The Pampean formation consists almost entirely of loose deposits, sand and clays of various sorts. There is no gravel.[80] Even in the vicinity of the sierras the beds of gravel, with round or angular pebbles, are almost always covered by clay, and are exposed only in the banks of the streams. Olascoaga mentions the surprise of the gauchos of General Roca's army when they found Patagonian pebbles on the ground during their stay at Choele Choel on the Colorado, in the course of the compaign campaign on the Rio Negro. Officers and soldiers dismounted to pick them up. Sand and clay form a thick bed of continental alluvia. The Tertiary maritime transgressions, which have left their mark in the clays and limestones of the left bank of the lower Paraná, and the layers of shells at San Pedro on the right bank, never penetrated far into the interior of the Pampean region, and one finds no trace of them when one leaves the coast or the river.
The source of the elements which compose the Pampean alluvia is very uncertain. Their composition does not clearly show their origin. The clays are comparatively rich in calcareous matter, which seems to indicate that they do not come from tropical America or the upper basin of the Paraná. Wright and Fenner insist upon the high proportion of siliceous glass of volcanic origin which they contain, which points to an intense eruptive activity during or before their formation.[81] Doering had already noticed in the Córdoba region the prevalence of beds of volcanic ash, which become thicker as one approaches the sierra. It is certain that the Pampean sierras have had their share in the formation of the Pampean beds. But the main mass is probably of Andean origin. However that may be, as soon as one gets away from the fringe of the mountains, the only variety noticeable in the lands of the Pampa is that which they owe to the conditions in which they have been deposited.
River deposits strictly so called, estuary deposits, lagoon deposits, æolian deposits, æolian deposits redistributed by water, river deposits redistributed by wind—all these different types are represented in the Pampean formation, but their relative importance is still disputed.[82]
When we confine ourselves to studying the actual conditions in which the deposits were formed, we are first struck by the poverty of the hydrographic network of the Pampa. It is slight except in the vicinity of the sierras, where the slope of the ground is pronounced, and in the eastern area, on the right bank of the Paraná and Entre Rios, where the climate is more humid, and the streams flowing over an impermeable soil more numerous. The only one of the streams born in the Pampean sierras that reaches the Paraná is the Rio Tercero or Carcaraña. All the others dwindle as they descend, and disappear in a low-lying district marked by lagoons which they only reach in time of flood. The floods themselves never bring the Rio Cuarto and the Rio Quinto and the Salado de Buenos Aires into touch with each other. The waters of the northern slope of the Sierra de Tandil, and even those of the Sierra de Curumalal, on the other hand, reach the Salado after the rains, either by way of streams which drain the strings of lagoons, or by flood-sheets, which spread over large areas.
The watercourses of the plain are unstable in their direction. The traces of their wanderings remain in the form of stretches of alluvial sand crossing the fine æolian clays. These river sands sometimes spread over extensive areas, the distribution depending upon a hydrographic scheme which is now partially effaced. The sands of the departments of General Lopez (south of the Santa Fé province) and General Arenales (Buenos Aires province), where the Salado is now developed, were probably brought by the Rio Cuarto, and mark an earlier junction of the Cuarto and the Salado. These sands run along the Salado as far as the confluence of the Saladillo, and the contrast between the light soil and the clay of the bank of the Paraná is so striking that the sand has long been regarded as a marine deposit, indicating an ancient shore. Along the Saladillo also, north-west of the Guamini lagoons, there is a sandy belt which corresponds with an important direction taken by the actual flow of the river, crossing the Bolivar and Veinte Cinco de Mayo departments.
While the agency of running water in transporting alluvia is confined to certain sections of the plain, the action of the wind is seen over its entire surface. The wind everywhere supplements or replaces running water. Like running water, it classifies the elements it conveys, and selects them according to their weight and size, the finest clays being deposited in the moist eastern zone and the coarsest sands in the sub-desert zone of the west. The mechanism of erosion explains this contrast. The grains of sand that are driven by the wind travel at the surface of the ground as long as the vegetation is too sparse to fix them. If one goes further east, to a moister district with a thicker vegetal carpet, the grains of sand no longer move at the surface of the ground, but the wind still carries fine particles of clay, which it bears to a great height. The bed of clay does not at all imply an arid climate, as is said sometimes, but corresponds to the region of the steppes, with moderate rainfall. It is, however, during dry seasons that the deposition of clay is at its greatest. Darwin mentions that after the droughts of 1827-1830 in the area round the Paraná, the marks were buried under dust to such an extent that one could no longer recognize the limits of the various lands. Apart, however, from these sorts of floods or storms of dust caused by the pampero, the summer atmosphere is clearly laden with dust, which colours the skies in the east of the Buenos Aires province, as far as Entre Rios.
The contour of the plain bears, like the soil, the double marks of erosion by running water and æolian erosion. The rivers of the Pampa, when they leave the sierras, flow between high cliffs, the height diminishing as one goes downward. Presently these barrancas become low, approach each other, and at last merely mark the banks of a larger bed which the floods fill. There is no trace of valleys. Bailey Willis, surprised at this weakness of watercourses that have, nevertheless, an appreciable fall, attributes it to the fact that the cycle of erosion opened by the last upheaval of the Pampa has not yet had time to penetrate into the interior. In reality, it means that here we are at the limit of the zone of erosion by running water, and that in this climate the essential factor in shaping the landscape is the wind.