Deserts of the World - Design with the Desert: Conservation and Sustainable Development - page 3

Mediterranean Sea

Tunisia

Morocco

Algeria

Libya

Sahara

Western
Sahara

Egypt

Mauritanica

Mali

Niger

Sahel

Sudan

Chad

Atlantic
Ocean

FIGURE 1.2
Map of North Africa showing the general distribution of the Sahara Desert and the Sahel.

striations, crescentic gouges, erratic boulders, and glacial lineations are still evident in the
present landscape. 8 In the early Tertiary, there was a long interval of intense weathering,
producing duricrusts on the southern side of the Sahara in tectonically stable lowlands, 9
while Tertiary and Quaternary uplift, with associated volcanism, produced major Saharan
massifs such as the Hoggar and Tibesti. In the late Tertiary, climatic deterioration and
tectonic movements between Africa and Europe led to the gradual diminution of the
Tethys Sea and the formation in the so-called Messinian salinity crisis (c. 6 million year
BP) of a large-closed depression in the vicinity of the present Mediterranean basin. The
northern Sahara must have been arid at that time, and large spreads of evaporites formed
before a marine transgression through the Straits of Gibraltar caused a reestablishment of
marine conditions. 10 The Nile cut down deeply to the low base level, forming a canyon
8200 ft deep, 813 miles long, and 6-12.5 miles wide, dimensions which comfortably exceed
those of the present-day Grand Canyon in Colorado (Figure 1.3). 11

FIGURE 1.3
Photograph of the eastern Sahara Desert in the Sudan. (Courtesy of J. Woodward.)