Chapter 4 THE EASTERN DESERT 4.1 GEOLOGY AND GEOMORPHOLOGY The Eastern Desert of Egypt occupies the area extending from the Nile Valley eastward to the Gulf of Suez and the Red Sea which is about 223000 km2, Le. 21% ofthe total area of Egypt (Figure 2.1). It is higher than the Western Desert as it consists essentially of a back­ bone of high, rugged mountains running parallel to and at a relatively short distance from the coast. The peaks of many of these mountains are more than 1500 m above sea level. These mountains are ßanked to the north and west by an intensively dissected sedimentary plateau. The folding and faulting that have occurred during geological history have caused these mountains to be dissected into several blocks of a series of mountain groups (Abu Al-Izz, 1971). The mountains of the Eastern Desert are of two types: igneous and limestone. The igneous mountains extend southward from about Lat. 28°N to beyond the Sudano-Egyptian border (Lat. 22°N). The highest peak, Gebel Shayeb EI-Banat (EI-Shayeb, near Lat. 2TN), reaches 2184 m above sea level. To the north of the igneous mountains are the extensive and lofty limestone mountains of South Galala (1464 m), North Galala (1274 m) and Gebel Ataqa (871 m) separated by broad valleys (wadis). To the west of the Red Sea mountains lie two broad plateaux, parted by the road of Qift to Qusseir (Lat. 26°N). This is a mountainous area which has the badland features typical of arid and semi-arid regions. The northern plateau is of Eocene limestone, the other is of Nubian sandstone and covers a broad area, approximately one-quarter of the total of the Eastern Desert. These plateaux differ from each other not only lithologically, but also geomorphologically. The sporadic rainfall of the Red Sea mountains has a differential infiuence depending on the nature of the formations on which it falls. On the Eocene lime­ stone narrow wadis are formed similar to canyons (Hume, 1925); in the Nubian sandstone area, running water produces broad wadis. This me ans that the Eastern Desert is greatly dissected by valleys and ravines and that all its drainage is external. Eastward drainage M. A. Zahran, The Vegetation of Egypt © M.A. Zahran 1992 118 The Eastem Desert to the Red Sea is by numerous independent wadis; the westward drainage to the Nile Valley, however, mostly coalesces into a relatively small number of great trunk channels. The dissection of the Eastern Desert by dense networks of wadis indicates that although the present time is a dry period, this could not have always been the case. Egypt must have witnessed some periods of pluviation. The range of the Red Sea coastal mountains thus divides the Eastern Desert into two main ecological units: the Red Sea coastal land and the inland desert. 4.2 ECOLOGICAL CHARACTERISTICS This section gives an account of the environmental characteristics and vegetation types of the Red Sea coastal land and the inland desert as being the main ecological units of the Eastern Desert of Egypt. 4.2.1 The Red Sea coastalland (a) General Features The Red Sea is an elongate trough extending NW-SE from the Sinai Peninsula (Lat. 29°50'N) to the Bab-EI-Mandab Strait (Lat. 12°35'N, Long. 43°3'E) and separates the Arabian Peninsula from the African continent. Throughout most of its length (2000 km), its opposing shore­ lines are remarkably parallel (Figure 2.1). In the south the width of the sea is only 175 km in the area between Jizan in Saudi Arabia (eastern co ast) and Massawa in Ethiopia (western co ast). It decreases to a minimum of 30-40 km at Bab-EI-Mandab Strait. The central re­ gion of the Red Sea has a depth of more than 2000 m (Zahran, 1977). Inland of the Red Sea shore-lines, there is a narrow coastal plain which is backed by a prominent escarpment (1500-3000 m high). The escarpment marks the uplift of the margins of the Arabian Shields and is the structural edge of the Red Sea Rift area (Chapman, 1978). The Asian (eastern) coast of the Red Sea extends from Aqaba southwards to the Bab-EI-Mandab Strait for about 2600 km from Jordan (at the north of the Gulf of Aqaba) to Saudi Arabia (2140 km) and northern Yemen (460 km). The African Red Sea coast (western) extends from Suez southwards to the Bab-EI-Mandab Strait for about 2860 km in Egypt (1100 km), Sudan (740 km), Ethiopia (1020 km) and Djibouti (Figure 1.1). Ecological characteristics 119 The Red Sea coastal land of Egypt extends from Suez (Lat. 300 N) to Mersa Halaib (Lat. 22°N) at the Sudano-Egyptian border (Figure 2.1). The land adjacent to the Red Sea in Egypt is generally mountainous, flanked on the western side by the range of coastal mountains. In the deep trough between the shore-line and the high­ lands extends a gently sloping plain which varies in width from 8 to 35 km. In certain parts of the west coast of the Gulf of Suez (e.g. Khashm EI-Galala, 60 km south of Suez), there is scarcely any plain, the mountains rising almost directly from the gulf. The coastal plain is covered with sand over which the drainage systems of wadis meander with their shallow courses. Along the Red Sea coast of Egypt, there are parallel lines of coral reefs between 50 and 100 m wide. They increase in density and width southwards and reach 250 m wide to the south of Mersa Alam (675 km south of Suez). The temperature of the Red Sea water is 21-22°C which is suitable for the formation of reefs and is further enhanced by such factors as the shallowness of the sea (in the coastal area its depth does not exceed 40 m), the high salinity (4%) and the purity of water near the coast. The climate of the Red Sea coastalland of Egypt is arid. The mean annual rainfall ranges from 25 mm in Suez, 4 mm in Hurghada to 3.4 mm in Qusseir (Figure 2.1). The main bulk of rain occurs in winter, i.e. Mediterranean affinity, and summer is, in general, rainless. Variability of annual rainfall is not unusual. In Suez, for example, the years 1949 and 1958 were very dry: total annual rainfall was 2 mm and 3.1 mm respectively; whereas in the years 1952 and 1956, which were relatively wet, rainfall was 56 mm and 55 mm respectively (Kassas and Zahran, 1962, 1965). Temperature is high and ranges between 14-21.TC in winter and 23.1-46.rC in summer. Relative humidity ranges from 43% in summer to 65% in winter. The Piche­ evaporation is higher in summer (13.7-21.5 mm/day) than in winter (5.2-10.4 mm/day). The effect of topography on precipitation is universal but more pro­ nounced in coastal regions. Within the arid and semi-arid countries, coastal mountains may cause ample orographie rain which is referred to as 'occult precipitation', 'horizontal precipitation' or 'fog precipita­ tion' (Moreau, 1938). This orographie rain may produce rich vegeta­ tion on slopes of high mountains - 'Nebelwald', 'Nebeloasen' (Troll, 1935); 'mist oases' (Kassas, 1956). When the whole range of the Red Sea coastal mountains of Egypt is considered, the southern blocks (represented by the Elba group) may be seen to receive greater amounts of water from orographie rain than the northern blocks. 120 The Eastem Desert (b) Vegetation types Apart from the valuable floristic studies by Ruprecht (1849), Schwein­ furth (1865a,b, 1896-1899), Ascherson and Schweinfurth (1889a,b) and Drar (1936), the plant ecology of the Red Sea basin has been the subject of several investigations. Schweinfurth (1865a,b, 1896-1899) accumulated valuable ecological observations on the Red Sea coastal land. Ferrar (1914) describes some of the mangrove swamps of the northem Red Sea coast. Troll (1935) gives a more detailed account of the southem part ofthe Red Sea coastalland. Vesey-FitzGerald (1955, 1957), Batanouny and Baeshin (1982), Younes et al. (1983), Zahran (1982b) and Zahran et al. (1983, 1985b) describe the vegetation types of the Saudi Arabian Red Sea coast. Hemming (1961) gives accounts of the plant cover of the coastal area of northem Eritrea. Kassas (1956, 1957, 1960) presents ecological information on the Red Sea coastal land of Sudan. Montasir (1938) provides an ecological de­ scription of the salt marsh vegetation, the mangrove vegetation and the desert vegetation of the Egyptian Red Sea coast. Hassib (1951) gives a life-form spectrum of the flora of the Red Sea region of Egypt, and detailed accounts of the vegetation and flora of the Red Sea coastalland of Egypt have been given by Kassas and Zahran (1962, 1965, 1967, 1971), and Zahran (1962, 1964, 1965, 1977) and Zahran and Mashaly (1991). These studies show that ecologically this coastal area may be considered under three principal ecosystems; (1) coastal salt marsh, (2) coastal desert and (3) coastal mountains. The environ­ mental conditions, notably tidal movement, sea water spray, sea water seepage and waves, characters of the substratum, land relief, and local and microclimates, are the main factors that limit the type and extent of the plant cover in each ecosystem. These systems are described below. (i) The littoral salt marsh The coastal (littoral) salt marshes comprise areas of land bordering the sea, more of less covered with vegetation and subject to periodic inundation by tides. They have certain features related to the prox­ imity of the sea that distinguish them from inland salt marshes (Chapman, 1974).