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56-574: The elevation of this region varies from 750 to 1,400 m and the annual rainfall ranges from 350 mm in the west to 600 mm in parts of the northeast. There are four significant mountain ranges in this region: the Magaliesberg which runs from Rustenburg in the west to Bronkhorstspruit in the east and forms the southern border of the Bushveld; the Drakensberg escarpment that forms

112-659: A Hertzian cone when struck with sufficient force. This results in conchoidal fractures, a characteristic of all minerals with no cleavage planes. In this kind of fracture, a cone of force propagates through the material from the point of impact, eventually removing a full or partial cone, like when a plate-glass window is struck by a small object such as an air gun projectile. The partial Hertzian cones produced during lithic reduction are called flakes , and exhibit features characteristic of this sort of breakage, including striking platforms , bulbs of force , and occasionally eraillures , which are small secondary flakes detached from

168-547: A curved prominence that intersects suburban Pretoria before it peters out some 50 km (31 mi) to the east, just south of Bronkhorstspruit . The highest point of the Magaliesberg is reached at Nooitgedacht , about 1,852 metres (6,076 ft) above sea level. A cableway reaching to the top of the mountain range is located at Hartbeespoort Dam , providing sweeping views of the Magaliesberg and surrounding area. The Magaliesberg has ancient origins. Its composition

224-448: A fraction of a micron to 20 microns, but most typically 8 to 10 microns. Chalcedony is a microfibrous variety of quartz, consisting of radiating bundles of very thin crystals about 100 microns long. Megaquartz is composed of equidimensional grains over 20 microns in size. Most chert is microcrystalline quartz with minor chalcedony and sometimes opal , but cherts range from nearly pure opal to nearly pure quartz chert. However, little opal

280-413: A microcrystalline form of silica composed mostly of bladed crystals of cristobalite and tridymite . Much opal-CT takes the form of lepispheres , which are clusters of bladed crystals about 10 microns in diameter. Opal-CT in turn transforms to microquartz. In deep ocean water, the transition to opal-CT occurs at a temperature of about 45 °C (113 °F) while the transition to microquartz occurs at

336-418: A source of dissolved silica, but they are sometimes found cutting across bedding surfaces, where the chert fills fossil burrows , fluid escape structures , or fractures. Nodules under a few centimeters in size tend to be egg-shaped, while larger nodules form irregular bodies with knobby surfaces. The outer few centimeters of large nodules may show desiccation cracks with secondary chert, which likely formed at

392-506: A spark that ignites a small reservoir containing black powder , discharging the firearm. Cherts can cause several problems when used as concrete aggregates. Deeply weathered chert develops surface pop-outs when used in concrete that undergoes freezing and thawing because of the high porosity of weathered chert. The other concern is that certain cherts undergo an alkali-silica reaction with high-alkali cements. This reaction leads to cracking and expansion of concrete and ultimately to failure of

448-870: A temperature of about 80 °C (176 °F). However, the transition temperature varies considerably, and the transition is hastened by the presence of magnesium hydroxide , which provides a nucleus for the recrystallization. Megaquartz forms at elevated temperatures typical of metamorphism . There is evidence that the variety of chert called porcelainite , which is characterized by a high content of opal-CT, recrystallizes at very shallow depths. The Caballos Novaculite of Texas also shows signs of very shallow water deposition, including shallow water sedimentary structures and evaporite pseudomorphs , which are casts of crystals of soluble minerals that could have formed only in near-surface conditions. This novaculate appears to have formed by replacement of carbonate fecal pellets by chert. Bedded cherts can be further subdivided by

504-472: Is a nature reserve above Rustenburg covering 4,257 ha (10,520 acres) of the Magaliesberg. Numerous smaller reserves, private and state, are to be found along the length of the range. After nearly a decade of lobbying and sustained efforts by a small committee of dedicated environmentalists, the Magaliesberg has been declared a World Biosphere Reserve . The announcement was made on 9 June 2015 in Paris by

560-480: Is ascribed to successive geological processes over a very protracted history. Its quartzites , shales , chert and dolomite were deposited as sediments in an inland basin on top of a 3 billion year old Archaean Basement Complex, known as the Kaapvaal Craton . This process of sedimentation lasted for about 300 million years. About 2 billion years ago, a massive upwelling of molten magma resulted in what

616-448: Is characteristically of biological origin, but may also occur inorganically as a chemical precipitate or a diagenetic replacement, as in petrified wood . Chert is typically composed of the petrified remains of siliceous ooze , the biogenic sediment that covers large areas of the deep ocean floor, and which contains the silicon skeletal remains of diatoms , silicoflagellates , and radiolarians . Precambrian cherts are notable for

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672-545: Is due to the Bushveld igneous complex , an extremely rich saucer-shaped geological formation that stretches over more than 50,000 square kilometers. This formation contains most of the world's reserves of minerals such as andalusite , chromium , fluorspar , platinum and vanadium . The complex includes the Merensky Reef , which is the world's biggest source of platinum as well as platinum-group metals. As most of

728-418: Is far below saturation, indicating that silica cannot normally be precipitated from seawater through inorganic processes. The silica is instead extracted from seawater by living organisms, such as diatoms, radiolarians, and glass sponges, which can efficiently extract silica even from very unsaturated water, and which are estimated to presently produce 12 cubic kilometers (2.9 cu mi) of opal per year in

784-885: Is little in the carbonate buildup zone itself. This may reflect dissolution of opal where carbonate is being actively deposited, a lack of siliceous organisms in these environments, or removal of siliceous skeletons by strong currents that redeposit the siliceous material in the deep basin. The silica in nodular chert likely precipitates as opal-A, based on internal banding in nodules, and may recrystallize directly to microquartz without first recrystallizing to opal-CT. Some nodular chert may precipitate directly as microquartz, due to low levels of supersaturation of silica. The banded iron formations of Precambrian age are composed of alternating layers of chert and iron oxides . Nonmarine cherts may form in saline alkaline lakes as thin lenses or nodules showing sedimentary structures suggestive of evaporite origin. Such cherts are forming today in

840-399: Is mostly traditional , with a small number of sport climbing venues and hundreds of routes across a wide range of grades . The relatively mild climate allows year-round climbing. Chert Chert ( / tʃ ɜːr t / ) is a hard, fine-grained sedimentary rock composed of microcrystalline or cryptocrystalline quartz , the mineral form of silicon dioxide (SiO 2 ). Chert

896-516: Is now known as the Bushveld Igneous Complex . The enormous weight of this intrusion depressed the sediments that lay beneath and tilted the sediments along the edges so that the broken escarpments faced outward and upward, and the gentler dip slopes inward. During the same period, these sediments were fractured and igneous intrusions of dolerite filled the cracks. With the passage of time these intrusions eroded, especially on

952-485: Is of only modest economic importance today as a source of silica (quartz sand being much more important.) However, chert deposits may be associated with valuable deposits of iron , uranium , manganese , phosphorite , and petroleum . In prehistoric times, chert was often used as a raw material for the construction of stone tools . Like obsidian , as well as some rhyolites , felsites , quartzites , and other tool stones used in lithic reduction , chert fractures in

1008-518: Is over 60 million years old. Opaline chert often contains visible fossils of diatoms , radiolarians , and glass sponge spicules . Chert is found in settings as diverse as hot spring deposits ( siliceous sinter ), banded iron formation ( jaspilite ), or alkaline lakes . However, most chert is found either as bedded chert or as nodular chert . Bedded chert is more common in Precambrian beds, but nodular chert became more common in

1064-476: Is the dry season throughout most of Southern Africa. The undisturbed portions of this habitat, such as much of the Waterberg Biosphere , are home to many large mammal species including white rhino , black rhino , giraffe , blue wildebeest , kudu , impala and a variety of additional antelope species and other game. The Bushveld is one of the most mineral -rich regions of the world. This

1120-435: Is the most abundant minor element, followed by iron and manganese or potassium , sodium , and calcium . Extracrystalline water (tiny inclusions of water within and around the quartz grains) make up less than 1% of most cherts. The Folk classification divides chert into three textural categories. Granular microquartz is the component of chert consisting of roughly equidimensional quartz grains, ranging in size from

1176-419: Is uncertain, but they may form from fossil remains that are completely dissolved in fluids that then migrate to precipitate their silica load in a nearby bed. Eolian quartz has also been suggested as a source of silica for chert beds. Precambrian bedded cherts are common, making up 15% of middle Precambrian sedimentary rock, and may have been deposited nonbiologically in oceans more saturated in silica than

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1232-506: Is usually black to green in color, and the full sequence of beds may be several hundred meters thick. The shale is typically black shale, sometimes with pyrite , indicating deposition in an anoxic environment. Bedded chert is most often found in association with turbidites , deep water limestone , submarine volcanic rock , ophiolites , and mélanges on active margins of tectonic plates . Sedimentary structures are rare in bedded cherts. The typically high purity of bedded chert, like

1288-659: The Cretaceous . Diatoms were the dominant siliceous organism responsible for extracting silica from seawater from the Jurassic and later. Radiolarite consists mostly of remains of radiolarians. When the remains are well-cemented with silica, it is known as radiolarian chert . Many show evidence of a deep-water origin, but some appear to have formed in water as shallow as 200 meters (660 ft), perhaps in shelf seas where upwelling of nutrient-rich deep ocean water support high organic productivity. Radiolarians dominated

1344-540: The Hartbeespoort Dam , situated in one of the valleys of the range, was completed. It became a popular holiday and weekend destination for the inhabitants of Johannesburg and Pretoria, and the villages of Hartbeespoort and Kosmos developed as a result. At present, the Magaliesberg area is still largely agricultural , although tourism is a rapidly growing industry in the area. Kgaswane Mountain Reserve

1400-591: The Matabele . As he feared an attack from Shaka if he returned home, he settled in the Magaliesberg region. On 17 January 1837, after some Voortrekkers had been attacked and killed by Mzilikazi's impis , they counter-attacked and, under the leadership of Hendrik Potgieter and Gerrit Maritz , and with the help of local Sotho- Tswana chiefdoms, drove the Matabeles north across the Limpopo River . Because

1456-600: The Phanerozoic as the total volume of chert in the rock record diminished. Bedded chert is rare after the early Mesozoic . Chert became moderately abundant during the Devonian and Carboniferous and again became moderately abundant from the Jurassic to the present. Bedded chert, also known as ribbon chert, takes the form of thinly bedded layers (a few centimeters to a meter in thickness ) of nearly pure chert separated by very thin layers of silica-rich shale . It

1512-730: The International Coordinating Council of the Programme on Man and the Biosphere (MAB). This is a UNESCO programme that aims to build a supportive and sustainable relationship between people and their environments. In effect, this means a specific focus on safeguarding natural ecosystems through innovative approaches to economic development. The World Network of Biosphere Reserves , which Magaliesberg now joins, counts 631 biosphere reserves in 119 countries. "We are delighted with this final acknowledgement of

1568-669: The Magaliesberg in the Highveld, leaving the Bushveld north of the Magaliesberg mostly to their Sotho-Tswana friends and allies; according to Sarah Heckford's memoir, A Lady Trader in the Transvaal , the Boers would move into the Bushveld to visit their Sotho-Tswana neighbors during the winter. According to oral testimony by Tswana headmen recorded around the turn of the 20th century, many individual Boers formed close friendships with prominent individual Tswana headmen and chiefs, especially for

1624-431: The Magaliesberg mountains is Dithaba Diloka . This is a local seTswana name in reference to the abundance of Heteropogon contortus which grows in the area. However it seems this name would have only been used by neighbouring Sotho-Tswana_peoples coming to and from the Magaliesberg mountains. Local resident Tswana tribes referred to it simply as Dithaba "The mountains" . The first European explorers who came to

1680-736: The Second Anglo-Boer War . The Boers, being extremely familiar with the mountains, used secret pathways across the mountains to launch guerrilla attacks on the British soldiers . In response, the British forces built blockhouses on top of the mountains in order to restrict the movement of the Boer forces; ruins of these structures are still to be seen on the mountain. Control of the Magaliesberg Mountain Range

1736-403: The alkaline Lake T'oo'dichi'. Chert may also form from replacement of calcrete in fossil soils ( paleosols ) by silica dissolved from overlying volcanic ash beds. The cryptocrystalline nature of chert, combined with its above average ability to resist weathering, recrystallization and metamorphism has made it an ideal rock for preservation of early life forms. For example: Chert

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1792-548: The alkaline lakes of the East African Rift Valley . These lakes are characterized by sodium carbonate brines with very high pH that can contain as much as 2700 ppm silica. Episodes of runoff of fresh water into the lakes lowers the pH and precipitates the unusual sodium silicate minerals magadiite or kenyaite , After burial and diagenesis , these are altered to Magadi-type chert. The Morrison Formation contains Magadi-type chert that may have formed in

1848-460: The area recorded the mountains being called the "Cashan" or "Kashan" mountains. This was a corruption of the name of a powerful chief of the BaKwena ba Mmatau whose name was Kgwasane wa Sekano (Kgwasane, son of Sekano) . In the mid-1800s the mountain range became known as Magaliesberg (lit. Mogale's mountain) after Kgosi Mogale . Mogale means "sharp" or "brave" person, but is also

1904-657: The common word for a warrior or Tswana soldier, in Sotho bogale means "sharp" and mogale is "brave" . Similarly, the mountain range to the north, near Sun City, Pilanesberg, was named after the local Bakgatla chiefs, who were called Pilane . In 1822, Shaka , the leader of the Zulu Kingdom , sent his most trusted commander, Mzilikazi , to conquer the Sotho tribes of the region. After accomplishing this task, Mzilikazi decided to break away from Shaka and found his own nation,

1960-684: The dip slopes, forming deep kloofs or ravines providing rock-climbing potential to modern man. This large dog-leg-shaped area is now termed the Transvaal Basin and includes the lofty escarpment of the Transvaal Drakensberg overlooking the Lowveld in the eastern part of the country. Massive outpourings of igneous material–volcanic lava–of the much younger Karoo Supergroup later covered the Transvaal Basin, but this

2016-679: The eastern border of the Bushveld and runs from Tzaneen in the north to Belfast in the south; the Waterberg range that is in the middle of the Bushveld and the Soutpansberg range just north of Louis Trichardt . The latter is the northernmost mountain range in South Africa. As implied by the region's name, the Bushveld's grassy plains are dotted by dense clusters of trees and tall shrubs. The grasses found here are generally tall and turn brown or pale in winter (May to August), which

2072-558: The extraction of silica from seawater prior to the Jurassic. Spicularite is chert composed of spicules of glass sponges and other invertebrates. When densely cemented, it is known as spicular chert . They are found in association with glauconite -rich sandstone , black shale , clay -rich limestone , phosphorites , and other nonvolcanic rocks typical of water a few hundred meters deep. Some bedded cherts appear devoid of fossils even under close microscopic examination. Their origin

2128-419: The flake's bulb of force. When a chert stone is struck against an iron-bearing surface, sparks result. This makes chert an excellent tool for starting fires, and both flint and common chert were used in various types of fire-starting tools, such as tinderboxes , throughout history. A primary historic use of common chert and flint was for flintlock firearms , in which the chert striking a metal plate produces

2184-490: The high purity of other chemically precipitated rock, points to deposition in areas where there is little influx of detrital sediments (such as river water laden with silt and clay particles.) Such impurities as are present include authigenic pyrite and hematite, formed in the sediments after they were deposited, in addition to traces of detrital minerals. Seawater typically contains between 0.01 and 11 parts per million (ppm) of silica, with around 1 ppm being typical. This

2240-484: The kinds of organisms that produced the silica skeletons. Diatomaceous chert consists of beds and lenses of diatomite which were converted during diagenesis into dense, hard chert. Beds of marine diatomaceous chert comprising strata several hundred meters thick have been reported from sedimentary sequences such as the Miocene Monterey Formation of California and occur in rocks as old as

2296-606: The modern ocean. The high degree of silica saturation was due either to intense volcanic activity or to the lack of modern organisms that remove silica from seawater. Nodular chert is most common in limestone but may also be found in shales and sandstones. It is less common in dolomite . Nodular chert in carbonate rocks is found as oval to irregular nodules . These vary in size from powdery quartz particles to nodules several meters in size. The nodules are most typically along bedding planes or stylolite (dissolution) surfaces, where fossil organisms tended to accumulate and provided

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2352-579: The most authoritative study of the mountain range, said that this announcement was the culmination of a campaign that began in 2006. "I'm most grateful there is now international recognition of this great mountain range that has witnessed the whole span of life, from its very origins," he said. "The Magaliesberg is almost 100 times older than Mount Everest and half the age of the earth , a unique treasure for us in this part of Africa." The Magaliesberg mountains offer excellent rock climbing opportunities, spread across its many kloofs and escarpments. The climbing

2408-527: The mountain, but almost never on the northern slopes. The area around the Magaliesberg range has seen extremely lengthy occupation by humans dating back at least 2 million years to the earliest hominin species (such as Mrs Ples ) in and around the Sterkfontein Caves, which lie at the Cradle of Humankind World Heritage Site , close to the town of Magaliesburg . The earliest known name for

2464-424: The ocean bottom and are buried, forming siliceous ooze that is 30% to 60% silica. Thus, bedded cherts are typically composed mostly of fossil remains of organisms that secrete silica skeletons, which are usually altered by solution and recrystallization. The skeletons of these organisms are composed of opal-A, an amorphous form of silica, lacking long-range crystal structure. This is gradually transformed to opal-CT,

2520-535: The presence of fossil cyanobacteria . In addition to microfossils , chert occasionally contains macrofossils . However, some chert is devoid of any fossils. Chert varies greatly in color, from white to black, but is most often found as gray, brown, grayish brown and light green to rusty red and occasionally as dark green. Its color is an expression of trace elements present in the rock. Both red and green are most often related to traces of iron in its oxidized and reduced forms, respectively. In petrology ,

2576-491: The purpose of forming hunting parties to gather ivory and other products from further north. Subsequently the Boers began settling in the valleys of the Magaliesberg Range and in the Bushveld north of the Magaliesberg, and turned the region into some of the most productive farmland in South Africa, while displacing their former allies, confining them to locations and reserves. The area saw some heavy fighting during

2632-629: The re-conquest of the region was a cooperative venture of the Boers and the Sotho-Tswana against the Matabele, the Boers and Sotho-Tswana had friendly relations at the beginning of white settlement . These friendly relations are reflected in the name of the main Boer town, Rustenburg ( lit . resting town), because it seemed to them that they would not have to engage in any more fighting against African communities. The Boers initially settled south of

2688-545: The region include: 24°S 28°E  /  24°S 28°E  / -24; 28 Magaliesberg The Magaliesberg (historically also known as Macalisberg or Cashan Mountains ) of northern South Africa , is a modest but well-defined mountain range composed mainly of quartzites. It rises at a point south of the Pilanesberg (and the Pilanesberg National Park ) to form

2744-424: The region tends to be dry, the Bushveld is mostly beef cattle and game farming country, with only a few drought-resistant crops such as sorghum and millet being farmed, usually under irrigation . The term Middleveld is sometimes used to describe land lying between an elevation of 600 and 1,200 m (2,000 and 3,900 ft) and has been treated as synonymous with the term Bushveld . Towns and cities in

2800-411: The same time as the nodule. Calcareous fossils are occasionally present that have been completely silicified. Where chert occurs in chalk or marl , it is usually called flint . Nodular chert is often dark in color. It can have a white weathering rind that is known in archaeology as cortex . Most chert nodules have textures suggesting they were formed by diagenetic replacement, where silica

2856-468: The term "chert" refers generally to all chemically precipitated sedimentary rocks composed primarily of microcrystalline , cryptocrystalline and microfibrous silica . Most cherts are nearly pure silica, with less than 5% other minerals (mostly calcite , dolomite , clay minerals , hematite , and organic matter.) However, cherts range from very pure cherts with over 99% silica content to impure nodular cherts with less than 65% silica content. Aluminium

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2912-679: The unique nature of the Magaliesberg and the powerful contribution it is making to our country, to the ecosystem services in Gauteng and the North West and the communities it nurtures over an extensive area," said Paul Fatti, chair of the Magaliesberg Biosphere Initiative Group (MBIG) that has been lobbying authorities since 2007 to support the establishment of the biosphere. Vincent Carruthers, past chair of MBIG and renowned author of The Magaliesberg ,

2968-408: The world's oceans. Diatoms can double their numbers eight times a day under ideal conditions (though doubling once per day is more typical in normal seawater) and can extract silica from water with as little as 0.1 ppm silica. The organisms protect their skeletons from dissolution by "armoring" them with metal ions. Once the organisms die, their skeletons will quickly dissolve unless they accumulate on

3024-578: Was deposited in place of calcium carbonate or clay minerals . This may have taken place where meteoric water (water derived from snow or rain) mixed with saltwater in the sediment beds, where carbon dioxide was trapped, producing an environment supersaturated with silica and undersaturated with calcium carbonate. Nodular chert is particularly common in continental shelf environments. In the Permian Basin (North America) , chert nodules and chertified fossils are abundant in basin limestones, but there

3080-446: Was of great importance to both the Boer and the British forces, especially the two routes between Pretoria and Rustenburg, which crossed it at Silkaatsnek and Kommandonek, respectively. As a result, many battles, such as the battles of Buffelspoort, Nooitgedacht and Olifantsnek were fought in the area. After the war, farms in the area were reoccupied and farming was resumed, tobacco and citrus being particularly successful. In 1923

3136-409: Was subsequently eroded so that it only remains along the Transvaal Basin's southern rim. The range forms a natural barrier between the lower lying Bushveld to the north and the cooler Highveld to the south. The range receives rainfall in summer in the form of thunderstorms, with an average of 650 mm (26 in) annually. In winter, frost occurs frequently in the valleys on the southern side of

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