Fig. 178—Plan and cross-sections of superimposed sand dunes of conventional outline. In the sections, dune A is supposed to have left only a small basal portion to be covered by dune B. In the same way dune C has advanced to cover both A and B. The basal portions that have remained are exaggerated vertically in order to display the stratification. It is obviously not necessary that the dunes should all be of the same size and shape and advancing in the same direction in order to have the tangential relations here displayed. Nor need the aggrading material be derived from true dunes. The results would be the same in the case of sand drifts with their associated wind eddies. All bedded wind-blown deposits would have the same general relations. No two successive deposits, no matter from what direction the successive drifts or dunes travel, would exactly correspond in direction and amount of dip.
Finally, we may note that a section of dune deposits has a distinctive feature not exhibited by water deposits. If the foreset beds of a cross-bedded water deposit be exposed in a plane parallel to the strike of the beds, the beds will appear to be horizontal. They could not then be distinguished from the truly horizontal beds above and below them. But the conditions of wind deposition we have just noted, and chiefly the facts expressed by [178] , make it impossible to select a position in which both tangency and irregular dips are not well developed in a wind deposit. I believe that we have in the foregoing facts and inferences a means for the definite separation of these two classes of deposits. Difficulties will arise only when there is a quick succession of wind and water action in time, or where the wind produces powerful and persistent effects without the actual formation of dunes.
The latest known deposits in the coastal region are found surmounting the terrace tops along the coast between Camaná and Quilca, where they form deposits several hundred feet thick in places. The age of these deposits is determined by fossil evidence, and is of extraordinary interest in the determination of the age of the great terraces upon which they lie. They consist of alternating beds of coarse and fine material, the coarser increasing in thickness and frequency toward the bottom of the section. It is also near the bottom of the section that fossils are now found; the higher members are locally saline and throughout there is a marked inclination of the beds toward the present shore. The deposits appear not to have been derived from the underlying granite-gneiss. They are distributed most abundantly near the mouths of the larger streams, as near the Vitor at Quilca, and the Majes at Camaná. Elsewhere the terrace summit is swept clean of waste, except where local clay deposits lie in the ravines, as back of Mollendo and where “tierras blancas” have been accumulated by the wind.
These coastal deposits were laid down upon a dissected terrace up to five miles in width. The degree of dissection is variable, and depends upon the relation of the through-flowing streams to the Coast Range. The Vitor and the Majes have cut down through the Coast Range, and locally removed the terrace; smaller streams rising on the flanks of the Coast Range either die out near the foot of the range or cross it in deep and narrow valleys. The present drainage on the seaward slopes of the Coast Range is entirely ineffective in reaching the sea, as was seen in 1911, the wettest season known on the coast in years and one of the wettest probably ever observed on this coast by man.
In consequence of their deposition on a terrace that ranges in elevation from zero to 1,500 feet above sea level, the deposits of the coast are very irregularly disposed. But in consequence of their great bulk they have a rather smooth upper surface, gradation having been carried to the point where the irregularities of the dissected terrace were smoothed out. Their general uniformity is broken where streams cross them, or where streams crossed them during the wetter Pleistocene. Their elevation, several hundred feet above sea level, is responsible for the deep dissection of their coastal margin, where great cliffs have been cut.