(Geomorphology)
A bornhardt is a type of inselberg, that is, a remnant of a previous landscape near the end of an erosional cycle in an arid region. An inselberg gets this special name if it is dome-shaped. They are probably named after German geologist Bergassor Bornhardt, who made a study of the geology of southern Africa.
Unlike inselbergs which develop in faulted sedimentary rocks, bornhardts are granite or gneiss domes which are particularly resistant to weathering. Bornhardts often form below ground, as chemical weathering and freeze/thaw weathering work their way into joints in granite covered by regolith. Sometimes, there is a particularly large resistant block, perhaps a particlularly intact unit of a batholith. The other blocks weather more quickly because the joints are closer together. Eventually, the regolith erodes to the point where the bornhardt is exposed. Some bornhardts will have a rocky pediment around the base, while others will stick straight up from the regolith. If the block weathers into smaller blocks before it is exposed, a pile of hill-sized boulders called a tor will appear instead.
Once exposed, the bornhardt is exposed to further weathering processes: water and wind erosion, exfoliation, further jointing assisted by salt crystal growth and frost cycles. Around the base of the bornhardt will be a talus slope of its own debris. Eventually the bornhardt will be reduced to a small isolated rocky hill (remember Tremors?) called a 'koppie' or 'kopje'.
Examples of bornhardts include
Stone Mountain in
Georgia,
Looking Glass Rock in
North Carolina,
Castle Rocks and the
City of Rocks in southwestern
Idaho,
Spitzkoppe in
Namibia,
Domboshawa in
Zimbabwe, and
Poondana/
Brazil Rock in
South Australia (as well as many others in south
Western Australia). By stretching the definition, we can include
Uluru as a bornhardt, although it's made of
conglomerate.
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Diagram inspired by Chorley, et al, Geomorphology, 1984 figure 18.2, page 36, inspired in turn from M. F. Thomas, Tropical Geomorphology, 1974.