170 THE PLAIN OF THE PAMPAS 



striking that the sand has long been regarded as a marine 

 deposit, indicating an ancient shore. Along the Saladillo 

 also, north-west of the Guamini lagoons, there is a 

 sandy belt which corresponds with an important direc- 

 tion taken by the actual flow of the river, crossing 

 the Bolivar and Veinte Cinco de Mayo departments. 

 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, 



