Progressos do conhecimento geográfico da África em finais do século XIX

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Classe de Letras

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Classe de Letras

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O Rio Cambongo-Negunza e os seus afluentes: um exemplo da complexidade de padrões de drenagem em Angola

THE CAMBONGO-NEGUNZA RIVER AND ITS TRIBUTARIES: AN EXAMPLE OF THE COMPLEXITY OF THE DRAINAGE PATTERNS IN ANGOLA. The hydrographical basin of River Cambongo-Negunza, situated in the coastal area of central Angola, was selected as an example of the complexity of drainage patterns in Angola. From its sources in the massifs of the Marginal Mountain (more than 2 000m) the river flows through the stepped planed erosion surfaces to the Ocean. In its lower course the river valley presents a narrow and V-shaped cross profile cut in sedimentary rocks (Cretaceous-Quaternary). The coastal erosion surface, including the raised beaches (mean altitudes from 40 to 100 m), levelling the rocks of the Sedimentary Belt and those of the ancient crystalline massif, gradually rises towards the east until a well marked topographical limit, the base (altitude approx. 280-300 m) of the steep western slopes of a NNE-SSW quartzitic ridge with a dissymmetrical cross-sectional profile: the serra do Engelo (altitudes between 1 400-1 700m), whose faults are peppered with thermal springs. Above the coastal surface, in the crystalline rock strip, some quartzitic hills with Inselberge characteristics rise out of the plains, leading to problems in relation to, for example, the western extension of the quartzite formations and their dismantling, and the behaviour of the same material in the complex genesis and evolution processes of the extensive regional line of N-S escarpments, against which the erosion surfaces of the coastal belt end. In the limestones near the coast, the river Cambongo-Negunza has a subterranean network of large caves and galleries surfacing near the small plain from which it flows into the ocean. Not far from the valley sinks and dolines of various sizes and lapies can be seen. The climatic conditions are semi-arid, with annual precipitation less than 700mm and high evaporation rates. Vegetation is a mix of that which can be found in savanna, steppe and xerophytic habitats, with few to no trees. A vast area of outcrops of granitic and related rocks, sometimes covered by regoliths of various thicknesses, can be found between the Serra do Engelo and the massifs of the Marginal Mountain chain. The altitudes of the general levelling range between 1 100 and 1 200m. The area is covered by forest and savannas with low trees, shrubs and tall grasses growing in rather thin and variable soils, fersialitic being among the most frequent. The average temperature is around 22ºC and annual rainfall is about 1 400mm. Lateritisation or bauxitisation lines and subaerean duricrust can be seen in many places. Above this surface, there are many Inselberge-type hills, isolated or in groups, mostly with square-shaped forms, linked or not to the geometrical patterns of the hydrographic networks, revealing the structural control of fractures and faults over these watercourses, whether small or large, permanent or temporary. Note that the majority of the Inselberge is not found near the escarpments of the marginal mountain chain, the eastern limit of the 1 100-1 200 metre-high plain, but rather quite a distance away to the west, near the Serra do Engelo quartzitic ridge. The same can be seen in the neighbouring basins of the River Cuvo-Queve to the north and to the south of the Cambongo-Negunza river basin. Fracture valleys and residual hills call for more detailed explanations, with suggestions of criteria for their study. Moreover, there has been the concern regarding how to correlate several factors relative to external dynamics (meteorisation and erosion in a broad sense) with those relative to internal dynamics (geology and tectonics) to explain the genesis and evolution of the landforms. Therefore, the study wraps up with some brief thoughts regarding a dynamic geo-chronological approach

Contribuição dos geógrafos portuguesas para o conhecimentos científico das regiões tropicais

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Luanda e os seus dois arcos complexos de vulnerabilidade e risco: o das restingas e ilhas baixas e o das escarpas abarracadas

Luanda e os seus dois arcos complexos de vulnerabilidade e risco: das restingas longas e ilhas baixas de areias siliciosas, e das escarpas moldadas em camadas sedimentares brandas com algumas intercalações de rochas mais resistentes, densamente abarrocadas. Clima caracterizado pela alternância de um período curto de chuvas fortes nos meses mais quentes, os primeiros do ano, e de um longo período seco, com meses menos quentes (cacimba); vegetação do tipo savana acusando ambiência de semi-aridez, bem representada sobre as areias vermelhas que cobrem o planalto. Modificações naturais e perturbações resultantes de intervenções humanas. A necessidade de medidas correctas para o acompanhamento e minoração dos efeitos negativos da erosão marinha e da erosão terrestre.Luanda and its two complex arches of vulnerability and hazard: that of the sandy and low restingas (spits) and small islands that protect the bay of still waters of the town against the oceanic waves, and that of the escarpments in bedded sedimentary rocks densely gullyed. Tropical littoral savanna climate with alternation of a short period of heavy rainfalls during the first months of the year, the hottest, and a long and dry period, with lower temperatures (the cacimbo); the vegetation cover is a grassland with interspersed trees and shrubs, well represented on the plateau coated with red sands. The natural changes and those resulting from men's actions. The necessity of appropriated policies to survey the negative effects of the erosional processes