Rio Grande do Sul Physical Geography
Geology and relief
According to ezinesports.com, the state of Rio Grande do Sul presents, for the most part, low relief, with seventy percent of its territory at less than 300m of altitude. The only elevated portion, with more than 600m of altitude, in the northeast, comprises 11% of the total surface. Four morphological units in the state can be described: the coastal plain, the dissected plateau of the southeast, the central depression and the basaltic plateau.
The entire eastern façade of the state is occupied by the coastal plain, which consists of sandy terrains about 500km long in the northeast-southwest direction and very variable width. The sands develop both on the eastern and western shores of the Patos and Mirim lagoons. These lagoons have a characteristic design, with a lobed cutout, due to the sand tips that on either side protrude into them. Contrary to what happens inside the lagoons, the coastline has a regular layout. The coastal plain is constituted by the juxtaposition of coastal strands (restingas), which sometimes leave empty spaces occupied by elongated or bathed lagoons (formerly clogged lagoons).
Dissected southeastern plateau
Also inappropriately called the southeastern mountains, the dissected southeastern plateau comprises a set of undulations whose highest level does not exceed 500m. It is an old plateau, whose tabular surface has only been preserved between a few rivers. These Precambrian lands constitute the so-called shield of Rio Grande do Sul and occupy the entire southeastern portion of the state, forming a triangular area whose vertices correspond approximately to the cities of Porto Alegre, Dom Pedrito and Jaguarão. The complex is divided, by the valley of the Camaquã River, into two large units, one to the north and the other to the south, called the Erval and Tapes mountains, respectively. It is the typical domain of the campinas, whose best expression is found in the Rio Grande do Sul campaign.
Consisting of lands from the Paleozoic era, the central depression forms an arch around the dissected plateau of the southeast, surrounding it on the north, west and south sides. It forms a wide corridor with approximately fifty kilometers of average width and 770 km of extension, of which 450 in the east-west direction, 120 in the north-south direction and 200 in the west-east direction. The smooth topography and the low altitude in relation to the level of the sea (less than a hundred meters), allow to classify the central depression as a gently undulating plain.
The northern and western portion of the state is occupied by the basaltic plateau, which describes a half moon around the central depression. This plateau, which has the geological structure as a striking feature, is formed by the accumulation or stacking of successive basaltic spills (that is, lava spills), interspersed with sandstone layers. They reach very variable thickness. In the northeast of the state the maximum thickness is registered, responsible for the highest elevation of the plateau in this area.
The surface of the plateau has a general slope from east to west. In the northeast, close to the coast, it reaches its highest elevation, between 1,000 and 1,100 m; in Vacaria it reaches 960m; in Carazinho, 602m. In Cruz Alta, 469m; in the extreme west of the state, next to the bank of the Uruguay River, it does not exceed one hundred meters. The topography is flat or slightly undulating, but the rivers, which bathe the highest part, have opened deep grooves or valleys in it, isolating tabular compartments.
A salient feature of the plateau is the way of transition to the lower lands with which it articulates. To the northeast, it falls directly on the coastal plain, with a steep wall or escarpment, with almost a thousand meters of unevenness: they are the so-called “trimmed of the mountains”. The rivers favored by the steep slope opened deep gorges or taimbés there. In this stretch, close to the border with Santa Catarina, the escarpment at the edge of the plateau runs parallel to the coast. At Osório’s height, it veers sharply to the west and from there it gradually decreases in height. In this stretch facing south, the rivers that flow into the central depression opened wide valleys. The edge of the basaltic plateau receives in Rio Grande do Sul, as in the other southern states, the name of Serra Geral.