O Relevo da Amazônia


  • Pedro de Moura


Relevo - Geografia, Geomorfologia


In this article, engineer Pedro de Moura, of the National Petroleum Council, in an original fashion clearly defines the major geomorphologic features of Brazilian Amazonia. He gives opportune explanations in regard to vegetation and to man's occupance in the region and is able to correct widely disseminated erroneous conceptions about the subject. In ending up he makes a few comments on facts concerning regional anthropogeography

Preliminarily the author refers to the configuration and space of the Amazonian basin which he considers distinct from the Amazonia properly speaking. Under a more restricted concept he encompasses it within a smaller area where uniformity in characteristics may be observed. Brazilian Amazonia is, in its dominant outlines, an immense plain slightly inclined toward the baselevel. It possesses an extensive network of navigable streams and to the valleys correspond gently rolling surfaces. With a relatively good climate though humid and warm, and clothed by a continuous tropical forest, it shows scanty density of population.

The relief varies in three gene1al aspects as follows: the p!ain, the plateau (firm land) and the crystalline peneplain. In the first two, terraces have been formed through epeirogenetic movements followed by cycles of erosion, mostly due to stream action Bare of morphogenetic details a distinction is made between two quaternary surfaces in the plain: the floodplain, consisting of two levels - the varzea (overflow prairie) and the Igapó (recent alluvial) -, and an area of dried quota, from 6 to 15 metres above the average water level, and which ls regionally termed teso (natural levee) Several times the author calls this dried terrace the level of Marajó; he characterizes it by a variety of clays containing a felluginous sandstone known as "stone of Pará" and recognizes in it a peistocene age With respect to both the varzea and the igapó, he gives an original interpretation defining varzea as the surface, subject to floods, which is closer to the liver, and igapó as a slightly higher ground also subject to flood but forest-covered. The igapó, whose typical region is the amazonian estuary, represents a farther advanced stage than the varzea which is a prairie zone utilized for cattle raising In time of flood the cattle take refuge at the tesos

In criticizing the literary excess of those who classify the Amazonia as an "immature land", he holds that such a concept is only applicable to the floodplain, that is, to a little over 1% of the area of the Amazonia. And later he adds that both the intricate water system and the ill-consolidated lands are not features peculiar to the Amazonia alone, but, on the contrary, they are aspects commonly observable in baselevel plains

The plateau - or film land - shows itself as a relief of early matuity, having a well-defined and dendritic drainage system in which the streams on the weakness of horizontal strata cut rapidly to the baselevel. The plateau, which ls uniform and low in central Amazonia, presents between the Parintins and the mouth of the Xingú a series of isolated swells or flat-toppled interfluves standing out conspicuously on the general surface and which are called serras (hills), like "Paranaquara, "Obidos", etc. These and other rounded elevations are developed by intense stream erosion which would have dissected and worn down the firm land It is on the central part of the Amazonia that the so-called "rios brancos" (white rivers) of clayish waters and meandering course are most suitable for steamer navigation through long stretches

In the region of the Lower Amazon between flim land and the peneplain lies a hilly-shaped zone of a paleozoic structure. Here overflows and basic levees occur and in the smaller streams they form waterfalls which do not, however, indicate any geological transition to the crystalline peneplain. Waterfalls of this kind are to be seen in contacting zones between film land and the peneplain, marking out the limit of navigation in large livers. The diastrophism observed in that paleozoic zone was the source of tectonic features such as the crown of hills in the neighborhood of Monte Alegre where Serra de Itajuli outstands 350 metres high Another example of the tectonic features is found in northwestern Acre where Serra do Divisor and Serra do Moa have spots above 600 m. Here, the livers cutting the latter into deep canions appear to be antecedent courses. The author prefe1s the name of "Divíso1" instead of "Contamana" because use the latter of these mountain ranges is located in Peruvian territory

In. passing to consider the peneplain, he presents it as the most irregular relief form of the Amazonia, with 'an old age structure, folded, extensively eroded and reduced to a peneplain with isolated hills and monadnocks In the peneplain zone streams have waterfalls and in spite of their waters being clear are called "black livers" To the north, on the international boundary line reliefs developed by erosion may be seen with striking forms like those appearing on the sandstone plateau of scarped wall of the Parima, Paracaima and Roraima mountain ranges, or with a peneplain mold or rounded hills and necks of low altitude as in Serra do Acarai and Serra Tumuc-Humac. Between Serra Roraima and Serra do Acaraí lies a low relief area affording connection of the Rio Branco basin with that of the relief Essequibo. To the south of the Amazon the peneplain exhibits the same gently rolling surfaces until contacting the higher sandstone deposits of the Central Plateau

Interesting observations are also presented when the author passes to analyse some anthropogeographic aspects. In dealing with the peopling conditions in relation to relief he points out that the plain and the firm land are areas where the Amazonian centers of population and other nucleuses are to be found sparsely. He refers still to the use of the rivers for penetration and settlement of the region and focuses the plain as the first and chief area of human occupance in connection with economical activities, as fishing, exploitation of some forest products, cattle breeding and incipient farming In the peneplain, however, a forest extractive economy with sporadic labor ls dominant, except the Rio Branco region where a farther advanced cycle of cattle raising occurs.