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The Fish River Canyon of southern Namibia is the second largest canyon in the World after the Grand Canyon of the USA. It consists of a northern upper and a southern lower canyon. From the first waterfall north of the northernmost viewpoint, to a point opposite the Chudaub trigonometrical beacon, the canyon is 56 km long, measured along the river course. The lower canyon is between 460 and 550 m deep and 5 km wide, whereas the upper canyon is only 160 to 190 m deep, but 8 km wide. A proto-Fish River Canyon 300 m above the present day level of the river and floored by sandstone and black limestone of the Kuibis Subgroup of the Nama Group formed as early as 300 million years ago, since at the end of the Dwyka glaciation sediments of the Dwyka Formation were deposited here. Erosion was directed mainly laterally, and the peneplain of the Huns Plateau formed. Major continental uplift, following the break-up of Gondwanaland some 130 million years ago, resulted in the deep incision of the river to its present day level. Judging from the numerous meanders; the river must have flowed very slowly on a very even surface before its dramatic incision. At first, it cut through the horizontal layers of the Kuibis Subgroup rocks, but later reached the underlying gneisses, amphibolites and migmatites of the Namaqualand Complex, which was metamorphosed some 1200 million years ago. These metamorphic rocks are cut by dark coloured, linear diabase dykes, which are about 770 million years old. There is a very obvious contact between the Nama sediments and the massive Namaqualand Complex rocks, which show a very different erosional pattern and display a typical metamorphic texture. The upper canyon is a tectonic trough bordered to the west by a sharp monoclinal fold, and on the eastern side by a linear reverse fault. The lower canyon also exposes a number of steeply inclined bounding faults, some of which are well visible from the main lookout point. These faults also affect Karoo Sequence rocks, and must therefore be younger than 300 million years. While the canyon without doubt formed in a rift valley, some of the faults actually indicate compressional tectonics. Groundwater is using these faults to reach the surface in the form of hot springs, of which Ai-Ais with 60°C is the most famous. The so-called sulphur spring upstream is also well-known and has a temperature of 56°C.