The Special Nature of Tropical Geomorphology
A large part of the southern continents is taken up by landform assemblages that seem strange and even full of contradictions to an observer coming from the humid mid-latitudes.
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The Special Nature of Tropical Geomorphology
A large part of the southern continents is taken up by landform assemblages that seem strange and even full of contradictions to an observer coming from the humid mid-latitudes. In the tropical lowlands, for example, we encounter steep-sided bare rock "inselbergs" or piles of giant boulders scattered in seemingly haphazard fashion, especially on poorly jointed granite and other particularly solid rocks (Fig. 1). In the mid-latitudes, such features are more characteristic of high mountains and barren upland plateaus. On the other hand, slopes so steep that they would be bare and rocky under humid mid-latitude conditions frequently carry a dense forest cover in the humid and seasonally humid tropics. Even on near-vertical walls the rocks may be hidden under a complete cover of regolith and low vegetation. Whereas the bare-rock inselbergs studding the plains often display no erosion rills at all, the weathered flanks of mountain valleys are often riddled by deeply incised gullies cut by tropical rains. This is not only true for extinct volcanoes with a long history of dissection (Fig. 2), but similarly for young fold mountains like the Himalayas or the Andes. Weathering and soil formation are rarely deeper than 1 m in the mid-latitudes. Soils in the humid tropics and their saprolitic
A. Wirthmann, Geomorphology of the Tropics © Springer-Verlag Berlin Heidelberg 2000
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1 The Special Nature of Tropical Geomorphology
Fig. 1. Inselberg in Namibia. The ancient crystalline basement of the southern continents has been plana ted by weathering and erosion since time immemorial. The more astonishing is the existence and preservation of weathering-resistant portions of bedrock standing out as single or groups of inselbergs or just barely rising above the surface as low shields. Shown here in semi-arid to arid Namibia, these landforms occur in a very similar way from the desert to the rainforest. (Drawing, like all others, by Bernadette Harder, after a photograph by Bliime11974)
zones, however, may be several tens of metres thick. They may also have developed enormously thick indurated horizons highly resistant to mechanical and even more so to chemical attack and thus to erosion. From our mid-latitude perspective, we generally expect to find plains and low relief landscapes close to sea level, but in many
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The Special Nature of Tropical Geomorphology
Fig. 2. V-Shaped Slope Channels on Oahu, Hawaiian Islands. Valley flanks and valley heads in Tertiary and Quaternary basalts (or comparably, though far less regularly in young fold mountains) are frequently dissected and worn back by closely spaced V-shaped slope channels. Prerequisite is the rapid progress of chemical weathering.
parts of the tropics extensive plains and gently rolling hill country also exist at heights between 1,000 and 2,000 m a. s.l., and major rivers flow for hundreds of kilometres across them without having carved any significant valleys. In the early days of tropical geomorphology Brandt (1917) c
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