166 THE PLAIN OF THE PAMPAS 



The ridge between the Pampa and the basin of the Salado 

 in the south of the San Luis province is about 450 metres 

 above sea-level. South of the province of Buenos Aires 

 the Sierras de Tandil and de la Ventana are joined 

 together by a ridge which does not fall below 200 metres. 

 Certain irregularities of the surface, such as the depression 

 of Mar Chiquita to the east of Cordoba, the thrust of 

 the plateau on the right bank of the Parana, south of 

 Villa Constitution and San Nicolas, can, apparently, 

 only be explained by recent tectonic movements. 



The Pampean deposits which cover the plain rest 

 upon a rocky base of which the salient representatives 

 are the sierras of the province of Buenos Aires and the 

 hills at Cordoba and San Luis. This base also appears 

 east of the Pampean basin in the granite island of 

 Martin Garcia, in the middle of the estuary of the Plata, 

 and in the hills on the coast of Uruguay. 1 



Underneath the even sheet of the alluvial deposits 

 the surface of the sub-Pampean platform is very irregular. 

 Its shape has been discovered by deep borings in search 

 of arterial waters. It has been warped and cut up by 

 faults, some of these deformations being probably 

 synchronous with the formation of the Pampean 

 deposits which have concealed them as they have been 

 produced. A subterranean rocky ridge continues the 

 Sierra de Cordoba southward and joins it with the 

 sierras of the Colorado. The granite emerges at 

 Chamaico, on the western railway, and on both sides 

 the borings have passed through great depths of clay 

 and sand. 2 This ridge isolates the eastern Pampa from 



1 While the Pampean deposits lie immediately on the crystalline 

 and Paleozoic formations in the sierras of the lower Colorado and 

 of the central Pampa, in the south of the province of Buenos Aires 

 and in Uruguay, they are, on the eastern edge of the Sierra de C6rdoba, 

 separated from it by red sandstones and conglomerates of uncertain 

 age, perhaps synchronous with the continental red sandstones of 

 Corrientes which outcrop east of the Parana and have been known 

 since D'Orbigny's time as " granitic sandstones." 



a At Rancul, in the east, 660 feet of loess overlying red sandstone : 

 at Telen, in the west, 2,800 feet of sand, marl, sandstone and gravel. 



