The only area where sand dunes are of any considerable magnitude or extent in South-Eastern Egypt is the tract called I Hubâl (lit. “the sand dunes”) which extends from Wadi Meisah to Wadi Aideib, fronting the hills of Ti Keferiai, Geror, Balatitda, Sul Hamid, and El Sela. The north faces of all these hills and the smaller hills north of them are swathed in thickly accumulated sand to such an extent as to make travelling over them difficult. The reason for the heavy sand accumulation at this particular place is that the north-west wind, which is the prevalent wind over Egypt proper, here meets with south-easterly winds coming up the Red Sea, causing a local calm and a consequent dropping of the sand borne from the Nubian sandstone areas by the north-west wind. Thus the same climatic conditions which give rise to the rich vegetation on the Elba mountains cause at the same time extreme desert conditions in the tract immediately north-west of them. Sand accumulations of less magnitude occur in and about the lower reaches of the Wadi Hodein, in the great valley to the north of Berenice, on the plain west of Gebel Um Harba, and at some few other places, but they are seldom so extensive as to cause a serious hindrance to travel. Besides transporting the ordinary dune-forming sand, the action of the wind is responsible for a considerable amount of redistribution of the coarser granitic and other sand produced by the disintegration of igneous rocks. In high winds this coarse sand, in angular grains as large as peas, is blown about on the coast-plain to considerable distances from the places where it was originally deposited by water transport. =Alluvial Deposits.=