Popular Science Monthly/Volume 81/September 1912/Wind-Graved Mesas and their Message
|WIND-GRAVED MESAS AND THEIR MESSAGE|
STRANGE and striking are the positive features of landscape presented by the continental divide in our southern arid country of New Mexico and Arizona. Wildest, least visited and most desolate section of our land is this, over which to-day still roams at will the aborigine in numbers greater for size of area than was ever known in any part of our realm since advent of European. Yet it was this very portion of our broad domain which was already settled by old-world men within fifty years after the landing of Columbus on Salvador. Before that time for more than twenty centuries there flourished within that region a peaceful and highly cultured race.
Throughout most of our desert lands the smooth illimitable plains are thickly studded by short, isolated, yet lofty mountain-ranges, which rise from the sea of earth as volcanic isles out of a glassy ocean. Desert-ranges form a distinctive mountain-type. As relief characters they attract wide attention from traveler and scientist alike, for they are the most impressive of the local features of topography.
In that part of the arid country of which we now speak there are none of these high mountains. The "Inselberglandschaft," as the Germans call it, still persists, but in different form. Instead of majestic peaks and lofty ranges there are lower truncated hills which rise even more abruptly from the general plains-surface. Mesas, or "tables," the Spanish-speaking settlers aptly denominate them. (Fig. 1.) The region is preeminently a mesa-land; therefore one of the most interesting and geologically one of the most instructive in all our domain.
That any part of a great continental divide should be a vast plain in place of a towering mountain ridge is primarily a result of geologic structure and secondarily of peculiarity of climate. In western New Mexico the broad plain occupying the divide is 7,000 feet above tide. So even is it that in crossing one is unaware of the time when he ceases to ascend on the Atlantic slope and begins to go down on the Pacific side. The ocean-to-ocean railway excavates its grade on the top of the continent only a scant half-dozen feet—one of the shallowest cuts on its entire line.Mesas of the mesa-land impart to the landscape features entirely novel. Nowhere else on the face of the earth do they reach such notable
development. They appear as even-topped surfaces more or less well elevated above the general plains-surface about. The margins of these truncated mounds form the brow of a precipitous escarpment which is one of their most characteristic features. Not infrequently the upper part of the escarpment is a vertical wall 100, 200 or even 500 feet in height. Mesa de Maya (armored mesa) and Llano Estacado (walled plain) are Spanish descriptive terms referring especially to this feature. The talus-like slopes below are the steepest of any angle of repose; and their meeting with the general plains-surface is as sharp as the strandline.
Mesa profiles and proportions are mainly functions of the geologic structure and of age. Some of these plateau-plains are so small in area and so high that they stand boldly out of the plain as conspicuous cones, or buttes. The Camaleon and Wagon-mound are illustrations. Others, as the Tooth of Time, the Enchanted Mesa (Fig. 2), the Covero, and the Sunset Tanks buttes are only a few acres in areal extent. The famous Toyalané (Fig. 1) and some of its neighbors are somewhat larger. From these to the great Chupadera Mesa and the Mesa Jumanes, which are a dozen miles across and a score of miles in length, or the vast Mesa de Maya, which extends along the northern border of Xew Mexico a hundred miles, there is every size.
Of the mesas of this description the foundation is generally some rock-layer more indurated than the rest of the section. Structurally they may be made up of (1) remnants of former plains worn out on the bevelled edges of folded strata, as in the case of the Mesa Jumanes; (2) slightly inclined strata of hard limestone or sandstone usually, which are intercalated in extensive beds of less resistant materials, as in the Chaca Mesa and other platform plains of the great Mesa Verde region; (3) almost horizontally disposed hard beds from which the soft superposed layers have been stripped, as the Toyalané, El Moro, the Tooth of Time (Fig. 3), and the Tucumcari; (4) old lava-sheets which cover soft shales and sandstones of which the Mesa de Maya, Mesa del Datil and Acoma Mesa (Fig. 4) are conspicuous examples (Fig. 5); and (5) surface-wash deposits locally hardened through the evaporation of moisture in the soil, leaving cemented lime-salts near the surface of the ground (Fig. 6), well represented by the Galisteo Ceja, south of Santa Fe.The origin of most flat-topped hills is commonly ascribed to circumdenudation effects on an upraised peneplain. All remnants of the old graded surface are on the same level. Throughout the arid region the mesas or plateau-plains, which rise above the general plains-surface, also appear to be the direct result of circumdenudation, but of a very different kind. In marked contrast to the humid-land effects the remnantal plains of the desert, whether their surfaces be formed of stratum-planes,
beveled tables of flexed strata, lava-sheets, or cemented regolith, are of quite different elevations even in the same district. In New Mexico, for instance, these plains attain all altitudes above the general plains-surface, from a few feet in the case of the very recently formed Malagro malpais, in the Hueco bolson northeast of El Paso, to the broad Mesa de Maya which is 3,500 feet above the general plains-surface, and 9,000 feet above the sea-level. The Sierra del Datil in western New Mexico has a magnificent northward-facing escarpment 1,000 feet high; and in sight of it is the Acoma Mesa 500 feet above the plains floor (Fig. 7).
Toyalané is a conspicuous flat-topped mountain situated just over the continental divide. The region is the largest, highest and driest desert plain in this country. Structurally and topographically it constitutes an essential section of the great Colorado dome, arching from the Rio Grande to the Rio Colorado. Save in one place—the Zuni Swell—its surface is unbroken by tectonic features. In this mesa-land the plateau-plains stand at many different heights above the general plains-surface. To this particular great genetic significance is attached.
Several notable peculiarities distinguish the Colorado dome. Around its southern slope there is, as Gilbert observes, one of the great lavatracts of the world, second in magnitude in our country only to the great northwestern lava-field, and fifteen times as large as the classical district of extinct volcanoes in central France. Sweeping in a broad crescent, 250 miles long, with the lofty cone of San Mateo on one horn and the towering San Francisco volcano on the other, the main body of lava-flows superposed in countless numbers, covers an area half the size of New York state. Beyond the borders of the crescent are numberless cinder-cones, coulees, and minor lava-sheets which spread out over the soft sedimentaries constituting the chief substructure of the plains in this part of the country. Farther west, in Arizona, the hard carboniferous limestone is the principal surface-rock, the shales once overlying it having been recently stripped off. Other volcanic evidences
are the denuded necks and dikes. These lava-sheets form the foundation of many a notable mesa.
Outside the limits of the lava-fields the massive and more indurated beds which are included between the thick sections of weaker strata take the place of the lava-flows in the formation of the mesas.
In the region under consideration extravasation of lavas has evidently gone on at frequent intervals from the very beginning of Tertiary times almost, it may be said, to within the memory of men still
living. The older trachytic and andesitic lava-sheet of the San Mateo, or Mt. Taylor district, now stand 1,000 feet above the country around, and upon this mesa rests the old volcanic cone itself, higher and more impressive than Vesuvius. To the north of this peak, which rises 13,000 feet above sea-level, there are abundant evidences of still earlier volcanic activities as shown in the forest of volcanic-necks of that area, from which is swept almost every vestige of their cones and the plains upon which they stood (Fig. 8). Cabazon, a huge volcanic pipe, stands 1,200 feet above its base and is a landmark for eighty miles about.
Much younger and 500 feet below the San Mateo plain is Acoma mesa, 30 miles long and 15 miles wide, capped by basalt. At its foot, another 500 feet down, is a great basaltic flow, 50 miles long by 20 miles broad, covering the present plains-surface. Even more recent are the coulèes from the Tintero, on the Mesa Redonda, west of the San Mateo, that finally enter the channel of the Rio San José at a level considerably below that of the great flow already mentioned.
Four distinct and notable periods of volcanic extravasation are thus recorded, between the first and last flows of which more than 1,000 feet of rock were removed from the entire region about. There are in the district many other lava-flows at other elevations; but between the four especially mentioned definite time-relations are readily established.
At the present time particular interest attaches to the mesas and their origin. Normal water-corrasion manifestly did not accomplish
the strange sculpturing of the country. In these relief features we seem to be introduced to an erosive force as potent as water but which we are just beginning fully to appreciate. Mesas appear to furnish the most direct and convincing testimony we have of the tremendous power of the wind in affecting general erosion under conditions of aridity.That water could not possibly produce such effects is shown in a number of ways. On the continental divide the streams are their smallest. On a vast plain so situated drainage features are necessarily insignificant. Rainfall is the scantiest. These three conditions combined with arid climate give water-action small opportunity to vigorously erode. On every hand the country clearly shows it. It is equally
manifest that notable leveling and lowering has gone on at a rapid rate. Since the San Mateo and Datil plains-surfaces were flooded with lava vast degradation has taken place.
Extensive erosion is everywhere manifest but of peculiar type. There is little of the sharp incision of the plains-surface such as normally characterizes stream-action, especially in a high-lying region having slopes of high gradients. Erosion is of the broad-basin type—wide, flat-bottomed, even plains between abruptly upturned rims against resistant rock masses. As the lava-flows and coulees became more and more numerous the separate basins became divided and smaller, but general lowering of surface went on without interruption. There can be no question but that the lava-capped mesas at varying heights represent former levels of the general plains-surface.
In the outpouring of the molten rock over the surface of the plains the lava-streams naturally flowed down the lowest lines of the plains, but as removal of the weaker beds on either side took place each flow was soon left as an elevation. A particularly instructive instance is shown near the Zuñi pueblo, where an old valley is exposed in a section filled with basalt, the level of the latter in the mesa-face being several hundred feet above the present floor of the plain.
There is further strong evidence of the strictly eolic nature of the landscape sculpturing. These summit plains of the continent are a region of continual high wind and constant sand-storm. Nowhere else in the arid region of the southwest is wind-scour in active operation so advantageously viewed. Nowhere else in this country are deflative effects and desert-leveling so well displayed. Nowhere else in all the world is general lowering of an elevated country by the winds so strikingly presented. Few places there are on this continent where stream action as a general erosional power is so manifestly utterly impotent.
That this high, dry, almost waterless waste on the continental divide should owe its landscape features chiefly to the incessant blowing away of the dry pulverulent soils seems to need little argument in this place. In the case of a desert district where the rocks alternate in hard and soft strata a mesa capped by a more indurated layer might not always offer conclusive evidence in support of this contention. Against the mesas surfaced by lava-sheets of diverse ages such objections can not be raised.
The existence of desert mesas whose surfaces stand at many different levels throughout the broad belts of the less resistant rocks appears to furnish one of the strongest proofs of the eolic character of the regional erosional activities; since in situations of this kind not only are rainfall and water-action very deficient and wholly inadequate to produce the relief effects presented without the time-element be vastly and unreasonably prolonged, but conditions are such in many cases as absolutely to preclude the intervention of stream-work.
By wind-action alone there appears now to be incontestable testimony that from the entire area of the vast arid region there has been lifted and exported by the winds in very recent geologic times a prodigious layer of rock not less than 5,000 feet in thickness. With a continuance of the present climatic conditions another volume of rock-materials of equal dimensions seems allotted to be similarly transported before the effects of the process shall become appreciably diminished.
Thus it is that the arid regions have introduced to us an erosive agent more potent than stream-corrasion, more constant than the washings of the rains, more extensive and persistent than the encroachments of the sea. The present conception of general eolic erosion, a sculpturing power in every way comparable to erosion by river and by ocean, appears destined to take its place among the first half-dozen great and new thoughts which shall especially distinguish geologic science of the twentieth century.