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But the Lochiel Granite is not the oldest rock in the reserve. The remaining three-quarters of Malolotja, including the whole of the Malolotja Valley, Ngwenya, and the high ground to the west, are rocks of the Swaziland Supergroup, amongst the oldest metamorphosed sedimentary rocks in the world. These rocks, which once included sandstones and mudstones as well as surface volcanics were laid down horizontally, layer by layer, in the deep ocean of the primaeval earth about 3500 million years ago. As such, they are thought to contain fossils of the earth's oldest life forms - blue-green algae. These are found in the green cherts exposed in the Nkomati gorges. After they were laid down as a series of horizontal strata, the whole group was drawn into the earth's crust and metamorphosed, cooked by the relatively low heat and pressure of the mantle, where they were metamorphosed into highly resistant quartzites and shales, gneissitic rocks and talc schists. Further, these transformed rocks were folded and cracked, faulted and tilted until now they have reappeared at the surface as a series of almost vertical but dislocated bands.
The Swaziland Supergroup is divided into three subgroups, the oldest of which is the Onverwacht Group. This is found in a broad hemicircle of the upper Malolotja Valley, Forbes Reef and the upper Mbuluzi Valley. The main components of the Group seen locally are talc-chlorite and talc-carbonate schists, which have been weakly metamorphosed from original lavas. The schists are extremely soft and they have a pearly appearance and noticeable "soapy" feel, hence the common name "soapstone". The locally made carvings and heads are made from this soft rock, although the finished carvings are often stained to alter the basic greenish white colour. Another similar kind of rock which is also found in the Onverwacht Group is serpentinite which tends to be more fibrous. The best known example of this is the fibrous chrysotile asbestos mined at Havelock near Piggs Peak since the 1930's, and Msauli Mine in South Africa. Waste from this mine is carried down the Nkomati River into the park and deposited as sands, distinctly green in colour. The Onverwacht rocks also contain minute quantities of gold, which has attracted prospectors to the Malolotja area for the best part of a hundred years. The gold is present in such small amounts that mining operations were never long lived, but long enough in one case to see the development of the village of Forbes Reef at the beginning of this century. Barite also occurs in the Onverwacht Group just to the west of Ngwenya in the Londozi Valley. It was discovered in 1937 and mining continued from 1945 to 1976 from small underground workings and shallow quarries.
The second set of rocks in the Swaziland Supergroup are the Fig Tree Group, a layer cake of metamorphosed rocks which include interbedded shales, quartzites, cherts and ironstones. These occur especially at the southern end of the reserve, south of the summit of Ngwenya and again in the north west of the reserve, either side of the Nkomati River. The quartzites were formed by the metamorphosis of original quartz sandstones which welded the granules together to make a rock which is extremely resistant to weathering or erosion. In the Fig Tree Group, the quartzites are usually a bluish black, grainy rock with a hard, glassy appearance. They alternate every few centimetres or metres with shales and ironstones, giving a hard "liquorice allsort" appearance. The contorted nature of these sedimentary beds has meant that the once horizontal rocks are now standing on end like books on a shelf.
At the southern end of the reserve, the rocks have been doubled over in a vertical cross-fold which has led to an enrichment of the haematite-chert ironstones and produced near the surface a pocket of very rich iron ore which has been mined at Bomvu Ridge, Ngwenya. The ore body is divided into 7 ore reserve blocks, each separated by a dike, a fault or by the major axis of the fold deformation. There are important differences in grade and chemical composition between the blocks as well as in thickness, the thickest being the Lion Block in the north. The origin of the iron deposits is uncertain. Goodwin (1964) infers that while the volcanics below were flat lying, fluids migrated up from these which caused chemical transport to the rocks above and the deposition of the carbonate iron formation. The origin of the fluids is uncertain.
The third and youngest set of rocks, still older than the adjacent granites, in the Swaziland Supergroup are yet more layers of ocean floor sediments, metamorphosed and twisted into an upright position. They form the largest area of the reserve and cover a broad zig-zag south from the Nkomati, west across the lower Malolotja Valley, to the summit of Ngwenya. Being the youngest of all the Supergroup, the Moodies Group was formed from the erosional debris of other rocks which now form a quartzite conglomerate. Now metamorphosed, they are blue-black in colour with a gravelly appearance.
Geomorphology
The dramatic nature of the scenery at Malolotja is a product of the underlying rocks and the different types of erosion which have acted upon the different rock types. The Lochiel Granite, near the eastern border of the reserve at the entrance has eroded in some cases into huge domes of rock and others into boulder heaps, or tors, scattered on the bare upland surface. Over the many millions of years since it was formed the vast mass of granite has become stressed and fractured, allowing moisture to penetrate into the rock and to rot its susceptible minerals through hydration and oxidation. Now it has finally appeared at the surface, streams and rivers are etching away the rotted weathered material which is being carried away to become the clays and sands of the river valleys, leaving the resistant "core" of the granite behind. The resistant edges of the rocks have formed massive convex domes which can be seen to the east of Nkomati Viewpoint. However, the incidence of gulleys along the east slope of Mbulukudvu Valley i.e. the edge of the granite may be the result of weathering along the baked margin of the granite. Around the entrance and picnic and cabin sites, boulders lie in heaps. These are the tors and represent the resistant core boulders lowered into their present position as the land surface has been eroded. They now form fire-resistant enclaves where trees can grow. These areas are in dolerite/gabbro dikes which in most cases is more resistant to erosion than the granite, an exception being the upper Majolomba valley below the road crossing where the stream follows the dike.
Turning to the Swaziland Supergroup rocks, the different hardness and softness of these rocks are the cause of the broken, mountainous terrain and deep, sheltered ravines which characterize the reserve. The chemically and physically weaker shales and schists are often deeply rotted and erode easily, whereas some of the quartzites are chemically extremely hard and resistant. The soapstones form gentle slopes and in comparison to the quartzites, are rarely exposed. Good exposures can be seen at the soapstone quarry above Malolotja Vlei. Because the two sorts of rock are often inter-bedded and are now sitting vertically out of the ground, the softer rocks are etched away into the deep valleys leaving the harder rocks like long walls in the landscape. These rock walls can be seen especially between the Malolotja River and Silotwane and again on the west side of Ngwenya. Usually they look like large ribs, running from north to south. Occasionally, the rivers have broken through one of these resistant ridges, creating pools and waterfalls, for example at the Mahulungwane Falls, Majolomba Gorge and Forbes Reef gorge.
As you stand on the high ground in the eastern part of the reserve, and look towards the west, ridge upon ridge of mountains can be seen. They appear now as separate, jagged, upstanding peaks. Ignoring the valleys in between they seem to form a flat, high plane, dipping towards the east. This phenomenon of accordant summits is all that is left of a very ancient, very widespread featureless flat plain. This ancient plain existed here some twenty five million years ago and is known as the "African Surface". Remnants can all be reconnected by an imaginary flat surface today. Even in Swaziland, you can see other remnants of the Surface in the north east of the country within Mlawula Nature Reserve on top of the Lubombo Mountains. The flat top of the Lubombo, if projected westwards, links with these remnants at Malolotja, to which it was once joined.
Beneath the African Surface, weathering was constantly taking place, deeply in the chemically weak rocks, but hardly at all in the resistant quartzites. During the last twenty five million years, enormous pressure in the earth's crust has pushed the African continent upwards, rejuvenating all the rivers and bringing about new cycles of erosion. The drainage has washed and scoured out the pre-weathered, rotten rock particles, leaving the ranges of the harder rocks as a skeleton, the remnants of the ancient surface of Africa. Evidence of rejuvenation is seen at Forbes Reef where the Malolotja River has captured the headwaters of the Mbuluzi River. The Malolotja generally flows south west from the entrance area of the park, makes a westward turn at Forbes Reef, the area of capture, and northwards below the gorge here to the Nkomati.
Soils
The only information currently available with regard to soil classification is the 1:250 000 soils map for Swaziland, with its associated information (Murdoch, 1970). Most of the soils in the reserve are classified as "rock outcrops and stony ground: raw mineral soil", and are thin, leached and not suited to agriculture. The area is very susceptible to soil erosion, and old roads and tracks have resulted in the formation of large gullies in some areas.
Hydrology
The reserve has innumerable perennial streams and rivers. In addition there are several upland vleis that retain water throughout the year, the most important of which is the Malolotja vlei. The reserve includes virtually the entire catchment area for the Malolotja, Mgwayiza and Mhlangamphepha rivers, and the Nkomati river cuts through the reserve, running from west to east. Prior to the establishment of the reserve a number of dams, weirs and barrages were built on the rivers. No boreholes have been sunk anywhere in the reserve and no artificial watering points for animals have been made, although various artificial impoundments within or on the boundary of the reserve are undoubtedly used by animals. Water quality is generally good and requires little or no treatment, apart from the Nkomati and Mkomazane Rivers. There is also some pollution of the Malolotja river from the portion of its catchment which lies outside the reserve boundary.
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