The Argentine Republic: Its Development and Progress

by Pierre Denis

The region of the right bank of the Paraná (east of the Salado), which alone has a complete hydrographic network, must be considered apart. From the latitude of Rosario to that of Buenos Aires it is cut by flat-bottomed valleys which are sometimes a hundred feet deep. The excavation of these valleys is due to an upheaval which raised this part of the Pampa above the base-level. The rapids of the lower Carcaraña also bear witness to this resumption of excavation. Farther on an inverse movement has put the bottom of the valleys below this level, and led to their being filled up (lagoon deposits of the Lujanense of Ameghino). South of Buenos Aires the upheaval has been less important, and the valleys are not so deep. Some of them (middle Salado and its tributaries on the left bank) are now occupied by long lagoons with steep banks, branching along the side-valleys, and these owe their origin to the same negative movement, subsequent to the excavation of the valleys. The upheaval did not extend to the eastern part of the province of Buenos Aires south of the Salado, a low-lying flat area, badly drained, exposed to floods, the contour of which has been minutely studied in connection with the construction of a great network of drainage-canals. North of Rosario, on the only slightly permeable clay, the water circulates, after rain, not by means of valleys in the proper sense, but along broad and almost imperceptible depressions (cañadas) where the current is slow, and the water dries up in the dry season. Their general relations are not yet known.

The loose deposits of the Pampean offer little resistance to erosion. The cycles are run through rapidly, and the traces of earlier cycles are faint, and are soon effaced.[83]

An ancient erosion-surface, dissected by the existing valleys, has survived in the south-west of the Pampean plain, thanks to the presence on the surface of a sheet of hard limestone, the tosca. The tosca is the result of the concentration of calcareous elements contained in clay at the surface in a dry climate. The formation of it implies a prolonged stability of the surface on which it has accumulated. Like the deep decomposition-soils in moister regions, it indicates a peneplain on which erosion has ceased. The bed of tosca covers the whole district between the Sierra de Tandil and the Sierra de la Ventana, the south-western slope of the Ventana, and most of the area of the central Pampa. In the north it does not go beyond the line from Buenos Aires to San Rafael. Its eastern limit goes almost by Ingeniero Malmen, Monte Nievas, and Atreuco, where it joins the southern bank of the lagoons of Carhue and Guamini in the east.[84] In some places the tosca is about forty feet thick.

To-day the region of the tosca forms a plateau cut by narrow valleys, sometimes 200 feet deep, west of the Sierra de la Ventana and in the central Pampa. These parallel valleys, with few ramifications, generally lying south-west to north-east, open to the east upon the Pampean plain about the frontier of the Buenos Aires province. On the other hand, the southernmost of them begin at the foot of the Ventana, and seem to blend in the south-west with a general depression that is still little known, though it appears to end at the bottom of the estuary of Bahía Blanca. None of them has permanent running water.[85] The origin of the dry valleys of the tosca is one of the most obscure problems of the morphology of the Pampean plain. Perhaps they are due to æolian erosion, like the depressions which are found on the plateau of the Colorado and the Rio Negro further south.

The action of the wind in shaping the landscape is more clearly seen in the formation of the dunes. When one starts from Buenos Aires or Rosario, and gets beyond the region of the level Pampas, the dunes are the first feature to meet the eye on the surface of the plain. The first fresh dunes are encountered at Carlota, on the line from the Rio Cuarto; at Lavalle on the line from Villa Mercedes; and at Trenque Lauquen on the line from Toay. The dunes spread northward as far as the latitude of Mar Chiquita, but do not enter the Chaco. They are also found in parts of the scrub on the west, but their proper domain is the western border of the steppe, the upper part of the plain at the foot of the Sierra de Córdoba, the south of the San Luis province, and the central Pampa.

Any accident that causes the vegetal covering to disappear, such as the tread of cattle near a drinking place or an enclosure, is enough to set æolian erosion at work. The wind raises the sand in a sort of tossing sea. Then the dune assumes a circular shape. A depression appears in the centre, and it deepens until it reaches the average level of the plain. Frequently there is a little lake in it. From this point onward the deformations are less rapid. The vegetation again creeps over the ground, and the dune falls a prey to the rains, which slowly reduce its mass.

In the central Pampa, where the elevation is considerable, the dunes do not form separate circular patches, but stretch in lines parallel to the valleys—sometimes in the heart of the valley, at other times backing against one of its slopes.

Far to the east of the zone of the quick dunes, in the south of the Córdoba province and the centre of the Buenos Aires province, there are certain soft undulations, covered with vegetation, with a sandier soil than that of the plain around them. These are dead dunes. The district of the dead dunes is characterized by the extreme irregularity of the surface-soil, the humus, which gains in richness and depth, as a general rule, as one goes eastward, because there it is in some places covered by recent æolian deposits.