The need of irrigation is due to the scarcity of rain, but it is accentuated by a number of causes which tend to increase the aridity. The _valles_ are the scene of scorching day-winds, the _zonda_, like the _Foehn_ of the Swiss Alps, which, there being no snow, dry up the water of the springs and of the irrigation trenches, or use the deposits left by the waters to form dunes, which they push southward, sometimes like veritable glaciers of sand. Moreover, the soil of the _valles_ is generally composed of coarse and permeable alluvial deposits, which absorb the rain-storms immediately. There is at the foot of both sides of the hills which enclose each _valle_ an immense and far-lying bed of imperfectly rounded shingle. This double zone of detritus is strangely desolate, for the vegetation on it is restricted to isolated bushes of _jarilla_ and _tola_. From the sheepfolds on the mountains to the oases in the valleys one hardly meets a single house. The bed of the valley is not so desolate. A broad ribbon of sand marks the dry bed of a torrent, and on the clays of its banks, if the sheet of water underground is not too deep, one finds, in spite of the goats and asses and charcoal-burners, little forests of _algarrobas_, which the foundries use for fuel. The modern alluvial beds, gravel and sand, represent the upper stratum of a considerable series of continental deposits which lie on the Paleozoic crystalline rock of the Andes.[16] They chiefly consist of red sandstone and coloured marls, which crop up here and there through the alluvial covering and give the landscape a rugged character, worn by water and wind. There is no trace of humus: nothing to soften the vivid colours of the rock. Bodenbender, to whom we owe the first general attempt to classify the series, points out the importance of distinguishing the different strata in connection with the question of water supply and the conditions of human life.[17] A complete geographical study would have to follow the geological description in detail. In places--on the eastern edge of the Sierra de los Llanos--the fine modern clays are in contact with the granites of the hills and form above them a thick bed that is rich in fresh water. In other places--south-westward of the Sierra de la Famatina, as far as the Bermejo--the outcrop is of red sandstone only. The tablelands of Talampaya and Ischigualasta, which are cut across by the gorges of the tributaries of the Bermejo, form one of the most conspicuously desert regions in the whole Republic. Wherever the gypsiferous marls of the Calchaqui are near the surface, the springs are saline. The undulations of the impermeable rocky substratum bring to light the water that gathers in the alluvial beds. Thus the streams which come down the Famatina range in the west disappear in the alluvial beds on the fringe of the Sierra, but re-appear presently in the oasis of Pagancillo.