The surface of the Patagonian tableland is very uneven, though it bears traces of having been much worn by the agencies of its desert climate, which seems to have lasted through the whole Tertiary Era. Going up the Rio Negro, one sees the grey sandstones and Tertiary tufas which form the cliffs, on both sides of the lower valley. They give place higher up to the variegated marls and red sandstones of the Cretaceous which form the tableland at the foot of the first Andean chains. The core of ancient granites and porphyries crops up at places from under the mantle of Cretaceous and Tertiary sandstones. The horizon of the peneplain passes from the Tertiary and Cretaceous tableland to level masses of crystalline rock, the contour of which has been almost entirely effaced. Volcanic eruptions have occurred until quite recent times, and so eruptive areas are the salient features of the tableland, at Añecon and at Somuncurra, south of the district of the Rio Negro, in the ridge on the left bank of the middle Senguerr, in the Chubut province. The basalts have spread out in sheets, the surface of which seems to have cooled not long ago. Basalt flows are found as far as northern Patagonia, south of Valcheta and Maquinchao; but their chief seat is in eastern Patagonia. They cover the inhospitable tablelands to the east of Lakes Buenos Aires and Pueyrredon. The Rio Chico and the Santa Cruz cross them for the upper two-thirds of their course, South of Coile and Gallegos they spread almost to the coast, and the Tertiary Pampas in this part are dominated by an archipelago of small volcanic cones.
The tableland is crossed from west to east by deep and broad valleys, enclosed between high cliffs, often strangled by ridges of basaltic or crystalline rock, and very little ramified. The ravines (cañadones), which make breaches in their cliffs on both sides, go only a little way into the sandstone Pampa or the lava tableland. Only a certain number of these valleys are occupied by important rivers (the Rio Negro and the Santa Cruz, for instance) which are born in the Andes, but receive little addition from the light rains of eastern Patagonia. Most of the valleys have only intermittent streams (Sheuen, Coile) or are altogether dry and sown with salt lakes (Deseado). The west wind is now the ruler of this network of fossil valleys. It carves their slopes, and brings into them sand, with which it makes dunes.
We must not confuse with these dead valleys the long depressions, with no outlet, which are scattered over the granite and sandstone tableland (bajos, valles, cuencas). Some have obstinately, but wrongly, sought in these the traces of rivers that have disappeared; and the bajos of Gualicho and Valcheta have wrongly been regarded as the former bed of the Rio Negro and the Limay. Erosion by wind seems to have had something to do with these depressions. Their persistence, at all events, is one of the effects of the aridity which prevents normal erosion from moulding the surface of the tableland. The chief of them are centres for collecting running water. There is a group of valleys all round them, and alluvial beds accumulate in them.
The climate determines the character of the soil in Patagonia. The rounded pebbles of granite and eruptive rock, so often described since the time of Darwin, sometimes free and sometimes embedded in red sand or limestone,[51] are spread over the tableland like aureoles round the masses of rock, and they are particularly abundant in the coast region. On the Rio Negro they seem to be confined to the vicinity of the valley; they disappear as one goes away from it. The progressive reduction in the volume of the Rio Negro gravels, as one goes downward, has been observed to begin in the Andean zone, and it is from the Andes that they come. South of Santa Cruz, in a moister climate, in which the circulation of the water is less localized, the bed is more continuous, and it covers the Tertiary sandstones and clays. It is of fluvio-glacial origin, and comes from the destruction of the old moraines, before the excavation of the actual valleys. But it is the wind that explains the concentration of the gravel at the surface. It separates the pebbles from the more mobile material about them. Wherever the outcrop-strata contain pebbles, the wind eventually converts the place into a field of shingle. It has done this with the terraces of the Limay. The Tertiary marine deposits of the coast region also are rich in pebbles torn from the rocky promontories of the shore; hence the extent of stony soils in the coast region. The wind similarly strips naked the angular stones, of local origin and incompletely worn, round the isolated rocks of the desert tableland or on the flanks of the secondary ravines.
On the other hand, the bedding action of the wind creates deposits consisting of small and uniform elements from the sands of the dunes to the finest dust. The lightest particles, caught up repeatedly by the squalls and carried to a great height in the atmosphere, go beyond the Patagonian region and reach the bottom of the Atlantic or the plain of the Pampa. Some of this, however, is deposited in the depressions of the tableland, where the moisture fixes it and prevents the wind from regaining it. These æolian deposits in the depressions, a dark-grey clay, which hardens when it is dry, but is softened by water, form two entirely different kinds of soil. If the depression is closed in, or if the circulation of the water is too slight, there is a concentration of the mineral salts; this is the salitral, either naked or sustaining a halophytic vegetation, which the saline efflorescences cover with a white coat during the dry season. If on the other hand, the underground waters have a free course, the æolian clay forms the mallin. Bushes and fine grasses grow on it, and, as they decay, gradually give it a darker shade and modify its composition. The soil above the mallin is rich in organic elements. It covers the bottom of the valleys between low terraces, covered with faceted pebbles, and dominated by the vertical cliffs of tufa and lava. The contrast between the verdure of the mallin and the arid, dusty, yellow steppe of the tableland is one of the most characteristic features of Patagonian scenery. The area in which mallin has been formed coincides with the most humid districts in the vicinity of the Andes and round the higher hills. On the road that runs along the right bank of the Limay, at some distance from the river, on the surface of the tableland, the limit between the country of the salitrales and that of the mallinas passes between Tricaco and Chasico, a hundred miles south-east of Neuquen; it almost tallies with the curve of a 200 millimetres rainfall.[52] Though the word mallin is not used at Santa Cruz, similar æolian soils are found in the western part of the tableland up to this latitude. Further south glacial deposits, clays with moraine-blocks, fill the valleys, and from Gallegos onward, cover the greater part of the tableland.
On the eruptive flows of recent date the rock is naked. The wind carries away the products of its decomposition, and the dust accumulates only in the fissures. Traffic is difficult, sometimes impossible.
Toward the west the tableland is separated from the Cordillera by a longitudinal depression, though the continuity of this has been exaggerated. This depression, which outlines the contact between the folded zone of the Andes and the flat zone of the tableland, is very important from the point of view of colonization. Just at the frontier of the steppe and the forest, it is the most hospitable part of Patagonia, the richest in natural resources. Amidst the glacial lacustrine deposits which are accumulated on it there rise masses of different kinds of rock which break it up into compartments, granitic ridges of laccolites exposed to view, eruptive structures that have been dismantled. In the south the sub-Andean depression forms a broad passage between Lake Maravilla and Puntá Arenas, about two hundred miles long, enclosed between the basalt cliffs of the tableland on the east and the mountains of the Brunswick Peninsula and William IV Land. The bottom of it is a singular glacial landscape, sown with lagoons, punctuated by scattered hills, with an impermeable soil of drift and mud. From Lake Argentina to Lake Buenos Aires the elevated tablelands, which rise to a height of 5,000 feet, back upon the Cordillera, and the sub-Andean depression is interrupted. Similarly, between Lake Buenos Aires and Lake General Paz the contour of the Patagonian tableland is not very marked above the sub-Andean zone. The glacial alluvia at the foot of the Cordillera rise to the level of the tableland, which sinks steadily eastward toward the Genua and the Senguerr. To the north, between Carrenleufu and Lake Nahuel Huapi, the retreat of the lakes has left long narrow beds right in the Cordillera, such as the Valle Nuevo del Bolson, the bed of which has been taken over by the Futaleufu west of the Cerro Situación. Further east the topographical features of the edge of the tableland (the valleys of the Chubut, Tecka, and Norquineo) lie from north to south. Hence within a space of little more than a hundred kilometres the sub-Andean zone has a series of parallel roads, communicating with each other by means of broad, transverse gaps, which at one time were occupied by the lower lobes of the glaciers. The sub-Andean depression does not go north of Lake Nahuel Huapi.