As part of the Cooperative Research Project 389 ACACIA (Arid Climate, Adaptation and Cultural Innovation in Africa), investigations were carried out in the Great Sand Sea of Egypt in spring 1996. Along 450 km of a selected dune ridge, 24 cross-sections reveal both a south-north change in modern and palaeo-processes and a change in palaeo-climate. The southern longitudinal draa (megadunes) were modelled by strong Pleistocene trade winds from sands most probably derived from weathered native sandstone, and were stabilized during an Early Holocene humid phase. Following aridisation, aeolian sands - probably in the form of barchan dunes - migrated some tens of kilometers farther south. The unique northern megadunes are probably transverse draa modelled during the Pleistocene by dry(!) extra-tropical westerlies. As a result of the aridisation following the humid phase, all of the draa sands, as opposed to the dune sands, are saline. The modem dunes (silk type), sitting between the draa in the north and on top of the draa in the south, reflect the bimodal wind system with a main component shifting from strong westerlies in the north to northern winds in the south. (C) 2000 Elsevier Science B.V. All rights reserved.
