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 sandstones 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 are replaced by granitic red sandstone.[125] On the right bank the height of the cliffs gradually diminishes up river. They are still conspicuous at the confluence of the Carcarana, but at Santa Fe they rise only about thirty-four feet. North of 31 deg. S. lat., and for some distance beyond Pilcomayo, the plain of the Chaco is very low, and it is impossible to define exactly the limit of the alluvial zone of the Parana. The fine clays, grey and white, which form the soil of the Chaco, reach the left bank north of Corrientes, in the _esteros_ of Neembucu. The red sandstone hills of the Asuncion district rise like an archipelago out of this level bed of lacustrine deposits. [125] In the space between the frontier of Entre Rios and the Rio Empedrado, south of Corrientes, the cliffs expose, above the red sandstone, beds of sand and clay, fluvial alluvia left by former beds of the Parana, the traces of which can be followed from the north-east to the south-west diagonally across the province of Corrientes. There is no obstacle to navigation in the entire stretch from Posadas to the falls of the Guayra on the Parana and the Salto Grande on the Yguassa. Sixteen miles below Posadas the Parana passes through a series of graduated rapids for about sixty miles (1,467 kil. to 1,558 kil. from Buenos Aires) wrongly called the Salto de Apipe. The current then rises to a speed of eight knots, and the depth is three feet at low water. These rapids are due to beds of melaphyre, which emerge amongst the granitic sandstone, and the water makes its way between large rocky islands. At Ituzaingo (1,455 kil.) the current loses force. There is, however, still a rocky bottom lower down, for ninety miles, at a depth of five feet. Below this the rock only appears on the left bank, and in a few ridges near the bank, or in isolated reefs which it has been easy to mark with buoys.