EGYPT

WESTERN DESERT

Like much of north-east Africa, the Western Desert is underlaid by level to gently dipping sedimentary rocks deposited on the Precambrian basement.

These rocks consist of continental sandstones with thin beds of marine limestones, and marls . This sequence is known as the Nubian sandstone (Issawi 1973) and was deposited between the Lower Paleozoic and the Upper Cretaceous, the marine beds dating from the Carboniferous to Lower Cretaceous. The Nubian sandstone thickens northwards from about 50 m near Aswan to 600 m in Sinai (Furon 1963).

The Western Desert consists of five main physiographic units. Three of these (the Libyan Plateau, the Great Sand Sea, and the eastern and southern areas) are flat to undulating surfaces, dominated by wind actions. They vary in altitude from 100 to 500 m above sea level (m a.s.l.).

The Libyan or El-Diffa Plateau in north-western Egypt extends from the Libyan border to the Qattara Depression, north of 29 N, reaching a height of 215 m a.s.l.. It is a hamada, a level to gently sloping surface covered mainly by stones and gravel, formed by the deflation of fine-grained sediment over thousands of years leaving behind a lag deposit of coarse stones. Towards the Mediterranean coast the plateau ends in an escarpment and the coastal plain varies in width from a few kilometers to about 50 km. The groundwater conditions, proximity to moisture-bearing winds, and cooler temperatures lead to moderate winter vegetation cover on the coastal plain, making it an important grazing area.

The area to the south of the Libyan Plateau is dominated by the Great Sand Sea. It sweeps down the western half of the desert joining the Great Selima Sand Sea in Sudan (El-Baz and Issawi 1982). This part of the desert is dominated by mobile sand and has little agricultural potential.

The eastern and southern parts of the desert have areas of mobile sand and stony plateau surfaces, which characterize the arid desert environment in this area and are formed by aeolian geomorphological processes. The plateaux exceed 500 m a.s.l.. and usually terminate in marked escarpments. The areas of mobile sand are less common and are mainly orientated north-west/south-east or north/south. The most significant of these is the Ghard Abu Muharik which exceeds 300 km in length.

The Western Desert is also characterized by a series of large depressions, the most famous being the Qattara which, at its lowest point is 134 m below sea level (m b.s.1.). It sweeps in an arc from approximately 50 km south of El Alamein on the Mediterranean coast to the Siwa Oasis, south of the Libyan Plateau; it extends further westwards into Libya. Other large depressions occur in the central and eastern parts of the Western Desert, and in areas adjacent to the Nile Valley and Delta (Table 1 and Fig. 1). All of these depressions have been formed by wind erosion and are termed deflation hollows; the areas of weaker rock in the Nubian sandstone sequence are preferentially eroded. The most important weakening factor is the geological structure (which is mainly faulted and jointed ), but the surface expression of rocks such as chalk, shales, and marls, which are not resistant to wind erosion, is also important (El-Etr and Moustafa 1982). Depressions such as Moghara Oasis, Wadi el Natrun, and El Faiyum have thick sequences of Lower Miocene and Pliocene sandstones, limestones, and gypsiferous marls with occasional basalt flows; the latter depression contains a well documented sequence of gypsiferous clays, limestones, and clayey sand. One of the longest sequence, however , from Cretaceous to the Oligocene, is found in Frafra Oasis , where it exceeds 225 m in thickness. Many of the depressions are geologically related along lines of extensive jointing or faulting, and smaller depressions can be found along these alignments. For instance, the Qattara and Siwa Depressions are formed along the same structural trend; the Bahariya and Farafra Depressions, and the farafra, Dakhla, and Kharga Depressions, form other structurally related groups. The large areal extents of these depressions leads to the presence of a number of geomorphological environments. First, there are stony hamada surfaces similar to those in the surrounding desert; second, areas of mobile sand dunes; and third, closed depressions with saline lakes or playas. Playas from when the wind erodes the rocks in the depressions, exposing aquifers. In some cases the groundwater from these aquifers can be a valuable, although slightly saline, water resource; in other depressions the salts are extracted for their mineral content. The combination of local groundwater resources for irrigation, and soils developed on sediments that have washed down or have been blown in, makes these closed basins suitable for oasis cultivation. A number of depressions (especially Dakhla and Kharaga) are being developed by the Egyptian Government at the present time.

while there are isolated hills and low plateaux throughout the Western desert formed by erosion of the Nubian sandstone, the only extensive mountainous area is found in the extreme south-west. This area can be divided into the more northerly sandstone Gilf el Kebir Plateau, and the granite Gebel Uweinat Mountains on Egypt's border with Libya and Sudan. The latter mountains are higher, Gebel Uweinat itself reaching 1893 m a.s.I. The area has been the site of many scientific investigations since its discovery in 1923, the most comprehensive work being that of El-Baz et al. (1980) and El- Baz and Maxwell (1982). Although this is hyperarid region (with rainfall occuring only once in 10-20 years or more), the plateaux and mountains exhibit strong evidence of Quaternary fluvial erosion, particularly in the from of wadis and gorges (the latter being known as kankurs). The kankurs are so deep that groundwater- bearing rocks are often exposed and springs are present. The groundwater in the Gebel Uweinat kankurs supports a perennial savannah-like vegetation of grasses, sedges, small herbaceous shrubs, and small trees reaching 4-5 m in height (Boulos 1982). The occasional flash floods in the wedis give rise to blooms of ephemeral desert vegetation, which can last for up to three years, based on the moisture trapped in the wadi- floor flood sediments. These outposts of vegetation in the hyperarid desert are exploited by nomadic herdsmen for sheep, goat, and camel grazing.