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Climatic Geomorphology
Throughout the history of geomorphology different theories have been proposed,
giving rise to terms such as peneplain (Davis, 1899), for humid zones, and pediplains, by
extension of pediments, for arid zones (King, 1942; 1953). Such plains form part of the
present landscape. These terms and their associated genetic implications have been
analysed in previous chapters.
In general, peneplains do not form perfect flat surfaces, but rounded and undulating
hills of low height (Vidal Romanf and Twidale, 1998). They develop on unaltered rocks
and some think that they can be recognized in South Africa, Transvaal, Namibia, and
Brazil. As will be discussed later, they are not exclusively limited to temperate zones.
Twidale (1983) indicated that peneplains are being progressively degraded and should be
denoted as ultiplains. Changes are so slow that the survival of antique forms occurs within
the present landform (Twidale, 1976a,b). Some authors consider that "pediments, and by
extension pediplains, are not genetically nor temporaly distinct from peneplains. They
usually coexist ... (and) are simply parts of a great group of landforms called planation
surfaces" (Vidal Romanf and Twidale, 1998, p. 101).
In the humid tropics there also exist exhumed plains or surfaces, generated by the
stripping of geological units that fossilize erosion surfaces. A classical example occurs in
the Valley of the Thousand Hills, Natal, South Africa, where the erosion of the
horizontally-bedded Table Mountain sandstone uncovers the underlying surface and some
dome inselbergs (Thomas, 1978).
Where the alteration mantle is eroded by fluvial action, the weathering front
outcrops as an etchplain surface (Wayland, 1933), referred to by other researchers as a
chemical corrosion surface (Vidal Romanf and Twidale, 1998). Numerous denominations have been proposed for these forms, which were compiled by Thomas (1989a).
Wayland estimated that the regolith thickness fluctuated by approximately 3 m and,
hence, during the next erosional cycle triggered by a rapid uplift, the regolith was
completely stripped giving rise to a new surface more or less parallel to the former.
These alteration and subsequent erosion processes can take place many times.
Wayland's ideas mainly differ in the weathering mantle thickness, which at present is
estimated in tens or hundreds of metres.
Btidel's (1957) concept of double planation surfaces (run-off surface and basal
weathering surface) is related to the etching idea. He applied his theories to zones with a
tropical seasonal climate and tectonic stability (Figure 21.30). His model starts from a
plain whose weathering depth fluctuates between 30 and 60 m that is eroded giving rise to
lower plains with a scatter of small rock domes (ruwares = whalebacks = knobs) 1 to 3 m
high or shield inselbergs. Rounded or parallepipedal blocks from the lower parts of the
weathering profile can remain detached during the surface lowering. Valleys with gentle
slopes or dambos drain these plains. In summary, it is deduced that soil erosion operates on
the run-off surface, whereas the development and deepening of the saprolite takes place in
the basal weathering surface. Under seasonal climates, where Btidel (1957, 1965, 1977,
1982) focused his research, these plains have a nearly null gradient (0.2 ~ in the Tamil Nadu
plains, southern India). Rainwater hardly finds routes for hierarchical drainage
organisation and hence a high water amount infiltrates into the regolith. During dry
seasons clay soils get cracked; at the same time, roots and other bioturbation conduits
constitute excellent ways for water penetration with the arrival of the rainy season. This
"cycle" favours weathering development along the whole saprolite and its basal deepening;
Climatic Geomorphology
Throughout the history of geomorphology different theories have been proposed,
giving rise to terms such as peneplain (Davis, 1899), for humid zones, and pediplains, by
extension of pediments, for arid zones (King, 1942; 1953). Such plains form part of the
present landscape. These terms and their associated genetic implications have been
analysed in previous chapters.
In general, peneplains do not form perfect flat surfaces, but rounded and undulating
hills of low height (Vidal Romanf and Twidale, 1998). They develop on unaltered rocks
and some think that they can be recognized in South Africa, Transvaal, Namibia, and
Brazil. As will be discussed later, they are not exclusively limited to temperate zones.
Twidale (1983) indicated that peneplains are being progressively degraded and should be
denoted as ultiplains. Changes are so slow that the survival of antique forms occurs within
the present landform (Twidale, 1976a,b). Some authors consider that "pediments, and by
extension pediplains, are not genetically nor temporaly distinct from peneplains. They
usually coexist ... (and) are simply parts of a great group of landforms called planation
surfaces" (Vidal Romanf and Twidale, 1998, p. 101).
In the humid tropics there also exist exhumed plains or surfaces, generated by the
stripping of geological units that fossilize erosion surfaces. A classical example occurs in
the Valley of the Thousand Hills, Natal, South Africa, where the erosion of the
horizontally-bedded Table Mountain sandstone uncovers the underlying surface and some
dome inselbergs (Thomas, 1978).
Where the alteration mantle is eroded by fluvial action, the weathering front
outcrops as an etchplain surface (Wayland, 1933), referred to by other researchers as a
chemical corrosion surface (Vidal Romanf and Twidale, 1998). Numerous denominations have been proposed for these forms, which were compiled by Thomas (1989a).
Wayland estimated that the regolith thickness fluctuated by approximately 3 m and,
hence, during the next erosional cycle triggered by a rapid uplift, the regolith was
completely stripped giving rise to a new surface more or less parallel to the former.
These alteration and subsequent erosion processes can take place many times.
Wayland's ideas mainly differ in the weathering mantle thickness, which at present is
estimated in tens or hundreds of metres.
Btidel's (1957) concept of double planation surfaces (run-off surface and basal
weathering surface) is related to the etching idea. He applied his theories to zones with a
tropical seasonal climate and tectonic stability (Figure 21.30). His model starts from a
plain whose weathering depth fluctuates between 30 and 60 m that is eroded giving rise to
lower plains with a scatter of small rock domes (ruwares = whalebacks = knobs) 1 to 3 m
high or shield inselbergs. Rounded or parallepipedal blocks from the lower parts of the
weathering profile can remain detached during the surface lowering. Valleys with gentle
slopes or dambos drain these plains. In summary, it is deduced that soil erosion operates on
the run-off surface, whereas the development and deepening of the saprolite takes place in
the basal weathering surface. Under seasonal climates, where Btidel (1957, 1965, 1977,
1982) focused his research, these plains have a nearly null gradient (0.2 ~ in the Tamil Nadu
plains, southern India). Rainwater hardly finds routes for hierarchical drainage
organisation and hence a high water amount infiltrates into the regolith. During dry
seasons clay soils get cracked; at the same time, roots and other bioturbation conduits
constitute excellent ways for water penetration with the arrival of the rainy season. This
"cycle" favours weathering development along the whole saprolite and its basal deepening;