On the right bank, on the contrary, the Parana has no tributaries of any importance except at the extreme north of the country. The scarcity of running water is, in fact, one of the characteristic features of the plain of the Pampas. Except in the east, along the Parana, where a network of permanent streams develops on a comparatively impermeable and fairly humid soil, and except at the foot of the mountains, where irregular torrents and streams, swollen after a storm and scanty in the dry season, disappear, as a rule, within sight of the hills that gave them birth, there is no superficial organized drainage. As a whole, the alluvial covering of the Pampas, the upper beds of which are cut through by the _barranca_ of the Parana, is not of river origin; it was brought and distributed by the wind, which took the place of running water. The clay of the Pampas is a present from the winds. The increasing dryness of the climate toward the west, as one approaches the Cordillera, explains the feebleness of the erosion by water and the extent of the erosion by wind. It is aridity, too, that gives their particular character to the Argentine Andes. They have little trace of perpetual snow, the lower limit of which approaches to within about four miles of the Bolivian frontier. There are no glaciers there; they reappear in the south only in the latitude of San Juan and Mendoza, on the flanks of the three giants of the southern Cordillera, Mercedario, Aconcagua, and Tupungato. Below the small number of steep furrows which the glaciers have carved, and usually up to the top of the mountain, there spreads what has been called, very expressively, "the zone of rubbish." In this the winter's snows, fretted by the sun in that clear atmosphere, form those multitudes of narrow pyramids which the Argentinians compare to processions of white-robed pilgrims. The underlying rock is rarely visible. It is covered with a thick cloak of rubbish, split off by the frost, which the slow-moving waters released by the melting of the snows heap up at the foot of the slopes, at the bottom of depressions. The half-buried summits are succeeded by basins of accumulation. In the valleys round the mountains there are immense beds of detritic, half-rounded shingle. The torrents have cut their way through the alluvial mass, and they flow at the foot of high terraces which mark the sites of former valleys.