The Andes of Southern Peru / Geographical Reconnaissance along the Seventy-Third Meridian

by Isaiah Bowman

Now while the alternations are as marked in Peru as in Chile, it is noteworthy that the Tertiary material in Peru is not only coarse throughout, even to the farthest limits of the piedmont, but also that the alternating beds are thick. Moreover, there are only the most feeble evidences of wind action in the lowermost Tertiary series. I was prepared to find curled plates, wind-blown sands, and muds and silts, but they are almost wholly absent. It is, therefore, concluded that the dryness was far less extreme than it is today and that full streams of great competency flowed vigorously down from the mountains and carried their loads to the inner border of the Coast Range and in places to the sea.

The fact that the finer material is sandy, not clayey or silty, that it almost equals in thickness the coarser layers, and that its distribution appears to be co-extensive with the coarser, warrants the conclusion that it too was deposited by competent streams of a type far different from the withering streams associated with piedmont deposits in a thoroughly arid climate like that of today. Both in the second Tertiary series and on the present surface are such clear examples of deposits made in a drier climate as to leave little doubt that the earliest of the Tertiary strata of the Majes Valley were deposited in a time of far greater rainfall than the present. It is further concluded that there was increasing dryness, as shown by hundreds of feet of wind-blown sand near the top of the section. But the growing dryness was interrupted by at least one period of greater precipitation. Since that time there has been a return to the dry climate of a former epoch.

Uplift and erosion of the earliest of the Tertiary deposits of the Majes Valley is indicated in two ways: (1) by the deformed character of the beds, and (2) by the ensuing coarse deposits which were derived from the invigorated streams. Without strong deformations it would not be possible to assign the increased erosion so confidently to uplift; with the coarse deposits that succeed the unconformity we have evidence of accumulation under conditions of renewed uplift in the mountains and of full streams competent to remove the increasing load.

It is in the character of the sediments toward the top of the Tertiary that we have the clearest evidence of progressive desiccation of the climate of the region. The amount of wind-blown material steadily increases and the uppermost five hundred feet is composed predominantly, and in places exclusively, of this material. The evidences of wind action lie chiefly in the fine (in places fluffy) nature of the deposits, their uniform character, and in the tangency of the layers with respect to the surface on which they were deposited. There are three diagnostic structural features of great importance: the very steep dip of the fine laminae; the peculiar and harmonious blending of their contacts; the manner in which the highly inclined laminae cut off and succeed each other, whereby quite bewildering changes in the direction of dip of the inclined beds are brought about on any exposed plane. Some of these features require further discussion.

It is well known that the front of a sand dune generally consists of sand deposited on a slope inclined at the angle of repose, say between 30° and 35°, and rolled into place up the long back slope of the dune by the wind. It has not, however, been generally recognized that the angle of repose may be exceeded (a) when there exists a strong back eddy or (b) when the wind blows violently and for a short time in the opposite direction. In either case sand is carried up the short steep slope of the dune front and accumulated at an angle not infrequently running up to 43° and 48° and locally, and under the most favorable circumstances, in excess of 50°. The conditions under which these steep angles are attained are undoubtedly not universal, but they can be found in some parts of almost any desert in the world. They appear not to be present where the sand grains are of uniform size throughout, since that leads to rolling. They are found rather where there is a certain limited variation in size that promotes packing. Packing and the development of steep slopes are also facilitated in parts of the coastal desert of Peru by a cloud canopy that hangs over the desert in the early morning, that in the most favorable places moistens even the dune surfaces and that has least penetration on the steep semi-protected dune fronts. Sand later blown up the dune front or rolled down from the dune crest is encouraged to remain near the cornice on an abnormally steep slope by the attraction which the slightly moister sand has for the dry grains blown against it. Since dunes travel and since their front layers, formed on steep slopes, are cut off to the level of the surface in the rear of the dune, it follows that the steepest dips in exposed sections are almost always less than those in existing dunes. Exceptions to the rule will be noted in filled hollows not re-excavated until deeply covered by wind-blown material. These, re-exposed at the end of a long period of wind accumulation, may exhibit even the maximum dips of the dune cornices. Such will be conspicuously the case in sections in aggraded desert deposits. On the border of the Majes Valley, from 400 to 500 feet of wind-accumulated deposits may be observed, representing a long period of successive dune burials.

The peculiar blending of the contact lines of dune laminae, related to the tangency commonly noted in dune accumulations, is apparently due to the fact that the wind does not require a graded surface to work on, but blows uphill as well as down. It is present on both the back-slope and the front-slope deposits. Its finest expression appears to be in districts where the dune material was accumulated by a violent wind whose effects the less powerful winds could not destroy.

It is to the ability of the wind to transport material against, as well as with, gravity, that we owe the third distinct quality of dune material, the succession of flowing lines, in contrast to the succession of now flat-lying now steeply inclined beds characteristic of cross-bedded material deposited by water. One dune travels across the face of the country only to be succeeded by another.[54] Even if wind aggradation is in progress, the plain-like surface in the rear of a dune may be excavated to the level of steeply inclined beds upon whose truncated outcrop other inclined beds are laid, [178] . The contrast to these conditions in the case of aggradation by water is so clearly and easily inferred that space will not be taken to point them out. It is also true as a corollary to the above that the greater part of a body of wind-drifted material will consist of cross-bedded layers, and not a series of evenly divided and alternating flat-lying and cross-bedded layers which result from deposition in active and variable currents of water.

The caution must of course be observed that wind action and water action may alternate in a desert region, as already described in Tarapacá in northern Chile, so that the whole of a deposit may exhibit an alternation of cross-bedded and flat-lying layers; but the former only are due to wind action, the latter to water action.

Finally it may be noted that the sudden, frequent, and diversified dips in the cross-bedding are peculiarly characteristic of wind action. Although one sees in a given cross-section dips apparently directed only toward the left or the right, excavation will supply a third dimension from which the true dips may be either observed or calculated. These show an almost infinite variety of directions of dip, even in restricted areas, a condition due to the following causes:

(1) the curved fronts of sand dunes, which produce dips concentric with respect to a point and ranging through 180° of arc; (2) the irregular character of sand dunes in many places, a condition due in turn to (a) the changeful character of the strong wind (often not the prevailing wind) to which the formation of the dunes is due, and (b) the influence of the local topography upon wind directions within short distances or upon winds of different directions in which a slight change in wind direction is followed by a large change in the local currents; (3) the fact that all combinations are possible between the erosion levels of the wind in successive generations of dunes blown across a given area, hence any condition at a given level in a dune may be combined with any other condition of a succeeding dune; (4) variations in the sizes of successive dunes will lead to further contrasts not only in the scale of the features but also in the direction and amount of the dips.