Tropical landforms
565
has been completely stripped and the Archoli surface developed upon fresh rock, with
numerous inselbergs along its rim. On the African surface, the most extensive one, a deep
saprolite is eroded and several inselbergs outcrop, indicating a very irregular basal
weathering surface. Several Miocene volcanoes are located upon some points of this
surface. In the opinion of Oilier, this last surface was formed, at least, in the Lower
Tertiary period while the Buganda Surface must be of a Mesozoic age or older.
5. Inselbergs
This is a German word that means island mountain and was first used by Bornhardt (1900)
to describe abrupt rocky hills rearing from the tropical plains of Tanganyika (today
Tanzania). They basically develop in cratonic tropical and subtropical regions, especially
on granites and metamorphic rocks. Nevertheless, they can also occur on sedimentary
rocks like the arkoses of the famous Ayers Rock (Uluru) monolith or the Olgas
conglomerates, both in the central desert of Australia (Twidale, 1978, 1982a; Vidal
Romanf and Twidale, 1998). The study of these forms is complex because it comprises of
factors like climate, lithology and rock structure, which at the same time determine the
degree of rock weathering. Similarly, they raise basic genetic questions, such as those
inherent to the exhumation of the basal weathering surface or to models of slope retreat.
Research on inselbergs must also include the effects of climatic changes and the
persistence or duration of these forms. As a consequence, many authors have focused their
studies on diverse aspects of inselbergs.
Inselbergs have raised numerous problems related to different definitions and
terminology. Twidale (1968a,b) defined inselbergs as isolated residual hills that stand
out in the tropical plains. Later, Twidale (1982a) and Vidal Romanf and Twidale (1998)
considered that inselbergs can occupy any topographic position. When studying the
morros and sugar loafs of the Rio de Janeiro zone, Birot (1958) already indicated that
crystalline domes are located in variable positions on the landscape. Thomas (1974, 1978)
agreed with the idea of not necessarily relating inselberg and plain. If this opinion is agreed
with, the initial definition of inselberg becomes rather distorted. Twidale (1968) also
indicated that inselbergs are located on tropical plains, although the existence of inselbergs
on the European temperate climates was known long ago (Penck, 1924). Therefore, either
inselbergs form under different climates, or they are paleoclimatic indicators. In order to
accurately define the situation of inselbergs in the world, Kesel (1973) analysed existing
publications and calculated the percentages of inselberg distribution under different
climates: 40% in savanna climates; 32% in arid and semiarid zones; 12% in humid
continental and subarctic climates and 6% for each of the humid tropics, subtropics and
Mediterranean zones. This author concluded, as did King (1957a), that inselbergs can
occur under nearly all climates. He recognized, however, that the humid and dry tropics
are favourable regions for inselberg formation, as previously suggested by Cotton (1961,
1962). The application of numerical parameters to the identification of inselbergs, as
compiled by Thomas (1978) from several studies, does not provide determinant values
and, following this author, can be applied to numerous hills.
The definition proposed by Young (1972) has a more general character and the
inselberg denotation is applied to isolated hills with steep slopes that arise abruptly in
565
has been completely stripped and the Archoli surface developed upon fresh rock, with
numerous inselbergs along its rim. On the African surface, the most extensive one, a deep
saprolite is eroded and several inselbergs outcrop, indicating a very irregular basal
weathering surface. Several Miocene volcanoes are located upon some points of this
surface. In the opinion of Oilier, this last surface was formed, at least, in the Lower
Tertiary period while the Buganda Surface must be of a Mesozoic age or older.
5. Inselbergs
This is a German word that means island mountain and was first used by Bornhardt (1900)
to describe abrupt rocky hills rearing from the tropical plains of Tanganyika (today
Tanzania). They basically develop in cratonic tropical and subtropical regions, especially
on granites and metamorphic rocks. Nevertheless, they can also occur on sedimentary
rocks like the arkoses of the famous Ayers Rock (Uluru) monolith or the Olgas
conglomerates, both in the central desert of Australia (Twidale, 1978, 1982a; Vidal
Romanf and Twidale, 1998). The study of these forms is complex because it comprises of
factors like climate, lithology and rock structure, which at the same time determine the
degree of rock weathering. Similarly, they raise basic genetic questions, such as those
inherent to the exhumation of the basal weathering surface or to models of slope retreat.
Research on inselbergs must also include the effects of climatic changes and the
persistence or duration of these forms. As a consequence, many authors have focused their
studies on diverse aspects of inselbergs.
Inselbergs have raised numerous problems related to different definitions and
terminology. Twidale (1968a,b) defined inselbergs as isolated residual hills that stand
out in the tropical plains. Later, Twidale (1982a) and Vidal Romanf and Twidale (1998)
considered that inselbergs can occupy any topographic position. When studying the
morros and sugar loafs of the Rio de Janeiro zone, Birot (1958) already indicated that
crystalline domes are located in variable positions on the landscape. Thomas (1974, 1978)
agreed with the idea of not necessarily relating inselberg and plain. If this opinion is agreed
with, the initial definition of inselberg becomes rather distorted. Twidale (1968) also
indicated that inselbergs are located on tropical plains, although the existence of inselbergs
on the European temperate climates was known long ago (Penck, 1924). Therefore, either
inselbergs form under different climates, or they are paleoclimatic indicators. In order to
accurately define the situation of inselbergs in the world, Kesel (1973) analysed existing
publications and calculated the percentages of inselberg distribution under different
climates: 40% in savanna climates; 32% in arid and semiarid zones; 12% in humid
continental and subarctic climates and 6% for each of the humid tropics, subtropics and
Mediterranean zones. This author concluded, as did King (1957a), that inselbergs can
occur under nearly all climates. He recognized, however, that the humid and dry tropics
are favourable regions for inselberg formation, as previously suggested by Cotton (1961,
1962). The application of numerical parameters to the identification of inselbergs, as
compiled by Thomas (1978) from several studies, does not provide determinant values
and, following this author, can be applied to numerous hills.
The definition proposed by Young (1972) has a more general character and the
inselberg denotation is applied to isolated hills with steep slopes that arise abruptly in