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107-888: The Cape Fold Belt is a fold and thrust belt of late Paleozoic age, which affected the sequence of sedimentary rock layers of the Cape Supergroup in the southwestern corner of South Africa . It was originally continuous with the Ventana Mountains near Bahía Blanca in Argentina , the Pensacola Mountains (East Antarctica), the Ellsworth Mountains (West Antarctica) and the Hunter-Bowen orogeny in eastern Australia. The rocks involved are generally sandstones and shales , with

214-556: A salient is formed. Between the bulges the areas are known as recesses , reentrants or sometimes embayments . Much of this table is adapted from Nemcok et al. , 2005 This tectonics article is a stub . You can help Misplaced Pages by expanding it . Weathering Weathering is the deterioration of rocks , soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases , sunlight , and biological organisms. It occurs in situ (on-site, with little or no movement), and so

321-588: A dissected limestone pavement . This process is most effective along the joints, widening and deepening them. In unpolluted environments, the pH of rainwater due to dissolved carbon dioxide is around 5.6. Acid rain occurs when gases such as sulfur dioxide and nitrogen oxides are present in the atmosphere. These oxides react in the rain water to produce stronger acids and can lower the pH to 4.5 or even 3.0. Sulfur dioxide , SO 2 , comes from volcanic eruptions or from fossil fuels, and can become sulfuric acid within rainwater, which can cause solution weathering to

428-640: A major earthquake , originating from movement of the northern end of the Worcester Fault. Another major (inactive) fault runs for 300 km along the southern edge of the Swartberg Mountains. The Swartberg Mountains were uplifted along this fault, to such an extent that in the Oudtshoorn region the rocks that form the base of the Cape Supergroup are exposed. These are locally known as the "Cango Group", but are probably continuous with

535-526: A maximum thickness of 2000 m, was laid down. These sandstones are very hard, and erosion resistant. They therefore form the bulk of mountains and steep cliffs and rugged crags of the Cape Fold Belt, including the upper 600 m of the 1 km high Table Mountain , below which Cape Town is situated. It contains no fossils. The Peninsula formation can be traced from 300 km north of Cape Town (i.e. about 50 km north of Vanrhynsdorp on

642-421: A maximum width of about 50 km (mostly only 15–30 km wide). Almost all of these ranges consist of hard erosion resistant Peninsula Group rocks. The valleys tend to be floored by Bokkeveld Group mudstones. A remarkable feature of these ranges is that the 1500 m high mountains (from base to crest) are cut through by very narrow, almost vertically walled defiles , not much more than 50–70 m wide at

749-691: A mineral crystal exposes ions whose electrical charge attracts water molecules. Some of these molecules break into H+ that bonds to exposed anions (usually oxygen) and OH- that bonds to exposed cations. This further disrupts the surface, making it susceptible to various hydrolysis reactions. Additional protons replace cations exposed on the surface, freeing the cations as solutes. As cations are removed, silicon-oxygen and silicon-aluminium bonds become more susceptible to hydrolysis, freeing silicic acid and aluminium hydroxides to be leached away or to form clay minerals. Laboratory experiments show that weathering of feldspar crystals begins at dislocations or other defects on

856-418: A more humid chemical microenvironment. The attachment of these organisms to the rock surface enhances physical as well as chemical breakdown of the surface microlayer of the rock. Lichens have been observed to pry mineral grains loose from bare shale with their hyphae (rootlike attachment structures), a process described as plucking , and to pull the fragments into their body, where the fragments then undergo

963-641: A mountain range of immense proportions to the south of the former rift valley. The folded Cape Supergroup formed the northern foothills of this towering mountain range. The weight of the Falkland-Cape Supergroup mountains caused the continental crust of Southern Africa to sag, forming a retroarc foreland system, into which the Karoo Supergroup was deposited. Eventually much of the Cape Supergroup became buried under these Karoo deposits, only to re-emerge as mountains when upliftment of

1070-403: A process of chemical weathering not unlike digestion. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical pressure as well as providing a pathway for water and chemical infiltration. Most rock forms at elevated temperature and pressure, and the minerals making up the rock are often chemically unstable in the relatively cool, wet, and oxidizing conditions typical of

1177-477: A result, thermal stress weathering is sometimes called insolation weathering , but this is misleading. Thermal stress weathering can be caused by any large change of temperature, and not just intense solar heating. It is likely as important in cold climates as in hot, arid climates. Wildfires can also be a significant cause of rapid thermal stress weathering. The importance of thermal stress weathering has long been discounted by geologists, based on experiments in

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1284-553: A road cutting between Durban and Pietermaritzburg , particularly at the Marian Hill Toll Plaza. Most of the group has been deeply eroded by numerous rivers creating the ragged plateaus and scarps of the Durban-Pietermaritzburg region. With the widening and deepening of the rift valley, the entire southern portion of what was to become South Africa, as far as a line extending from Calvinia in

1391-407: A slower reaction kinetics , this process is thermodynamically favored at low temperature, because colder water holds more dissolved carbon dioxide gas (due to the retrograde solubility of gases). Carbonate dissolution is therefore an important feature of glacial weathering. Carbonate dissolution involves the following steps: Carbonate dissolution on the surface of well-jointed limestone produces

1498-513: A thin layer (on average only about 60 m thick) in the Peninsula Formation rocks of the more inland mountains to the west of a line between Swellendam and Calvinia . These diamictite rocks are composed of finely ground mud, containing a jumble of faceted pebbles. They can easily be recognized at a distance as this formation readily erodes into fertile, gently sloping, green swaths in a landscape where this contrasts starkly with

1605-469: A threat to the environment and occupant safety. Design strategies can moderate the impact of environmental effects, such as using of pressure-moderated rain screening, ensuring that the HVAC system is able to effectively control humidity accumulation and selecting concrete mixes with reduced water content to minimize the impact of freeze-thaw cycles. Granitic rock, the most abundant crystalline rock exposed at

1712-406: Is Afrikaans for mountain; its plural is berge .) 33°24′S 22°00′E  /  33.4°S 22°E  / -33.4; 22 Fold and thrust belt Fold and thrust belts are formed of a series of sub-parallel thrust sheets, separated by major thrust faults. As the total shortening increases in a fold and thrust belt, the belt propagates into its foreland. New thrusts develop at

1819-426: Is acid hydrolysis , in which protons (hydrogen ions), which are present in acidic water, attack chemical bonds in mineral crystals. The bonds between different cations and oxygen ions in minerals differ in strength, and the weakest will be attacked first. The result is that minerals in igneous rock weather in roughly the same order in which they were originally formed ( Bowen's Reaction Series ). Relative bond strength

1926-550: Is 14 megapascals (2,000 psi). This is still much greater than the tensile strength of granite, which is about 4 megapascals (580 psi). This makes frost wedging, in which pore water freezes and its volumetric expansion fractures the enclosing rock, appear to be a plausible mechanism for frost weathering. Ice will simply expand out of a straight open fracture before it can generate significant pressure. Thus, frost wedging can only take place in small tortuous fractures. The rock must also be almost completely saturated with water, or

2033-704: Is a crucial part of the rock cycle ; sedimentary rock , the product of weathered rock, covers 66% of the Earth's continents and much of the ocean floor . Physical weathering , also called mechanical weathering or disaggregation , is the class of processes that causes the disintegration of rocks without chemical change. Physical weathering involves the breakdown of rocks into smaller fragments through processes such as expansion and contraction, mainly due to temperature changes. Two types of physical breakdown are freeze-thaw weathering and thermal fracturing. Pressure release can also cause weathering without temperature change. It

2140-403: Is a less well characterized mechanism of physical weathering. It takes place because ice grains always have a surface layer, often just a few molecules thick, that resembles liquid water more than solid ice, even at temperatures well below the freezing point. This premelted liquid layer has unusual properties, including a strong tendency to draw in water by capillary action from warmer parts of

2247-440: Is also important, acting to oxidize many minerals, as is carbon dioxide, whose weathering reactions are described as carbonation . The process of mountain block uplift is important in exposing new rock strata to the atmosphere and moisture, enabling important chemical weathering to occur; significant release occurs of Ca and other ions into surface waters. Dissolution (also called simple solution or congruent dissolution )

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2354-715: Is also known as sheeting . As with thermal weathering, pressure release is most effective in buttressed rock. Here the differential stress directed toward the unbuttressed surface can be as high as 35 megapascals (5,100 psi), easily enough to shatter rock. This mechanism is also responsible for spalling in mines and quarries, and for the formation of joints in rock outcrops. Retreat of an overlying glacier can also lead to exfoliation due to pressure release. This can be enhanced by other physical wearing mechanisms. Salt crystallization (also known as salt weathering , salt wedging or haloclasty ) causes disintegration of rocks when saline solutions seep into cracks and joints in

2461-551: Is attested to by the fact that the 1 km high Table Mountain on the Cape Peninsula is a syncline mountain , meaning that it formed part of the bottom of a valley when the Cape Supergroup was initially folded. The anticline , or highest elevation of the fold between Table Mountain and the Hottentots-Holland Mountains (1.2 to 1.6 km elevation), on the opposite side of the isthmus connecting

2568-612: Is chemically weathered to iron(II) sulfate and gypsum , which then crystallize as salt lenses. Salt crystallization can take place wherever salts are concentrated by evaporation. It is thus most common in arid climates where strong heating causes strong evaporation and along coasts. Salt weathering is likely important in the formation of tafoni , a class of cavernous rock weathering structures. Living organisms may contribute to mechanical weathering, as well as chemical weathering (see § Biological weathering below). Lichens and mosses grow on essentially bare rock surfaces and create

2675-442: Is distinct from erosion , which involves the transport of rocks and minerals by agents such as water , ice , snow , wind , waves and gravity . Weathering processes are either physical or chemical. The former involves the breakdown of rocks and soils through such mechanical effects as heat, water, ice and wind. The latter covers reactions to water, atmospheric gases and biologically produced chemicals with rocks and soils. Water

2782-577: Is in equilibrium with kaolinite. Soil formation requires between 100 and 1,000 years, a brief interval in geologic time. As a result, some formations show numerous paleosol (fossil soil) beds. For example, the Willwood Formation of Wyoming contains over 1,000 paleosol layers in a 770 meters (2,530 ft) section representing 3.5 million years of geologic time. Paleosols have been identified in formations as old as Archean (over 2.5 billion years in age). They are difficult to recognize in

2889-472: Is likely the more important mechanism in nature. Geomorphologists have begun to reemphasize the importance of thermal stress weathering, particularly in cold climates. Pressure release or unloading is a form of physical weathering seen when deeply buried rock is exhumed . Intrusive igneous rocks, such as granite , are formed deep beneath the Earth's surface. They are under tremendous pressure because of

2996-484: Is relatively poor in potassium, the basalt weathers directly to potassium-poor montmorillonite , then to kaolinite . Where leaching is continuous and intense, as in rain forests, the final weathering product is bauxite , the principal ore of aluminium. Where rainfall is intense but seasonal, as in monsoon climates, the final weathering product is iron- and titanium-rich laterite . Conversion of kaolinite to bauxite occurs only with intense leaching, as ordinary river water

3103-641: Is represent only by the Natal Group and a trace of the Peninsula Formation (without the intervening Graafwater Formation). The Witteberg sediments were laid down in what remained of the Agulhas Sea – a shallow, much reduced expanse of water compared to its size during the Bokkeveld period. Immediately following the Witteberg period (about 330 million years ago) much of Gondwana (particularly what

3210-449: Is shown in the following table: This table is only a rough guide to order of weathering. Some minerals, such as illite , are unusually stable, while silica is unusually unstable given the strength of the silicon–oxygen bond . Carbon dioxide that dissolves in water to form carbonic acid is the most important source of protons, but organic acids are also important natural sources of acidity. Acid hydrolysis from dissolved carbon dioxide

3317-421: Is sometimes described as carbonation , and can result in weathering of the primary minerals to secondary carbonate minerals. For example, weathering of forsterite can produce magnesite instead of brucite via the reaction: Carbonic acid is consumed by silicate weathering, resulting in more alkaline solutions because of the bicarbonate . This is an important reaction in controlling the amount of CO 2 in

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3424-427: Is the principal agent behind both kinds, though atmospheric oxygen and carbon dioxide and the activities of biological organisms are also important. Biological chemical weathering is also called biological weathering. The materials left after the rock breaks down combine with organic material to create soil . Many of Earth's landforms and landscapes are the result of weathering, erosion and redeposition. Weathering

3531-417: Is the process in which a mineral dissolves completely without producing any new solid substance. Rainwater easily dissolves soluble minerals, such as halite or gypsum , but can also dissolve highly resistant minerals such as quartz , given sufficient time. Water breaks the bonds between atoms in the crystal: [REDACTED] The overall reaction for dissolution of quartz is The dissolved quartz takes

3638-431: Is usually much less important than chemical weathering, but can be significant in subarctic or alpine environments. Furthermore, chemical and physical weathering often go hand in hand. For example, cracks extended by physical weathering will increase the surface area exposed to chemical action, thus amplifying the rate of disintegration. Frost weathering is the most important form of physical weathering. Next in importance

3745-426: Is wedging by plant roots, which sometimes enter cracks in rocks and pry them apart. The burrowing of worms or other animals may also help disintegrate rock, as can "plucking" by lichens. Frost weathering is the collective name for those forms of physical weathering that are caused by the formation of ice within rock outcrops. It was long believed that the most important of these is frost wedging , which results from

3852-663: The Cape Supergroup , which is more extensive than the Fold Belt. The Supergroup is divided into several distinct Groups. The western and southern extents of the Supergroup have been folded into a series of longitudinal mountain ranges, by the collision of the Falkland Plateau into what would later become South Africa (see diagrams on the left). However, the entire suite in this region slopes downwards towards

3959-721: The Cape Town City Bowl and Table Bay. The Pakhuis Formation is also well exposed on the road along Michell's Pass just below the Tolhuis, and especially on the Pakhuis Pass near Clanwilliam , from which the formation derives its name. The glaciers which formed the Pakhuis and Cederberg formations came from the north west, in the direction of the South Pole which was located in the neighborhood of Cameroon at

4066-494: The Carboniferous and early Permian periods. After closure of the rift valley, and rucking of the Cape Supergroup into a series of parallel folds, running mainly east-west (with a short section running north-south in the west, due to collision with eastward moving Patagonia), the continued subduction of the paleo-Pacific Plate beneath the Falkland Plateau and the resulting collision of the latter with Southern Africa, raised

4173-646: The Cederberg 200 km to the north of the Cape Peninsula , and then along the south coast as far as Port Elizabeth , 650 km to the east (see the two maps one above the other on the right). The rocks were laid down as sediments in a rift valley that developed in southern Gondwana , just south of Southern Africa, during the Cambrian - Ordovician Periods (starting about 510 million years ago, and ending about 330–350 million years ago). (See

4280-584: The Gondwana rifting when South America and the Falkland Plateau separated from Africa. Patagonia was to the west of Cape Town and the Falkland Islands were to the south during the Jurassic Period prior to separation. Most of these faults are inactive today, but on 29 September 1969 the towns of Ceres and Tulbagh , about 160 km north-east of Cape Town, were severely damaged by

4387-593: The Ou Kaapse Weg (road) goes up the slope from Westlake on to the Silvermine plateau. In the cutting one can also see the abrupt and obvious transition into the Peninsula Formation above it. Looking up the slope from below to the first hairpin bend, the granite basement on which the Graafwater formation rests is visible. And in the cutting at the first hairpin bend, the ocher-colored, gritty clay into which

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4494-544: The Tradouws Pass , Kogmanskloof, Garcia's pass, the Gourits River gorge (which does not have a road running through it), and Michell's Pass , which is, however, more V-shaped than the others, but impressive nevertheless. In addition several roads and highways crest the mountains over cols, or go through them via tunnels. A number of parallel faults still run roughly parallel with the coast, having formed during

4601-546: The " Malmesbury Group " that forms the base of Table Mountain on the Cape Peninsula , and similar outcrops in the Western Cape . In the Little Karoo the outcrop is composed of limestone , into which an underground stream has carved the impressively extensive Cango Caves . The following is a list of the largest individual ranges within the Cape Fold Belt include from west to east. (Translations in brackets; berg

4708-493: The Agulhas Sea. The group contains fewer fossils than the Bokkeveld Group, but the assemblage that is preserved includes primitive fish, an extinct species of shark, brachiopods, bivalves , and a meter long sea scorpion . There are also plant fossils and numerous animal tracks. The Witteberg Group is truncated by the overlying Dwyka sediments of glacial origin. The latter forms part of the Karoo Supergroup . Therefore,

4815-500: The Cape Fold Belt, being made up of the local lowest (oldest) strata of the Cape Supergroup, composed predominantly of quartzitic sandstone which forms the impressive, almost vertical cliffs which characterize the mountain and the rest of the range which constitutes the backbone of the Cape Peninsula . The degree to which the original Cape Fold mountains (formed during the Carboniferous and early Permian Periods) have been eroded

4922-600: The Cape Supergroup sediments, but also the southern portions of the Dwyka and Ecca Groups of the Karoo Supergroup. By the time Gondwana broke up about 150 million years ago, the Falkland Mountains had been all but eroded away, before drifting south-westwards to their present position off the coast of southern South America , close to Cape Horn , leaving behind only the submarine Agulhas Bank along

5029-408: The Earth's surface, begins weathering with the destruction of hornblende . Biotite then weathers to vermiculite , and finally oligoclase and microcline are destroyed. All are converted into a mixture of clay minerals and iron oxides. The resulting soil is depleted in calcium, sodium, and ferrous iron compared with the bedrock, and magnesium is reduced by 40% and silicon by 15%. At the same time,

5136-411: The Earth's surface. Chemical weathering takes place when water, oxygen, carbon dioxide, and other chemical substances react with rock to change its composition. These reactions convert some of the original primary minerals in the rock to secondary minerals, remove other substances as solutes, and leave the most stable minerals as a chemically unchanged resistate . In effect, chemical weathering changes

5243-585: The Fold Belt into the Eastern Cape and northern KwaZulu-Natal , where no folding took place. The initial sedimentation into the rift valley which developed in southern Gondwana (see diagram at top left) was confined to the western and eastern ends of the rift. Rivers diverted into these early rifts deposited sand and gravel to form the Klipheuwel Group in the west, and the Natal Group in

5350-521: The Karoo's Ecca Group . During these Ecca times continued collision of the Falkland Plateau into Southern Africa, and subduction of oceanic crust under the plateau caused the formation of a range of mountains of Himalayan proportions to the south of South Africa. The Cape Fold Mountains became fully formed during this period to become the northern foothills of this enormous mountain range. The folding that occurred during this period therefore involved not only

5457-475: The Peninsula Formation sediments, the western portion of the region was covered for a geologically brief period by glaciers. The diamictite sediments that were laid down by these glaciers, or in glacial lakes, are known as the Pakhuis and Cederberg Formations . A small patch of Pakhuis tillite occurs on the top of Table Mountain at Maclear's Beacon , but most of the Pakhuis and Cederberg Formations are found as

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5564-764: The Peninsula to the Mainland, has been eroded away. The Malmesbury shale and granite basement on which this anticline mountain rested also formed an anticline; but being composed of much softer rocks, readily eroded into a 50 km wide flat plain, (now covered with dune sands) called the "Cape Flats". The Cape Fold Belt (i.e. the mountain ranges) extends from about Clanwilliam (approximately 200 km north of Cape Town), to about Port Elizabeth (approximately 650 km east of Cape Town). The Cape Supergroup sediments beyond these points are not folded into mountain ranges, but do, in places, form steep cliffs or gorges, where

5671-593: The West Coast), southwards to Cape Town and then eastwards to northern KwaZulu-Natal , a total distance of approximately 1800 km, roughly along the South African coastline. Only the section between Clanwilliam (approximately 200 km north of Cape Town) and Port Elizabeth (approximately 650 km east of Cape Town) is folded into the Cape Fold Mountains. During the laying down of

5778-473: The Witteberg Group forms the uppermost layer of the Cape Supergroup. It tends to form the most inland outcrops of the Cape Supergroup, and can be traced eastwards as far as the Bokkeveld Group can be traced (i.e. to Port Alfred), about 120 km beyond the Cape Fold Belt. The Bokkeveld and Witteberg groups do not occur in the north-eastern Eastern Cape and KwaZulu-Natal , where the Cape Supergroup

5885-638: The Worcester-Robertson valley in the middle of the Fold Belt. Although the Dwyka and Ecca sediments adjoining the Cape Fold Mountains were subjected to the same compression forces that gave rise to the Cape Mountains, they do not form the same mountain ranges as do the Cape Fold Mountains. This is because they are composed of much softer rocks than the Peninsula Formation Sandstone, and were thus soon eroded into

5992-409: The affected rocks a reddish-brown coloration on the surface which crumbles easily and weakens the rock. Many other metallic ores and minerals oxidize and hydrate to produce colored deposits, as does sulfur during the weathering of sulfide minerals such as chalcopyrites or CuFeS 2 oxidizing to copper hydroxide and iron oxides . Mineral hydration is a form of chemical weathering that involves

6099-466: The atmosphere and can affect climate. Aluminosilicates containing highly soluble cations, such as sodium or potassium ions, will release the cations as dissolved bicarbonates during acid hydrolysis: Within the weathering environment, chemical oxidation of a variety of metals occurs. The most commonly observed is the oxidation of Fe ( iron ) by oxygen and water to form Fe oxides and hydroxides such as goethite , limonite , and hematite . This gives

6206-416: The bare rocky surfaces of the quartzites above and below. In several locations the quartzites below the glacial horizon have been rucked into a series of folds. This is believed to have been caused by the movement of ice ploughing into the underlying unconsolidated sands. A good example of this can be seen on a ridge of rocks near Maclear 's Beacon on Table Mountain, close to the edge of the plateau overlooking

6313-446: The bottom, through which rivers flow from the inland Great Escarpment to the sea. It is from within these narrow defiles, many of which can be traveled by road, that one is afforded a cross sectional view of the mountains, and can appreciate their intense folding and distortion (see the photographs on the top right above). Their origin is as follows: After the fold mountains had been formed they became buried under sediments derived from

6420-819: The carbon dioxide level to 30% of all soil gases, aided by adsorption of CO 2 on clay minerals and the very slow diffusion rate of CO 2 out of the soil. The CO 2 and organic acids help break down aluminium - and iron -containing compounds in the soils beneath them. Roots have a negative electrical charge balanced by protons in the soil next to the roots, and these can be exchanged for essential nutrient cations such as potassium. Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering. Chelating compounds, mostly low molecular weight organic acids, are capable of removing metal ions from bare rock surfaces, with aluminium and silicon being particularly susceptible. The ability to break down bare rock allows lichens to be among

6527-553: The depression that had been the Agulhas Sea, and rucking the Cape Supergroup sediments into folds running roughly parallel to what would ultimately become the south-western and southern coastlines of South Africa. This mountain building continued into the next phase of the Karoo sedimentation, which began about 260 million years ago, after the ice sheets had melted, leaving a large lake (the Karoo Sea) extending over much of South Africa. The resulting marine or lacustrine deposits form

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6634-530: The early 20th century that seemed to show that its effects were unimportant. These experiments have since been criticized as unrealistic, since the rock samples were small, were polished (which reduces nucleation of fractures), and were not buttressed. These small samples were thus able to expand freely in all directions when heated in experimental ovens, which failed to produce the kinds of stress likely in natural settings. The experiments were also more sensitive to thermal shock than thermal fatigue, but thermal fatigue

6741-585: The early Devonian Period) there was further subsidence of the rift valley floor. This brought about the deposition of deeper-water, fine grained sediments of the Bokkeveld Group . This is in marked contrast to the predominantly sandy sediments of the Table Mountain Group. The Bokkeveld Group consists predominantly mudstones. After the Cape Supergroup had been folded into the Cape Fold Mountains, these soft mudstones readily washed away from

6848-500: The east. These formations contain no fossils. Today the Klipheuwel Group is exposed in several small patches near Lamberts Bay, Piketberg, and to the south-west of Paarl Rock . The Natal Group, which is similar to the Table Mountain Group (see below), is found in several elongated patches near the coast of the northern Eastern Cape and KwaZulu-Natal . It forms the impressive cliffs of Oribi Gorge , and can also be seen in

6955-513: The ebb and flow of tidal currents, as well as polygonal sand-filled mud cracks that indicate occasional exposure to desiccation. This layer, known as the Graafwater Formation , reaches a maximum thickness of 400 m, but on the Cape Peninsula it is only 60–70 m thick. No fossils have been found in the Graafwater rocks, but shallow-water animal tracks have been found. A particularly good example of these tracks can be viewed in

7062-428: The eroding African landscape to form the parallel ranges of mountains that run for 800 km along the southern and south-western Cape coastline today. In fact, they form the coastline, either sloping steeply directly into the sea, or are separated from it by a relatively narrow coastal plain. The Falkland Mountain range had probably eroded into relative insignificance by the mid-Jurassic Period, and started drifting to

7169-427: The expansion of pore water when it freezes. A growing body of theoretical and experimental work suggests that ice segregation, whereby supercooled water migrates to lenses of ice forming within the rock, is the more important mechanism. When water freezes, its volume increases by 9.2%. This expansion can theoretically generate pressures greater than 200 megapascals (29,000 psi), though a more realistic upper limit

7276-404: The expense of silica, titanium, aluminum, ferrous iron, and calcium. Buildings made of any stone, brick or concrete are susceptible to the same weathering agents as any exposed rock surface. Also statues , monuments and ornamental stonework can be badly damaged by natural weathering processes. This is accelerated in areas severely affected by acid rain . Accelerated building weathering may be

7383-527: The extinct blastoids and cystoids ), foraminifera and fish with jaws ( placoderms ). The upper layers of the Bokkeveld Group become increasingly more sandy, grading into the sandstone of the Witteberg Group , named for the range of mountains to the south of Matjiesfontein and Laingsburg in the southern Karoo . These rocks were laid down 370 – 330 million years ago in the silted up, and therefore shallow marine conditions of what remained of

7490-488: The first colonizers of dry land. The accumulation of chelating compounds can easily affect surrounding rocks and soils, and may lead to podsolisation of soils. The symbiotic mycorrhizal fungi associated with tree root systems can release inorganic nutrients from minerals such as apatite or biotite and transfer these nutrients to the trees, thus contributing to tree nutrition. It was also recently evidenced that bacterial communities can impact mineral stability leading to

7597-415: The flat plains of the "Lower Karoo" , except where they were protected by hard, erosion resistant dolerite or turbidite caps, to form isolated mountains that stand out from the plain. The mountains are not particularly ancient, despite their old-looking appearance. They are considered middle-aged in geologic terms. They were created when the Falkland Plateau collided with Southern Africa, when Pangaea ,

7704-463: The form of silicic acid . A particularly important form of dissolution is carbonate dissolution, in which atmospheric carbon dioxide enhances solution weathering. Carbonate dissolution affects rocks containing calcium carbonate , such as limestone and chalk . It takes place when rainwater combines with carbon dioxide to form carbonic acid , a weak acid , which dissolves calcium carbonate (limestone) and forms soluble calcium bicarbonate . Despite

7811-611: The foyer of Geology Department of the University of Stellenbosch, where a slab of Graafwater rock from the Cederberg mountains has been built into the wall. The cutting for Chapman's Peak Drive, on the Cape Peninsula, is carved into the Graafwater Formation which overlays the Cape Granite basement rock below the road. The Graafwater Formation can also be clearly seen in the cutting on the second hairpin bend as

7918-415: The front of the belt, folding the older thrusts that have become inactive. This sequential propagation of thrusts into the foreland is the most common. Thrusts that form within the belt rather than at the thrust front are known as "out-of-sequence". In map view, fold and thrust belts are generally sinuous rather than completely linear. Where the thrust front bulges out in the direction of tectonic transport,

8025-669: The geologic record. Indications that a sedimentary bed is a paleosol include a gradational lower boundary and sharp upper boundary, the presence of much clay, poor sorting with few sedimentary structures, rip-up clasts in overlying beds, and desiccation cracks containing material from higher beds. The degree of weathering of soil can be expressed as the chemical index of alteration , defined as 100 Al 2 O 3 /(Al 2 O 3 + CaO + Na 2 O + K 2 O) . This varies from 47 for unweathered upper crust rock to 100 for fully weathered material. Wood can be physically and chemically weathered by hydrolysis and other processes relevant to minerals and

8132-567: The granite weathers is clearly displayed. With further subsidence of the rift valley floor, and possibly breaking through to the ocean, the sediments abruptly become more sandy, indicative of a sudden increase in the depth of the Agulhas Sea (see photograph on the right). A deposit, known as the Peninsula Formation (also often referred to as Table Mountain Sandstone ), consisting of thickly layered quartzitic sandstone , with

8239-439: The ice will simply expand into the air spaces in the unsaturated rock without generating much pressure. These conditions are unusual enough that frost wedging is unlikely to be the dominant process of frost weathering. Frost wedging is most effective where there are daily cycles of melting and freezing of water-saturated rock, so it is unlikely to be significant in the tropics, in polar regions or in arid climates. Ice segregation

8346-551: The massive Himalaya-sized Falkland Mountains to the south of the Cape Fold Belt. Sediments eroded from these mountains provided the bulk of the 6 km thick Beaufort sediments in the Karoo Basin, but they also covered the Cape Fold Belt, thereby protecting them from erosion. At the end of the Karoo Period about 180 million years ago, the subcontinent was covered by a thick layer of Drakensberg lavas , an event that

8453-546: The most effective biological agents of chemical weathering. For example, an experimental study on hornblende granite in New Jersey, US, demonstrated a 3x – 4x increase in weathering rate under lichen covered surfaces compared to recently exposed bare rock surfaces. The most common forms of biological weathering result from the release of chelating compounds (such as certain organic acids and siderophores ) and of carbon dioxide and organic acids by plants. Roots can build up

8560-591: The mountain tops, and only remain in the valleys. Here they form the fertile soils on which the vineyards and fruit orchards of the Western Cape flourish with the help of irrigation from the rivers that have their sources in the surrounding mountains. The Bokkeveld Group does not extend on to the Cape Peninsula or its isthmus (the Cape Flats). Here the Stellenbosch , Franschhoek , Paarl , Durbanville , Tulbagh and Constantia vineyards have been planted on

8667-494: The north and east, so that the oldest rocks are exposed in the south and west, while the youngest members of the Supergroup are exposed in the north, where the entire Cape Supergroup dives beneath the Karoo rocks . Drilling in the Karoo has established that Cape Supergroup rocks are found below the surface up to approximately 150 km north of their northernmost exposure on the surface. The Cape Supergroup extends eastwards beyond

8774-551: The original set of minerals in the rock into a new set of minerals that is in closer equilibrium with surface conditions. True equilibrium is rarely reached, because weathering is a slow process, and leaching carries away solutes produced by weathering reactions before they can accumulate to equilibrium levels. This is particularly true in tropical environments. Water is the principal agent of chemical weathering, converting many primary minerals to clay minerals or hydrated oxides via reactions collectively described as hydrolysis . Oxygen

8881-443: The overlying rock material. When erosion removes the overlying rock material, these intrusive rocks are exposed and the pressure on them is released. The outer parts of the rocks then tend to expand. The expansion sets up stresses which cause fractures parallel to the rock surface to form. Over time, sheets of rock break away from the exposed rocks along the fractures, a process known as exfoliation . Exfoliation due to pressure release

8988-402: The production of weathering agents, such as protons, organic acids and chelating molecules. Weathering of basaltic oceanic crust differs in important respects from weathering in the atmosphere. Weathering is relatively slow, with basalt becoming less dense, at a rate of about 15% per 100 million years. The basalt becomes hydrated, and is enriched in total and ferric iron, magnesium, and sodium at

9095-412: The protective layer over the Cape Fold Belt eroded away, exposing their mountain tops. The rivers breached these ridges, after possibly being dammed back for a short period, creating a narrow passage through the low rocky barrier. Continued erosion exposed more and more of these quartzitic mountain ranges, but the rivers, now confined to narrow, fast flowing gorges, continued breaking through each barrier as

9202-416: The release of inorganic nutrients. A large range of bacterial strains or communities from diverse genera have been reported to be able to colonize mineral surfaces or to weather minerals, and for some of them a plant growth promoting effect has been demonstrated. The demonstrated or hypothesised mechanisms used by bacteria to weather minerals include several oxidoreduction and dissolution reactions as well as

9309-448: The rigid attachment of water molecules or H+ and OH- ions to the atoms and molecules of a mineral. No significant dissolution takes place. For example, iron oxides are converted to iron hydroxides and the hydration of anhydrite forms gypsum . Bulk hydration of minerals is secondary in importance to dissolution, hydrolysis, and oxidation, but hydration of the crystal surface is the crucial first step in hydrolysis. A fresh surface of

9416-455: The rock surface, which gradually pry the rock apart. Thermal stress weathering results from the expansion and contraction of rock due to temperature changes. Thermal stress weathering is most effective when the heated portion of the rock is buttressed by surrounding rock, so that it is free to expand in only one direction. Thermal stress weathering comprises two main types, thermal shock and thermal fatigue . Thermal shock takes place when

9523-413: The rock. This results in growth of the ice grain that puts considerable pressure on the surrounding rock, up to ten times greater than is likely with frost wedging. This mechanism is most effective in rock whose temperature averages just below the freezing point, −4 to −15 °C (25 to 5 °F). Ice segregation results in growth of ice needles and ice lenses within fractures in the rock and parallel to

9630-408: The rocks and evaporate, leaving salt crystals behind. As with ice segregation, the surfaces of the salt grains draw in additional dissolved salts through capillary action, causing the growth of salt lenses that exert high pressure on the surrounding rock. Sodium and magnesium salts are the most effective at producing salt weathering. Salt weathering can also take place when pyrite in sedimentary rock

9737-425: The rocks on which it falls. Hydrolysis (also called incongruent dissolution ) is a form of chemical weathering in which only part of a mineral is taken into solution. The rest of the mineral is transformed into a new solid material, such as a clay mineral . For example, forsterite (magnesium olivine ) is hydrolyzed into solid brucite and dissolved silicic acid: Most hydrolysis during weathering of minerals

9844-574: The shales ( Bokkeveld Group ) persisting in the valley floors while the erosion resistant sandstones (of the Peninsula Formation) form the parallel ranges, the Cape Fold Mountains, which reach a maximum height of 2325 m at Seweweekspoortpiek ('Seven Weeks Defile Peak' in Afrikaans). The Cape Fold Mountains form a series of parallel ranges that run along the south-western and southern coastlines of South Africa for 850 km from

9951-536: The soil is enriched in aluminium and potassium by at least 50%; by titanium, whose abundance triples, and ferric iron, whose abundance increases by an order of magnitude compared with the bedrock. Basaltic rock is more easily weathered than granitic rock due to its formation at higher temperatures and drier conditions. The fine grain size and presence of volcanic glass also hasten weathering. In tropical settings, it rapidly weathers to clay minerals, aluminium hydroxides, and titanium-enriched iron oxides. Because most basalt

10058-422: The south-west soon after Gondwana began to break up 150 million years ago, leaving the Cape Fold Belt to edge the southern portion of the newly formed African continent. Even though the mountains are very old by Andean and Alpine standards, they remain steep and rugged due to their quartzitic sandstone geology (see below) making them very resistant to weathering . The famous Table Mountain forms part of

10165-489: The southern coastline of Africa. The Cape Fold Mountains possibly survived erosional obliteration, firstly because of the extremely hard rocks (the Peninsula Formation Sandstone) that form the backbone of the mountain chains, but also possibly because they had become buried under the Karoo deposits which originated in the Falkland Mountains. Thus traces of Karoo deposits can, for instance, be found in

10272-419: The stresses are so great that the rock cracks immediately, but this is uncommon. More typical is thermal fatigue, in which the stresses are not great enough to cause immediate rock failure, but repeated cycles of stress and release gradually weaken the rock. Thermal stress weathering is an important mechanism in deserts , where there is a large diurnal temperature range, hot in the day and cold at night. As

10379-399: The subcontinent, about 180 million years ago, and again 20 million years ago, started an episode of continuous erosion that was to remove many kilometers of surface deposits from Southern Africa. Although the tops of the original Cape Fold Mountains were eroded away, they eroded much slower than the considerably softer Karoo deposits to the north. Thus the Cape Fold Belt "erupted" from

10486-469: The supercontinent formed during the Cambrian - Ordovician periods (from 510 to about 330–350 million years ago),. Their stature, with heights varying from 1000m to 2300m, is mostly due to the weather-resistant rocks of quartzitic sandstone of the Peninsula Group (see above). They occur in long parallel ranges each not much more than 10 km wide, separated by equally long valleys with

10593-454: The surface of the crystal, and that the weathering layer is only a few atoms thick. Diffusion within the mineral grain does not appear to be significant. Mineral weathering can also be initiated or accelerated by soil microorganisms. Soil organisms make up about 10 mg/cm of typical soils, and laboratory experiments have demonstrated that albite and muscovite weather twice as fast in live versus sterile soil. Lichens on rocks are among

10700-534: The surrounding landscape eroded to lower and lower levels, particularly during the past 20 million years. These 150‑million-year-old rivers therefore cut the defile, starting by flowing over, and then through the gradually erupting Cape Fold Mountains, to form the spectacular "poorte" and "klowe" (plural of "poort" and "kloof", the Afrikaans for defile or chasm) that characterize these mountains today. The best known defiles are: Meiringspoort , Seweweekspoort ,

10807-510: The surrounding sediments have been eroded away (see, for instance, Oribi Gorge in KwaZulu-Natal ). The mountains, although only of moderate height, are majestic and dramatic. This is due in part to numerous geological factors; The ranges usually have few to no foothills and rise directly from valley floors. The bases of the mountains are usually at or near sea level. The mountains of the Cape Fold Belt are composed of rocks belonging to

10914-583: The time. The Upper Peninsula Formation, above the Pakhuis and Cederberg Formations, consists of much softer sandstone than the Lower Peninsula Formation, and is often referred to as the Nardouw Formation . In the Cederberg this formation has been eroded by the wind into a wide variety of "sculptures", caves, and other fascinating structures for which these mountains have become well-known. About 400 million years ago (in

11021-516: The weathered Cape Granite and Malmesbury shale soils, which form the basement rocks on which the Cape Supergroup rocks in this region rest. The Bokkeveld Group extends eastwards to Port Alfred (near Grahamstown ), approximately 120 km beyond the eastern extent of the Cape Fold Belt. The bulk of the fossils found in the Cape Supergroup occur in the Bokkeveld mudstones. They include a variety of brachiopods , as well as trilobites , molluscs , echinoderms (including starfish , crinoids , and

11128-638: The west to East London in the east, plus the KwaZulu-Natal coastline, became flooded, leading to a seaway across the southern and eastern parts of the country, called the Agulhas Sea. The sea floor consisted of the Klipheuwel Group in the west, the Natal Group in the east, and eroded pre-Cambrian rocks in between. The first sediments into the initially still shallow, possibly inland, sea were alternating layers of maroon-colored mudstones and buff-colored sandstones, each mostly between 10 and 30 cm thick. The mudstone units commonly display ripple marks from

11235-445: The yellow block labeled C on the Earth's geological timeline diagram on the right.) An 8-km-thick layer of sediment, known as the Cape Supergroup (see below), accumulated on the floor of this rift valley. Closure of the rift valley, starting 330 million years ago, resulted from the development of a subduction zone along the southern margin of Gondwana , and the consequent drift of the Falkland Plateau back towards Africa, during

11342-402: Was accompanied by upliftment or bulging of Southern Africa, ushering in an almost uninterrupted period, continuing to the present, of erosion removing many kilometers of surface rocks from the entire subcontinent. Rivers running off this bulging interior into the seas that were forming around South Africa as Gondwana was breaking up 150 million years ago, eventually encountered rocky ridges as

11449-489: Was to become Africa and Antarctica, but also parts of South America and India) became covered in a kilometers thick layer of ice as the super-continent drifted over the South Pole. The diamictite deposits left by these glaciers form the first layer of the Karoo Supergroup , called the Dwyka Group . During the glaciation period the Falkland Plateau started to move northwards into what was to become southern Africa, closing

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