♦Basal surface: A deep sitted and unweathered rock usually of granitic type. ♦Crest: A peak or summit of a hill.
Etchplanation or stripping of regolith is believed to be responsible for small to medium granitic protrusion called inselbergs. These inselbergs include bornhardts, whalebacks, tors, nubbins and castle kopjes. Usually these inselbergs rise abruptly from a vast extensive land called a pediplain or found near each other along hill or slope crests. Some of these inselbergs such as dwalas emerge as unweathered basal surfaces. Deep weathering can act along joints underground which creates inselbergs such as tors. They are then exposed to the surface by stripping of regolith (etching). Granitic inselbergs are found in humid and sub-humid tropics, a climate most native to Africa. Stripping of regolith (etching) after regolith has been produced by deep weathering.
Bornhardts are one of the commonest granitic inselbergs. Bornhardts are dome shaped inselbergs rising abruptly from a pediplain which are formed from stripping of regolith or scarp retreat (the latter is rare). When these landforms are exhumed from deep underground, pressure release acts on them creating cracks causing them to be prone to further weathering by other weathering agents. The continued sub-aerial weathering further breaks down the rocks into other distinct landforms such as castle kopjes and or tors. Bornhardts are abundant across the plains of Africa e.g in Zimbabwe, Tanzania and Kenya.
Whalebacks/ Dwala/ Ruware
Whalebacks are gently sloping granitic inselbergs emerging as unweathered basal surfaces. They are usually emerging bornhardts. Dwalas can be seen protruding in areas such as the Matopos in Zimbabwe.
Tors are small to medium blocks or boulders of rock piling on top of another resulting from etchplanation. Tors can also be formed from sub-aerial weathering of bornhardts. Tors can form intricate balancing structures called balancing rocks e.g Balancing Rocks in Epworth, Harare, Zimbabwe.
Castle kopjes have a castle like appearance resulting from sub arial bornhardt weathering. Castle kopjes can be created from tors. NB: castle kopjes can be classified as tors.
Nubbins are small to medium rounded corestones that emerge on top of a small dome (basal surface). Deep weathering acts along joints which creates rounded corestones. As the dome gets weathered on the surface it gradually gets gentler and the corestones spread apart.