692
Climatic Geomorphology
1975) and Kesel (1973) consider that inselbergs as subaerial remains are generated from
scarp retreat. On the other, several researchers who stand out from the others; Btidel
(1957), Ollier (1960), Thomas (1965, 1994), Twidale (1964, 1982a,b), and Vidal and
Twidale (1998) maintained that inselbergs are landforms exhumed from thick chemicallyweathered mantles.
Pediplanation, which commonly occurs together with these landforms, in the first
hypothesis is due to the intrinsic development of the landform and, therefore, weathering
is superimposed on the inselberg-pediment system once this generated. In the
exhumation hypothesis, the development of the regolith is the dominant process in the
genesis of the inselbergs, playing in the accompanying pediment process a secondary and
belated role.
Obviously, the development of inselberg landscapes, according to the theory on slope
evolution, occurs in semiarid morphoclimatic zones. The genesis of these landscapes by
exhumation occurs in morphoclimatic systems of humid tropics. The latter explains the
small domal inselbergs (bornhardts), up to 50 m high, simply by denudation of a thick
saprolite. When this hypothesis is applied to the origin of large bornhardts many problems
rise. These difficulties can be solved if a continuous weathering is supposed at the foot of
the slope, as the weathering allows the surrounding plains to be lowered.
In savanna areas, the history is not monoclimatic (Birot, 1978), at least not in
Quaternary times, and, precisely, in those areas is where inselbergs proliferated (Kesel,
1973). The alternation of wet and dry climate stages is what facilitates the development of
these landforms. Furthermore, it is necessary to take into account that once the bornhardtplain was established, the contrast in the velocity of the weathering between both stages
assures the persistence of the system (Thomas, 1978).
Inselbergs, in some cases, can have been formed in quite old periods of the geologic
past and have remained until present. This interpretation has been proposed by Twidale
(1978) for the famous Ayers Rock (Uluru), in the desert plains of central Australia. The
author, on the basis of correlative sediment analyses and geomorphologic interpretations,
concluded that the development of that monolith started at the boundary of the MesozoicCenozoic period.
2.3.3. Fluvial systems
The morphology of the channels and fluvial sediments in the humid tropics records the
effects of environmental fluctuations, data on fluvial regimes as well as the evolution of the
system, because many of these sediments can be dated (Thomas, 1994; Thomas and Thorp,
1995).
The Amazon River, which represents 15% of the total discharge of all the rivers in the
world, has a slope gradient of 0.1 m/km near the Peruvian Andes, decreasing drastically to
0.03 m/km from Manaus, in the centre of the basin, to the mouth, 1400 km distant. These
contrasts in altimetry and gradient clearly affect the morphology of the channels. Rivers that
plough through the Andes, as with the Solimoes, the principal course of the Amazon River,
transport coarse-grained material but mainly sediments held in suspension (Figure 21.4)
and in the alluvial plains channels present low sinuosity. On the other hand, in rivers in the
centre of the fluvial basin, as with the river Juru~i, the sediment load transportation is almost
zero and the channels have a high sinuosity. All that can be interpreted is a function of the
Climatic Geomorphology
1975) and Kesel (1973) consider that inselbergs as subaerial remains are generated from
scarp retreat. On the other, several researchers who stand out from the others; Btidel
(1957), Ollier (1960), Thomas (1965, 1994), Twidale (1964, 1982a,b), and Vidal and
Twidale (1998) maintained that inselbergs are landforms exhumed from thick chemicallyweathered mantles.
Pediplanation, which commonly occurs together with these landforms, in the first
hypothesis is due to the intrinsic development of the landform and, therefore, weathering
is superimposed on the inselberg-pediment system once this generated. In the
exhumation hypothesis, the development of the regolith is the dominant process in the
genesis of the inselbergs, playing in the accompanying pediment process a secondary and
belated role.
Obviously, the development of inselberg landscapes, according to the theory on slope
evolution, occurs in semiarid morphoclimatic zones. The genesis of these landscapes by
exhumation occurs in morphoclimatic systems of humid tropics. The latter explains the
small domal inselbergs (bornhardts), up to 50 m high, simply by denudation of a thick
saprolite. When this hypothesis is applied to the origin of large bornhardts many problems
rise. These difficulties can be solved if a continuous weathering is supposed at the foot of
the slope, as the weathering allows the surrounding plains to be lowered.
In savanna areas, the history is not monoclimatic (Birot, 1978), at least not in
Quaternary times, and, precisely, in those areas is where inselbergs proliferated (Kesel,
1973). The alternation of wet and dry climate stages is what facilitates the development of
these landforms. Furthermore, it is necessary to take into account that once the bornhardtplain was established, the contrast in the velocity of the weathering between both stages
assures the persistence of the system (Thomas, 1978).
Inselbergs, in some cases, can have been formed in quite old periods of the geologic
past and have remained until present. This interpretation has been proposed by Twidale
(1978) for the famous Ayers Rock (Uluru), in the desert plains of central Australia. The
author, on the basis of correlative sediment analyses and geomorphologic interpretations,
concluded that the development of that monolith started at the boundary of the MesozoicCenozoic period.
2.3.3. Fluvial systems
The morphology of the channels and fluvial sediments in the humid tropics records the
effects of environmental fluctuations, data on fluvial regimes as well as the evolution of the
system, because many of these sediments can be dated (Thomas, 1994; Thomas and Thorp,
1995).
The Amazon River, which represents 15% of the total discharge of all the rivers in the
world, has a slope gradient of 0.1 m/km near the Peruvian Andes, decreasing drastically to
0.03 m/km from Manaus, in the centre of the basin, to the mouth, 1400 km distant. These
contrasts in altimetry and gradient clearly affect the morphology of the channels. Rivers that
plough through the Andes, as with the Solimoes, the principal course of the Amazon River,
transport coarse-grained material but mainly sediments held in suspension (Figure 21.4)
and in the alluvial plains channels present low sinuosity. On the other hand, in rivers in the
centre of the fluvial basin, as with the river Juru~i, the sediment load transportation is almost
zero and the channels have a high sinuosity. All that can be interpreted is a function of the