Sugarloaf Mountain as seen from Corcovado (crop)

A bornhardt () is a dome-shaped, steep-sided, bald rock

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial ...

ping at least in height and several hundred metres in width. They are named after Wilhelm Bornhardt (1864–1946), a German geologist and explorer of

German East Africa German East Africa (GEA; german: Deutsch-Ostafrika) was a German colony in the African Great Lakes region, which included present-day Burundi, Rwanda, the Tanzania mainland, and the Kionga Triangle, a small region later incorporated into Mozam ...

, who first described the feature. While ''bornhardt'' was originally used to sometimes denote a type of

inselberg An inselberg or monadnock () is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa a similar formation of granite is known as a koppie, a ...

(literally island mountain—an isolated dome in an otherwise flat landscape), the term ''bornhardt'' is used in modern literature to refer to domed hills and mountains regardless of isolation; thus, not all bornhardts are inselbergs and not all inselbergs are bornhardts. Bornhardts are commonly composed of

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...

s, often

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

s, but examples of

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tect ...

and arkose bornhardts exists. The

Sugarloaf Mountain Sugarloaf Mountain ( pt, Pão de Açúcar, ) is a peak situated in Rio de Janeiro, Brazil, at the mouth of Guanabara Bay on a peninsula that juts out into the Atlantic Ocean. Rising above the harbor, the peak is named for its resemblance to ...

Rio de Janeiro Rio de Janeiro ( , , ; literally 'River of January'), or simply Rio, is the capital of the state of the same name, Brazil's third-most populous state, and the second-most populous city in Brazil, after São Paulo. Listed by the GaWC as a b ...

is a typical example of this landform and is the origin of the common bornhardt nickname "sugar loaf". Bornhardts are most easily seen in arid and semi-arid regions, but occur over a wide range of climates. Found in diverse topographic settings, they mainly occur in multicyclic landscapes. Over

geological time The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochron ...

, bornhardts may degrade to form other landforms such as

nubbins This is a list of characters that appear in '' The Texas Chainsaw Massacre'' horror film franchise. Cast overview Continuity A (1974–1995) Platinum Dunes continuity (2003–2006) Continuity B (1974, 2013–2017) Killer families T ...

(or

knoll In geography, knoll is another term for a knowe or hillock, a small, low, round natural hill or mound. Knoll may also refer to: Places * Knoll Camp, site of an Iron Age hill fort Hampshire, England, United Kingdom * Knoll Lake, Leonard Canyon, A ...

s) and castle koppies. Bornhardts were previously thought of as characteristically tropical landforms, but have been shown to be more related to

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...

and rock structure than climate. Depending on their shape, they are also referred to as sugar loaves, whalebacks, turtlebacks or domes. In certain locales they may be known as '' dwalas'', half-oranges, ''

matopos The Matobo National Park forms the core of the Matobo or Matopos Hills, an area of granite kopjes and wooded valleys commencing some south of Bulawayo, southern Zimbabwe. The hills were formed over 2 billion years ago with granite being forced ...

'', etc. Often, the underlying geological fracture pattern is shown by the surface arrangement of bornhardts, as can be seen in the

Kamiesberge The Kamiesberg or Kamiesberge (Khoikhoi ''"Th'amies"'' = ''"jumble"''), is a mountain range of jumbled granite inselbergs or bornhardts dotted over sandy plains and centered on Kamieskroon in Namaqualand in South Africa. This range is very like th ...

Namaqualand Namaqualand (khoekhoe: "Nama-kwa" meaning Nama Khoe people's land) is an arid region of Namibia and South Africa, extending along the west coast over and covering a total area of . It is divided by the lower course of the Orange River into ...

and the

Everard Ranges The Everard Ranges, officially known as The Everard Ranges, is a range of low rounded granite hills located in the Australian state of South Australia in the Aṉangu Pitjantjatjara Yankunytjatjara lands about west of Mintabie. It is of Pa ...

Central Australia Central Australia, also sometimes referred to as the Red Centre, is an inexactly defined region associated with the geographic centre of Australia. In its narrowest sense it describes a region that is limited to the town of Alice Springs and i ...

. Bornhardts commonly show extensive sheet jointing (fracture systems in the rock parallel to its surface). There are two main theories on the origins of bornhardts, each relating to jointing. The ''exogenetic'' theory asserts that sheet jointing is formed in response to the emerging topography, while the ''endogenetic'' theory suggests that the topography in bornhardts reflects already existing sheet jointing.

See also

References

Citations

Sources

External links