While the agency of running water in transporting alluvia is confined to certain sections of the plain, the action of the wind is seen over its entire surface. The wind everywhere supplements or replaces running water. Like running water, it classifies the elements it conveys, and selects them according to their weight and size, the finest clays being deposited in the moist eastern zone and the coarsest sands in the sub-desert zone of the west. The mechanism of erosion explains this contrast. The grains of sand that are driven by the wind travel at the surface of the ground as long as the vegetation is too sparse to fix them. If one goes further east, to a moister district with a thicker vegetal carpet, the grains of sand no longer move at the surface of the ground, but the wind still carries fine particles of clay, which it bears to a great height. The bed of clay does not at all imply an arid climate, as is said sometimes, but corresponds to the region of the steppes, with moderate rainfall. It is, however, during dry seasons that the deposition of clay is at its greatest. Darwin mentions that after the droughts of 1827-1830 in the area round the Parana, the marks were buried under dust to such an extent that one could no longer recognize the limits of the various lands. Apart, however, from these sorts of floods or storms of dust caused by the _pampero_, the summer atmosphere is clearly laden with dust, which colours the skies in the east of the Buenos Aires province, as far as Entre Rios. The contour of the plain bears, like the soil, the double marks of erosion by running water and aeolian erosion. The rivers of the Pampa, when they leave the sierras, flow between high cliffs, the height diminishing as one goes downward. Presently these _barrancas_ become low, approach each other, and at last merely mark the banks of a larger bed which the floods fill. There is no trace of valleys. Bailey Willis, surprised at this weakness of watercourses that have, nevertheless, an appreciable fall, attributes it to the fact that the cycle of erosion opened by the last upheaval of the Pampa has not yet had time to penetrate into the interior. In reality, it means that here we are at the limit of the zone of erosion by running water, and that in this climate the essential factor in shaping the landscape is the wind.