Popular Science Monthly/Volume 74/January 1909/Lineaments of the Desert
|LINEAMENTS OF THE DESERT|
DES MOINES, IA.
OUR notions of the genesis of desert landscapes have lately undergone complete revision. In land-sculpturing under conditions of aridity we are led to recognize some entirely new phases of geologic operations. The principles deduced are not alone applicable to countries with excessively dry climates, but likewise to all lands of the earth.
In the moister regions of the globe, or those parts with which the majority of us are most familiar, moving water is so universally regarded as the chief agent of denudation that other erosive means are seldom more than barely considered. In the arid districts there is a reversal of the relative efficiencies of the erosion processes. Water-action is of quite secondary consequence. Wind-scour, or deflation, is not only the most vigorous, but often almost the sole, erosive power.
The tremendous efficiency of wind as an erosive agent has been lately brought to general notice mainly through the results of Passarge's investigations in the South African deserts. His principal deduction is far-reaching in its scope and significance, and seems destined to stand in geology as one of the grand generalizations of the new century. In our own country it opens up vast and fertile fields of geologic inquiry.
The desert regions of earth have given to modern geography its most suggestive and fundamental concepts. This is a fact that is all but forgotten by most of us who are accustomed daily to apply these basic principles in the more familiar moist tracts in which we live. Yet the definite cycle of evolution which land-forms pass through, the base-level to which all erosion tends, and a general plains-leveling, without regard to sea-level, that goes on in dry countries, are deductions of the desert.True desert conditions prevail over a much larger proportion of our earth than most of us appreciate. Southwestern United States, the greater part of central Mexico, western South America, South Africa, northern Africa, southwestern, central and east-central Asia, eastern Europe and central Australia, all present vast areas of territory which have rain-fall insufficient to raise ordinary grain crops without artificial watering. The desert, however, is not the repulsive land that its name whenever mentioned suggests to the layman. Most of it is not
the barren waste of popular fancy. Vast tracts of it are certain to be reclaimed to the uses of mankind. Some of it is doubtless to become the most eagerly sought of all estates. Its beauties and its treasures are only beginning to be understood. Even scientists have only commenced to turn their attention seriously to the make-up, features and resources of the so-called desert regions.
The idea of a base-level of erosion, lying but slightly above tide, but below which stream-corrasion can not go, has done for geography what the principle of evolution has accomplished for biology. It is, in fact, the evolutionary principle applied to land sculpture. This theory of Powell's is justly regarded as one of the three grand deductions which geology of the century just past has bequeathed to science. Conceived midst the arid regions of the West, its widest influence has been in the moister countries of the earth. The second great geographic principle, that of the verity of a distinctly staged cycle of erosion comparable in a way to the periods of growth in the human individual, has done more than anything else to advance and place the study of geography on a truly genetic basis. It is to Davis we are chiefly indebted for devising for us a practical working scheme.
What the base-level of erosion is to the general theory of land degradation under conditions of a normal moist climate Passarge's great deduction of the possibility of general land leveling and lowering without regard to sea-level is to land sculpture under conditions of a dry climate. Until within the last lustrum the lineaments of the great desert regions of our globe have remained without adequate or satisfactory explanation. The genesis of the grander features of the landscape on the basis of ordinary tectonics, or of normal erosion during former wet climatic periods, or of water-action under present conditions, has always met with seemingly unsurmouutable obstacles. The origin of the salient features of the desert, its peculiar mountains, its smooth plains, its strange plateaus, its streamless surface, its remarkable rock-floor, and its many other unexpected features, are only beginning to be fully appreciated in their proper relationships. To us of the moister countries they present many novelties. They make us acquainted with the vigorous workings of geologic processes to which we are as yet almost complete strangers.
In the operations of the geologic processes under conditions of an arid climate the most noteworthy effects as compared with those under normal conditions are the prevalency, the constancy and efficiency of
wind-scouring, the very subordinate, local and sporadic character of water-action, and the remarkable plains-forming tendency which deflative erosion effects.
Since most of our conceptions of landscape genesis are derived from our experiences in a normal moist or wet climate, the erosion agency with which we are best acquainted is running water. In the desert regions actually and necessarily water plays but small part in erosion. With less than ten inches of annual rainfall, most of which sinks into the earth as soon as it touches it, as in the dry regions of southwestern United States and the northern part of the Mexican tableland, or less than one inch as in the Nubian and Libian deserts of north Africa, the erosive influences of water must be all but a negligible quantity.
With water-action reduced to relative impotency in the desert region, wind-scour assumes a rôle the denuding power of which has been heretofore little considered. Its action is general and constant. Its effects are probably even more vigorous than the work of water under normal climatic conditions. In the effort to reduce the land surface to a low-lying plain the belts of hard and soft rocks are brought into somewhat stronger contrast than in the case when water is the chief agency of. The geologic structures are more sharply accentuated. The rock-floors are cleaner swept. The belts of weak rocks are faster removed. At all times the plain is more characteristically the main relief feature.
Contrary to popular belief the great desert tracts of the earth are mountainous regions. The mountain character has many novel and instructive peculiarities. Yet so dominant is the plains feature locally that the mountain ranges, bold and lofty as they often are, rise sharply from the level expanse as do volcanic isles out of the sea. So characteristic is this aspect that it is, in the South African deserts, appropriately denominated the "Inselgeberglandschaft."
In regard to the manner of their development the salient lineaments of the desert deserve much more attention than ever has been given them. They acquire new meanings when their peculiarities are considered in the light of an origin eolian in nature. Notwithstanding the fact that some of us, whose lifelong experiences have been mainly with the workings of the geologic processes in the moister parts of the globe, may find it a little difficult to fully appreciate at first the direct significance of many of the details of the relief features, a visit to the desert soon convinces us of their verity. There are at least a score of these physiographic characteristics of the diy lands that are especially striking.
The dominant feature of such desert regions as the western part of our own country and of Mexico is the interrupted plain the general surface of which is 5,000 to 7,000 feet above the sea. Out of it rise abruptly the numerous mountain ranges to like heights above the plains-surface. Of this region four fifths are plain; one fifth highland. The vastness and evenness of the intermont plains is a matter of much speculation with all who travel the region, scientist and layman alike. Extensive desiccated lake-bottoms they are usually regarded. They are sometimes considered to be intermont basins deeply filled with wash from the peripheral highlands. At the present time the only adequate explanation of their physiognomy is that they are fashioned mainly by agencies with some slight modification by water.
The complete isolation of the different mountains is one of the most remarkable facts concerning the desert features. In most parts of the world there is some more or less close structural relationship between neighboring mountains which are often united to one another by foot-hills. In the desert there is no such tectonic connection. The mountain ranges are all independent individuals without relationship of any kind to one another. Structural mountains, volcanic mountains, laccolithic mountains, fault-block mountains and high residual plateau mountains are neighbors distinctly separated by stretches of level plain.
Mountain ranges throughout the dry regions of western America are completely and evenly surrounded by level plains as if by the sea. They are numerous, short and narrow. Upon the map Button has likened them to an army of caterpillars crawling northward out of Mexico, dividing as it enters the United States, the main body turning
westward and then northward again until it passes into the British possessions. The simile is as striking as it is apt.
To the average mountaineer, one of the strangest characteristics of the desert ranges is the absence of foot-hills. The main eminences of the mountains rise directly from the plains. The line where plain and mountain meet is as sharply defined as if drawn with a pen. Often this line is represented by high mural faces that can be scaled at but few points. For this reason it is, chiefly, that the mountains appear to be half buried by the drifting sands. It is on this account largely that the plains areas appear to be leveled by the waters of former seas, of which the mountains formed the coastal cliffs. The illusion is all but completed by the fact that the phenomenon is perfectly independent of geologic structure. This surprising feature is really one of the most novel peculiarities ofaction under conditions of aridity.
That the substructure of the intermont plains is made up of the softer or non-resistant rocks is an observation the full significance of which has been only lately appreciated. Under conditions of a moist or wet climate it is not an unlooked-for fact that the belts of weak rocks coincide with the valleys or lowlands. It is unexpected, however, that this is also true in an arid country, especially since the intermont plains are commonly considered as areas of extensive accumulations of mountain waste. On the whole they are now thought to be areas of most rapid degradation. The much stronger contrasts of belts of soft and hard rocks observable in the arid lands than in the moist lands appear to be due to this very fact. Then, too, wind-scour and not water-action must be reckoned with as the main erosional means, another fact that has not been usually taken into account.
Without exception the mountain rocks of the western deserts are very hard and resist the attacks of erosive action to an eminent degree. The plains rocks being mainly non-resistant rocks, as we have seen, there thus appears a notable alternation of hard and soft rock-belts. In the geologic succession the rocks composing the mountains are principally ancient crystallines and Paleozoic limestones which are followed by enormous thicknesses of soft sandstones and shales that frequently
attain a vertical measurement of 10,000 feet or more. Tertiary faulting on a grand scale has brought the soft strata to the same level as the resistant beds. In the general and profound wearing down of the surface of the country towards sea-level, marked contrasts of relief are produced between the various rock-belts. In the moist countries the ipolated residual eminences of general erosion, known to geographers by the special name of monadnocks, after the New England mountain regarded as the type, are of rare occurrence. In the desert region the majority of the mountains are of this character.
One of the most peculiar of the many odd features of the desert is the beveled rock structure of the plains-surface. In the moist regions a high-lying plain with the substructure beveled is taken as an indication of former peneplanation, the lowest level to which water can wear down the land. The intermont plains of the arid region of western America have beveled rock-substructures, but their surfaces are 5,000 to 8,000 feet above sea-level. The beveled character of the strata is almost conclusive evidence that these plains are not areas of extensive aggradation. In the absence of adequate water-action their origin must rest mainly in long-continued and effective deflation.
Critical examination of the substructure of the plains, in favorable places, shows clearly that the rock-floor itself is a plain. Although not always apparent at first glance this rock-floor is commonly only thinly mantled by wash débris and soil. Hence the rock-floor and the present plains-surface are not very different in their detailed relief characters.
A phenomenal feature of the desert plains is the plateau-plain. Mesas they are called in southwestern United States and Mexico. These mesas, as their Spanish name signifies, are extensive, fiat-topped, table-like areas rising abruptly from the general plain to heights of from one or two hundred feet to a thousand feet or more. The great Mesa de Maya, in northeastern New Mexico, is 3,500 feet above the next lower plain.The surface of the plateau-plain is usually found to be composed of some hard rock layer, as in the case of the vast Llano Estacado, or "walled plains," or staked plains as it is called by the Texans; or is made up of an extensive lava flow, as, for example, the Mesa de Maya, the Ocate mesa and the majority of the plains of this kind. The surface beneath the lava flows of the mesas is itself a plain worn out on
the beveled edges of the strata. The plateau-plain thus represents a former position of the general plains-level. It is the best example of circumdenudation through vigorous wind-scour.
The soil mantle of the interment plains is everywhere relatively thin. These plains instead of being areas of great accumulations of recent rock-waste, as might very naturally be expected, appear to be, as a rule, only thinly veneered. Often extensive areas are swept clean by the winds so that the rock-floor is exposed.
Nearly all of the finer surface detritus is transported from a greater or less distance. It is rare for the surface materials of the desert plains to give any suggestion of the rock-composition immediately beneath.
The gravely character of the intermont plains of the desert, to which travelers commonly allude, is largely only apparent. Most of the gravel-surfaced areas when upturned by the plow give excellent loamy fields. It is not generally recognized that the great abundance of pebbles on the surface of many plains is due to the fact that the strong and persistent winds blow away the finer materials, leaving a pebble mosaic behind.
While the rock-floor of the plains is itself a plain there are many inequalities in the surface. Between sheet-flood erosion and winddrifted sands all local depressions are quickly filled. The tendency of the surface mantle is thus merely to make the plains smoother than thev otherwise might be.
One of the most remarkable features which at once attracts the attention of the traveler is the general absence of distinct waterways in the valleys or intermont basins. Notwithstanding the fact that the gradients are high, no drainage systems are developed. Channel-ways that are corraded by unusual freshets, which sporadically occur, are quickly obliterated by the drifting sands and soils.
The degradation of the desert regions is not to be regarded as all accomplished by wind-scour. At times water plays a minor but important part locally. In the loftier mountain ranges normal torrential water-action takes place, much the same as it does in humid regions. The streams which are occasionally formed by heavy rainfalls soon sink into the ground on reaching the plain and become lost rivers. At other times, the stream-ways are without water for the greater part of their courses; arroyos, or dry creeks, the Spanish term them. McGee's vivid description of the advance of a flood-sheet over a piedmont slope is in reality the running of an arroyo as it enters a plain.
Water sometimes assumes another strange phase. The excessive local rainfalls which occur at rare intervals, "cloud-bursts" they are called, often form in the intermont valleys extensive shallow lakes. Most of these bodies of water are of short duration. When they are so situated as to be periodically renewed for a part of each year the areas become vast mud-flats, forming the playas of the Mexicans. Notwithstanding the fact that most of these lakes are ephemeral in character, they are sometimes long-lived. In the valley of the Rio Conchos, in the state of Chihuahua, the rainfall was once so heavy that in a single night a great lake was formed that lasted for more than eighty years.
A most singular phenomenon resulting directly from water-action in the desert is sheet-flood erosion. The vigor with which it often acts makes it locally an important plains-forming agent. Instead of the water from the rainfall tending to concentrate along certain lines of lowest level, as in humid regions, it spreads out over the sloping plains-surface. The railroads crossing the desert have yet to discover an effective method to overcome the disastrous work of the flood-sheet.
In the desert, however, extensive water-action is exceptional and relatively unimportant.
Not the least striking characteristic of desert regions is the marked absence of rock-weathering as we ordinarily know it. All exposed rock surfaces present an aspect of wonderful freshness. Chemical decomposition of rock-masses is all but unknown under the influences of a dry climate. The breaking down of the rocks is mainly mechanical, assisted somewhat by sand-blast action.
During the next decade the features of the arid portions of our earth promise to give us many novel ideas concerning the workings of certain of the geologic processes. To the scientist the desert will become one of the most interesting fields of research.