The Great Sand Sea

Sand seas exist in at least 18 important locations in the world, four of these in the North African Sahara (Vivian, 2002). The Calanscio Sand Sea in Libya and Great Sand Sea of Egypt begin as one near the northern Mediterranean coast and at the latitude Siwa and Jalo they break up in two. They form the second biggest sand sea in the world after the Rub al Khali (empty quarter) in southern Saudi Arabia. The Great Sand Sea alone covers 72,000 km2, almost 1.5 times the size of Holland!

The great Sand Sea on Google Earth (the red lines show our route)

The Great Sand Sea is steadily moving south and has begun to infiltrate the northern eastern corner of the Gilf Kebir. Dunes are already encroaching on Gebel Almasy, and it is clear that even a plateau the size of Gilf Kebir will eventually not be able to stop the dunes.

Dunes encroaching on the south side of Gebel Almasy

After a brief encounter with the awesome Great Sand Sea in Siwa, we fell in love with this magnificent stretch of desert. Our recent expedition is probably one of the most challenging possible in Great Sand Sea; crossing the ±350 km of sand sea from east to west and back from west to east. It is a route rarely taken, and we were told that a Paris-Dakar rally champion attempted it once and had to give up (but we did not know this beforehand…). We are very proud that we made it, thanks to our excellent guide and friend Mohammed. We drove ten days, nearly 1600 km off road, with no place to resuply fuel, water or food on the way.

The Great Sand Sea is a complex of various dune forms and sand sheets. A lot of the early classification of windblown sand was actually done in the Great Sand Sea by Major Ralph Bagnold (1896-1990), a British desert explorer who formed the Long Range Desert Group in WWII (Kelly, 2002; Bagnold, 1941; Pannekoek 1976). Below some of the main components are described.

Simply said, dunes form when wind erodes sand on one side and deposits it on the other side. A slip face is where the sand falls down when wind has transported it over the dune crest. As such, these are as steep as dry sand grains can physically be sacked up (this the angle of repose, generally about 30° for dry sand). Because of their steepness, you can drive/fall off slip faces, but you cannot drive up slip faces. Also if you fail to drive exactly parallel to the slope of a slip face you car can get into a roll in a matter of seconds. This makes dune driving probably the most dangerous form of off-road driving, and therefore adrenaline levels can get pretty high!

Corine is just about to descent a slip face

Mohamed just drove down a major slip face

Parallel straight dunes or Seif or sword dunes are formed by unidirectional wind. In the Great Sand Sea moving Seif dunes sit astride, relatively unmoving 2 km wide whaleback dunes (Vivian, 2002). While the long Seif dunes move, the base, the whaleback dunes remain in place (Vivian, 2002). Some whaleback dunes have multiple Seif dune crests with multiple slip faces to cross, something you cannot see from the foot of the dune. In the Sand Sea these are parallel to the prevailing NNW wind direction, and some are as high as 100m and as long as 145 km! The parallel dune slip faces of the Great Sand Sea generally face eastwards. So, crossing east to west is very difficult. Despite the fact that between the dunes ground is often nearly void of sand, the only way to get across is driving over them (Vivian, 2002). Trying to go around is often impossible (Vivian, 2002). In order drive over them in the wrong direction, you have to drive through narrow gaps without a slip face. Thanks to Mohammed’s preparation with detailed satellite map reconnaissance (he studied the maps for 6 nights, 3 hours per evening) and his extensive experience we found most corridors for crossing smoothly. If you get it wrong and you cross at the wrong point you can get stuck or in a far more dangerous situation very quickly.

Seif dune

Parallel wavy dunes or barchan dunes are crescent-shaped with a width equal to their length and a single curved slip face, roughly perpendicular to the wind direction. They can go as high as 100m and extent for 400m (Vivian, 2002). They can move very fast, as much as 19 m per year, and change direction when the wind pattern changes (Vivian, 2002). At the tips of a crescent –shaped “mother” dune new “child” barchan dunes form, eventually forming a barchan dune field. These are extremely hard to cross as they change rapidly and their curved slip faces can easily roll your car if you approach them the wrong way.

On the right hand side a big barchan dune has formed

Star dunes are created when the wind often changes direction, and have the shape of a star in plan view. They are rare in the Great Sand Sea, and usually found in isolation. Because of the irregular shape of the crests, they are very hard to climb or cross by car. We saw (and climbed) one on an earlier trip near Siwa.

Sand sheets are flat hard sheets of sand that go on for many kilometres. Here you can drive 80-90 km/h easily. However, sometimes there are major soft patches. If you slow down to much, and you do not have enough momentum to cross them, you get stuck. Because they are flat there is no direction you can drive in to get the help of gravity to get unstuck. Therefore, it is almost impossible to pick up speed and momentum again to drive out. You could end up driving out sand plate by sand plate (these are only 1.5 m long); a slow and lengthy process. Fortunately, when never got into this situation, but we where close a couple of times.

Driving over a sand sheet

References

Bagnold, R.A. 1941. The physics of blown sand and desert dunes. London, Methuen, 265p.

Kelly, S., 2002. The hunt for Zerzura –The Lost Oasis and the Desert War. Published by John Murray, London, 302p.

Pannekoek, A.J. 1976. Algemene Geologie. Tjeenk Willink, Groningen, 531p.

Vivian, C., 2002. The Western Desert of Egypt. The american University in Cairo Press. Cairo, 426p.