Though gravel plains, limestone pans and scarps account for sixty percent of Egypt’s Western Desert, it is the region’s dunes that captivate the imagination. Lifeless yet restless, they shift and reproduce, burying palm groves, roads and railways in their unstoppable advance. Their shape is determined by prevailing winds, local geology and whatever moisture or vegetation exists. Where sand is relatively scarce and small obstructions are common, windblown particles form crescent-shaped barchan dunes, which advance horns first, moving over obstacles without altering their height. Baby dunes are formed downwind of the horns, which produces parallel lines of barchans with flat corridors between them, advancing up to 20m each year. Barchans can grow as high as 95m, extend for 375m, and weigh up to 450 million kilos. However, their mass is nothing compared to parallel straight dunes, or seif dunes (from the Arabic word for “sword”) – some in the Great Sand Sea are 140km long. Formed by a uni-directional wind, they have slipfaces on both sides and a wavy, knife-edged crest along the top. When seif dunes fall over an escarpment they reform at the bottom as crescent dunes, which is why barchans are the prevailing form in Dakhla and Kharga. Occasionally, they pile one on top of another to create mountainous whalebacks or mega-barchans. Seif and whaleback dunes can combine to form huge sand seas or ergs. Egypt’s Great Sand Sea extends from Siwa Oasis to the Gilf Kebir and far into Libya, where it merges with the Calanscio Sand Sea. When the wind direction alters constantly, it can even form star-shaped dunes. These are rare in Egypt, but one has been recorded at Wadi al-Bakht in the Gilf Kebir. Another type of formation is the flat, hard-packed sand-sheet, found in the Darb al-Arba’in Desert.
Much of the science of dune formation was discovered by the explorer Ralph Bagnold, whose classic book The Physics of Blown Sand and Desert Dunes (1939) later helped NASA to interpret data from its Martian space probes. The book was written with the benefit of five years’ experimentation with a home-made wind tunnel and builder’s sand; after his desert journeys of the 1920s, Bagnold felt “it was really just exploring in another form”.