Note 50 — 11. The Sand-Dunes.

The steep Miocene escarpment, some 200 metres high, which bounds the Qattara depression on the north, probably produces a great eddy in the wind sweeping southwards over it, causing a whirl round a horizontal axis south of the scarp; this may be very influential both in excavation and in lifting the sand to great heights. As a large portion of the floor of the depression at the foot of the escarpment is formed of a salty sludge (sabakha), it is likely that a considerable quantity of finely divided salt crystals are carried up with the sand in summer, and this may conceivably act to some extent as a binding material in the dunes.