6
Blowing in the Wind
Desert Landscapes
The desert is the Garden of Allah, from which he removed all superfluous human and animal life, so that there might be one place where he can walk in peace.
A dense, stinging fog of low-flying sand grains wholly obscured not only our cars but ourselves up to our shoulders, while our heads stuck out against a clear blue sky. One after the other, our feet dropped an inch as sand was scoured from beneath.
The whole landscape was on the move.
JINNS
When sand moves under a gathering desert wind, it seems to take on a life of its own, to become a different form of matter—like a gas, like liquid nitrogen spilling and spreading, following the ground surface. Spraying off the crest of a dune, shimmering in the light, veils of sand race and ripple, spread and vanish, their place continually taken by the next gossamer sheet, dancing, playing, celebrating. Are these jinns, the spirits of the desert? The sight is beautiful and hypnotizing in the evening sun, but if the wind gathers speed, beauty rapidly vanishes as the violence and menace of a sandstorm grows. Suddenly, it seems as if the entire mass of desert sand has sprung from the ground to hurtle with the wind. On the surface, everything is moving, even the largest grains, rolling, tumbling, kicking smaller grains into the rushing current. The sky disappears, and the howl of the wind seems amplified by its cargo of sand. The air is filled with flying sand, unbreathable.
WIND
The desert is a stage on which wind and sand are actors and dancers, with everything else the backdrop. The sound of the wind is the sound of the desert. The winds have names: in North Africa, the simoun, the “poison wind,” is searingly hot and dry, blasting everything in its path—it is the carrier of jinns. The sirocco, the ghibli, the khamsin, and the harmattan are the regional winds of the Sahara; the names themselves sound threatening. Out of those voices of the desert have come great religions and storm gods—Set for the ancient Egyptians, Jehovah, Baal, and Hercules, the ancient storm god of the Atlas Mountains. In his short story “A Passion in the Desert,” Honoré de Balzac concludes: “In the desert, you see, there is everything and nothing. . . . It is God without mankind.” The desert is the home of deities and strange and terrifying beings. Herodotus, in his wide-ranging but often fanciful descriptions of the arid lands beyond civilization, wrote: “There are enormous snakes there . . . donkeys with horns, dog-headed creatures, headless creatures with eyes in their chests (at least, that is what the Libyans say), wild men and wild women.” The nisnas are mythical half-people of the Arabian deserts, running on their single leg, seeing with their single eye. Dorothy and Toto are blown to the land of Oz, which is isolated by the four forbidding expanses of the Deadly Desert, the Shifting Sands, the Impassable Desert, and the Great Sandy Waste, uncrossable by even the Winged Monkeys. The desert is, traditionally, the opposite of heaven, the character of hell for Dante. But today this has changed; as Robert Twigger observes in Lost Oasis: In Search of Paradise, the real lost oasis is the desert itself, “an oasis of light and contemplative beauty that replenished our inner reserves.”
The winds are not only the voices of the desert, but are also often its cause. The major deserts of the world lie along the low latitudes north and south of the equator; these are the zones of subtropical high pressure, where the trade winds, having lost their moisture in the tropics, descend as dry air masses, precluding cloud formation and desiccating the land below. The Sahara and the Kalahari owe their origins to these winds. Elsewhere, the winds rise over mountain ranges, where they drop their moisture, to descend, dried out, on the other side. Rain shadow deserts result—in the western United States, east of the Sierra Nevada and the Cascades; in Patagonia, east of the Andes; and in Central Asia, helped by the seasonal reversal of the monsoon winds, north of the Himalayas. Other major deserts lie along coasts where cold ocean currents, themselves driven in part by the winds, come to the surface, sucking the moisture out of the air. The Namib in Africa and the Atacama Desert in South America are so formed, the Atacama being officially the driest place on Earth—some parts had no rain from 1570 to 1971 (and when the rains did come, they caused devastation). The Gobi and Taklimakan Deserts of Asia are so-called continental deserts—places so far from sources of moisture that hot summers and cold winters generate desert conditions. Distinctions between desert types are not necessarily clear-cut, however, and many deserts result from a conspiracy of circumstances. For example, the Australian deserts—the Great Sandy, the Little Sandy, the Gibson, and the Great Victoria, among others—are influenced by the trade winds, rain shadows, and remoteness from ocean moisture.
But all deserts share one thing—extreme dryness. In strict terms, that dryness, the definition of a desert, is not simply a matter of low precipitation, but also takes into account the theoretical capacity to return whatever water is received from rain back into the atmosphere. It is the ratio between actual precipitation and the amount of moisture that could potentially be returned through evaporation and the activity of vegetation that determines whether a region is semiarid, arid, or hyperarid. Deserts fall into the last two categories, where the ratio is less than 0.2 (one to five) for arid regions and less than 0.03 for hyperarid. Much of the Sahara has the capacity to return to the atmosphere two hundred times the amount of precipitation that actually falls on it (a ratio of 0.005). Arid regions typically receive less than 200 millimeters (8 in) of rain per year, hyperarid regions less than 25 millimeters (1 in), and together they cover around 20 percent of the planet’s land surface.
It should be mentioned here that the polar regions—deserts by definition and in reality the Earth’s largest arid areas—are generally excluded. The entire continent of Antarctica receives an average of 50 millimeters (2 in) of precipitation per year, and there are sand dunes there, although on a far smaller scale than those of the classic deserts of the world.
SAND
Sand dunes cover only about 20 percent of the deserts (the sandiest being the Australian desert, which is half-covered in sand), but it is in the interplay between sand, wind, and ground surface that the dynamics of the arid landscape lie (Plate 9). The rest of the desert, the backdrop and the stage, is made up of mountains and badlands, the vast gravel plains of the regs, the ephemeral river valleys of the wadis, and the bare rock and boulder-strewn plateaus of the hamadas. The Sahara is the largest desert of all—it could comfortably cover the United States or Australia. The expanses completely covered in sand and dunes are the ergs, of which the Rub’ al-Khali, the Empty Quarter, of the Arabian Peninsula is the largest, an area the size of France, covered in sand. The ergs are the image of the desert—vast, timeless, apparently lifeless seas of sand, the waves rolling and breaking in extreme slow motion, their spray carried by the wind—landscapes that have their own extraordinary beauty. Watch the opening sequence of the film The English Patient (sometimes referred to as Gone with the Sand), as the plane flies over the deeply shadowed dunes in the evening sun—it is a landscape of sensual beauty, nature as art.
The ergs of today are tangibly mobile, ever changing, but there are larger areas of ergs past that are now fixed by vegetation. Most of today’s active sandy deserts are surrounded by vast stretches of old stabilized dunes, formed as the trade-wind belts and arid regions expanded in the cold, dry climate of the last ice age and immobilized as the climate changed. However, continuing shifts in the climate may bring these fixed ergs, granular reserves awaiting activation, back to life. We know that small changes in climate coupled with shifts in wind direction can create and remobilize dunes. The largest area of sand dunes in the Western Hemisphere covers around a quarter of the state of Nebraska. There, the Sand Hills were active and mobile between A.D. 1000 and 1200, formed originally from the debris of the glacial erosion of the Rocky Mountains. The hills were stabilized eight hundred years ago but have had episodes of reincarnation since: a long drought toward the end of the eighteenth century resuscitated dunes on the Great Plains, whose activity caused problems for the westbound wagon trains decades later.
Deserts, like rivers and oceans, are engines, with wind and sand doing the work. Sand is supplied by the desert, deposited in the desert, and, occasionally, exported from the desert: a sediment budget again, on a huge scale. Where does all the sand come from? Essentially, all of it is provided internally, by the wind reworking ancient river and lake beds and by the sandblasting of the exposed rock, mechanical weathering, and erosion. Most deserts are topographical depressions, largely surrounded by higher areas, and so most of the sand stays in the desert, following the complex flow lines of the winds. A map of the sand highways of the Sahara reveals a number of points away from which the sand flows, sometimes in straight lines, sometimes in great whorls and arcs, weaving around the mountain massifs, pouring down through the ergs. A significant number of the flow lines end at the Atlantic or Mediterranean coasts, the export terminals for desert sediment.
Most of the sediment exported from the Sahara is not sand. As we saw in chapter 1, even the finest sand will settle from the air and not be carried huge distances in suspension; we will return to this later. The only Saharan sediments that can be carried great distances from the coasts are particles smaller than sand—silt and dust. And they are carried in enormous quantities. It is estimated that more than ten million dump truck loads of Saharan dust are exported each year, with the occasional giant “sand” storm carrying up to 100 million tons. The dust has been found coating the Greenland ice and is a probable source of nutrients to the rainforests of the Amazon. Florida receives perhaps 50 percent of African dust exports to the United States, and with the dust come microbes that have been accused of damaging coral and other marine organisms. The natural export terminals along the Mediterranean have resulted in sand being deposited in great quantities close to the coast, burying the ancient Roman ports and cities of Leptis Magna and Sabratha; the dust continues on to Europe, causing the “blood rain” reported throughout history.
These are huge global movements of sediment, but not of sand—although there is the occasional exception. Fuerteventura in the Canary Islands, today only 100 kilometers (60 mi) off the desert coast of West Africa, is famed for its sand dunes; the sand probably arrived at the height of the last ice age, when the lower sea level meant that the distance from the desert was much shorter and the erg much larger. Sand is today again being exported from Western Sahara—for artificial beach nourishment of the islands.
Most sand stays in the desert—where it occupies itself energetically. Aeolian activity—after Aeolus, the Greek god of the winds, whose domain was the desert—is the dominant process, and the only sound. Wilfred Thesiger, one of the great desert explorers and writers, described in Arabian Sands “a silence in which only the winds played, and a cleanness that was infinitely remote from the world of men.” As the sun heats the desert in the morning, the air rises; more air rushes in to take its place, creating a rising wind, which gathers force during the afternoon. By the following morning, the sand of the erg is swept clean, the landscape rejuvenated by shifting sand. Every day, countless tons of sand are moved on by the wind. But how does this work? And why is it important to know?
While the word desert often connotes a landscape devoid of people, this is by no means true. Except in the most extremely hyperarid regions, people make a way of life in the desert, developing agriculture and communities around the desert margins. Upward of a billion people live in arid or semiarid environments, and their way of life is dependent on coexisting with sand. Regardless of whether or not, on the global scale, “desertification” is happening, the day-to-day struggle to keep houses, villages, roads, and fields free of sand is a real one. And just as modern coastal communities have developed in ways that were long considered unwise, so have we spread into arid lands that are not naturally our home and built commercial infrastructure there. If we want to understand climate change and changing aridity, we need to look into the Earth’s past to interpret those changes over long periods of time. The only way to do that is to learn to read the record of ancient desert sediments through understanding those of today. It’s important to know how the desert engine works.
The mechanical fundamentals of aeolian sediment transport and deposition are the same in many ways as those for the actions of water in rivers and oceans—air and water are, after all, both fluids. But they are very different fluids, and there are therefore some important and basic differences between the ways in which they work. Their densities, buoyancies, and viscosities are vastly different: the effective weight of a quartz sand grain in air, for example, is around two thousand times greater than it is in water, which means that its settling velocity is far more rapid. To start a sand grain moving, the wind speed must be considerably greater than that of a current of water. But, very influentially, wind, unlike water, can flow uphill.
THE SAND MAN
The journey of a sand grain tumbling in the wind is a complex one, and while many of the aspects of that journey are understood, there is much, again, that is not. The foundation of what we do know, and of the research that continues today, is entirely the result of the pioneering work of one man (of whom we have already heard)—Ralph Bagnold. Today’s academic textbooks on sand transport often include advice along the lines of “for inspiration, read Bagnold (1941).”
Bagnold’s early encounters with sand occurred after he was posted to Egypt in 1926. Shortly after his arrival in Cairo, he watched the first successful excavation of the Great Sphinx: “I watched the lion body of the Great Sphinx being slowly exposed from the sand that had buried it. For ages only the giant head had projected above the sand. As of old, gangs of workmen in continuous streams carried sand away in wicker baskets on their heads, supervised by the traditional taskmaster with the traditional whip, while the appointed song leader maintained the rhythm of movement” (Sand, Wind, and War). It was never an ideal place to construct one of the world’s great monuments.
Arguments about the age and meaning of the Sphinx still rage—there are limitations to the wisdom of that which, according to the Sphinx’s riddle, goes on four legs in the morning, on two legs at noon, and on three legs in the evening (the answer being humankind). However, its link with the building of the pyramids is clear, and King Khafre (or Chephren) was the likely builder. The Great Sphinx has spent most of its existence largely covered by the continuously drifting sand, with only its head, blasted and worn by that sand, protruding. The ravages suffered by the head confirm that the sand that buried the body has been its salvation, preserving it from abrasion. In spite of its role over the centuries as an inspiration for archaeologists, poets, travelers, and those who believe it was built by refugees from Atlantis, its life has largely been like that of an iceberg, demurely hiding its bulk beneath the surface.
It was not until early in the nineteenth century that serious attempts at excavating the Great Sphinx were made, but these were defeated by the enormous volumes of sand involved. Further efforts in 1858 and 1885 revealed a good part of the body and some of the surrounding structures, but these attempts were again abandoned. The Great Sphinx had to wait until 1925 and the arrival of the French archaeologist Emile Baraize for its full glory to be revealed. Removal of the vast quantities of sand required eleven years of labor.
FIGURE 27. Ralph Bagnold as the young desert explorer in 1929 and as the distinguished scientist later in life. (Photos courtesy of Stephen Bagnold)
In watching the results of natural sand movements on a staggering scale, Bagnold perhaps had an inkling of the way in which his future would be intimately driven, grain by grain, by sand. The insatiable curiosity that had possessed him since childhood—from an early age he was “aware of an urge to see and do things new and unique, to explore the unknown or to explain the inexplicable in natural science” (Sand, Wind, and War)—would carry him through an extraordinary diversity of accomplishments until his death in 1990 at the age of ninety-four, still in full stride (Figure 27).
Bagnold was one of those larger-than-life characters, but he was also, unusually, deeply modest. In spite of his scientific achievements and accolades, he always regarded himself as an amateur. His obsession was with seeking the truth through well-designed scientific experiment and observation, unsullied by conventional wisdom or tradition. Peacetime soldiering, as Bagnold observed soon after arriving in Egypt, left time for extracurricular activities, and so he set off on adventurous excursions into the desert that, over the next twelve years, through 1938, would become more and more ambitious and extensive, earning him his place among the pioneering explorers of the desert. The Western Desert of Egypt in the 1920s was a little-known place outside the great oases; the maps were conspicuously empty, the words “limit of sand dunes unknown” providing the only description to be found in otherwise large areas of blankness. Bagnold regarded these words as a challenge, and whereas journeys by previous explorers had largely taken place on the backs of camels, he set out by car—first the Model T Ford and later the Model A, painstakingly adapted for the desert and the need to be entirely self-sufficient, particularly in water supplies. In determining that the giant dunes of the erg of the Great Sand Sea could be crossed by car, Bagnold made close and meticulous observations of the nature and quality of sand—early work on the physics of granular materials.
Bagnold’s method of crossing dunes required careful selection of the right location and then full-frontal assault, flying up the side of the dune in a cloud of sand. He recognized that while the sand might be quite soft, the way in which the grains were packed together created a firm enough foundation for a truck to leave only shallow tracks from its deflated tires. Nevertheless, he recognized equally that the sand was “unreliable” and, depending on its location within the dune system, could contain “pools” of unconsolidated sand into which a vehicle could sink, instantly and deeply.
In battling his way across the Great Sand Sea, Bagnold realized that there were three fundamental questions about sand dunes and sand transport that had perhaps never been asked and certainly had never been answered:
What determines the distinct shape of the different kinds of dunes, and how do they retain that shape while moving inexorably across the desert?
Why does sand gather itself into dunes at all, rather than being spread evenly over the desert floor? Why is sand self-accumulating?
How do individual sand grains interact with each other and with the wind to feed the dunes?
Being of an analytical and inquisitive nature, Bagnold decided that the only approach was to go back to basics, ignore the few theories that had been put forward to that point, and conduct some carefully designed laboratory experiments. He was provided space at Imperial College in London, where he proceeded to design and build a wind tunnel within which he could blow (or suck) sand grains under controlled conditions and carefully collect the quantitative data that he needed. It was an exquisite piece of apparatus, and the results were revolutionary. The experimental data, combined with the thousands of painstaking analyses of sand-grain sizes that Bagnold had assembled through endless sieving, culminated in The Physics of Blown Sand and Desert Dunes, finally published in 1941 while Bagnold was causing logistical havoc behind enemy lines in North Africa through the activities of his Long Range Desert Group. The book remains a masterpiece of scientific inquiry and analysis—it was used as a reference by NASA for planning Moon and Mars missions.
HOW SAND MOVES
Among Bagnold’s key findings were that the basic laws of physics could be applied to sand movement, that the interaction between grains and the wind could be described quantitatively, and that the predictions that result could be tested by experiment and then verified by measurements in the desert.
When the wind blows over the desert floor, its flow is influenced by the nature of that surface, its roughness on all scales. Surface irregularities disrupt the smooth flow of air, causing turbulence and eddy currents. These in turn interact with the sand grains on the surface, which may be moved along or temporarily kicked up by the wind, which modifies its movement—a constant series of feedbacks between the wind and the grains. The act of moving sand grains removes energy from the wind and transfers it to the grains, which, colliding with their colleagues, transfer that energy in turn to them. The result is that, close to the ground surface, where most of the action is going on, the wind speed is reduced. There is a velocity gradient, whereby the wind speed increases with the height. Velocity gradients cause pressure gradients, and pressure gradients mean planes—and grains—can fly. What happens on a very small scale very close to the surface of the ground in the desert is critical to the grand-scale results.
A wind moving with the speed of a violent hurricane—perhaps 300 kilometers (190 mi) per hour—can pick up and transport pebbles, but typical winds deal with sand and smaller grains. Clearly, the wind speed needed to start sand grains moving depends on the size of the grains, but the minimum wind speed necessary to move the fine sands of the desert is around 16 kilometers (10 mi) per hour. Look closely at the sand dunes the next time you are at the beach: it’s remarkable how even a light wind can nudge sand grains along the surface. This nudging is referred to as surface creep, but if the wind picks up a bit, it will lift grains very briefly off the surface. They fall back quickly, but when they do, they bang into other grains and kick them into the air. Very rapidly, the whole ballistic process gathers momentum, and in a moderate wind there will be a cloud close to the surface, comprising sand grains traveling by leaping and jumping, kicking off other grains as they land (Figure 28, top).
This is the process that we saw taking place in rivers in a less dramatic way, the impacts between grains being cushioned by water. In air, there is virtually nothing to dampen the impacts, and saltation—movement by jumping—is a violent business. Bagnold observed that a single high-speed saltating grain can move a surface grain more than six times its own diameter and two hundred times its own weight—saltation of fine sand can maintain movement of grains too large to be moved by the wind alone. This is fundamental. In a moving cloud of sand grains, the majority, perhaps 75 percent, are moving by saltation. As illustrated in Bagnold’s diagram in Figure 28, the more pebbles there are on the surface, the more violent the grain impacts and the higher they bounce; if a saltating sand grain lands on a bed of sand, it splashes rather than bounces and much of the energy is lost. Recent work by Jasper Kok and Nilton Renno at the University of Michigan has added a further, influential, player to the game of saltation: electricity. As soon as the grains start moving, their collisions generate static electrical charges, which in turn facilitate the initial picking up of grains from the surface, adding significantly to the amount of saltating sand on the move but keeping it close to the ground.
FIGURE 28. Bagnold’s high-speed photographs of saltating sand grains in a wind tunnel (top); his sketches showing sand grains saltating more energetically from impacts with pebbles than with loose sand (bottom). In the photographs, the “wind” is blowing from left to right; a falling grain ejects another almost vertically into the flow. Ripples are beginning to form in the bottom image. (Images courtesy of Stephen Bagnold)
Bagnold’s second question, as to why sand is self-accumulating, was answered by his observations of saltating sand. A blowing grain will continue to bounce energetically off pebbles but will splash into even a small pile of sand and have more difficulty moving on: the pile grows and a dune is born. At the same time, a rock-strewn surface between dunes will continue to bounce sand grains along. This accounts for the long “streets” of clear ground commonly occurring between dunes (Plate 10); covered with rock and pebbles but little sand, they allow a car access into the erg, but often alarmingly close off with no warning.
As with all forms of sediment transport and deposition, the laws of supply and demand prevail in the desert. As long as there is a supply of sand and sufficient wind, then the demands of the dunes will be satisfied. A wind will carry more sand over rocky ground, feeding the dunes, than over the dunes themselves—and the higher the wind velocity, the more sand it will carry. Because of the physics of what’s going on, the ability of the wind to shift sand increases by the cube of the amount that its velocity exceeds the threshold velocity needed to start movement—exactly the same relationship seen in the moving fluid of a river. For the same grain size, a wind blowing at 16 meters per second (35 mph) will move as much sand in a day as a wind at half that velocity will in three weeks.
So, as we saw in chapter 1, size is critical. Larger grains that normally would not move are pushed along by the ballistics of saltation, and saltating grains bounce more energetically off larger grains than smaller—because of this interaction, there will be more mass movement if the sand is composed of different grain sizes (if it is poorly sorted) than if it’s of a uniform size. But the range of size is limited by the processes of the wind, and the predominant size in that range is roughly 0.08 to 0.15 millimeters (1/300 to 1/170 in): much of the sand in all of the deserts of the world is in this size range.
Even in a strong wind, gravity wins and the height of the cloud of saltating grains doesn’t reach your knees (although it does a painful job of sandblasting everything lower). But turbulence and eddy currents can pick up fine grains and carry them in suspension for some distance, and so the total height of the cloud of whirling sand may well be up to your shoulders, while your head is in clear air.
In a particularly violent sandstorm (and I can vouch for this), the air is charged with flying sand grains above head height and surprising quantities of sand can be driven into uncomfortable places. Typically, suspended sand grains remain aloft for only a matter of seconds, while dust can be blown around for years without ever coming back to earth. However, again as we saw counterintuitively in chapter 1, smaller grains are more difficult to start moving than larger grains because of their being protected by larger grains, the way they can pack themselves together, and, with even the smallest amount of moisture, the effects of surface tension between the particles. It takes a fair wind to start dust moving, but once it’s in the air, it’s gone, leaving the desert to the sand.
FIGURE 29. Rounded sand grains of the Sahara. (Photo by author)
The sheer violence of the endless impacts between moving sand grains has another important consequence: it knocks off any rough edges on the grains, even the hardest ones, and they become more rounded. Our sand grain journeying down the Susquehanna traveled a long distance, rolling, tumbling, and bouncing along the riverbed, but because of the cushioning effect of the water, it suffered little damage. In the wind, however, there is no cushioning, and a grain that started off with rough edges will be abraded and become rounded very effectively. Once a grain has become relatively round and smooth, it is resistant to further abrasion—the process of making the grain smaller is a very slow one for quartz sand. Blowing in the wind is by far the most effective way of making angular sand grains rounded; desert sand is typically rounded and smooth (Figure 29), and this is diagnostic, as we shall see, for the forensics of ancient desert sandstones.
Bagnold’s meticulous work answered his second and third questions, but what about the first, the life and behaviors of the dunes themselves? As he observed in the introduction to The Physics of Blown Sand and Desert Dunes, in the dunes, “instead of finding chaos and disorder, the observer never fails to be amazed at a simplicity of form, an exactitude of repetition and a geometric order unknown in nature on a scale larger than that of a crystalline structure.” Why?
MOVING MOUNTAINS
Lie down on the desert sand, across the direction of the wind, and sand will gradually drift up against you. In a modest wind, your body weight in sand may well accumulate in an hour; in the gale of a sandstorm (remember the cube of the velocity rule), you may be buried in a ton of sand. Burial by sand makes for a good story, and there are many. The history of Arabia and North Africa features tales of armies dispatched to fight the sand and the wind, only to be overwhelmed. One such story that contains real characters and therefore might be closer to truth than myth is told by Herodotus in his history of the Persian wars, written around 450 B.C. Cambyses, the Persian conqueror of Egypt, was being given a great deal of trouble by the Ammonites, the guardians of the Oracle at Siwa (whom Alexander would later consult). In 525 B.C., Cambyses assembled an army and began the march across the desert to Siwa. The army departed the oasis of Kharga and was never seen again: forty thousand men, together with their animals and equipment, “lost at sea,” buried in the sand.
In Mark Twain’s Tom Sawyer Abroad, Tom, Huck Finn, and Jim take off on a Jules Verne–like voyage in “the boat,” a science-fiction hot air balloon. Sailing over the Sahara, they watch a camel caravan plodding over the dunes below them:
Pretty soon we see something coming that stood up like an amazing wide wall, and reached from the Desert up into the sky and hid the sun, and it was coming like the nation, too. Then a little faint breeze struck us, and then it come harder, and grains of sand begun to sift against our faces and sting like fire, and Tom sung out:
“It’s a sand-storm—turn your backs to it!”
We done it; and in another minute it was blowing a gale, and the sand beat against us by the shovelful, and the air was so thick with it we couldn’t see a thing. In five minutes the boat was level full, and we was setting on the lockers buried up to the chin in sand, and only our heads out and could hardly breathe.
Then the storm thinned, and we see that monstrous wall go a-sailing off across the desert, awful to look at, I tell you. We dug ourselves out and looked down, and where the caravan was before there wasn’t anything but just the sand ocean now, and all still and quiet. All them people and camels was smothered and dead and buried—buried under ten foot of sand, we reckoned, and Tom allowed it might be years before the wind uncovered them, and all that time their friends wouldn’t ever know what become of that caravan.
Realizing that “the boat” (apparently defying the laws of physics) contains several tons of “genuwyne sand from the genuwyne desert of the Sahara,” they consider the commercial potential of putting it into vials and selling it back home (presumably to early arenophiles), “because it’s over four million square miles of sand at ten cents a vial.” But the scale of the operation—and that of the duties they would have to pay—leads them to abandon the idea.
Because of the sheer scale and inaccessibility of the sand seas, the way in which they and their dunes formed was completely unknown before Bagnold got to work. An ability to see the relationships between minute and gigantic scales was part of Bagnold’s genius: he simply took his work on the behavior of individual and small groups of sand grains and applied it to the larger scale. Until then, sand dunes had simply been thought of as very large ripples, for ripples form on the surface of desert sand just as they do at the beach or on a channel bar.
We have seen that ripples formed by water are enigmatic features, not fully understood, and the same applies to those made by wind. Bagnold suggested that small initial irregularities of the ground surface interact with the jumps of saltating grains to form ripples, with the distance between the crests of the ripples being directly related to the lengths of the jumps. Grains impacting the windward side nudge other grains of a certain size over the top, and these are then sheltered on the lee side while coarser grains become marooned on the crest of each ripple. But profound complexity can characterize even small features: while this process fits many of the facts, it is not consistent with all of them, and other mechanisms have been suggested. It is certainly true that under the right circumstances, very large ripples, termed, appropriately, mega-ripples or sand waves, can form. The wavelengths of these can be 25 meters (82 ft), and they can be 30 centimeters (1 ft) high; they make for appalling driving conditions, very much like moguls on a ski run. Figure 30 illustrates small-scale ripples forming on the flanks of mega-ripples. The latter seem to form differently than the smaller ripples; the grain size is larger and the internal structure is different, and they are common where the wind is funneled violently between rock outcrops. But however ripples, large or small, form, they are not simply small dunes. The largest mega-ripples are smaller than the smallest properly formed dunes, and no gradual transitions are seen. So what constitutes a properly formed dune?
FIGURE 30. Sand ripples, small and mega. (Photo by author)
It doesn’t matter whether you are in the Sahara, the Namib, the Sonoran, or any of the world’s deserts, there are distinct types, or “species,” of sand dunes that can form, their shapes depending on sand supply and wind directions. There are species that form transverse to the wind direction and those that form parallel to it, those that form in response to complex winds and those that result from the interaction with topography and vegetation. And, of course, there are mixtures of all of the above—the classification of dunes is an inexact science. In fact, even today, the understanding of how dunes form and behave has real limitations. No one has ever made a dune, even a small-scale laboratory dune. Piles of sand, yes, but they never evolve into a working, moving, living sand dune. Computer modeling helps, but much of what we understand today is still based on Bagnold’s fascination with dunes. Plates 11 and 12 illustrate the most common “species”: barchan and seif dunes.
Perhaps the simplest and the most elegant dunes are the individual crescent- or horn-shaped barchans (from the Arabic word for a ram’s horn). Many of the dunes in the Nebraska Sand Hills are barchans, as are those in the Great Sand Dunes National Park in Colorado. Bagnold saw in barchan dunes some of the attributes we ascribe to organisms, living things with inherent shapes and behaviors:
Here, where there existed no animals, vegetation, or rain to interfere with sand movements, the dunes seemed to behave like living things. How was it that they kept their precise shape while marching interminably downwind? How was it that they insisted on repairing any damage done to their individual shapes? How, in other regions of the same desert, were they able to breed “babies” just like themselves that proceeded to run on ahead of their parents? Why did they absorb nourishment and continue to grow instead of allowing the sand to spread out evenly over the desert as finer dust grains do? (Sand, Wind, and War)
Barchans are the simplest form of transverse dunes, forming at right angles to the wind direction and depending on that direction being constant, without seasonal variations. It is this constancy of the wind that leads to the beautiful symmetry of the barchan (whose form is also seen in underwater dunes where currents are constant). An initial patch of sand grows, as we have seen, by attracting saltating grains. As it grows, developing into a mound of sand, it begins to react with the wind that is forming it. Grains build up a gradual slope on the windward side, while a wind shadow develops on the lee side. A crest forms above the steepening lee slope, and the dune develops its typical asymmetrical profile. As sand is carried up off the windward slope, avalanches (the natural behavior of granular materials) start pouring down the steeper lee side, the slip face, and so the dune migrates. In a strong wind, the avalanching is continuous and dramatic, and the crests of the dunes are shape-shifting folds, curtains of moving sand, compared by Bagnold to the gale-driven motion of the mane of some huge animal.
Figure 6 showed sand avalanching in a liquid-like action down the slip face of a dune, like a still from a movie of masses of moving sand pouring down a slope, peeling off across the face. This process of avalanching not only results in the dune migrating, but produces huge-scale cross-bedding in its internal structure, a diagnostic feature for reading the records of deserts from the Earth’s past.
Around the edges of the dune, where there is more bouncing and less splashing, the rate of sand transport is greater, and so the flanks of the dune extend into the characteristic “horns” of the barchan shape; the dune moves in the direction in which the “horns” are pointing. These dunes can grow up to 300 meters (1,000 ft) wide and 100 meters (330 ft) high, but smaller ones are common—isolated individuals marching in a herd across the desert floor. And march they do—at rates of up to 30 meters (100 ft) every year. During a far-reaching 1930 expedition, Bagnold and his colleagues were making their way across the Selima sand sheet of Northern Sudan, a flat expanse of sand the area of Wyoming. The only shelter they could find for the night was a single barchan dune, apparently the scout out in front of the rest of the herd. They camped on its sheltering lee side, out of the wind, and in the morning moved on, leaving their empty cans to be buried beneath its sand. This was Camp 18, whose location was carefully determined astronomically; the vital statistics of the dune were also carefully measured.
In February 1980, an American geologist, Vance Haynes, was traversing the Selima sand sheet and, in the middle of nowhere, happened upon a small pile of empty tins—Bagnold’s rubbish. He confirmed their identity with the brigadier and noted that, though Bagnold’s idea of the tins being covered by the sand might have worked for a while, the sand had continued to move on and had now left the pile in its wake—the stern of Bagnold’s barchan was now over 150 meters (490 ft) away. Haynes made careful measurements and set out markers that he monitored for the next seven years. In the end, thanks to Bagnold’s rubbish, he had measurements of the dune’s movement over a period of fifty-seven years. It had moved 7.5 meters (25 ft) per year. Individual barchans move at different rates, the smaller ones often being more skittish than their larger relatives. A small one will often rear-end a larger one and grow; the larger one in front is starved of sand, diminishes in size, but picks up speed, eventually detaching itself and moving off. This leads to the impression that the original small dune has merged with and passed entirely through the larger one, but modern computer modeling of the process shows that this is not the case. Occasionally, the horns of a barchan will separate from the body of the dune, grow into small self-contained dunes, and move off—the reproduction mechanism that Bagnold referred to. Very commonly, because of their different rates of advance, there are large pile-ups of barchans, forming a complex series of barchanoid ridges, transverse to the wind but complex in their flowing shapes and continuously shape-shifting crests and slip faces.
Where there is more than one significant wind direction, different species of dunes form, typically long linear forms, in reality often sinuous, joining and bifurcating, sometimes building up into huge relatively stable platforms (called “whalebacks” by Bagnold), on the top of which elongate seif (“sword”) dunes are the active players. These dunes can be gigantic, both in height and length; all require a constant and plentiful supply of sand. Linear dunes are by far the most common type in the Australian deserts, where many are now stabilized, but they are common to all the great ergs of the planet.
Linear and seif dunes form broadly parallel to the dominant wind direction but seem to result from seasonal, often stormy winds from a different direction; these set up large-scale eddies and vortices between the dune chains, clearing sand out of the intervening “streets” or redistributing grains from the broad foundations of sand into the elongated dunes (Plate 12). Transport of sand alternates direction, building up downwind along one side of the dune for part of the time and then on the other, extending the dune in the average direction of the winds: the dune gradually migrates. This is also true of star dunes, which show central peaks with radiating arms and multiple slip faces; they often form into networks on, again, a broad plinth of sand and may result from multiple wind directions.
Once topography—cliffs, wadis, mountains—becomes involved, dune shapes and processes can become yet more complex and mysterious. The Great Sand Dunes National Park is a spectacular example. The southwesterly winds pick up sand from the old flood plain of the Rio Grande and transport it until they run into the towering obstacle of the Sangre de Cristo Mountains. Funneling into an embayment in the range, the winds lose energy and have to dump their load in order to gain altitude and cross the mountains—hence the dunes. However, storm winds come from the northeast, and the combination of variable, swirling winds and mountain topography creates a mass of complicated, merging, ever-changing dune types, more varied than those we have mentioned here. The result is a landscape of staggering complexity and beauty.
But for communities living with the threat of ever-moving mountains of sand, that beauty has limitations.
SURGING SANDS
The beleaguered character in Kobo Abe’s Woman in the Dunes may be fictional, but the sand women of Arawan are a fact. The once-thriving oasis north of Timbuktu used to serve the camel caravans from the salt mines farther north, but for a long time now the visitors have been dunes rather than traders. And the women of Arawan shovel sand, every day when the wind is not blowing too hard. It’s not that they haven’t always lived with sand, being, after all, in the middle of the Sahara; but in the 1960s and early 1970s a series of devastating droughts destroyed much of the vegetation, and the sand began to move. The mosque was buried, trees were engulfed, and the wells had to be reexcavated. Dunes encroach on the roofs of houses, the owners keeping only the doorway clear. The sand women are professionals, often paid in rice or sugar to shovel the surrounding dunes with the same energy and futility as Abe’s “woman in the dunes.”
Arawan is by no means alone in suffering this invasion. In towns and villages in Algeria and elsewhere in the Sahara, houses engulfed by sand are abandoned, to be reoccupied, perhaps by the next generation of the family, after the sand dune has moved over and onward. Fields and palm groves are treated the same way—but the palms will be dead and new ones will need to be planted. Even in the great oases of Egypt, isolated herds of barchans march relentlessly over oasis villages. Huge amounts of money have to be spent keeping roads and railways clear of sand or rebuilding them. Outside the oasis town of Kharga in Egypt’s Western Desert, a family group of perhaps nine individual barchans plays havoc with the main road (Plate 11). The highway has had to be rebuilt along different routes five times in the past forty years; lines of telegraph poles, only their tops protruding from the sand, mark old routes. Today, a dune threatens the newest segment of road but has moved past an older one, which can be brought back into service.
Encroaching sand is a constant menace to agriculture, oases, and infrastructure in all arid regions of the world. Even late in his life, Bagnold found himself “in some demand as a sage on the subject” (Sand, Wind, and War) and was summoned to various parts of the Middle East to assess the threat posed by migrating dunes.
The threat, of course, is not new. Populated oases have been dealing with the problem of sand for thousands of years. But those problems seem to be getting worse, and waves of sand are breaking over villages, towns, fields, and infrastructure every day. So how to deal with them? Shutting off the supply at its source is as impracticable as it is for the oceans, so intervention and defense are the only options. The traditional defense has always been building fences of date palm fronds to capture the encroaching sand, a temporary solution at best. Surface coatings have also been used to stick the grains together and stop sand movement. Water can be sprayed on the surface of a dune, but water is a scarce and valuable commodity in the areas where such problems arise. The dunes at Kharga have been partially covered in tarmac—unsightly and expensive. Since there tends to be more oil than water in desert regions, oil has been sprayed on dunes—expensive, polluting, and potentially toxic. Today, there are specialty chemicals that avoid the pollution and toxicity, but they remain far from cheap.
Other defenses have been devised that use what seems to be an innate knowledge of the physics of moving sand. In the early twentieth century, engineers constructing Peru’s railways spread pebbles and gravel over threatening dunes, a solution based on the natural process of deflation: wind erosion lowers the surface of the lands, winnowing out sand from between larger fragments until all that is left is a layer of pebbles and gravel, creating a desert pavement that resists any further erosion. The solution also drew on the process of saltation. As we know from the principle of bouncing versus splashing, sand blown across pebbles will saltate with more energy, and so it moved on across the railway tracks rather than stopping at the dune—and the dune was starved of sand.
Many physical remedies along the lines of palm frond fences have been tried by different communities, largely on an ad hoc basis, but Jean Meunier, a retired French agriculture teacher, was the first person to attempt a coordinated approach using the basics of blown sand and desert dunes. In the early 1990s, he applied his interventionist thinking around Nouakchott (“place of the winds”), the capital of Mauritania, a city whose growth has put it on a direct collision path with the surrounding dune fields. Meunier’s approach was simple: harness the wind energy itself to destroy the dunes. To do this, he developed two types of fencing structures, semipermeable and impermeable. The semipermeable he would place on a dune upwind from a threatened structure. The wind could pass through the barrier, but the sand grains could not. On the downwind side, the wind, relieved of its load, would have the energy to scour and remove sand already deposited. It worked: for example, he saved a school from burial this way. His other method was to use solid, impermeable barriers to divert the wind and cause it to attack a dune directly. Most of the dunes threatening Nouakchott are linear, piled up by seasonally alternating winds from the northeast and northwest. Meunier would build what he called guillotines, V-shaped barriers, each arm at right angles to one of the winds. They forced the wind around and over them, disrupting the natural flow and causing large eddies that scoured the sand. The dune would be split in two in a matter of weeks, its natural form destroyed, its ability to move crippled. He developed a similar method for self-destruction of a barchan. As Meunier remarked: “If you destroy the shape of the dune, it becomes simply a pile of sand and can no longer migrate” (Zandonella, 2003).
Once he had reduced a dune to a mere pile of sand, Meunier then applied the coup de grâce—he vegetated it. Planting vegetation has long been an approach used to stabilize dunes, with varying degrees of success. The kinds of grasses used to plant coastal dunes are ineffective in the desert—their roots are too shallow for the lack of moisture, they become quickly buried, and their leaves are inadequate for trapping sand. Bushes and trees are more effective in the desert (as long as the planting pattern does not generate a wind-funneling effect, which only exacerbates the problem); however, relatively small areas need very large numbers of them. Meunier used small desert shrubs and an innovative watering system that encouraged the roots to rapidly reach depths where there was sufficient natural moisture.
Meunier’s methods were initially greeted with skepticism, but his success brought collaboration, and after ill health forced him to leave the Nouakchott project in 2002, his methods, supported by new computer modeling of dune dynamics, continue to be the focus of activities by nongovernmental organizations.
If one were to try to guess which country is most desperately in need of innovative solutions to combat the surging seas of sand, one might think of one of the countries of the Sahara, or possibly the Middle East. But that country is China, not only because of the scale of the problem, but because of the scale of the threatened population and infrastructure. More than a quarter of the total area of China, over 2.5 million square kilometers (a million sq mi)—about four times the area of the state of Texas—is covered by deserts. Four of these—the southern Gobi, the Alashan, the Taklimakan, and the Tengger—are more or less connected to form the vast arid interior. The total area of China’s deserts is growing at around 200 square kilometers (80 sq mi) every month, and every year tens of thousands of tons of sand and dust are blown into Beijing. China’s capital has always suffered from dust storms, helped again by the ice ages, when grinding glaciers wore rocks down to flour, technically known as loess, which, once airborne, blankets huge areas for long periods of time. But Beijing’s dust storms are turning into sandstorms. It’s not necessary to travel to the Gobi Desert to find encroaching sand; it’s a mere hour’s drive out of Beijing. The Great Wall, built to defend against invaders from the west, is proving no match for the onslaught of sand: whole sections are being destroyed by the storms.
In the village of Longbaoshan, dunes are consuming the houses, and digging has become a way of life. In the region where forests and lakes once provided the hunting grounds for emperors, sand dunes move across the landscape at 20 meters (65 ft) per year. Entrepreneurs have benefited from tourists and filmmakers traveling to Longbaoshan from Beijing in quest of desert landscapes, but the villagers’ narrative could, again, have been taken straight out of Woman in the Dunes: “Sometimes I dream of the sand falling around me faster than I can dig away. The sand chokes me. I worry that in real life, the sand will win” (Gluckman, 2000). In a few years, Beijing will be facing not only the airborne assault of sand, but also its ground troops.
In the heart of its deserts, moving sand has long been a challenge for China. Whenever roads and railways are built, sand clearance becomes an ongoing necessity. Vast stretches of rail tracks and roads are constantly blocked by sand. The Shapotou Desert Experimental Research Station, on the edge of the Gobi, has an international reputation in desert research. The defense methods developed there vary. Grids of straw laid out across the dunes have been successful saltation inhibitors, at the same time allowing vegetation to be reestablished. Revegetation—on the huge scale characteristic of so many Chinese projects—is the focus of current efforts. Villagers, schoolchildren, farmers, and herdsmen all receive incentives to plant trees, shrubs, bushes, and grasses—as fast as possible. What has been called the “Green Great Wall,” an arc of vegetation bordering the entire southern margin of the Gobi Desert, is already well on its way to completion. Gigantic plantations designed to protect Beijing are also underway. But all this consumes huge quantities of water that is no longer available for other uses. Is this the solution?
Today’s inhabitants of villages like Longbaoshan can point to dusty, bare hill-sides and valleys that they remember being verdant. It is estimated that firewood collection, excessive grazing, and overcultivation account for close to 90 percent of recent “desertification.” Wind patterns can change on a human time scale—in the Sand Hills of Nebraska, for example—and, on a longer scale, climate can change, but for many areas of the world where sand is threateningly on the move, it is not nature but humans that are the cause.
THE GRANULAR ORCHESTRA
The Gobi is not the largest desert in the world, nor the sandiest, but nevertheless it is vast and difficult. The first Western traveler to penetrate the Gobi was Marco Polo; in his description of his journey, he mentions some of the many spirits of the desert, Asian jinns, among which are those that create sounds like musical instruments, a terrifying and mysterious drumming in the dunes. Ninth-century local records also talk of the “Hill of Sounding Sand,” spontaneously emitting noises or doing so under the inducement of local villagers sliding en masse down its slopes as part of their ritual worship of the dune.
Such stories are common in arid lands—and in the great tracts of coastal dunes. Sand dunes around the world spontaneously give voice, emitting a wide range of sounds, booming ominously or simply singing to a weary traveler. Charles Darwin described the Chilean sand hill known as El Bramador, “the bellower,” and noted the “chirping” sounds made by horses’ hooves in the sand. Guy de Maupassant, traveling through Algeria, described how “somewhere, close to us, in an undefined direction, a drum was beating, the mysterious drum of the dunes.” Wilfred Thesiger and many other desert explorers were startled by the chorus of the dunes, ascribed by their terrified guides to jinns, sirens, the bells of buried churches, or the drummer of death. And finding himself having to shout in order to converse with his colleagues during one such performance, Bagnold took a particular interest in “the weird chorus.”
The repertoire of sand grains is extensive and varied. Dunes have been described as booming, roaring, thundering, whining, squeaking, and singing. Comparisons have been made to the sounds of a drum, a zither, a horn, a cello, a trumpet, a didgeridoo, bees, a foghorn, and low-flying aircraft. Dozens of dunes worldwide have voices, from the coasts of the Scottish islands to the Kelso Dunes of California, as does every deep desert. Sand Mountain, Nevada, emits a low C, while dunes in Chile have been noted to play an F, and dunes in Morocco a G sharp. The phenomenon—referred to as “dune tunes,” “the sand of music,” “music of the spheres”—has considerable scientific and popular appeal. But what causes it?
Scientific opinion varies widely, but it is clear that, as the ancient Chinese could induce sound by running or sliding down the face of a dune, it is caused by sand movement, or avalanching. Bagnold experimented with inducing sound both in the desert and in the laboratory with different types of sand under different conditions. My own experiences of attempting to seduce a dune into singing to me have been extremely frustrating. I have watched countless natural avalanches cascading in complete silence. Since the orchestra is often reported to perform most commonly at night, I have spent some time perched on top of a slip face in the moonlight, trying various devices (Bagnold succeeded with pencils, bottles, and his hands), with no success at all. Then finally, as I ran and plowed down the face of a Western Desert dune in broad daylight, suddenly through the avalanches at my feet emerged creaking sounds that built in resonance to a satisfying though modest booming sound.
The physics of granular materials—avalanching and the interactions of grains in motion—has led to significant research efforts into sand sounds. But there are several camps, each with a particular theory and often at quite dramatic personal loggerheads with one another. A fundamental division arises from whether the phenomenon results from behaviors of individual grains or the large-scale structure of the dune itself. If the sounds come from individual grains, what is the importance of sorting, rounding, dryness, polishing, and mineral coatings? Or does the avalanche set up a standing wave that resonates with the internal structure, effectively turning the dune into a loudspeaker? Is it the dilation and compression of the mass of grains, or does it have something to do with “triboelectrification,” static electricity charges among the grains? What are the characteristics that result in high-frequency squeaking versus deep booming? Why does a particular dune change its tune from summer to winter?
Data from nature and the laboratory can be found to support essentially every theory. For the moment, dune voices remain among nature’s most elusive and haunting secrets.
DENIZENS
The intrinsic acoustic properties of sand may be entertainingly mysterious to us, but they are vital to some of our fellow creatures. The desert environment, described as a “mineral world” by Bagnold and as an “iron landscape” by Antoine de Saint-Exupéry, will often seem to be devoid of life, but it is far from inorganic. Even in the depths of the hyperarid desert that supports no human life, when you wake up in the morning, the sand around you will be patterned with a network of tracks, large and small, left by inquisitive creatures of the night. Once, when photographing dune avalanches, I looked down at my hand and perched there was that stalwart of suburban gardens, a ladybug.
One such creature—and one that you would prefer not to encounter—is the sand scorpion. Like many desert residents, the scorpion solves the problems of extreme temperatures by burrowing and then listening to the sand. The acoustic background in the desert is blank, save when the wind blows. To us, there is utter silence, but against this apparent silence the blind scorpion has developed an exquisitely sensitive detection system. Drop a sand grain a short distance from it, and the scorpion will react. The movement of a small insect, the scorpion’s dinner, will set up microscopic low-frequency acoustic waves in the sand that are instantly detected by sensors at the ends of the scorpion’s eight legs. Not only does it detect the insect’s presence, but it also immediately knows in which direction it can be found—the sound reaches the legs closest to the insect first, those farther away a millisecond later. And that millisecond is enough for the scorpion to whirl and pounce. These scorpions are also very good at detecting airborne vibrations and have a sense of smell orders of magnitude better than ours—they can sniff a san d grain and detect a mate.
There are two basic challenges for life in the sand: temperature and moisture. Many desert denizens, like the sand scorpion, solve the first by burrowing to levels where the temperature is stable, protected from the surface heat. Desert mammals, reptiles, and insects demonstrate some remarkable adaptations to help with digging in the sand. Australia is the home of twenty species of burrowing frogs, mice that hop—and the itjaritjari. The itjaritjari is a strange little marsupial mole that lives throughout Australia’s deserts. It seems to have no eyes or ears, but put it down on the sand and it will disappear, as the Aboriginal people say, “like a man diving into water.” No one knows how the itjaritjari navigates or senses, but it has a hard nose and front feet well adapted to excavating, the back ones being webbed to help push through the sand; the young are carried in a pouch that cleverly faces backward so as not to fill up with sand. Lizards too have different front and back feet designs for efficient excavation, and often very short legs. The sand skink, a lizard also known as the sand swimmer or the sand fish, seems to have taken a kind of nanotechnology approach, the microscopic design of its scales reducing friction and minimizing abrasion as it swims effortlessly beneath the surface of the sand for much of its existence.
Lizards have transparent eyelids that protect their eyes when burrowing or moving around on the surface. Some geckos have notched eyelids that interlock to keep out the sand—and a long tongue to clean them off. Snakes, however, have no eyelids; they can’t blink or close their eyes. Sidewinders and some other snakes have enlarged horns above their eyes, so that when they burrow, the pressure of the sand against the horns effectively closes their eyes protectively. And the sidewinder, as its name signifies, is, together with other species, a snake that adopts an unusual form of locomotion across the surface of the sand, arching, twisting, and propelling itself in a series of sideways jumps. This is a highly effective way for a snake to move over granular material and, by keeping segments of the body off the hot ground, may also help it avoid overheating. Sand skinks, sidewinders, and sand vipers are also exquisite examples of sand camouflage. The viper will slither into the sand until only the top part of its head is exposed; essentially invisible, it then waits for its dinner to pass by.
Life in the sand desert is carried on largely underground. Ants carry individual grains from their nest to build a protective rampart around the entrance; wasps cover their immobilized prey with sand to hide it while they excavate a nest, lay an egg, and then bury the prey as food for the larva; and beetles inter themselves by stimulating small avalanches of sand. Fennec foxes, whose huge ears act as coolers and whose hairy feet help them travel through soft sand, can burrow 10 meters (more than 30 ft) into a dune. For the ancient Egyptians, the scarab beetle, emerging from beneath the desert sand, was a potent symbol of rebirth.
Underground living may solve the temperature problem, but it does little for the challenge of getting something to drink. There are beetles in the Namib that seem to use sand to solve the water problem. The same cold offshore current that dries out the coastal Namib also creates fogs that roll in over the dunes. Button beetles excavate furrows in the sand at right angles to the fog-carrying wind; the furrows disrupt the airflow, causing eddies from which moisture condenses and the beetles can drink. There is often dew in even the driest desert, but for many creatures it is simply consuming each other that provides sufficient moisture.
And then, of course, there are the ships of the desert, Camelus dromedarius and Camelus bactrianus. With a third sand-wiping eyelid, luxurious sand-filtering eyelashes, ears filled with sand-inhibiting hairs, closeable nostrils, and broad feet for sand walking, they are the ultimate desert sand machines. Their height also keeps their heads well above all but the worst sandstorms. Despite their ugliness, their foul smell, and their cantankerous nature, camels are highly valued and remain the means by which the Tuareg, the Bedouin, and all nomadic tribes of the Sahara and the Arabian deserts make a living. Today, there are around twelve million dromedaries in the world, all of them domesticated; their wild ancestors are long extinct. It was the camel that allowed Western eyes to be opened to the desert, that provided the means of access for the great early journeys of desert exploration. These journeys began with the epic travels of Marco Polo and Ibn Battuta in the thirteenth and fourteenth centuries and continued with the exploits of Wilfred Thesiger and T. E. Lawrence, the search for Timbuktu, and the intrepid wanderings of various Victorian British women.
THE INFINITE PRESENCE
Through these journeys, the desert was revealed and many of its myths dispelled, but it never lost its sense of vastness, its mystery, and its power—over human flesh and over the human imagination. It has created mystics, heretics, hermits, saints, prophets, and philosophers. It remains a place where god is, and where humans are only temporarily—“Look on my works, ye Mighty, and despair!” reads the inscription on the crumbled sculpture of Rameses the Great in Percy Bysshe Shelley’s “Ozymandias.” It is a place where human senses are distorted or redundant—for centuries, blind desert guides have navigated by the smell of the sand. It is a place to which we are drawn but within which we do not belong. In The Sacred Desert: Religion, Literature, Art, and Culture, David Jasper describes “a landscape that both kills and redeems and is absolutely indifferent and pure. It is never and always.” Edmond Jabès, a Jewish writer and poet who was raised in Egypt but fled to Paris in 1956, was one of the great twentieth-century philosophers of the desert, merging the idea of the book and the process of writing with the mystical qualities of the desert: “We saw later how the book was but the letters of each word and how this alphabet, re-used thousands and thousands of times in different combinations, slipped through our fingers like grains of sand. Thus we became aware of the infinite presence of the desert.” And: “What is a book but a bit of fine sand taken from the desert one day and returned a few steps further on?”