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 Cordoba, 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 aeolian 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 Cordoba 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 aeolian deposits.