576
Climatic Geomorphology
subsurface weathering. These forms also reveal the amplitude of lateral corrosion on
slopes of massive bornhardts during different stages of base-level standstill.
Regardless of whether bornhardts may have originated by scarp retreat or by
exhumation, they require a long time to be formed. Their age, however, is a function of
the velocity of geomorphic processes involved in their genesis and of the size of the
form. Hence, we can find dome inselbergs recently exposed as well as bornhardts tens
of millions of years old. Another problem arises when trying to explain the survival of
great bornhardts for long periods of geological time. It has been interpreted that, since
the inselberg domical form is controlled by sheeting, precipitation water flows as if on
an umbrella, producing very weak alteration and low erosion (Thomas, 1974).
Mesozoic or Early Tertiary ages have been proposed for many dome inselbergs.
Savigear (1960) considered that inselbergs in Nigeria were exhumed from a Cretaceous
sedimentary cover. Barbier (1967) indicated that domes (sugarloafs) in the Tassili
Massif, South Algeria, have also been exhumed from below an Upper Paleozoic
sandstone. Chaput (1971) pointed out that the pre-Triassic surface in eastern Sierra
Morena in Spain is partially exhumed, with the outcrop of quartzitic inselbergs.
Twidale and Bourne (1975) and Twidale (1978) used stratigraphical and geomorphological arguments to suggest that the famous Ayers Rock (Uluru) monolith dates back
to the Mesozoic age.
As the age of inselbergs is sometimes very old, these forms have been exposed to
endogenic and exogenic activity during long periods of time. They may have been
affected by the motion of lithospheric plates, with the modification of their latitude and
even climate, as for example, in India. As far as the origin of inselbergs is concerned,
climatic changes affect the balance between weathering and erosion velocities. Periods
during which weathering prevails correspond to geomorphological and ecological
stability (biostatic stages) and when this equilibrium is interrupted then erosion sets in
(rhexistatic stages) (Erhart, 1967). In the European middle latitudes a wide and deep
ferralitic weathering has been recognized, especially during Eocene times (Millot,
1964). This alteration is confirmed in the sedimentary record of Tertiary basins and
gives convincing arguments for an old deep weathering of zones where inselbergs have
been formed.
6. Tropical karst
Landforms developed on limestones present clear analogies with forms developed on
granites in humid tropical climates (Twidale, 1990). The activity of the etching processes
can be also recognized in carbonate rocks and is extensive in karstic corrosion plains and
their tower forms. There is even a clear similarity between the half-orange morphologies
on granite landforms and the domical forms of the tropical cone karst. These
circumstances induce one to consider a matter of form convergence or equifinality
(Thomas, 1994).
Lehmann (1936) was probably the pioneer in studying the climatic differentiation of
karstic landforms. In the Karst Symposium held in Frankfurt, Germany, in 1953, Lehmann
indicated that every climate has its own karstic development, although factors like
composition, thickness and carbonate rock fracturing must be also taken into account.
Climatic Geomorphology
subsurface weathering. These forms also reveal the amplitude of lateral corrosion on
slopes of massive bornhardts during different stages of base-level standstill.
Regardless of whether bornhardts may have originated by scarp retreat or by
exhumation, they require a long time to be formed. Their age, however, is a function of
the velocity of geomorphic processes involved in their genesis and of the size of the
form. Hence, we can find dome inselbergs recently exposed as well as bornhardts tens
of millions of years old. Another problem arises when trying to explain the survival of
great bornhardts for long periods of geological time. It has been interpreted that, since
the inselberg domical form is controlled by sheeting, precipitation water flows as if on
an umbrella, producing very weak alteration and low erosion (Thomas, 1974).
Mesozoic or Early Tertiary ages have been proposed for many dome inselbergs.
Savigear (1960) considered that inselbergs in Nigeria were exhumed from a Cretaceous
sedimentary cover. Barbier (1967) indicated that domes (sugarloafs) in the Tassili
Massif, South Algeria, have also been exhumed from below an Upper Paleozoic
sandstone. Chaput (1971) pointed out that the pre-Triassic surface in eastern Sierra
Morena in Spain is partially exhumed, with the outcrop of quartzitic inselbergs.
Twidale and Bourne (1975) and Twidale (1978) used stratigraphical and geomorphological arguments to suggest that the famous Ayers Rock (Uluru) monolith dates back
to the Mesozoic age.
As the age of inselbergs is sometimes very old, these forms have been exposed to
endogenic and exogenic activity during long periods of time. They may have been
affected by the motion of lithospheric plates, with the modification of their latitude and
even climate, as for example, in India. As far as the origin of inselbergs is concerned,
climatic changes affect the balance between weathering and erosion velocities. Periods
during which weathering prevails correspond to geomorphological and ecological
stability (biostatic stages) and when this equilibrium is interrupted then erosion sets in
(rhexistatic stages) (Erhart, 1967). In the European middle latitudes a wide and deep
ferralitic weathering has been recognized, especially during Eocene times (Millot,
1964). This alteration is confirmed in the sedimentary record of Tertiary basins and
gives convincing arguments for an old deep weathering of zones where inselbergs have
been formed.
6. Tropical karst
Landforms developed on limestones present clear analogies with forms developed on
granites in humid tropical climates (Twidale, 1990). The activity of the etching processes
can be also recognized in carbonate rocks and is extensive in karstic corrosion plains and
their tower forms. There is even a clear similarity between the half-orange morphologies
on granite landforms and the domical forms of the tropical cone karst. These
circumstances induce one to consider a matter of form convergence or equifinality
(Thomas, 1994).
Lehmann (1936) was probably the pioneer in studying the climatic differentiation of
karstic landforms. In the Karst Symposium held in Frankfurt, Germany, in 1953, Lehmann
indicated that every climate has its own karstic development, although factors like
composition, thickness and carbonate rock fracturing must be also taken into account.