DUNES AND ISLANDS 245 



to alternate beds of clay and sand. Below Rosario 

 the river gradually loses its power. The islands become 

 more stable and flatter. Clumps of willow and spiny 

 ceibos {Erythrina cristagalli) still cover the edges of 

 them, and sometimes spread over the interior. But 

 as the climate is now less humid, the vegetation fixes 

 the soil less firmly, and the wind becomes the chief 

 sculptor of the landscape. It heaps up the sand 

 during the low-water season, and makes dunes which 

 rise above the level of the greatest floods. These 

 dunes form an unbroken line along the land in the 

 southern part of Entre Rios, in the north of the main 

 arm, with ridges at right angles, advancing toward 

 the south, which rest upon the river clay ; like the 

 one which the Ibicuy railway follows across the flood- 

 able area. The cattle of the district take refuge on 

 the dunes during floods. During periods of drought, 

 on the other hand, they retain a quantity of water, 

 and this is drawn from surface-wells at their base. 



The limits of the zone of the river are clearly 

 marked on the whole of the lower Parana. It is 

 enclosed on both sides by high barrancas (cliffs), 

 vertical in places where the main current washes 

 their feet, but sloping slightly where there is only a 

 secondary arm with little erosive power. The cliff 

 is broken only at the confluences of small valleys, 

 the flat, filled-up bottoms of which are on the level 

 of the alluvial plain of the Parana. The cliffs are 

 at their highest in the district of Villa Parana, where 

 they rise in places to a height of 300 feet. On the 

 right bank the cliffs show a section of the upper 

 layers of the Pampean clays. On the left bank there 

 are aeolian clays only at the top of them. Below 

 these are Tertiary marine strata (marls and sand- 

 stones with beds of shells). The cliffs of the left 

 bank stretch northwards, with a few breaks, as far 

 as Corrientes, and even into Misiones. Their height 

 gradually diminishes, and the Tertiary marine strata 



