These various shades of the Pampean climate are of essential importance in the history of colonization and the spread of cultivation. The belt of summer rain is the belt of maize-growing, whereas the cultivation of wheat requires spring rain and a comparatively dry summer.
While the isohyetic curves, which represent the precipitation for the whole year, are orientated from north-west to south-east, the curves of rainfall during the cold season, from April to September (dry season in the north), cut diagonally across the preceding, and are oriented directly north and south. Bahía Blanca receives in winter as much rain as Rosario, and General Acha (in the district of the central Pampa) as much as Córdoba. Unless one attends to this, one cannot explain the extension of wheat-growing, in the south-west, as far as the 400 millimetre curve, and even beyond it on the Atlantic coast.
The relief of the Pampean plain is known fairly accurately, thanks to the observations made along the railways. The ground rises slowly toward the west. The 100-metre curve describes a deep gulf some 300 miles west-south-west of Buenos Aires. The belt comprised between 100 and 150 metres above sea-level is more than sixty miles broad in the latitude of Santa Fé, and 130 miles in the latitude of Buenos Aires. Beyond the 150-metre curve the land rises rapidly toward the west and north-west, and reaches 400 metres in the Córdoba district and 500 in the Villa Mercedes district. It is at the altitude of 150 metres, and the break in the inclination which this marks, that the Rio Quinto is lost, near Amarga, south of General Lavalle.
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 Córdoba, the thrust of the plateau on the right bank of the Paraná, south of Villa Constitución 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 Córdoba 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.[78]
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 Córdoba 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.[79] This ridge isolates the eastern Pampa from the sub-Andean chains, and marks the limit of the area with sheets of underground water. In the north of the Pampean region, between the Sierra de Córdoba and the Paraná, the loose continental formations are more than 2,000 feet thick at Bellville, and more than 3,500 feet north-west of Santa Fé (fodder farms of San Cristobal and El Tostado). At Buenos Aires the granite has been found 985 feet below the surface.
THE PAMPEAN PLAIN. TRES ARROYES (BUENOS AIRES PRAIRIE BETWEEN THE SIERRA DE TANDIL AND THE SIERRA DE LA VENTANA).