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. [84] In the vicinity of San Luis and Cordoba the hard strata which are called _tosca_ are beds of eruptive ashes. 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 Bahia 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 aeolian erosion, like the depressions which are found on the plateau of the Colorado and the Rio Negro further south. [85] The surface of the _tosca_ tableland is further punctuated by a great number of closed depressions of various depths: long tunnels (_dolines_) which can only be explained, apparently, as an effect of the dissolving of the limestone by water.