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. Bahia Blanca receives in winter as much rain as Rosario, and General Acha (in the district of the central Pampa) as much as Cordoba. 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 Fe, 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 Cordoba 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 Cordoba, the thrust of the plateau on the right bank of the Parana, south of Villa Constitucion 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.[78] [78] 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 Cordoba, 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."