SUN AND WIND IN THE LANDS OF INFREQUENT RAINS

The law of the desert.—It is well to keep ever in mind that there is no universal law which dominates Nature’s processes in all the sections of her realm. Those changes which, because often observed, are most familiar, may not be of general application, for the reason that the areas habitually occupied by highly civilized races together comprise but a small portion of the earth’s surface. In the dank tropical jungle, upon the vast arid sand plains, and in the cold white spaces near the poles, Nature has instituted peculiar and widely different processes.

The fundamental condition of the desert is aridity, and this necessitates an exclusion from it of all save the exceptional rain cloud. Thus deserts are walled in by mountain ranges which serve as barriers to intercept the moisture-bringing clouds. They are in consequence saucer-shaped depressions, often with short mountain ranges rising out of the bottoms, and such rain as falls within the inclosure is largely upon the borders. Of this rainfall none flows out from the desert, for the water is largely returned to the atmosphere through evaporation.

The desert history is thus begun in isolation from the sea from which the cloud moisture is derived, a balance being struck between inflow and evaporation. Yet if deserts have no outlets, it is not true that they have no rivers. These are occasionally permanent, often periodic, but generally ephemeral and violent. The characteristic drainage of deserts comes as the immediate result of sudden cloudburst. As a consequence, the desert stream flows from the mountain wall choked with sediment, and entering the depressed basin, is for the most part either sucked down into the floor or evaporated and returned to the atmosphere. The dissolved material which was carried in the water is eventually left in saline deposits, and the great burden of sediment accumulates in thick stratified masses which in magnitude outstrip the largest deltas in the ocean.

The self-registering gauge of past climates.—From the initiation of the desert in its isolation from the lands tributary to the sea, its history becomes an individual and independent one. An increasing quantity of rainfall will be marked by larger inflow to the basin, and the lakes which form in its lowest depression will, as a consequence, rise and expand over larger areas. A contrary climatic change will bring about a lowering of the lakes and leave behind the marks of former shorelines above the water level (Fig. 205). Deserts are thus in a sense self-registering climatic gauges whose records go back far beyond the historic past. From them it is learned that there have been alternating periods of larger and smaller precipitation, which are referred to as pluvial and interpluvial periods.

From such records it is learned that the Great Basin of the western United States was at one time occupied by two great desert lakes, the one in the eastern portion being known as Lake Bonneville (Fig. 206). With the desiccation which followed upon the series of pluvial periods, which in other latitudes resulted in great continental glaciers and has become known as the Glacial Period, this former desert lake dried up to the limits of Great Salt Lake and a few smaller isolated basins. Between 1850 and 1869 the waters of Great Salt Lake were rising, while from 1876 to 1890 their level was falling, though subject to periodic fluctuations, and in recent years the waters of the lake have risen so high as to pass all records since the occupation of the country. As a consequence the so-called Salt Lake “cut-off” of the Union Pacific Railway, constructed at great expense across a shallow portion of the lake, has been overflowed by its waters. The Sawa Lake in the Persian Desert, which disappeared some five hundred years ago, again came into existence in 1888 so as to cover the caravan route to Teheran.

The record in the rocks of the distant past reveals the fact that in some former deserts barriers were, in the course of time, broken down, with the result that an invading sea entered through the breached wall. The result was the sudden destruction of land life, the remains of which are preserved in “bone beds”, now covered by true marine deposits. A still later episode of the history was begun when the sea had disappeared and land animals again roamed above the earlier desert. Such an alternation of marine deposits with the remains of land plants and animals in the deposits of the Paris Basin, led the great Cuvier to his belief that geologic history was comprised of a succession of cataclysms in which life was alternately destroyed and re-created in new forms—a view which later, under the powerful influence of Lyell and Darwin, gave way to that of more gradual changes and the evolution of life forms.

Some characteristics of the desert wastes.—The great stretches of the arid lands have been often compared to the ocean, and the Bedouin’s camel is known as “the ship of the desert.” Though a deceptive resemblance for the most part, the comparison is not without its value. Both are closed basins, and it is in this respect that the desert and the ocean may be said to most resemble each other, for none of the water and none of the sediment is lost to either except as boundaries are, with the progress of time, transposed or destroyed. Flatness of surface and monotony of scenery both have in common, and the waters and the sand are in each case salt; yet the ocean, from the tropics to the poles, has the same salts in essentially the same proportions, while in the desert the widest variations are found both in the salts which are present and in their relative quantities.

Upon the borders of the ocean are found ridges of yellow sand heaped up by the wind, but these ramparts are small in comparison to those which in deserts are found upon the borders (plate 7 A).

The desert is a land of geographic paradoxes. As Walther has pointed out, we have rain in the desert which does not wet, springs which yield no brooks, rivers without mouths, forests preserved in stone, lakes without outlets, valleys without streams, lake basins without lakes, depressions below the level of the sea yet barren of water, intense weathering with no mantle of disintegrated rock, a decomposition of the rocks from within instead of from without, and valleys which branch sometimes upstream and sometimes down.

Within the deserts curious mushroom-like remnants of erosion afford a local relief from the searching rays of the desert sun. Pocket-like openings large enough for a hermit’s habitation are hollowed out by the wind from the disintegrated rock masses. Amphitheaters open out from little erosion valleys or wadi, and isolated outliers of the mountains stand like sentinels before their massive fronts.

Because of the general absence of clouds above a desert, no shield such as is common in humid regions is provided against the blinding intensity of the sun’s rays. Sun temperatures as high as 180° Fahrenheit have been registered over the deserts of western Africa. Every one is familiar with the fact that a blanket of thick clouds is a prevention of frosts at night, for, with the setting of the sun and the consequent radiation of heat from the earth, these rays are intercepted by the clouds, returned and re-returned in many successive exchanges. Over desert regions the absence of any such blanket of moisture is responsible for the remarkable falls of temperature at sunset. Though shortly before temperatures of 100° Fahrenheit or greater may have been measured, it is not uncommon for water to freeze during the following night. Much the same conditions of sudden temperature change with nightfall are experienced in high mountains when one has ascended above the blanketing clouds.

Dry weathering—the red and brown desert varnish.—In desert lands the fierce rays of the sun suck up all the available moisture, and the water table may be hundreds of feet below the surface. Roots of trees a hundred feet or more in length have been found to testify to the fierce struggle of the desert plant with the arid conditions. In humid regions the meteoric water dissolves the more soluble sodium salts near the surface of the rock and carries them out to the ocean, where they add to the saltness of the sea. In the desert the rare precipitations prevent an outflow, but the sun’s strong rays suck out with the moisture the salts from within the rock, and evaporating upon the surface, the salts are left as a coat of “alkali”, which is in part carried away on the wind and in part washed off in one of the rare cloudbursts. In either case these constituents find their way to the lowest depressions of the basin, where they contribute to the saline deposits of the desert lakes (Fig. 207).

Certain of the saline constituents of the rocks, as they are thus drawn out by the sun’s rays, fuse with the rock at the surface to form a dense brown substance with smooth surface coat, known as desert varnish. Within the interior a portion of the salts crystallize within the capillary fissures, and like water freezing within a pipe, they rend the walls apart. As a direct consequence of this disintegrating process the interior of rock masses may crumble into sand; and if the hard shell of varnish be broken at any point, the wind makes its entrance and removes the interior portion so as to leave a hollow shell—the characteristic “pocket rock” (Fig. 208) of the desert. The nummulitic limestone of Mokkatan and many of the great hewn blocks of Egyptian limestone sound hollow under the tap of the hammer, and when broken, they reveal a shell a few inches only in thickness (Fig. 209).

The brown desert varnish is one of the most characteristic marks of an arid country. It is found in all deserts under much the same conditions, and is especially apt to be present in sandstone. When scratched, the surface of the rock becomes either cherry-red, indicating anhydrous ferric oxide, or it is yellowish, due to the hydrated iron oxide which we know as iron rust. Thus it is seen that the sands of deserts, in contrast to those yielded by other processes within humid regions, have a characteristic red color, and this may vary from brownish red upon the one hand to a rich carmine upon the other.

The mechanical breakdown of the desert rocks.—The chemical changes of decomposition within desert rocks are, as we have seen, largely due to the action of concentrated solutions of salts at high temperatures. That there is a certain mechanical rending of these rocks, due to the “freezing” of salts within the capillary fissures, has been already mentioned. A further strain effect arises in rocks like granite, which are a mixture of different minerals. Heated to a high temperature during the day and cooled through a considerable range at night, the different minerals alternately expand and contract at different rates and by different relative amounts, so that strains are set up, tending to tear them apart. The effect of these strains is thus a surface crumbling of rocks.

But rock is, as already pointed out, a relatively poor conductor of heat, and hence it is a relatively thin skin only which passes through the daily round of wide temperature range. This outer shell when heated is expanded, and so tends to peel off, or exfoliate, like the outer skin of an onion. The process is therefore described as exfoliation. In all rocks of homogeneous texture the continued action of this process results in convexly spherical surfaces, the material scaled off in the process remaining as a slope or talus which surrounds the projecting knob (Fig. 210). Naked, these projecting domes rise above the rim of dÉbris at their bases. Not a particle of dust adheres to the fresh rock surface—no dirt interferes with its glaring whiteness. Yet close at hand lie masses of dÉbris into which wells may be carried to depths of more than six hundred feet without encountering either solid rock or ground water. The bare walls of granite sometimes mount upwards for thousands of feet into the air, as steep and as inaccessible as the squared towers of the Tyrolean Dolomites.

Rock is such a poor conductor of heat that special strains are set up at the margin of sunlight and shade. This localization of the disintegration on the margin of the shaded portions of rock masses is known as shadow weathering (see Fig. 215, p. 206).

There is, however, still another mechanical disintegrating process characteristic of the desert regions, which is likewise dependent upon the sudden changes of temperature. Rains, though they may not occur for a year or more, come as sudden downpours of great volume and violence. Rock masses, which are highly heated beneath the desert sun, if suddenly dashed with water, may be rent apart by the differential strains set up near the surface. That rocks may be easily rent as a result of sudden chilling is well known to our Northern farmers, who are accustomed to rid themselves of objectionable bowlders by first building a fire about them and then dashing water upon their surface. Thus split into fragments, even the larger bowlders may be handled and so removed from the farming land. The natural process of rock rending by the occasional cloudburst may be described as diffission. Blocks as much as twenty-five feet in diameter have been observed in the desert of western Texas, soon after being broken into several fragments at the time of a downpour of rain (Fig. 211).

The natural sand blast.—Because of the saucer-like shape, the vast expanse, and the absence of wind breaks, the potency of wind as a geological agent is in desert areas not easily overestimated. While most of its work is accomplished with the aid of tools, it has been proven that even without this help, considerable work is done through the friction of the wind alone, particularly when moving as powerful eddies in cracks and crannies. This wear of the wind, unaided by cutting tools, is known as deflation.

The greater work of the wind is, however, accomplished with the aid of larger or smaller rock particles, the sand and dust, with which it is so generally charged above the deserts. Unprotected by any mat of vegetation the materials of the desert surface are easily lifted and are constantly migrating with the wind. The finest dust is raised high into the air, and is carried beyond the marginal barriers, but none of the sand or coarser materials ever passes beyond the borders.

The efficiency of this sand as a cutting tool when carried by the wind is directly proportioned to the size of the grain, since with larger fragments a heavier blow is struck when carried at any given velocity. These more effective grains are, however, not lifted far above the ground, but advance with a squirming or hopping motion, much as do the larger pebbles upon the bottom of a river at the time of a spring freshet. To quote Professor Walther: “Whoever has had the opportunity to travel over a surface of dune sand when a strong wind is blowing has found it easy to convince himself of the grinding action of the wind. At such times the ground becomes alive, everywhere the sand is creeping over the surface with snake-like squirmings, and the eye quickly tires of these writhing movements of the currents of sand and cannot long endure the scene.”

A direct consequence of this restriction of the more effective cutting tools to the layer of air just above the ground, is the strong tendency to cut away all projecting masses near their bases. The “mushroom rocks”, which are so characteristic of desert landscapes, have been shaped in this manner (Fig. 212). Another product of the desert sand blast is the so-called Windkante (wind-edge) or Dreikante (three-edge), a pebble which is usually shaped in the form of a pyramid (Fig. 213).

Whenever a rock face, open to direct attack by the drifting sand, is constituted of parts which have different hardness, the blast of sand pecks away at the softer places and leaves the harder ones in relief. Thus is produced the well-known “stone lattice” of the desert (Fig. 214). Particularly upon the neck of the great Sphinx have the flying sand grains, by removing the softer layers, brought the sedimentary structures of the sandstone into strong relief.

When guided both by planes of sedimentation and planes of jointing, forms of a very high degree of ornamentation are developed. Some of the most remarkable forms are due to the protection afforded to the sun-exposed surfaces by the shell of desert varnish. In the shaded portions of projecting masses there is no such protection, and here the sand blast insinuates itself into every crack and cranny. In this it is aided by shadow weathering due to the differential strains set up at the border of the expanded sun-heated surface. As a result, projecting rock masses are sometimes etched away beneath and give the effect of a squatting animal. These forms, due to shadow erosion, have also been likened to projecting faucets. (Fig. 215).

Worn by its impact upon neighboring sand grains while in transport, but much more as it is thrown against the ground or hard rock surfaces, the wind-driven or eolian sand is at last worn into smoothly rounded granules which approach the form of a sphere. Compared to the surface which sea sand acquires by attrition, this shaping process is much the more efficient, since in the water the beach sand is buoyed up and is more effectively cushioned against its neighboring grains. The grains of beach sand when examined under a microscope are found to be much more irregular in form and usually display the original fracture surfaces only in part abraded.

The dust carried out of the desert.—When, standing upon the mountain wall that surrounds a desert, the traveler gazes out to windward over the great depression, his field of view is generally obscured by the yellow haze of the dust clouds moving across the margins. Upon the mountain flanks and extending far outside the borders, this cloud of dust settles as a shrouding mantle of impalpable yellow powder, which is known as loess. These deposits are continually deepening, and have sometimes accumulated until they are hundreds or even thousands of feet in thickness. Before reaching its final resting place the dust of this deposit may have settled many times, and has certainly been in part redistributed by the streams near the desert margin. In it are the ingredients which are necessary for the nourishment of plants, and it constitutes the most important of natural soils. Continually fed by new deposits from the desert, and refertilized from below by a natural process so soon as the upper layers become impoverished, it requires no artificial fertilization. Without artificial aids the loess of northern China has been tilled for thousands of years without any signs of exhaustion.

Though easily pulverized between the fingers, loess is none the less characterized by a perfect vertical jointing and stands on vertical faces as does the solid rock (Fig. 216), but it is absolutely devoid of layers or bedding. Its capacity of standing in vertical cliffs the loess owes to a never failing content of lime carbonate which acts as a cement, and to a peculiar porous structure caused by capillary canals that run vertically through the mass, branching like rootlets and lined with carbonate of lime. This texture once destroyed, loess resolves itself into a common sticky clay.

By the feet of passing animals or by wheels of vehicles, the loess is crushed, and a portion is lifted and carried away by the wind. Thus in the course of time roadways sink deep into the mass as steep-walled caÑons (Fig. 217). A portion of the now structureless clay remaining upon the roadway is at the time of the rains transformed into a thick mud which makes traveling all but impossible, though before its structure has been destroyed the loess is perfectly drained to the bottom of its deposits.

The particles which compose the loess are sharply angular quartz fragments, so fine that all but a few grains can be rubbed into the pores of the skin. Fine scales of mica, such as are easily lifted by the wind, are disseminated uniformly throughout the mass. The only inclosures which are arranged in layers consist of irregularly shaped concretions of clay. These show a striking resemblance to ginger roots and are called by the Chinese “stone ginger”, though they are elsewhere more generally known by their German name of LoessmÄnnchen, or loess dolls. These concretions are so disposed in the loess that their longer axes are vertical, and they were evidently separated from the mass and not deposited with it.

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