THE FEATURES IN DESERT LANDSCAPES

The wandering dunes.—Over the broad expanse of the desert, sand and dust, and occasionally gypsum from the saline deposits, are ever migrating with the wind; on quiet days in the eddying “sand devils”, but especially during the terrifying sand storms such as in the windy season darken the air of northern China and southern Manchuria. This drift of the sand is halted only when an obstruction is encountered—a projecting rock, a bush, or a bunch of grass, or again the buildings of a city or a town. The manner in which the sand is arrested by obstacles of different kinds is of great interest and importance, and is utilized in raising defenses against its encroachments. If the obstacle is unyielding but allows some of the wind to pass through it, no eddies are produced and the sand is deposited both to windward and to leeward of the obstruction to form a fairly symmetrical mound (Fig. 218 a). An obstruction which yields to the wind causes the sand to deposit in a mound which is largely to leeward of the obstruction (Fig. 218 b). A solid wall, on the other hand, by inducing eddies, is at first protected from the sand and mounds deposit both to windward and to leeward (Fig. 218 c and Fig. 219).

Except when held up by an obstruction, the drifting sand travels to leeward in slowly migrating mounds or ridges which are known as dunes. Their motion is due to the wind lifting the sand from the windward side and carrying it over the crest, from where it slides down the leeward slope and assumes a surface which is the angle of repose of the material. In contrast with this the windward slope is notably gradual, being shaped in conformity to the wind currents.

The dunes which are raised upon seashores, like those of the desert, are constantly migrating, those upon the shores of the North Sea at the average rate of about twenty feet per year. Relentlessly they advance, and despite all attempts to halt them, have many times overwhelmed the villages along the coast. Upon the great barrier beach known as the Kurische Nehrung, on the southeastern shore of the Baltic Sea, such a burial of villages has more than once occurred, but as in the course of time further migration of the dune has proceeded, the ruins of the buried villages have been exhumed by this natural excavating process (Fig. 220).

The forms of dunes.—The forms assumed by dunes are dependent to a very large extent upon the strength of the wind and the available supply of sand. With small quantities of sand and with moderate winds, sickle-shaped dunes known as barchans (Fig. 221) are formed, whose convex and flatter slopes are toward the wind and whose steep concave leeward slopes are maintained at the angle of repose. The barchan is shaped by the wind going both over and around the dune, constantly removing sand from the windward side and depositing it to leeward. With larger supplies of sand and winds which are not too violent a series of barchans is built up, and these are arranged transversely to the wind direction (Fig. 222 b). If the winds are more violent, the minor depressions in the crests of the dunes become wind channels, and the sand is then trailed out along them until the arrangement of the ridges is parallel to the wind (Fig. 222 c). The surfaces of dunes are generally marked by beautiful ripples in the sand, which, seen from a little distance, may give the appearance of watered silk (plate 7 A).

Under normal conditions dunes are not stationary but continue to wander with the prevailing winds until they have reached the outer edge of the zone of vegetation near the base of the foothills at the margin of the desert. Here the grasses and other desert plants arrest the first sand grains that reach them, and they continue to grow higher as the sands accumulate. Some of the desert plants, like the yuccas, have so adapted themselves to desert conditions that they may grow upward with the sand for many feet and so keep their crowns above its surface.

The cloudburst in the desert.—Such clouds as enter the desert through its mountain ramparts, and those derived from evaporation from the hot desert soil, usually precipitate their moisture before passing out of the basin. Above the highly heated floor the heavy rain clouds are unable to drop their burden. The rain can sometimes be seen descending, but long before it has reached the ground it has again passed into vapor, and through repetition of this process the clouds become so charged with moisture that when they encounter a mountain wall and are thus forced to rise, there is a sudden downpour not equaled in the humid regions. Desert rains are rare, but violent beyond comparison. Often for a year or more there is no rainfall upon the loose sand or porous clay, and the few plants which survive must push their roots deep down until they have reached the zone of ground water. When the clouds burst, each small caÑon or wed (pl. wadi) within the mountain wall is quickly occupied by a swollen current which carries a thick paste of sediment and drowns everything before it. Ere it has flowed a mile, it may be that the water has disappeared entirely, leaving a layer of mud and sand which rapidly dries out with the reappearance of the sun.

As the mountains upon seacoasts are generally rising with reference to the neighboring sea bottom, so the mountains which hem in the deserts are generally growing upward with reference to the inclosed desert floor. The marginal dislocations which separate the two are often in evidence at the foot of the steep slope (Fig. 223), and these may even appear as visible earthquake faults to indicate that the uplift is more accelerated than the deposition along the mountain front.

The zone of the dwindling river.—The rapid uplift so generally characteristic of desert margins gives to the torrential streams which develop after each cloudburst such an unusual velocity that when they emerge from the mountain valleys on to the desert floor, the current is suddenly checked and the burden of sediment in large part deposited at the mouth of the valley so as to form a coarse delta deposit which is described as a dry delta (Fig. 224). Dependent upon its steepness of slope, this delta is variously referred to as an alluvial fan or apron, or as an alluvial cone. Over the conical slopes of the delta surface the stream is broken up into numerous distributaries which divide and subdivide as do the roots of a tree. In the Mohammedan countries described as wadi, these distributaries upon dry deltas are on the Pacific coast of the United States referred to as “washes” (Fig. 225).

Fast losing their velocity after emerging from the mountains, the various distributaries drop first of all the heavy bowlders, then the large pebbles and the sand, so that only the finer sand and the silt are carried to the margin of the delta. As they enlarge their boundaries, the neighboring deltas eventually coalesce and so form an alluvial bench or “gravel piedmont” at the foot of the range. Only the larger streams are able to entirely cross this bench of parched deposits with its coarsely porous structure, for the water is soon sucked up by the thirsty materials. Encountering in its descent more clayey layers, this water is conducted to the surface near the margin of the bench and may there appear as a line of springs. At this level there develops, therefore, a zone of vegetation, though there is no local rain.

The alluvial bench grows upward by accretion of layers which are thickest at the mountain end, so that the steepness of the bench increases with time.

Erosion in and about the desert.—The violent cloudburst that is characteristic of the arid lands is a most potent agent in modeling the surface of the ground wherever the rock materials are not too firmly coherent. Under the dash of the rain a peculiar type of “bad land” topography is developed (plate 5 B and Fig. 226). Such a rain-cut surface is a veritable maze of alternating gully and ridge, a country worthless for agricultural purposes and offering the greatest difficulty in the way of penetrating it. When composed of stiff clay with scattered pebbles and bowlders, the effect of the “rain erosion” is to fashion steep clay pillars each capped by a pebble and described as “demoiselles” (Fig. 226).

Behind the mountain front the valleys out of which the torrents are discharged are usually short with steep side walls and a relatively flat bottom, ending headward in an amphitheater with precipitous walls (Fig. 227). In the western United States such valleys are referred to as “box caÑons”, but in Mohammedan countries the name “wed” applies to the river valley within the mountains and to the distributaries as well.

Characteristic features of the arid lands.—It is characteristic of erosion and deposition within humid regions that all outlines become softened into flowing curves, due to the protective mat of vegetation. In arid lands those massive rocks which are without structural planes of separation, partly as a consequence of exfoliation, develop broad domes which are projected upon the horizon as great semicircles, broken in half it may be by displacement. The same massive rocks where intersected by vertical joint planes yield, on the contrary, sharp granite needles like those of Harney Peak (plate 8 A). Similarly, schistose or bedded rocks, when tilted at a high angle, may yield forms which are almost identical. Examples of such needles are found in the Garden of the Gods in Colorado.

At lower levels, where the flying sand becomes effective as an eroding agent, flat bedded rocks become etched into shelves and cornices, and if intersected by joints, the shelves and cornices are transformed into groups of castellated towers and pinnacles of a high degree of ornamentation. These fantastic erosion remnants are usually referred to as “chimneys” and may be seen in numbers in the bad lands of Dakota, as they may in Colorado either in Monument Park or in the new Monolithic National Park (plate 8 B).

Where wind erosion plays a smaller rÔle in the sculpture, but where after an uplift a river has made its way, horizontally bedded rocks are apt to be carved into broad rock terraces, nowhere shown upon so grand a scale as about the Grand CaÑon of the Colorado. Each harder layer has here produced a floor or terrace which ends in a vertical escarpment, and this is separated from the next lower layer of more resistant rock by a slope of talus which largely hides the softer intermediate beds. The great Desert of Sahara is shaped in a series of rock terraces or steppes which descend toward the interior of the basin.

A single harder layer of resistant rock comes often to form the flat capping of a plateau, and is then known as a mesa, or table mountain. Along its front, detached outliers usually stand like sentinels before the larger mass, and according to their relative proportions, these are referred to either as small mesas or as the smaller buttes (Fig. 228).

The war of dune and oasis.—In every desert the deposits are arranged in consecutive belts or zones which are alternately the work of wind and water. Surrounding the desert and upon the flanks of the mountain wall there is found (1) the deposit of loess derived from the dust that is carried out of the desert by the wind. Immediately within the desert border at the base of the mountains is (2) the zone of the dwindling river with its sloping bench of coarse rubble and gravel.

Next in order is (3) the belt of the flying sand, a zone of dune ridges often separated by narrow, flat-bottomed basins (Fig. 229) into which the strongest streams bring the finer sands and silt from the mountains. Lastly, there is (4) the central sink or sinks, into which all water not at once absorbed within the zone of alluviation or in the zone of dunes is finally collected. Here are the true lacustrine deposits of clay and separated salts (Fig. 230 and Fig. 207, p. 201). The lake deposits fill in all the original irregularities of the desert floor, out of which the tops of isolated ranges of mountains now project like islands out of the surface of the sea. The several zones of deposits in their order from the margin to the center of the desert are given schematically in Fig. 231.

The zone of vegetation, as already stated, lies near the foot of the alluvial bench, so that here are found the oases about which have clustered the cities of the desert from the earliest records of antiquity until now. Just without the line of oases is the wall of dunes held back from further advance only by the vegetation which in turn is dependent upon the rains in the neighboring mountains. With every diminution in the water supply, the dunes advance and encroach upon the oases (plate 7 B); while with every considerable increase in this supply of moisture the alluvial bench advances over the dunes and acquires a strip of their territory. Thus with varying fortunes a war is continually waged between the withering river and the flying sand, and the alternations of climate are later recorded in the dovetailing together of the eolian and alluvial deposits at their common junction (Fig. 231).

In addition to the smaller periodic alternations of pluvial and interpluvial climate—the “pulse of Asia”—the record of the Asiatic deserts indicates a progressive desiccation of the entire region, which has now given the victory to the dune. The ancient history of the cities of the plains supplies the records of many that have been buried in the dunes. To-day, where once were prosperous cities, nothing is to be seen at the surface but a group of mounds (Fig. 232). Exhumed after much painstaking labor, the walls and palaces of these ancient cities have once more been brought to the light of day, and much has thus been learned of the civilization of these early times (Fig. 233). Quite recently the mounds which cover between one and two hundred buried villages have been found upon the borders of the Tarim basin of central Asia, where they were lost to history when they were overwhelmed in the early centuries of the Christian Era.

The origin of the high plains which front the Rocky Mountains.—To the eastward of the great backbone of the North American continent stretches a vast plain gently inclined away from the range and generally known as the High Plains region (plate 9). The tourist who travels westward by train ascends this slope so gradually that when he has reached the mountain front it is difficult to realize that he has climbed to an altitude of five thousand feet above the level of the sea. That he has also passed through several climatic zones—a humid, a semiarid, and an arid—and has now entered a semiarid district, is more easily appreciated from study of the vegetation (Fig. 234). The surface of the High Plains, where not cut into by rivers, is remarkably even, so that it might be compared to the quiet surface of a great lake.

The materials which compose the surface veneer of these plains are coarse conglomerates, gravels, and sands, and the so-called “mortar beds”, which are nothing but sands cemented into sandstone by carbonate of lime. The pebbles in all these deposits are far-traveled and appear to have been derived from erosion of those crystalline rocks which compose the eastern front of the Rocky Mountains. These different materials are not arranged in strictly parallel beds, as are the deposits of a lake or sea; but the beds are made up of long threads of lenticular cross section which are interlaced in the most intricate fashion and which extend down the slope, or outward from the mountain front (Fig. 235). It is thus shown that the High Plains are a bench or plain of alluviation formed at the front of the Rocky Mountains during an earlier series of pluvial periods, and that subsequent uplift has produced the modern river valleys which are cut out of the plain. The plexus of long threads of the coarser materials are the courses of dwindling rivers which interlaced over the former plain, and which in time were buried under other channel deposits of the same nature but in different positions (Fig. 236). The pluvial periods in which this bench was formed produced in other latitudes the great continental glaciers which wrought such marvelous changes in northern North America and in northern Europe.

Character profiles.—In contrast with the profiles in the landscapes of humid regions (see Fig. 187, p. 177), those of arid lands are marked by straighter elements (Fig. 237). Almost the only exception of importance is furnished by the domes of massive granite monoliths, which are sometimes broken in half by great displacements. Below the horizon the secondary lines in the landscape betray the same straightness of the component elements by the gabled slopes of talus which are many times repeated so as almost to reproduce the lines in a house of cards, since the sloping lines are maintained at the angle of repose of the materials (Fig. 482, p. 443). Wherever the waves of desert lakes have made an attack upon the rocks and have retired the projecting spurs, other gables characterized by slightly different slopes are introduced into the landscape.