The basins of the Parana and Paraguay are separated by low mountain ranges extending north from the sierras of Paraguay.
The apparently uniform level of the pampas is much broken along its southern margin by the Tandil and Ventana sierras, and by ranges of hills and low mountains in the southern and western parts of the territory of La Pampa.
Other small rivers rising in the Cordoba sierras are the Primero and Segundo, which flow into the lagoons of north-east Cordoba, and the Quinto, which flows south-easterly into the lagoons and morasses of southern Cordoba.
The Cordillera, which bounds them on the west, is formed of folded beds, while the Sierras which rise in their midst, consist mainly of gneiss, granite and schist.
In the western Sierras, which are more or less closely attached to the main chain of the Cordillera, Cambrian and Silurian fossils have been found at several places.