Popular Science Monthly/Volume 82/May 1913/Great Erosional Work of Winds
|GREAT EROSIONAL WORK OF WINDS|
DES MOINES, IA.
AS in the eighteenth century marine planation was one of the notable discoveries in earth-study, and as in the last century the theory of general peneplanation through stream-corrasion was one of the grander conceptions of the age, so the recognition of desert wind scour as the principal among erosional agencies seems destined to take its place among the first half-dozen great and novel thoughts which shall especially distinguish geologic science of the twentieth century. Under conditions of arid climate, by which more than one half of the land-surface of our globe is profoundly influenced, eolian erosion appears to become, as recently aptly stated, more potent than stream-corrasion, more constant than the washings of the rains, more extensive and persistent than the encroachments of the sea. Both as a sculpturing power and as a sedimentative agent the wind is thus in every way comparable to erosion and deposition by river and by ocean.
That it is possible for the universal disintegration of the rocks to go on by means of insolation instead of through ordinary chemical decay, that general and rapid exportation of rock-waste takes place through the agency of the winds instead of through the movement of waters, and that on the land deposition of wind-borne dusts in terranes
as mighty as any swept into the seas by streams or laid down on the floor of the ocean, are new and important generalizations belonging distinctly to the first lenstrum of our new century.
Prior to the year 1900 wind-action had been always regarded as merely one of the minor geologic agents of erosion—a mere idler in its manifestations, and a denuding power at all times negligible. That its real role in geologic economy had been so long so completely overlooked
appears to be due to the recency and enthusiasm with which by the scientific world the great law of the base-level of erosion had been received and to the vast dynamic possibilities which it had opened up.
Scant indeed is the attention given in the text-books on earth-history to the geologic effects of wind-action. A good and concise summary of our prevailing notions on the subject a decade ago is given by Udden. The great significance and value of the newer generalization lies not alone in the recognition of the geologic potency of wind-power as an agency of erosion, or as a means of forming such vast continental deposits as the loess, but of its tremendous efficiency as a general or regional denuding force. In far-reaching importance it compares favorably with the enunciation of the glacial theory of the last century.
It has long been the custom not only to treat the subject of general land-sculpturing independently of climatic considerations, but as if the molding of all landscape features was controlled by the same laws. The fertility of suggestion arising from the conception of a definite cycle of development through which all land-forms must pass has tended to exaggerate the evolutionary aspects of the theme at the expense of the genetic means by which the physiographic changes have taken place. Even the latest and most authoritative treatise on physical geography has premised the same derivation of physiognomy for the glacial Alps and the arid-high plateaux of western America, for the forest-clad Appalachians and the barren South African veldt, for the jungle-matted eastern Andes and the desert Australian interior. Ordinary stream-corrasion is made to account for all. Rain is regarded as the universal and sole graving-tool of land-sculpturing.
A full comprehension of the pregnant idea that wind-action under the favorable physical conditions imposed by arid climate is a general erosional agent may be said to date from the year 1904—the time of the appearance of Passarge's brief but quite remarkable essay on "Die Inselberglandschaften im tropischen Afrika." In various parts of the world during the decade previous the conception had in one way or another begun to assume form. The Trans-Caspian region had already furnished some facts bearing upon the new generalization. The vast deserts of the Dark Continent had supplied others. Our American arid lands had brought forth a host of still different suggestions. Indeed, as a definite working hypothesis the general scheme appears to have been first successfully formulated and applied in the great dry region of our own Southwest.Whether first definitely outlined by American on the Girghiz steppes, by German on the South African plateau, or by Yankee on the Mexican tableland, it is certain that, as McGee astutely observes, the satisfactory disposal of the rock-waste of the desert by prodigious wind exportation furnishes the missing link to a rational explanation of all the long puzzling phenomena presented by arid regions throughout the world.
The distinctive feature of this great new conception of regional eolation is that under the favorable climatic conditions of aridity such as effect more than one half of the entire land-surface of our globe windscour is the chief agency of provincial lowering and leveling, far more rapid and efficacious than any general work by rain, river or ocean. To it are ascribed all the larger lineaments characteristic of arid lands. By it are graved the majority of desert details. It is the dominant sculpturing power in all excessively dry regions.
In a district undisturbed by mountain-making forces even plains are produced, smoother than any peneplain possibly can be, and yet
standing at a level high above that of the sea; such are the Kalihari and elevated South African veldt, recently so graphically described by Passarge, Bornhardt and others. Elsewhere, when open-patterned orogenic structure prevails, broad valleys and lofty flat-topped highlands persist, as in Turkestan, lately noted by Davis, Huntington and Friederichsen. Our own southwestern country, with its close-patterned structure, presents still other phases, remarkable as the most perfect of all typical Inselberglandschaften.Singularly enough, the great law of the base-level of erosion, the most useful in all geologic science, had its birth under the cloudless skies of desiccated lands where in reality no vestige of its operations is discernible. The grand generalization applies strictly to land-surfaces under humid climates. Doubtless for this reason it is that none of our numerous government experts, in their fifty years' experience covering every part of the vast arid domains of the West, failed to perceive anything
of the potency of wind-action in the general leveling and lowering of the country. Of late years others who have traversed this field have caught occasional glimpses of deflative activity. Spurr notes certain minor aspects of it in the intermont basins of Nevada. Davis mentions others in Utah and Arizona. Cross calls attention to some notable phenomena in the San Juan district of southwestern Colorado that he mainly ascribes to wind-action. Hill describes still other features in northern Mexico. Free summarizes the literature on the action of the wind in the formation of soils. In none of these records of observation is the great principle of regional eolation recognized or even suggested. Through all of them the influence of the idea of peneplanation by water is overpowering.
No phase of land-sculpturing by water explains the peculiarities of desert relief. Where in humid lands are there such vast and even surfaces as the intermont plains of arid regions? Where under conditions of moist climate do such lofty mountains stand out so isolated as in our southwestern country—ideal monadnocks only theoretically and faintly suggested elsewhere? Where but in a dry climate does entire absence of foothills characterize the mountain ranges? Towering desert eminences rise out of elimitable expanse of level plain as volcanic isles jut from the sea. Plain meets mountain as sharply as the strand-line of the ocean. The rock-floor of the desert is often a plain itself worn out on the beveled edges of the strata beneath. The remarkable plateau-plains clearly represent former plains-levels. The soil-mantle is generally thin and gravelless; and all surface materials are transported. There is almost total absence of distinct waterways in the broad valleys. None of these relief characters bespeak of water-action of any kind. They all bear testimony of some erosive agency other than the one with which most of us are most familiar. Water can not do such geologic work. It seems to be a great advance in earth-study to be able at last to account satisfactorily for the formation of all those wonderful expressions on the face of the desert that have been so long so manifestly little understood or misinterpreted.
As in the case of ordinary general erosion, there are involved the three major processes of rock-weathering, transportation of rock-waste, and deposition of sediments, so in eolation there are the three corresponding phases termed insolation, deflation and aeroposition. Rock weathering in the desert is peculiar in that there is practically no chemical decay going on at the surface. The destruction of rock-masses is accomplished by means of a process known as insolation—a constant flaking-off of rock fragments due to the great diurnal range of temperature so prevalent in dry climates.
The movement and exportation of fine rock-waste through deflation
is now measurable to something of its true proportions. A "sand-storm" or "dust-storm" is really a strong desert air-current two or three hundred miles in width instead of a mile wide as in the case of the largest rivers, running forty miles an hour instead of three or four miles, and sweeping along a thousand times as much sedimentative materials. Only by such comparison is the enormous erosive potency of deflative action fully comprehended.
Wind-formed deposits are mainly laid down far outside the confines of deserts—in the moist verdure-clad lands, or in the sea. Their magnitude is very great, as the enormous loess formations, the vast expanses of black soils of the steppes and prairies, and the extensive adobe clays
of many parts of the world amply attest. Continental deposits of this origin are just beginning to receive from scientists the attention which they merit.
The law of regional eolation will rank high among modern geological discoveries. What William Smith's discovery of characteristic fossils for identifying geologic terranes was to stratigraphy and historical geology, what Bunsen's theory of magmatic differentiation was to modern petrology, what Agassiz's hypothesis of continental glaciation was to recent geologic history, and what Powell's law of the base-level of erosion was to physiography of to-day, so the general theory of deflation, or regional eolation, is to the sciences of desert landscape sculpturing, and the formation of continental deposits as vast as any laid down on ocean borders. The theory adequately explains a grander host of perplexing phenomena concerning the larger features of earth than any one of the great themes mentioned and perhaps more than all of them combined. It projects the imagination backward to the beginnings of geologic history; and it carries it forward to the end of time. In the lineaments of our dead moon it may be we behold the final effect of eolic powers.
Although perhaps not wholly the unaided work of any one man or group of men, the generalization of regional eolation is first distinctively American in origin. As such it seems not too much to say that it is allotted to stand as one of the far-reaching achievements of our century. It is doubtless the last of the great discoveries in geologic science to be attained by purely observational methods. The future advancements in earth-study must be quantitative instead of qualitative in character. They must be the direct outcome of mathematical investigation, of the rigid application of the new physico-chemical laws, and of the complete evolution which the discovery of radio-activity has imposed.
This, then, is briefly a statement of the theory of regional eolation.