Fig. 177—Composite structure section at Aplao.
For the later history it is necessary to assemble the various Tertiary sections described on the preceding pages. First of all we recognize three quite distinct types of accumulations, for which we shall have to postulate three sets of conditions and possibly three separate agents. The first or lowermost consists of even-bedded deposits of red and gray sandstones, the former color predominating. The material is in general well-sorted save locally, where lenses and even thin beds of conglomerate have been developed. There is, however, about the whole series a uniformity and an orderliness in striking contrast to the coarse, cross-bedded, and irregular material above the unconformity. On their northeastern or inner margin the sandstones are notably coarser and thicker, a natural result of proximity to the mountains, the source of the material. The general absence of wind-blown deposits is marked; these occur entirely along the eastern and northern portions of the deposits and are recognized (1) by their peculiar cross-bedding, and (2) by the fact that the cross-bedding is directed northeastward in a direction contrary to the regional dip of the series, a condition attributable to the strong sea breezes that prevail every afternoon in this latitude.
The main body of the material is such as might be deposited on the wide flood plains of piedmont streams during a period of prolonged erosion on surrounding highlands that served as the feeding grounds of the streams. The alternations in the character of the deposits, alternations which, in a general view, give a banded appearance to the rock, are produced by successions of beds of fine and coarse material, though all of it is sandstone. Such successions are probably to be correlated with seasonal changes in the volume and load of the depositing streams.
To gain an idea of the conditions of deposition we may take the character of the sediments as described above, and from them draw deductions as to the agents concerned and the manner of their action.
We may also apply to the area the conclusions drawn from the study of similar deposits now in process of formation. We have between the coast ranges of northern Chile and the western flanks of the Cordillera Sillilica, probably the best example of piedmont accumulation in a dry climate that the west coast of South America affords.
Along the inner edge of the Desert of Tarapacá, roughly between the towns of Tarapacá and Quillagua, Chile, the piedmont gravels, sands, silts, and muds extend for over a hundred miles, flanking the western Andes and forming a transition belt between these mountains and the interior basins of the coast desert. The silts and muds constitute the outer fringe of the piedmont and are interrupted here and there where sands are blown upon them from the higher portions of the piedmont, or from the desert mountains and plains on the seaward side. Practically no rain falls upon the greater part of the desert and the only water it receives is that borne to it by the piedmont streams in the early summer, from the rains and melted snows of the high plateau and mountains to the eastward. These temporary streams spread upon the outer edge of the piedmont a wide sheet of mud and silt which then dries and becomes cracked, the curled and warped plates retaining their character until the next wet season or until covered with wind-blown sand. The wind-driven sand fills the cracks in the muds and is even drifted under the edges of the upcurled plates, filling the spaces completely. Over this combined fluvial and æolian deposit is spread the next layer of mud, which frequently is less extensive than the earlier deposits, thus giving abundant opportunity for the observation of the exact manner of burial of the older sand-covered stratum.