Sand formation in sandy deserts. Sand
The ancient Greek philosopher-mathematician Pythagoras somehow puzzled his students by asking them how many grains of sand there are on Earth. In one of the tales told by Scheherazade to King Shahriyar during 1001 nights, it is said that "the troops of the kings were countless, like grains of sand in the desert." It is difficult to calculate how many grains of sand on Earth or even in the desert. But on the other hand, it is quite easy to establish the approximate number of them in one cubic meter of sand. Having calculated, we will find that in such a volume the number of grains of sand is determined by astronomical figures of 1.5-2 billion pieces.
Thus, the comparison of Scheherazade was at least unsuccessful, because if the fairy-tale kings needed as many soldiers as there are grains in only one cubic meter of sand, then for this the entire male population would have to be called under arms the globe. Yes, and that would not be enough.
Where did myriad grains of sand come from? To answer this question, let's take a closer look at this interesting breed.
The vast continental expanses of the Earth are covered with sands. They can be found on the coasts of rivers and seas, in the mountains and on the plains. But especially a lot of sand has accumulated in the deserts. Here it forms mighty sandy rivers and seas.
If we fly in an airplane over the deserts of Kyzylkum and Karakum, we will see an immense sandy sea (Fig. 5). Its entire surface is covered with mighty waves, as if frozen "and petrified in the midst of an unprecedented storm that engulfed colossal spaces." In the deserts of our country, sandy seas cover an area exceeding 56 million hectares.
Looking at the sand through a magnifying glass, you can see thousands of sand grains of various sizes and shapes. Some of them have a rounded shape, others differ in irregular outlines.
Using a special microscope, you can measure the diameter of individual grains of sand. The largest of them can be measured even with a regular ruler with millimeter divisions. Such "coarse" grains have a diameter of 0.5-2 mm. Sand, consisting of particles of such sizes, is called coarse-grained. The other part of the sand grains has a diameter of 0.25-0.5 mm. Sand consisting of such particles is called medium-grained.
Finally, the smallest grains of sand have a diameter of 0.25 to 0.05. mm. It can only be measured with optical instruments. If such grains of sand predominate in the sands, then they are called fine-grained and fine-grained.
How are sand grains formed?
Geologists have established that their occurrence has a long and complex history. The progenitors of sand are massive rocks: granite, gneiss, sandstone.
The workshop in which the process of turning these rocks into sand accumulations takes place is nature itself. Day after day, year after year, rocks are exposed to weathering. As a result, even such a strong rock as granite breaks up into fragments, which are more and more crushed. Part of the weathering products dissolves and is carried away. The minerals most resistant to the action of atmospheric agents remain, mainly quartz - silicon oxide, one of the most stable compounds on the surface of the Earth. The sands may contain feldspars, micas and some other minerals in much smaller quantities.
The story of grains of sand does not end there. For the formation of large clusters, it is necessary that the grains turn into travelers.
Sand is a material that consists of loose stone grains with a grain diameter of 1/16 mm to 2 mm. If the diameter is greater than 2 mm, it is classified as gravel, and if less than 1/16, then as clay or silt. Sand is mainly created as a result of destruction rocks which, over time, accumulate together to form grains of sand.
Sand weathering process
The most common way sand is formed is by weathering. This is the process of transformation of rocks under the influence of such factors as: water, carbon dioxide, oxygen, temperature fluctuations in winter and summer period. Most often, granite is destroyed in this way. The composition of granite is quartz crystals, feldspar, and various minerals. Feldspar in contact with water disintegrates faster than quartz, which allows granite to crumble into fragments.
sand denudation process
The rock being destroyed moves down from the hills under the influence of the force of the wind, the influence of water and under the force of gravity. This process is called denudation.
Under the influence of weathering, denudation and accumulation processes minerals for a long time, you can observe the alignment of the land relief.
Sand fragmentation process
Fragmentation - is the process of crushing something into many small fragments, in our example it is granite. When the crushing process is fast, the granite breaks down even before the feldspar breaks down. Thus, the resulting sand is dominated by feldspar. If the crushing process is slow, then, accordingly, the content of feldspar in the sand decreases. The process of rock fragmentation is influenced by the flow of water, which enhances crushing. And as a result, we have sands with a low content of feldspar on steep slopes.
Sand grain shape
Sand grains start out angular and become more rounded as they are polished by abrasion during transport by wind or water. Grains of quartz sand are the most resistant to wear. Even long stay near the water, where it washes it, is not enough for a thorough rolling of the angular grain of quartz. The refining time is on the order of 200 million years, so the quartz grain that first weathered from granite 2.4 billion years ago may have gone through 10-12 cycles of burial and re-erosion to reach its state of the art. Thus, the degree of roundness of an individual quartz grain is an indirect indicator of its antiquity. Feldspar grains can also be rolled, but not as well, so the sand that has been moved several times is mostly quartz.
The influence of the ocean and wind on the process of sand formation
Sand can be formed not only by weathering, but also by explosive volcanism, as well as by the impact of waves on coastal rocks. As a result of the impact of the ocean, the sharp corners of the rocks are polished and crushing occurs over time. Thus, sea sand familiar to us is obtained. During a storm in the cold season, the water that has fallen into the splits of the rocks becomes ice, which leads to a split. Thus, over time, sand is also obtained. Nothing would have happened without the intervention of the wind. The wind sharpens the grains of sand on the rocks and disperses them.
Scope of sand
Sand is all around us. Most of all it is used in construction. Combining it with water and cement, we get a concrete solution. Sand is added to dry building mixtures, in the manufacture of artificial stone and tiles. Sand has found application even in alternative medicine for the prevention of sciatica and problems with musculoskeletal system. No playground is complete without a sandbox. Sand is also widely used for making glass; filling in sandblasters to clean the surface from rust, different kind corrosion; for backfilling football fields; as a substrate for an aquarium; .
Details about the origin of quartz sand can be emphasized from the article: Big choice fractionated quartz sand can be found on our website.
The desert near the basin of the Lena River and its tributary, the Vilyuy River, caused at least surprise among many: where did such volumes of sand come from in this place? Sand is a clear product of erosion, and it is safe to say that it is water erosion. Such a fraction (without large impurities) can be obtained only with water erosion of the movement (peeling, precipitation) of the masses.
Here's what readers wrote in the comments to the article YAKUT TUKULANS :
l1000 In the Belarusian Polesie, in the Pripyat River basin, there are similar sandy deposits. Moreover, they have a layer of peat layers of various thicknesses.
Light areas are sands. It can be seen that these are areas where oil and gas exploration and production of these natural resources is taking place. To do this, remove the upper part of the soil, sod. The sand is exposed. But this is not done in all areas. It can be seen that not a single road approaches part of the sandy areas.
Here are the views:
63° 32" 16.31" N 74° 39" 25.26" E
River south. High sandy shores. Purovsky District, Yamalo-Nenets Autonomous Okrug
Opened turf on the site. 63° 38" 31.17" N 74° 34" 57.89" E
Here is the next sand outcrop, a little to the north:
The diameter is approximately 1.3 km. Link https://www.google.com/maps/@63.88379,74.31405,2109m/data=!3m1!1e3
Link
Sites of geologists are visible. And everywhere the light color of the sand.
The same picture, the light color of the sand under a thin layer of tundra vegetation.
Moving northeast:
Drill site. Sand. Link into place
Komsomolskoye deposit. Here the satellite took a higher resolution, you can see the details. Link
Do you think that this snow is so white? I thought so too. But moving east, to the river:
It can be seen that the water is not frozen, shooting in the warm season.
Sand embankment of the road
p. Gubinsky
High sandy bank of the river near the town
Several photos of sites where a person damaged a thin layer of vegetation in these places:
64° 34" 6.06" N 76° 40" 45.91" E
62° 19" 50.31" N 76° 43" 17.63" E
63° 7" 35.72" N 77° 54" 31.28" E
The conclusion is that the vast expanses of the Yamalo-Nenets Autonomous Okrug are swamps, rivers and huge layers of sand under a thin layer of vegetation. Sands ancient
Let's move to the Moscow region:
Lyubertsy sand quarries
The Lyubertsy sand deposit is located 5 km. south of the Lyubertsy railway station near the city of Dzerzhinsky near Moscow. This is one of the largest high-quality quartz sand deposits in Russia. The thickness of overburden rocks is from 0.3 to 22.6 m, usually 5-8 m. km.
Geological information:
Quartz sands of the Moscow region were formed in the coastal zones of the ancient seas and are found mainly in the deposits of the Upper Jurassic and Lower Cretaceous. The Upper Jurassic sands of the Lyuberetsky and Eganovsky deposits are mainly used. The second largest in the Moscow region is the Chulkovskoye field, located 17-18 km. south of the city of Lyubertsy. The thickness of the sands at the deposit reaches 35 m.
If these layers are so ancient, millions of years old, then why is there such a thin layer of chernozem and other deposits above them?
In the thickness of the Upper Jurassic quartz sands, there are significant interlayers, slabs, and pillow-shaped nodules of dense sandstones. Genetically, these are large bedded concretions formed due to cementation of sand with silica (cement is predominantly quartz). Some of them are so dense and strong that they correspond to the designation "quartzite" rather than "sandstone".
Outcrop of quartz sands of the eastern wall of the Dzerzhinsky quarry
Sand washing by a dredger in the near (Dzerzhinsky) quarry of the Lyubertsy GOK
Sandstone outcrops in the second, Forest Quarry
petrified geoconcrete
Can be mistaken for destroyed megaliths or remnants
There are such patterns on the stones. Maybe it was cut out when these rocks were still unhardened? Sharp corners and cuts speak for themselves. If so, then it was clearly in the recent past. And then what to do with all geochronological data?
On the steep slopes and cliffs above the quarry bushes of wild sea buckthorn grow picturesquely. For some reason, this shrub loves to grow in quarries. Somehow this was noticed to me in Krasnoyarsk places.
***
So what cataclysmic events or huge maritime epochs in the geochronology of the Earth's past provoked these sand accumulations? Official science speaks about the ancient seas in these territories. But a thin layer of vegetation in the Yamao tundra suggests otherwise. There was no accumulation of humus or inorganic soil above the sand. This indicates the very recent presence of sea water or water streams there. Maybe it was the melting of the glacier and the big streams pure water flowed south from it. Was this glacier also quite recent? Who else is thinking?
Sources:
European scientists initially got acquainted with sands far from deserts - on the banks of rivers, moraines and oceans. The sands brought by the rivers are exposed from under the water only in low water and in climatic conditions Europe is almost not overwhelmed. Ancient river sands European countries distributed in small strips, overgrown with forests, and therefore river sands in Europe do not bring much harm and are not afraid of anyone.
Another thing is the sands on the shores of the oceans. Storm waves and tidal waves throw more and more masses of sand ashore every time. Winds walking over the ocean easily pick up dried sand and carry it deep into the mainland. It is not easy for vegetation to establish itself on such constantly shifting sand. And then goats will come from the village and pit, trample, or even uproot fragile shoots. And more than once it happened that the villages of fishermen, and even large villages and towns, turned out to be buried under sand dunes on the coast of Europe. Centuries passed, and only the top of the high spire of the old gothic cathedral, sticking out of the sands, reminded people of the death of the village that had once occurred.
Almost the entire western Atlantic coast of France was covered with sand for centuries. Many areas of the northern coasts East Germany and the Riga seaside also suffered from them. Raging Atlantic, North and Baltic Sea and the thrusting of the sands generated by them were the most formidable picture of nature, familiar to the inhabitants and scientists of Europe.
And it is natural that when the Europeans got into the deserts and got acquainted with their huge, like the sea, sandy massifs, they involuntarily considered that the sands of the deserts were the brainchild of the sea. This is how the “original sin” appeared in the study of deserts. The usual explanation was applied both to the sands of the Sahara, supposedly the bottom of the recent ocean, and to the sands Central Asia, which, they say, were in ancient times covered by the inland Khanhai Sea.
Well, what can we say about our deserts, where indeed the Caspian Sea used to flood spaces that rise 77 meters above its current level?
And, however, it is precisely Russian researchers who have the honor of overthrowing these incorrect views, according to which sea waves were considered the only powerful creator of sand on earth.
In this regard, many of our researchers of the 19th century, who for the first time began to study various regions of Central and Central Asia, turned out to be on the right track. Among them, first of all, it is necessary to name Ivan Vasilyevich Mushketov, a pioneer of geological study Central Asia, and his student Vladimir Afanasyevich Obruchev, who made many difficult and long journeys in Central and especially Central Asia. These two researchers, combining geologists and geographers, showed that, along with truly marine sands, sands of other origin are widely developed in deserts.
I. V. Mushketov believed that, in addition to sea and river sands, in many areas of deserts, including the Kyzyl-Kum, sands are formed during the destruction of various rocks under conditions of sharp continental climate deserts. One of the merits of V. A. Obruchev was the substantiation by a number of facts of the position that the sands of another empty Central Asia - the Kara-Kums - were formed due to the deposits of the ancient Amu-Darya, which previously flowed from the area of the city of Chardzhou directly to the west to the Caspian Sea.
He also proved that in the deserts of the eastern part of Central Asia, in Ordos and Ala-Shan, the main creator of sands is the destructive forces of the atmosphere.
The arguments of these scientists were logical and convincing, but they had too few facts to completely resolve the questions of the origin of each mass of sand in the deserts.
AT Soviet period comprehensive study of the sands was given incomparably more research. As a result, it was possible to establish the sources and ways of accumulation of a wide variety of sand massifs, although it was not always easy to restore their biography.
In western Turkmenistan alone, we counted twenty-five sand groups of different origin. Some of them were formed due to the destruction of ancient rocks of different age and composition. This group of sands is the most diverse, although it occupies a relatively small area. Other sands were brought by the Syr Darya to the area of the modern Khiva oasis. The third sand was brought by the Amu Darya and deposited on the plains, now located at a distance of 300 - 500 kilometers from the river. The fourth sands were carried by the Amu Darya into the sea, the fifth, very special sands, accumulated in the sea due to shells crushed by waves sea shellfish. The sixth sands were formed in the now waterless, but formerly lake-like Sarykamysh depression. They contain a mass of calcareous and silicic skeletons of microorganisms.
sea of sand. From the northern Aral Sea to the south, along the eastern shores of the Aral Sea, through the entire Kyzyl-Kum desert and further, through the expanses of the Kara-Kum to Afghanistan and the foothills of the Hindu Kush, and from east to west, from the foothills of the Tien Shan to the shores and islands of the Caspian, there is a huge, covered waves of the sea, above which only individual islands rise. But this sea is not blue, its waves do not splash, and it is not filled with water. This sea shimmers now red, then yellow, then gray, then whitish tones.
Its waves, in many places immeasurably higher than the breakers and waves of the ocean, are motionless, as if frozen and petrified in the midst of an unprecedented storm that has engulfed colossal spaces.
Where did these enormous accumulations of sands come from and what created their motionless waves? Soviet scientists have studied the sands well enough to be able to answer these questions definitively.
In the Aral Sea Kara-Kum, in the sands of Big and Small Badgers and on the eastern shores of the Aral Sea, the sands have a dull white color. Each grain of them is rounded and polished like the smallest pellet. These sands consist almost exclusively of quartz alone - the most stable of minerals - and a small admixture of smaller black grains of ore minerals, mainly magnetic iron ore. These are old sands. They were long life path. It is difficult to find the remains of their ancestors now. Their family originates from the destruction of some ancient granite ridges, the remains of which are now preserved on the surface of the earth only in the form of the Mugodzhar mountains. But since then, many times these sands have been redeposited by rivers and seas. So it was in the Permian, and in the Jurassic, and in the Lower and Upper Cretaceous. Last time the sands were washed, sorted and redeposited at the beginning of the Tertiary period. After that, some layers turned out to be so tightly soldered with solutions of silicic acid that the grains were merged with cement, and a hard, fatty in a fracture, pure as sugar, quartzite was formed. But even this strongest stone is affected by the desert. Loose layers of sand are blown out, hard stones are destroyed, and again the sands are redeposited, this time not by sea or river water, but by the wind.
Our studies have shown that during this last "air travel" of the sands, which began as early as the late Greek time and continued throughout Quaternary period, they were carried by the wind from the northern Aral Sea region, along the eastern shores of the Aral Sea up to the shores of the Amu Darya, and possibly further south, that is, approximately 500 - 800 kilometers.
How Red Sands Happened. It is not for nothing that the Kazakhs and Karakalpaks call their largest sandy desert Kyzyl-Kumami, that is, the Red Sands. Its sands in many areas really have a bright orange, reddish-red, and even brick-red color. Where did these layers of colored sand come from? From the destroyed mountains!
The ancient mountains of the Central Kyzyl-Kum are now low, rising 600 - 800 meters above sea level. Millions of years ago they were much higher. But for the same amount of time, the destructive forces of wind, hot sun, night cold and water act on them. Remaining hills, like islands, rise above the surface of the Kyzyl-Kum. They, like trains, are surrounded by strips of gently sloping gravelly drifts, and then sandy plains stretch.
In the Middle Ages of the history of the earth, and the Mesozoic and at the beginning of the Tertiary period, the climate here was subtropical and red earth soils were deposited on the slopes of the mountains. The destruction of the remnants of these soils, or, as geologists say, "ancient weathering crusts," in some places paints the sands of Kyzyl-Kum in red tones. But the sands of this desert are far from being of the same color everywhere, since their origin is different in different regions. In places where the ancient sea sands were re-wept, the sands of these plains are light yellow. In other areas, these yellowish-grayish sands are the ancient deposits of the Syr Darya. Take a look at the diagram on page 64 and you will see that we have been able to trace these sediments both in the southern and in the central and western parts of the desert. In the south of the Kyzyl-Kum, their sands are dark gray and they were brought by the Zeravshan River, and in the west of this desert the sands are bluish-gray and contain many sparkles of mica - they were brought here by the Amu Darya to one of the standards of her wanderings. Thus, the history of the Kyzyl-Kums is far from simple, and the biography of their sands is perhaps more complex and diverse than most other deserts in the world.
How did the Black Sands form? . The most southern desert USSR - Kara-Kum. This name - Black Sands - was given to them because they are heavily overgrown with dark saxaul bushes and the horizon in many places darkens, like the edge of a forest. In addition, the songs here are dark - grayish.
In those depressions between the ridges, where the wind opens up fresh sands that have not been overwhelmed before, their color is steel-gray, sometimes bluish-gray. These are the youngest sands - baby sands in the history of our planet, and their composition is very diverse. 42 different minerals can be counted in them under a microscope. Here, in the form of small grains, there are also garnets and tourmalines, familiar to many from necklaces and rings. Large plates of shiny mica, quartz grains, pink, greenish and cream feldspar grains, black-green sand grains from hornblende are visible to the eye. These grains are so fresh, as if they had just ground and washed granite. But where the wind has managed to winnow the sands, their color changes, taking on a grayish-yellow color. And along with this, the shape of sand grains slowly, gradually begins to change: from the angular, characteristic of young river sands, it increasingly takes on the rounded shape of the so-called “eolian” sands blown by the wind.
The composition of the Kara-Kum sands, the shape of their grains, the good preservation of unstable minerals, their grey colour, the conditions of occurrence and the nature of the stratification indisputably testify to their riverine origin. But the question is, what kind of river can we talk about if the Kara-Kums begin in the south from the very foothills of the Kopet-Dag, and the nearest large river - the Amu-Darya - flows at a distance of 500 kilometers? And where can such an amount of sand come from in the river to wash up a huge desert - more than 1300 kilometers long and 500 kilometers across?
Every time I visited various regions of the deserts of Central Asia, I took samples of their sands and gave them for microscopic analysis. These studies showed that the Kara-Kums were indeed deposited by the Amu-Darya, and partially, in its southern part, by the Tejen and Murghab rivers (see the map on p. 69). The composition of the sands of these rivers, carried directly from the mountains, turned out to be exactly the same. as well as in the areas of deserts created by them, lying a hundred kilometers from the current channels of the Murgab and Tejen and 500-700 kilometers from the modern Amu Darya. But one wonders where it comes from mountain rivers such a huge amount of sand? To get an answer to this question, I had to get to the area of the origin of the Amu Darya - in the highlands of the Pamirs.
Tract of upland sands. In 1948 I had the opportunity to visit the Pamirs. And here among mountain ranges and impregnable rocky cliffs, almost a thousand kilometers from the sandy deserts, I came across a small tract lost in the mountains, which turned out to be a genuine natural laboratory for the formation of sands.
The Nagara-Kum tract, which we called by consonance "The tract of highland sands", is located at the junction of three intersecting valleys, at an altitude of 4-4.5 thousand meters above sea level. One of the valleys stretches in the meridional direction, while others in the latitudinal direction. These valleys are not particularly long, their width does not exceed 1 - 1.5 kilometers, but they are deep. The flat, undivided bottom of the valleys is not indented by traces of water streams or ancient channels. And therefore, perhaps, the contrast between the even and flat bottoms of the valleys and the steep dissected rocky, bare slopes of the mountains is so striking. It seems as if someone cut deep and wide corridors in the mountains.
Everything testified to the fact that these valleys, geologically relatively recently, were the bed of powerful glaciers sliding down from snow-capped mountains. And the smoothed, unweathered rocks of the slopes of the amphitheater, located in the eastern part of the latitudinal valley, indicated that quite recently they were buried under a layer of firn snow.
A whole series of data led to the assumption that with the disappearance of glaciers, lakes took possession of the valleys. However, now in this cold mountainous kingdom there is too little precipitation, so little that even in winter the snow does not cover the area with a continuous cover. Therefore, over time, the lakes also disappeared.
In neighboring valleys, thick ice does not melt even in summer. Here, around the tract, the peaks exceeding Kazbek and Mont Blanc turn black against the background of a clear blue sky, - they are almost not covered with snow in summer, but sometimes there is little of it in winter.
We were in Harapa-Kum during the warmest time of the year - in mid-July. In the afternoon, when there was no wind, the sun burned so strongly that the skin on our face (and we had been in Kyzyl-Kum for a month before) cracked from burns. During the day it was so hot in the sun that I had to take off both my coat and jacket, and sometimes even my shirt. But it was extremely rarefied air of the highlands, and as soon as the sun went down and its last rays disappeared behind the tops of the mountains, it instantly became cold. Temperatures plummeted, and were often well below freezing throughout the night.
The significant height of the terrain, dry rarefied air and cloudless skies lead to extremely sharp temperature changes.
The transparent rarefied air of the highlands almost does not prevent the sun's rays from heating the earth and rocks during the day. At night, intense radiation is emitted from the earth heated during the day back into the atmosphere. However, rarefied air itself hardly heats up. It is equally transparent to both sunlight and night radiation. It heats up so little that it was enough for a cloud to pass through during the day or a wind to blow, as it immediately became cold. This sharp change in temperature is perhaps the most characteristic and, in any case, the most active climatic factor. high mountain areas.
It is also important that at these heights night frosts in summer occur almost daily, and if the stone does not crack from rapid cooling, then water will continue to complete this work. It seeps into the smallest cracks and, freezing, tears them apart and expands more and more.
The rocks of the eastern slopes of the tract are composed of rounded blocks of coarse-grained gray granite-porphyry with well-faceted greenish feldspar crystals up to 4-5 centimeters long. The mountain slopes formed by these rocks seem at first glance to be a grandiose accumulation of large moraine boulders, a heap of perfectly round glacial boulders rising above the plain. And only the contrast between steep heaps and table-smooth bottoms of the valleys, where there is not a single such boulder, makes one more cautious about the assumption that these are glacial boulders.
Carefully looking at the slopes of the tract, we discovered an amazing thing. Many boulders of gray granite-porphyry turned out to be dissected by white stripes of veins, consisting of only feldspars - the so-called aplites. It would seem that aplite veins should have been located in the boulders brought by the glacier in the most disorderly way. But why is it absolutely clear that the vein of one boulder is, as it were, a continuation of the vein in another boulder? Why, despite the heap of boulders, do aplite veins retain a single direction and structure along the entire slope, although they cross dozens and hundreds of granite blocks?
After all, no one would have been able to diligently lay all these boulders in this order, strictly making sure not to change the direction of the veins. If a glacier had dragged them along, it would certainly have piled up the boulders in the most chaotic way, and the veins of the aplites could not have had the same direction in the neighboring boulders.
For a long time I examined the large rounded boulders, until I became convinced that many of them were only half separated from the mountain, like a bump on the lid of a porcelain teapot. This means that these are by no means glacial boulders, but the result of the destruction in place of bedrock, from which, over many centuries, nature has made these blocks under the influence of sudden changes in temperature, or, as geologists call them, spherical weathering units. This was also evidenced by the fact that many balls had a shell exfoliating from them, which is typical for the processes of mechanical destruction - peeling of rocks.
Granite round logs, the most diverse in size, from 20-30 centimeters to 2-3 meters in diameter, were half buried under a layer of gruss and sand formed during granite peeling, crumbling from them. These decay products turned out to be mineralogically so fresh that the sand grains retained completely their original appearance; they had not yet been touched by chemical decomposition or abrasion, and sharp-cut crystals of feldspars - a mineral chemically the least stable - lay here in the sand, shining in the sun with completely fresh surfaces of the faces.
Many of these blocks crumbled into grains at the very light touch. The whole area was a clear proof of the strength, power and inevitability of the processes of destruction of rocks that change and shape the earth's surface over the course of millennia.
"Hard as granite" - who does not know this comparison! But under the influence of sunlight, night cold, freezing of water in cracks and wind, this hard granite, which has become synonymous with a fortress, crumbles into sand under a light touch of the fingers.
In high-mountain regions, the process of thermal destruction proceeds so rapidly that the chemical decomposition of minerals does not have time to affect the decay products at all. The destruction is so intense that the slopes of the mountains are almost half covered with stone scree and sand.
Often breaking here strong winds they pick up the smallest decay products of granites and blow out all the dust and sand from them. Dust air flow carried far beyond the boundaries of the tract; sand, heavier than dust, is discharged here, in all those places where the strength of the wind falls due to obstacles encountered.
Over time, along the entire meridional valley for 13 kilometers, a sandy rampart was formed. Its width ranges from 300 meters to one and a half kilometers. In some places it is quite flat, smoothed, overgrown with grassy vegetation. To the north, at the intersection of the valleys, where the sand is open to latitudinal winds blowing in opposite directions, the shaft is completely bare and the sand is collected in several dune chains parallel to each other.
These chains are high, up to 14 meters, their slopes are steep, the ridges constantly change their shape, obeying the blowing wind, and the wind blows from the east, then from the west.
Naked, free-flowing, high and steeply uplifted sands, the burning sun and the "smoking" crests of dunes - all this involuntarily took us to the hot deserts of Asia.
But the tract of upland sands lies in the realm of permafrost. Around the dunes, wherever you look, the tops of the ridges, covered with eternal snows and sparkling ice. And in the valleys lying a little lower, huge icings of thick ice appeared white, formed from the freezing of spring waters in winter.
The most powerful accumulation of sands in the tract is located at the southern intersection of the valleys. The winds here are the strongest.
Reflecting in all directions from the surrounding steep slopes, the winds experience powerful eddies. The relief of the sands therefore turns out to be the most complex and most rearing. Dune chains either scatter in different directions, or merge with each other, forming huge knots of pyramidal uplifts rising tens of meters above the depressions.
The array of these clean, wind-blown sands covers an area of only 14.5 square kilometers in the tract, but nevertheless, the thickness of these sandy accumulations is quite large, about one and a half hundred meters.
Having experienced these turbulences, the wind rushes further to the east. Rising to the nearby pass, air jets uplift the sand and pull it up the slope. The sand is drawn out in the direction of the prevailing winds in a strip narrowing to the east. This strip stretches upwards for almost 500 meters and goes from the main massif of sands not along the lowest and widest main valley, but in a straight line to the pass, while climbing a rather steep slope.
So, high in the mountains of the "Roof of the World" and "Foot of the Sun" - the snow-covered Pamir - there was a corner of the sandy desert! A corner in which nature from beginning to end carries out the entire process of formation and development of sands! First, the emergence of igneous rocks on the surface, their destruction by temperature fluctuations, the formation of scree, its crushing into sandy grains, and, finally, powerful heaps of sands blown away by the wind. And not only blown away, but also reared up by him into dune pyramids with a height of a twenty-story house, collected in a sandy relief typical of deserts!
All these processes took place within a comparatively short geological time interval. However, the strength and power of these processes are such that everything that takes thousands of years in the deserts, in the tract of upland sands, took place literally ten times faster.
It is important, however, that this destruction of rocks and their transformation into sands is not an exceptional phenomenon, but, on the contrary, is very typical for all dry high-mountain regions. On the greatest uplands of the world - Tibet - there are many such sandy tracts. In the Pamirs and Tien Shan, sands rarely accumulate in massifs due to the conditions of the relief, but they form there constantly and continuously for several million years. Lake Kara-Kul, located in the Pamirs in the permafrost region, is bordered from the east by solid sands. And almost every grain of sand of these highlands, formed under the influence of sudden changes in temperature, melting and freezing of water, soon becomes the property of scree, and then the mountain stream. That is why the rivers of the highlands carry gigantic amounts of sand to the foothill plains. This is where up to 8 kilograms of sand comes from in the Amu Darya during floods, and on average it carries 4 kilograms of sand in every cubic meter of water. But there is a lot of water in it, and in just one year it brings a quarter of a cubic kilometer of sediment to the shores of the Aral Sea. Is it a lot? It turned out that if we take the duration of the Quaternary period as 450 thousand years, consider that during this period the Amu Darya carried out the same amount of sand, and distribute it mentally in a uniform layer over all those areas where the mighty Amu wandered during this time, then the average thickness only its Quaternary deposits would be equal to three quarters of a kilometer. But the removal of sand was carried out by the river before, in the second half of the Tertiary period. That is why there is nothing surprising in the fact that in its former mouths, in southwestern Turkmenistan, oil wells penetrate this stratum of sands and clays to a depth of up to 3.5 kilometers.
Now it is clear to us that most of the foothill sandy deserts of Asia are the brainchild of the highlands. Such are the Kara-Kums, which are the result of the destruction of the high-mountainous Pamirs. Such are many areas of the Kyzyl-Kum, formed as a result of the destruction of the Tien Shan. These are the sands of the Balkhash region carried from the Tien Shan by the river Ili. This is the greatest sandy desert of the world Takla Makan, the sands of which are deposited by rivers from the Himalayas, the Pamirs, the Tien Shan and Tibet. Such is the great Indian Thar desert, created by the sediments of the Indus River flowing from the Hindu Kush.
Abrupt change temperatures in deserts and highlands destroys rocks and creates sands. Above - flaky layers of sandstones in Western Turkmenistan. Below - dune sands in the Nagara-Kum tract in the Pamirs, formed from the destruction of granites. (Photo by the author and G. V. Arkadiev.)
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