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 Parana 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 Parana is the Rio Tercero or Carcarana. 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 aeolian 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 Fe 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 Parana 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.