The distribution of the dead dunes is connected with the stretches of river sand across the Pampa, which have offered an easy victim to the winds. A line of dead dunes follows the upper course of the Salado in the district of Junin and Bragado. On the line from Buenos Aires to San Luis one crosses it between Chacabuco and Vedia, and then one comes again upon the horizontal plain, which has fresh dunes, only further west, at 120 miles from Villa Mercedes. Its elevation is so conspicuous on the level plain that the first breeders who used its pasturage gave it the emphatic name of the _cerillada_. D'Azara correctly appreciated the nature of it. "It is," he says, "only a dune of very fine sand." It is only a few yards high. The dead dunes of the Bolivar and Veinte Cinco de Mayo departments, which Parchappe described, have a more conspicuous relief, and in their disposition sometimes remind us of the fresh circular dunes with a central lagoon. The lines of coastal dunes in the eastern part of the Buenos Aires province obstruct the proper flow of the water there, and form a group apart, which must be clearly distinguished from the dunes on the plain.[86] [86] Outside the districts with quick and dead dunes, a frequent type of landscape on the Pampa is a plain thinly sown with very small lagoons, generally circular, between which develop a series of barely perceptible undulations. The inequality is at times so slight that one only notices it by the contrast between the vegetation of the lower and the higher ground. This type of landscape, which is especially seen in the district of Lincoln or of Nueve de Julio, is due to the action of the wind on a plain where the level of the underground water is near the surface. This level marks a limit below which aeolian erosion does not take place: a sort of base-level. The periodic variations of level of the underground water reduce or enlarge the undulations of the surface. Thus the impression of monotony which the Pampa makes in us is corrected to some extent by close observation. High and low land alternate on it. Parchappe himself had noticed the contrast between the area that stretches from Buenos Aires to the Salado, with its soft undulations and its well-developed hydrographic network, the horizontal plains on the right bank of the Salado, with their irregular dunes, and the southern plateau of the _tosca_ between the Sierra de Tandil and the Sierra de la Ventana. * * * * * We may now distinguish the following regions in the Pampa as a whole: