Brazil Geology 2

Brazil Geology and Morphology Part 2

The Goyan system is connected to the maritime system by means of the relief improperly called Serra das Vertentes, a low threshold or rather fraction of the plateau limited and engraved to the north by the spring valleys of San Francisco and to the south by the Rio Grande valley (Paranȧ), which extends between the Serra da Mantiqueira to SE. and that from Canastra (1287 m.), the latter continued to N. by the Serre Matta do Corda and dos Pilões which divide the San Francisco itself from the Paranahyba basin.

At the center of the Goyan system is the Chapada dos Couros (“plain of the leathers”), or Serra dos Pyreneos, located in the heart of the Brazilian plateau (between 15 ° and 16 ° lat. S.) and from which the tributary Corumbá descends del Paranahyba (Paraná) towards S. and the Maranhão (Tocantins) towards N. According to an article of the constitution voted in 1891, the Districto Federai should be carried on this Chapada(see the rectangle, to the east of Pyrenopolis, whose sides, of about 150 and 100 km. represent the limits of the Federal District: AT, 155-156). The reliefs that from the orographic node dos Pyreneos extend towards the north, subsequently take the names of Serra do Paraná, São Domingos, Taguatinga, das Mangabeiras, up to the 10th southern parallel: the series of reliefs are then connected to the Serra das Mangabeiras, with the names of Serra do Gurgueia, do Piauhy and dos Dous Irmãos, follow the middle course of the S. complex of hills known as Planalto da Borborema, archaic skeleton of the north-eastern cusp of Brazil. To the west of the Serra dos Pyreneos the plateau of southern Goyaz widens (the watershed between the Paranahyba and Araguaya basins) and then the plateau of the Matto Grosso which has an average height between 600 and 900 meters and which extends towards the south up beyond the route of the S. Paolo-Corumbá railway, and to the north it forms the source region of Paraguay e. Cuyabȧ on one side, Tapajóz and Araguaya on the other. The plateau of the Matto Grosso extends towards the NW. flanking the entire course of the Guaporé (Madeira) on whose right bank it descends with a very steep slope, forming the so-called Serra or Cordilheira dos Parecis, consisting of red sandstones which between 12 ° and 10 ° lat. S. give way to the mica schists, gneisses and granites of the middle valleys of Tapajóz,

Considerably varied appears the action exerted by erosion in each of the three areas of high lands and in the individual parts of them, given the diversity of geological constitution and the diversity of climate It is evident above all in areas of constant humidity, characterized by large atmospheric precipitation and consequent chemical decomposition of crystalline rocks. This decomposition gives products so minute, so fine, that they cease to be perceptible at a first consideration: this happens for the Pão de Assucar which dominates with the Gavia and the typical Corcovado the blue waters of the Bay of Guanabara, and in other isolated reliefs at dome of the surroundings of Rio de Janeiro. This chemical decomposition is sometimes found up to a depth of 100 meters from the surface of the soil (southern part of the state of Minas). Hence the presence of a real clayey soil at considerable depth, for which the Agassiz was able to suppose that the action of the glaciation had been exerted on a large part of the Brazilian area; hence also the small resistance to the wind opposed by the slope forest, where the roots do not reach the rock in place.

According to, the three lowland areas extend, one near the SW edge, the other (by far wider) to the north of the central massif, while the last surrounds the maritime slope.

The first, which can be said to be a continuation of the great Argentine plains and the Gran Chaco (see argentina), is the plain of the Paraguay River, limited to the north and east by the slope of the Matto Grosso plain and to the south by the Serra da Bodoquena: it is an almost horizontal alluvial plain: the altitude of the plain in Porto Esperança south of Corumbá is just 107 m. and slightly exceeds 150 m. both at the northern limit, in S. Luiz de Cáceres, and at the edge of SE. at Aquidauana. This depression in Paraguay is made up of tertiary and Quaternary deposits that covered the archaic massif of the Matto Grosso, which emerges at the northern limit in the Serra Tombador and to the west of the river, in Bolivian territory, in the Chiquitos gneissic plateau. The Pantanal de Xaraes which offers a strong analogy with the triangular lake area circumscribed by the Amazons, the Negro and the Japurá,

The depression area between the Guianas rialto and the central massif, that is the Amazonian depression, interposed between the eastern slope of the Andean relief and the Atlantic, can appear as a whole vast alluvial plain only to a superficial consideration, since in the vast Amazon basin (calculated by Le Cointe equal to 5,594,000 sq km, leaving aside the Tocantins, and of which more than half – about 3,600,000 sq km – is in Brazilian territory) the ancient formations are considerably represented. The gneissic platform on the right bank of the lower Amazon River (see Amazons) covered, between Pará and Gurupí, by a light blanket of sandstones and tertiary, Quaternary and alluvial clays, attests to the geological continuity with the gneisses and the granites of the left bank pertaining to the ridge of the Guianas, and extending up to Carvoeiro and Moura, not far from the confluence of the Rio Branco with the Negro.

Brazil Geology 2