They're hills or small mountains made entirely from very small grains of sand that are formed by the wind and can move for tens of meters during major desert storms but still never leave their known areas! They've fascinated many non-Saharan people as the most exotic if not dangerous of all desert landforms! Myths have been told about them and offroaders —in their shining armored 4x4s— have feared them for long.
Although they appear on coastal zones in many places around the world (England, Africa, China, Egypt (elBorollos), etc), we'll be mostly talking here about the inland desert ones . Dunes types, names, locations, strange characteristics will be explained.
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How do dunes form in the desert?
All a sand dune needs is an obstacle like a bush, rock to start forming. In some cases an abandoned car where wind can deposit sand would do. It also needs--of course--lots of sand particles carried in the air around it to get its material from. When wind hits the obstacle and goes from both sides around it, it finds a spot behind the object (leeward) where speed of wind comes down to zero. The wind then drops any sand it was carrying at this leeward side of the obstacle.
Now after some short time, the obstacle is completely covered with sand and a small and a nice dune is formed. This little dune itself becomes the obstacle and keeps growing and take one of the forms that all dunes must take like Seif (linear) or Barchan (crescent) for example.
Various name for the fields of sand dunes in the Sahara
When Sand Dunes gather in a large group covering maybe hundreds or thousands kilometers, they're are called "Ghoroud" غرود in Egyptian Western desert. Ghard Abu elMaharik is a famous one that appears on maps in a very thin straight line (around 5-10 kilometers) and hundreds of kilometers long parallel to Nile Valley west of Asyout starting east of Bahareya oasis and going south.
"Ghoroud" is not likely to be Arabic (none is called so in Arabia or Eastern desert), it could be ancient Egyptian name of dunes (like 'wa7at' is), or maybe Berber too. Ghoroud is the word used for dunes in Egyptian Sahara. Ghard seems to be a singular form of Ghoroud and is used interchangeably (like water and waters).
The way the word ghoroud is used in our Arabic language is a bit confusing. Abul-Maharik could be sometimes called ghard indicating that the entire field is one 'ghard' and in other occasions "ghoroud" word in plural form is used indicating either a group of dunes or group of dunes fields. My best guess is that a ghard is a group of dunes various in size that could be existing singly or in group of ghoroud close to each other. Anyway, it's almost as hard to define where a dune end and another start so the need for accurate use of the word is not really strong.
"Erg" is more common in English language for "Ghoroud". "Erg" is originated from Arabic (3irq عرق ) and means literally "artery". It seems to describe the sharp crests of dunes or perhaps of the longitudinal 'seif' dune (see later) that appear like a long artery of the Earth.
"Sand Sea" بحر رمال is another name of area covered almost entirely by sand dunes. Since we have never heard it from a native of the Oases, we think it's an European word (perhaps English) indicating a large and wide field of dunes. Erg Occidental in Algeria is a case in point: it fits perfectly what a Sand Sea is but remains to be called Erg by the French. The Mapping Agency of Egypt for a long time has been chaired by British in early 20th Century when it was a British Protectorate (such as John Ball), so the name has stuck and even translated to Arabic as "BaHr Remaal" (بحر رمال). Sahara desert of North Africa has many Sand Seas, and other fields were called so around the world. In Egypt, the Great Sand Sea (baHr elremaal ela3zam بحر رمال الأعظم ) is situated on the far west of Western Desert south of Siwa region. It is 72,000 km2. Compare to Erg Chech-Adrar in the Maghreb countires of 319,000 km2. Even Selima sandsheet in Egypt is 100,000 km2.
By 'face', the mean the sides of the dunes. All sand dunes have what is called a "windward" side (facing the wind or facing "upwind") and an opposite "leeward" side (facing "downwind"). The sand blown on the windward side blows along the top until it gets deposited on the "crest" of the dune. This accumulation of sand grains is unstable and when too much is built up, it falls (or "slips" down) in the form of a small avalanche down the leeward side. This leeward side (other side of the wind) is normally called in English a "slip face".
Ripples are the small waves of sand you find on the surface of dunes sides. Some are as small as 5 cm apart (wave length) and 5 mm high (amplitude). Some ripples (giant ripples) can get couple of meters apart (25 in some books) and as high as 70 cm high! Some even think that sand dunes themselves are gigantic ripples.
From desert riding experience, ripples indicate not so loose sand surface that one can drive a relatively heavy 4x4 car on without getting stuck. This doesn't apply all the time, though. Ripples also are uncomfortable to drive on and if they got larger suddenly you might crash into one of the giant ripples.
One slip after another for thousands of times over years and the dune will seem to be moving. Ghoroud motion is called migration and they normally move of course along with the direction of the prevalent wind.
When the tiny particles of sand jumps over the windward face, this motion is called saltation (word of French origins). Saltation is one of the major factors of ghoroud migration and is studied as part of grains-flow mechanics to be able to see how (under known wind and particles nature) dunes can move and how fast. This is important for countries that has migrating ghoroud near its towns/villages or their agricultural fields and threatening to cover it all if it changes its direction of migration. Another good reason why those who believe in Zarzora oasis in Egypt think that it was covered by ghoroud and should appear again someday with all its wealth.
The migration is part of what is called Eolian geology. In English, 'eolian' means the part of geology that study how air shapes sand.
Types of dunes
The form or type of the dune seems to be determined by the wind directions and how it changes among few other things. Many forms are not easily recognized from the ground but some distinguishable factors can help the very experienced ones. If so, they can tell you what to expect on the other unseen side of the dune.
The forms remain a plenty and very different, but could be characterized by few things such as: count of slipfaces, orientation from the wind-direction, how the form seems from plan-view (helicopter view).
So far in SaharaSafaris, we've recognized 17 types and they are:
Sometimes, there are some secondary features that if added after the Dune form name above explains a lot of how the shape will be like, and those are called "Sub-forms". Some of the Sub-forms are:
Also known as "Barchan/Barkhan", "Transverse", and "Crescent". These are the most common of all dunes types on Earth as mentioned in one of the sources. They are crescent in shape with their arms called horns. Horns always point along the direction of wind where they move. Ripples form as usual at their backs where winds blow. Between the arms it's always a slipface specially in the middle.
Barchans form when wind blows in one direction all year long with no change.
Also known as "longitudinal", and "linear" ghoroud. They're particularly uniform in their straightness on the long axis with beautifully winding but mostly sharply edged crests.
The left photo above shows from a plane how linear ghoroud seem from high above. The right photo is one I took on the back of a Seif looking north toward the lower lake of elRayan. It shows the seif I am standing on (ghoroud samuel) and another one running parallel to it to my right (there are many more out of the photo to right and left). When on picnic to the place near the highway, a nice walk before sunset on the crest till you reach the lake and back is advised.
Seif ghoroud are made by winds that shifts its direction a little on seasonal bases slightly to the east then slightly to the west. The longitudinal direction of ghard will take the middle direction between these two directions of the wind. Climate doesn't change that fast so but if a wind started blowing in the same direction all year long, then the seif might change into separated barchans (crescent) ghoroud.
Seif ghoroud move (migrate) along their longitudinal axis.
As it appears in photo taken from a plane (on the right), they're very large dunes in the form similar to a pyramid with slipfaces on its arms (arms of the star).
They appear in areas with multidicretional winds. Those star dunes appear from far like a high pointed mountain. They tend to grow upward which enables them to reach some of the highest recorded peaks of any sand dunes and up to 500 meters as some examples in China.
A peculiar type of sand dunes that resembles a dome with plenty of crests on its back.
The domed dunes are different from Stars in the fact that they have no arms and are covered with crested dunes all over its back while stars may have smooth arms with slipfaces.
In the photo, this is D18 near Sitra with a diameter of 1 km and over 100m elevation at its central peak. The tiny dot on its back is a 4x4 attempting climb while other 4x4s in the foreground are watching.
Transverse dunes are linear dunes that is formed in lines that are --unlike Seif-- perpendicular to the direction of wind. Unlike Seif, they're short (200m or so) and appear to be placed away from each other leaving no clear corridors although still parallel to each other
This type of a dune has steep leeward faces that are --unlike slipfaces-- quite solid and consolidated but remains very steep and without careful use of breaks it may cause 4x4s to land ungracefully at the bottom. They form in broken crested form (3urf) but may form long lines of whalebacks. The photo is between Sitra and Baharia where there's a large field of Transverses.
Sand-Sea type of dunes
All these dunes types may occur in three forms: "simple", "compound", and "Sand-sea". "Sand-sea" here is not to be confused with Great Sand Sea to be found south of Siwa oasis in Sahara which is mostly Seif type with small territories occupied by the type "Sand Sea". Simple dunes are basic forms as explain above. Compound dunes are large dunes on which smaller dunes of mostly similar type. Complex dunes are forms that are combinations of any number of types and are difficult to identify as any of the above. Dome may seem to be a complex or even Helaly Compound but they always take the shape of a lens so they were identified separately. Extended ghard field of Complex are called 'Sand-Sea'. At the Ghard itself, Sand-sea seems like a criss-crossing field of crested waves of sand frozen in time.
In nature nothing is straightforwardly simple. However, Sand-Sea are rare and usually are making no more than 20% of the entire ghard field.
Sandsheet (Arabic: فرش)
A flat field of sand. The hilly dunes are not there although sand layers are thick. Selima Sheet is the largest in the world covering area between Egypt, Sudan and Libya with 100,000 km2.
Friends going to elGilf elKebir through the Selima Sheet have said that it was the easiest part of the drive because of the flatness.
It seems they are so flat because the particles of the sand are slightly bigger than other sand dunes'.
Whalebacks or Zibar dunes
They've no slipface and look from far exactly like a whaleback. They're the gigantic in size like typical dunes and are the most suited to cruise on with your 4x4. If you ask a British how to describe them they'd say, they're 'rolling' hills of sands because of their smooth surfaces with no crests.
They're called whaleback by the famous British Sahara expert Bagnold active in Egypt at the first half of the 20th century (and God father to scientists working in desert geomorphology) but Zibar now is becoming more used in science. The word Zibar is 'borrowed' from Arabic by D. A. Holm in 1953 when he was discussing dunes in Nejd, Saudia but the Arabic source of the word is not clear.
According to some, the Zibars are the same as granule-armoured dunes, mréyé in Mauritania, giant ripples, giant undulations, and low-rolling dunes with no slipfaces, and maybe they're the same as dome dunes of Iraq, and wanderrie banks of central Australia. They're not all exactly the same shape and dimensions, but could be included under Zibar.
Next time you're at Borg elBorollos village at the Mediterranean or in the wadi elRayan, climb a sand dune in bare feet on a windy day. Stand still in various places on the gently sloping windward side. Watch how wind-driven sand grains appear to jump an inch or two above the dune, stinging your ankles and making the dune's surface appear to be in constant motion ever upward toward the crest.
At the dune's crest, kneel to examine closely what's happening. Watch how airborne sand grains fall and cascade down the steep lee slope in tiny avalanches. Start hiking down the lee side; notice how suddenly still the air feels, especially just past the dune's crest.
You've just observed how dunes grow.
In the movies, you see the very dry quick-sand (elremal elmota7arrekah الرمال المتحركة ) and how can it swallow a man in few minutes. If you mean quicksands that is dry, then we have never seen any of it among the circles of Egyptian travelers in SaharaSafaris or Bedouins that we mingle with and might be a complete myth. But if it has some wetness to it (even if it's hiding under very dry sand) then Yes, we have seen it and Yes, you sink in it but much faster than you see in the movies.
The dry quicksand is just a myth! The quicksand you see swallowing in Hossam's Land Cruiser (the photo below) is very wet underneath because it's very near the desert lake of Qaroun in Fayoum.
Science books seem to have completely avoided the word quicksand too and was sticking to the scientific or native names. Perhaps it's like trying to describe the scientific classification of Ness monster of Scotland based on the 'nice' stories of inexperienced people.
Andrew Goudie and his colleagues (in Encyclopedic Dictionary of Physical Geography) have been more bold about it and said about quicksand that it is: "Water-saturated sand which is semi-liquid and cannot bear the weight of heavy objects." In same page (p.408) they have added also a definition for 'quick clay' which is more or less the same as quicksand but in finer size of grains. Just remember that wherever there are sea shores or desert lakes, there might be some quicksand nearby.
The photo above was taken near a water rich depression in Egyptian Western desert.
If the travelers --mostly cannot speak native languages of the Sahara-- have asked their Bedouin guides what they call it, they probably would have told them the name 'Sabkha'.
Sabkha سبخة (That's Our Quicksand)
Sabkha are stretches of wet 'silt' (Arabic: tafla طَفلة ) in Arabic which is between sand and clay). Water doesn't seem to cover them but the wet soil take a color clearly darker than surrounding soil. In most cases, salt is very obvious with white color on the surface but varies greatly in quantities from none obvious to fully covering.
The soft layer of salty silt of sabkha is viscous. Your feet will sink if you step in it for few centimeters depending on the depth of the sabkha. Some cars have sunken in our safaris up to a meter.
So if you like to find some quicksands, just check any maps in north Africa for the words, Sabkha and Chott. You'll find many place to experiment on your own. Qattara depression probably has the worst and most dangerous of all. North of Lake Qaroun has the easiest to spot. Watch it near sand dunes though; a thin layer of 'innocent' dry sands might cover the sabkha and embarrass you and your driving skills if you don't have a winch in the other 4x4s to pull you out (from experiences of others on SaharaSafaris, there's no other option)
Sabkhet elBardawil is an example from Mediterranean coasts of Sinai. Another name could be malla7aat like ones near Alexandria. Sabkha is Arabic word but in North Africa, the word Chott appears instead for the same thing. Sabkha has become an English word with that spelling although other spellings are sabkhah, sebcha, sebjet, sebchet, sebkha. Other alternative names are pan (south africa), nor (mongolia), dry lake and playa (North America). Also, zahrez, and garaet. This is for those near sea coasts. For inland ones far from any sea, playa seems to be the now more used name although it is the same as clay pan in Australia, takyr in Russia, khabra in Saudia, qu in Jordan, salar in Chile and tsaka in Mongolia.
Called AlHayal (not sure if 'h' هـ'' or '7' 'ح' in the reference I read) by some bedouins (Al Murra in Arabia) and are very loud voices of sand dunes that could go as far as 10 kms.
People camping among ghoroud sometimes hear loud and continuous sounds that come out of the ghard. The sound appears to be the produce of the inner friction of the sand particles of the dune. The 'cleaner' and drier the sand (no dust), the easier it produces sound. Wet dunes near coasts are said to cause squeaks (>500Hz) but ours in this article are very deep and sometimes called 'booming' because they're around 80Hz and sometimes down to 10Hz. That's really really 'base'.
Marco Polo (1295) wrote of evil desert spirits which "at times fill the air with the sounds of all kinds of musical instruments, and also of drums and the clash of arms." References can be found dating as far back as the Arabian Nights, and as recently as the science fiction classic Dune.
Sounds have been noticed to have a certain pattern to it. Booming in the deserts dunes and squeaking near the beaches. Sometimes, dry booming dunes can produce sounds that can be heard up to 10 km away and which resemble hums, moans, drums, thunder, foghorns or the drone of low-flying propeller aircraft.
None of the authors of this article has come across this singing sands yet in my travels in Egypt, but they do exist.
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