their passage into surface water and ground water. The tailings ponds contain an average of 100 mg/kg of U 3 O 8 , and uranium levels are measureable in human hair. Sulfuric acid also erodes concrete and cement structures, resulting in structural damage to buildings and bridges. 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

2408-505: The scarp, overlooking the rest of northern Johannesburg with views up to the Magaliesburg (although locals refer to segments of the scarp using area-specific names, such as Linksfield Ridge , Parktown Ridge or Observatory Ridge ). The entire plateau-like structure is also often called the Witwatersrand. The plateau's elevation above sea-level is between 1,700 and 1,800 metres (5,600–5,900 ft). The Witwatersrand plateau forms

2464-535: The sedimentation of the fine grain mud was affected by cyclic episodes with more or less precipitation of iron oxides. The result was alternating red and beige layers of mud which, when consolidated, became banded ironstones . As the sea deepened, finer grained and muddy sediments accumulated. Changing geographical conditions resulted in the accumulation of a wide variety of sediments, ranging from mud, to sand, to gravel, and banded ironstones . Tillite deposits, dating from 2.95 billion years ago, are indicative of

2520-647: The south eastern portion of this microcontinent below sea level. The floor of this newly formed "Witwatersrand sea" consisted of smoothly eroded granites . Sandy sediments brought in by rivers from the north started being deposited on the granite about 2.97 billion years ago. This sandy layer eventually became compressed to form the Orange Grove Quartzite, the lowermost layer of the Witwatersrand Supergroup. This quartzite layer can be seen lying on its granite base in Johannesburg , where it forms

2576-468: 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 . The abundance of this gold is without a natural equal anywhere else in the world. Over 40,000 tonnes (44,000 short tons) have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 22% of all

2632-421: The surface of rocks that had been laid down in oxygen-free conditions had unforeseen effects. Iron pyrite (FeS 2 ), which is relatively plentiful in the gold ores of the Witwatersrand, oxidises to insoluble ferric oxide (Fe 2 O 3 ) and sulfuric acid (H 2 SO 4 ). Thus, when mine waste comes into contact with oxygenated rainwater, sulfuric acid is released into the ground water. Acid mine drainage , as

2688-441: The total world output. Silver , uranium , and iridium are recovered as gold-refining by-products. The Witwatersrand basin was created during the Archean Eon, and is therefore amongst the oldest geological structures on Earth. It was laid down in two stages, over the course of 260 million years starting just short of 3 billion years ago. The first phase, lasting 60 million years, consisted of sedimentary deposits in

2744-510: The uninterrupted removal of a several kilometres thick layer of deposits from the surface of the Southern African Plateau in the relatively recent geological past: i.e. the past 150 million years, but especially during the last 20 million years. The vast majority of the Earth's gold and other heavy metals are locked up in the earth's core . Evidence from tungsten isotope studies indicates that most gold in

2800-426: The upper, younger layers of the Witwatersrand Supergroup of rocks, on the southern side of the Witwatersrand plateau. The Witwatersrand Supergroup strata which reach the surface in Johannesburg dip downwards to the south at an angle of about 30°. From there on they are almost everywhere, with very few exceptions (see below), covered by younger rocks. Gold mining in these buried portions of the Witwatersrand Supergroup

2856-908: The world, the Highveld is excellent agricultural land and most of the area has been converted for farming. The grassland areas now remaining in the natural state are in various nature reserves, which, although a small portion of the Highveld, are still the largest areas of remaining grassland in South Africa . The protected areas include Suikerbosrand Nature Reserve , Verloren Valei Nature Reserve , Nooitgedacht Dam Nature Reserve , Bronkhorstspruit Dam Nature Reserve, Vaal Dam Nature Reserve, and Koppies Dam Nature Reserves and Willem Pretorius Game Reserve . 27°S 28°E  /  27°S 28°E  / -27; 28 Witwatersrand The Witwatersrand ( UK : / w ɪ t ˈ w ɔː t ər z r æ n d , - r ɑː n d / , US also / ˈ w ɪ t w ɔː t ər z -/ ; Afrikaans pronunciation: [ˌvətˌvɑːtəɾsˈɾant] ; locally

SECTION 50

#1732779695066

2912-534: Was alluvial gold in an old riverbed, that had been tilted as a result of earth movements. However, when it was found that, traced downdip, the conglomerate was not merely developed for the narrow width of a river, but continued in depth, there came the realisation that this conglomeratic zone was part of a sedimentary succession. Harrison had stumbled on the Main Reef conglomerate (part of the "Johannesburg Subgroup" of rocks — see illustration above). The conglomerate

2968-532: Was brought to an abrupt end by massive outpourings of lava, which form the Ventersdorp lavas which erupted 2.715 billion years ago. The cause of these lava outpourings is a matter of speculation. It might be related to the collision of the Kaapvaal Craton with the Zimbabwe Craton , eventually to become knitted together to form a single continental unit. A final event that had a major impact on

3024-419: Was embedded in a conglomerate, it was first assumed that this was alluvial gold in an old riverbed, that had been tilted as a result of earth movements. However, when it was found that, traced downdip , the conglomerate was not merely developed for the narrow width of a river, but continued in depth, there came the realisation that this conglomeratic zone was part of a sedimentary succession . The conglomerate

3080-548: Was quickly traced east and westward for a total continuous distance of 50 km (31 mi) to define what became known as the "Central Rand Gold Field". Harrison declared his claim with the then-government of the Zuid Afrikaanse Republiek (ZAR), and in September 1886 President Paul Kruger issued a proclamation declaring nine farms public mining diggings, starting on 20 September 1886. This heralded

3136-497: Was quickly traced east and westward for a total continuous distance of 50 km (31 mi) to define what became known as the "Central Rand Gold Field". It has since been established that the rocks that make up the Witwatersrand Ridge dip downwards and southwards to form the largely underground "Witwatersrand Basin" which covers an elliptical area with a 300 km (190 mi) long major axis from Evander in

#65934