In the Serra di Paranaquara, not far from the confluence of the Parú (left bank, near Almeirim), tertiary and Quaternary sandstones and clays have been raised en masse up to 300 meters, forcing the river to deepen its bed under the high terraces of the left bank. Going up the Trombetas, which reaches the great river to the west of the typical Obidos strait, you will first encounter sandstone, limestone and carbonic schist, while at 100 km. gneisses appear, and diorite and diabase outcrops form rocky dams in the river bed. Finally, still on the left bank, near Monte Alegre, rises the massif of Ereré, which includes a group of chains formed of devonic schists which reach a height of 350 meters in the Serra de Tajuri.
The tectonic uplift of the eastern Amazon, bringing the erosive action to the surface of the tertiary bumps of sand and sandstone, has had the consequence that downstream of the Madeira mouth the volume of debris does not gradually decrease, as great river current approaches the Atlantic. In fact, large sands are in the estuary, upstream of which the high lands appear, emerging a few meters, or even just a few feet, on the level of the floodwaters in the period in which the varzea, or alluvial plain, is more or less flooded (it was completely flooded by the floods of the years 1857, 1898, 1904, 1909, 1918).
According to shoefrantics.com, there is an insensitive transition from the Quaternary deposits to the older ones of a similar nature in the central Amazon, from the mouth of the Madeira to the Peruvian border, where Tabatinga rises at 80 m. above sea level (see amazons) and where the ground, engraved by numerous river furrows, generally rises towards the SW. and O. Hilly undulations, which consist of sands, clays and sandstones cemented by iron oxides under the action of the tropical climate, are between the Juruá basin, which flows downstream of Fonte Bôa, and that of Javary, which marks for much of its course the border with Peru, and rise for about 200 m. at the bottom of the valley the hilly ridges which divide the Acre basin (v. acre) from that of the Abuná, which marks the border with Bolivia.
In the Brazilian Amazon area, various natural regions can be distinguished: the Solimões plain, between the Peruvian border and the Madeira mouth, domain of the “forest par excellence”, as the name caa – eté sounds (in the Tupi language); the Rio Negro basin, with the typical pastoral area of the Rio Branco; the plain of the lower Amazons, from the mouth of the Madeira to the shore of the Atlantic, an area for fishing and agriculture, and also for raising livestock on the islands of the estuary; the coastal area characterized by fish-rich depressions and a low, sandy, marshy and generally unstable modeling shore line, which represents, north of the estuary, the eastern edge of the Guianas rialto.
From the extreme southern limit of the plain of the low Amazons to the bay on which Rio de Janeiro stands, the third lowland area extends, that is the coastal plain, variously wide, which only rarely has relatively extensive coastal dune formations (Sergipe and of Ceará). Sometimes, as in the southern section of the State of Bahia (where there is no lack of reliefs of 500-600 meters, such as Mount Paschoal), a lowering of the coast with respect to the sea level is clearly evident, followed by a slight rise, a phenomenon of particular economic importance, due to its influence on the creation of natural harbors. In the eastern section of the State of Rio de Janeiro, from the mouth of the Parahyba to near C. Frio, from which the general course of the coast in the NE.-SO direction begins, there are numerous lagoons, including the vast and stormy Feia. In the southernmost section of the Atlantic plain, numerous granite mounds emerge, mostly rounded; these give variety and richness to one of the most admirable terrestrial landscapes.
It is estimated that of the 8 and a half million square kilometers that make up the area of Brazil, about 3 million are located at an altitude of less than 200 meters and about 5.5 million at an altitude varying, approximately, between 200 and 3000 meters. At one time it was inclined to assign a higher altitude to the internal relief, and there was also talk of an average elevation of 2000-3000 meters, while now the average altitude is estimated to oscillate between 1000 and 1200 meters, or at most, between 1000 and 1400 meters. Otto Bürger assigns an area of 3,485,000 sq km to the complex of lowlands, somewhat higher than the figure referred to above, accepted instead by Theodoro Sampaio (see Hydrography of Brazil, in the Diccionarioof the Instituto Historico e Geografico Brasileiro, 1922), according to which, the isoipsa of 200 meters starting from Cape S. Rocco goes to the Cachoeira (cataract) de São Antonio do Madeira, follows the Guaporé valley and that of Paraguay: therefore, passing through Cuyabá, Miranda, Villa Rica, Salto Grande do Uruguay, São Borja and San Leopoldo, it approaches the Atlantic coast towards the 30th southern parallel, subsequently curves, with general direction NE., between the reliefs of the Serra do Mar and the coast, and finally to the north of Capo Frio, leaves to the east towards the ocean, a strip of lowlands from 60 to 100 kilometers wide.