Provável origem das depressões observadas no Sertão do Nordeste

Resumo

            The author, geographer of the Conselho Nacional de Geografia, (National Council of Geography) presents a paper which resulted from field observations dealing with depressions.

            To the author, the traditional explanation according to which the depressions are the result of dissolution caused by subterranean circulation in soluble rocks, as limestone for instance, is not completely satisfying as thees depressions are also found in sienite and identical

to the ones observed in limestone regions.

            It becomes necessary, thus, the seeking of new explanations, considering that there are various manners by which a depression can be formed.

            The solution can be found after the different types of depression are examined through the study of the nature of the rock climate, topography, and all processes that could influence the formation and evolution of depressions.

            A peculiar type is the one found by the author in the Brazilian northeast: depressions directly over granites and gneisses. Of course, the processes which acted here are of various origins. As a first stage in the formation of a depression of this type, the author admits disaggregation.

            This initial depression was then worked over by the continuous action of weathering and its evolution completed. Water containing humid acids and other salts has an important and active role in the attack of minerals in the rocks; its action is sometimes a dissolving action.

            Depressions constitute one of the processes by which the evolutions of the topography in semi-arid regions or those with a tendency to semi aridity. The author considers a flat region - with lowered water divides where depressions are numerous and broad valleys - as the final stage in the evolution of these forms. Hartt describes these "lake - planes" in his "Geology and Physical Geography of Brasil".

            In zones where water is scarce, these depressions are points a thin population can settle.

            Sometimes men increase the capacity of these depressions by removing the clay from its bottom or constructing dams so as to minimize lack of water during the dry season.

            Finally, the author presents a preliminary classification of depressions according to their genesis. According to this classification, four main categories are recognized:

            1) Depressions formed by simple local displacement of the terrain.

            2) Depressions formed by the removal of the material of the terrain.

            3) Depressions formed by barrages and 4) Special cases. Each of these categories comprehends a series of genetical types in a total of 25 different species.