Tropical landf orms
537
(expressed in volume) (Gibbs, 1967). The greatest erosion values correspond to islands
of SE Asia, Indonesia and Papua-New Guinea, where slopes and runoff are very high.
The Cilulung River of Indonesia annually transports 8000 m3/km 2, the highest value ever
recorded. In areas of low relief, represented by the cratonic regions of Brazil, Africa, India,
and Australia, recorded values are much lower, about 30 m3/km2/yr. Finally, in the lower
Amazon tributaries, where relief is very low, suspended load is remarkably lower and
ranges between 0.3 and 3.8 m3/km2/yr.
Studies on plots and small basins carried out in Malaysia indicate that dissolved load
represents 27.4 per cent of the total sediment discharge (Douglas and Spencer, 1985b). In
fluvial basins chemical denudation varies between 143 T/km2/yr for the Yamuna River,
affluent of the Ganges River, and 9 T/km2/yr for the Congo or Zaire River. The Amazon
River presents intermediate values, of 35 T/km2/yr (Sarin et al., 1989). Nevertheless,
average values of dissolved load for the affluents of the low Amazon basin are of 5 to
8 mg/1, very small if compared with the global average concentration in rivers, which is
about 120 mg/1 (Walling and Webb, 1986). The highest dissolved concentrations appear
in rivers crossing evaporitic formations, with values of up to 60,000 mg/1.
By considering the average values from all rivers of the world, the resulting relationship
between suspended and dissolved load is 3.5:1 (Meybeck, 1979). The highest values are
those of the Ganges-Brahmaputra rivers, which bears a relationship of 11:1 because of
the great supply given by the Himalayan range. On the other hand, the Congo River
basin has little altitudinal contrast and hence the relationship of this river is of 0.9:1 and
dissolved load exceeds that transported in suspension (Walling and Webb, 1986).
2.2. Slopes and stone lines
Despite the great extension occupied by slopes within these environments (Young, 1972),
few rigorous studies exist about them. This can be due to the high vegetation density,
which hampers their study, and that a great part of the observations made on them are
based on visual estimations rather than on accurate measurements of angles and forms in
the field (Chorley, 1964).
Relief in shield areas is sometimes interrupted by long and deep escarpments linked
to different factors and processes. Differential erosion is recognizable on escarpments
on sandstones, of which the most spectacular ones are those of Roraima in southeast
Venezuela, with plunging walls several hundred metres high. In Africa, some of the great
escarpments are associated with the stepping of extensive erosion surfaces of different
ages (King, 1962). Escarpments of tectonic origin, generated by endogenetic activity
during the Cenozoic period, reach their utmost expression along the rift valley system of
East Africa (Figure 21.5) with vertical flights greater than 1500 m. In general, they form
on basic volcanic rocks and have been mostly generated in recent times because they
appear relatively unaltered by erosion (Nyamweru, 1980). Within the prelittoral Tertiary
graben system of the region between Sao Paulo and Rio de Janeiro in Brazil important
escarpments can be recognized in the Da Mantiqueira and Do Mar ranges, with heights
close to 1000 m (Petri and Fflgaro, 1983).
In the cratonic areas, bared hills on crystalline rocks are plentiful, commonly forming
inselbergs. In some cases they constitute grouped hemispheric domes, named half-oranges
537
(expressed in volume) (Gibbs, 1967). The greatest erosion values correspond to islands
of SE Asia, Indonesia and Papua-New Guinea, where slopes and runoff are very high.
The Cilulung River of Indonesia annually transports 8000 m3/km 2, the highest value ever
recorded. In areas of low relief, represented by the cratonic regions of Brazil, Africa, India,
and Australia, recorded values are much lower, about 30 m3/km2/yr. Finally, in the lower
Amazon tributaries, where relief is very low, suspended load is remarkably lower and
ranges between 0.3 and 3.8 m3/km2/yr.
Studies on plots and small basins carried out in Malaysia indicate that dissolved load
represents 27.4 per cent of the total sediment discharge (Douglas and Spencer, 1985b). In
fluvial basins chemical denudation varies between 143 T/km2/yr for the Yamuna River,
affluent of the Ganges River, and 9 T/km2/yr for the Congo or Zaire River. The Amazon
River presents intermediate values, of 35 T/km2/yr (Sarin et al., 1989). Nevertheless,
average values of dissolved load for the affluents of the low Amazon basin are of 5 to
8 mg/1, very small if compared with the global average concentration in rivers, which is
about 120 mg/1 (Walling and Webb, 1986). The highest dissolved concentrations appear
in rivers crossing evaporitic formations, with values of up to 60,000 mg/1.
By considering the average values from all rivers of the world, the resulting relationship
between suspended and dissolved load is 3.5:1 (Meybeck, 1979). The highest values are
those of the Ganges-Brahmaputra rivers, which bears a relationship of 11:1 because of
the great supply given by the Himalayan range. On the other hand, the Congo River
basin has little altitudinal contrast and hence the relationship of this river is of 0.9:1 and
dissolved load exceeds that transported in suspension (Walling and Webb, 1986).
2.2. Slopes and stone lines
Despite the great extension occupied by slopes within these environments (Young, 1972),
few rigorous studies exist about them. This can be due to the high vegetation density,
which hampers their study, and that a great part of the observations made on them are
based on visual estimations rather than on accurate measurements of angles and forms in
the field (Chorley, 1964).
Relief in shield areas is sometimes interrupted by long and deep escarpments linked
to different factors and processes. Differential erosion is recognizable on escarpments
on sandstones, of which the most spectacular ones are those of Roraima in southeast
Venezuela, with plunging walls several hundred metres high. In Africa, some of the great
escarpments are associated with the stepping of extensive erosion surfaces of different
ages (King, 1962). Escarpments of tectonic origin, generated by endogenetic activity
during the Cenozoic period, reach their utmost expression along the rift valley system of
East Africa (Figure 21.5) with vertical flights greater than 1500 m. In general, they form
on basic volcanic rocks and have been mostly generated in recent times because they
appear relatively unaltered by erosion (Nyamweru, 1980). Within the prelittoral Tertiary
graben system of the region between Sao Paulo and Rio de Janeiro in Brazil important
escarpments can be recognized in the Da Mantiqueira and Do Mar ranges, with heights
close to 1000 m (Petri and Fflgaro, 1983).
In the cratonic areas, bared hills on crystalline rocks are plentiful, commonly forming
inselbergs. In some cases they constitute grouped hemispheric domes, named half-oranges