III. PHYSICAL AND VEGETATIONAL ASPECTS OF THE SAHARA
109
In the Saharan Central Massif, the Paleozoic regions encircle the core
of the Tuareg Shield with the "tassilian enclosure." The tassilis are essentially sandstone plateaus, slightly inclined in an outward direction. As
outlined in the complete diagram established by Kilian (1925) they are
composed of the following monoclinal elements: (1) internal tassilis (ordovician sandstone), (2) intratassilian depressions (gotlandian schists)
(Upper Silurian, U.S.A.), (3) external tassilis (early Devonian sandstone),
and (4) pretassilian zone (more recent Paleozoic). Very often the tassilis
exhibit sharply curved folds and submeridian-oriented fractures.
Farther east, the folds are replaced by large undulations with a large
curvature radius, and in the eastern Sahara, the primary formations (entirely continental) are often intermingled with the subhorizontal Mesozoic
structural surface.
As a consequence of the hercynian movements, the Paleozoic sea withdraws completely from the Sahara leaving in its place a vast sedimentary
lacune intercalated between the formations of the primary and secondary.
Following a few maritime encroachments in the northern part of the
central Sahara during the Triassic and Jurassic periods, the great phase
of the Mesozoic and Tertiary sedimentation then begins with the Cretaceous, spreading mostly over the northern part of the Sahara, with some
extensions arising from the Sudanese regions infringing over the Nigerian
Basin, in the meridional part of the Sahara.
These formations, generally almost completely devoid of any fault, constitute, in the north of the Sahara, a series of vast tabular regions called
hamadas (the Tademait, the Tinghert Hamada, and the Mzab) (Bernard,
1939; Joly et al., 1954; Lavocat, 1955).
The alpine movements (Atlasic) causing the development of folded
chains in Barbary, have only resulted, on the platform, in the formation
of vast alterations with moderate degree of curvature.
3. Paleoclimate and Prehistory
The Quaternary is a particularly interesting period owing to the climatic
variations sustained at that time by the Sahara (Balout, 1952; Chavaillon,
1964;Alimen, 1963).
According to Conrad (1971) : "The evolution of the Sahara is governed
by a succession of climatic pulsations which apparently occurred at a
rather rapid rhythm, since, following the Neogene sedimentary formation
of the hamadas, no less than five different cHmatosedimentary periods have
been established over a 2-3 millions of years time."
"These climatic changes have affected regions located in the desertic
belt centered on the Tropic of Cancer. The direction, duration and interactions of these variations which have determined the climate of the
109
In the Saharan Central Massif, the Paleozoic regions encircle the core
of the Tuareg Shield with the "tassilian enclosure." The tassilis are essentially sandstone plateaus, slightly inclined in an outward direction. As
outlined in the complete diagram established by Kilian (1925) they are
composed of the following monoclinal elements: (1) internal tassilis (ordovician sandstone), (2) intratassilian depressions (gotlandian schists)
(Upper Silurian, U.S.A.), (3) external tassilis (early Devonian sandstone),
and (4) pretassilian zone (more recent Paleozoic). Very often the tassilis
exhibit sharply curved folds and submeridian-oriented fractures.
Farther east, the folds are replaced by large undulations with a large
curvature radius, and in the eastern Sahara, the primary formations (entirely continental) are often intermingled with the subhorizontal Mesozoic
structural surface.
As a consequence of the hercynian movements, the Paleozoic sea withdraws completely from the Sahara leaving in its place a vast sedimentary
lacune intercalated between the formations of the primary and secondary.
Following a few maritime encroachments in the northern part of the
central Sahara during the Triassic and Jurassic periods, the great phase
of the Mesozoic and Tertiary sedimentation then begins with the Cretaceous, spreading mostly over the northern part of the Sahara, with some
extensions arising from the Sudanese regions infringing over the Nigerian
Basin, in the meridional part of the Sahara.
These formations, generally almost completely devoid of any fault, constitute, in the north of the Sahara, a series of vast tabular regions called
hamadas (the Tademait, the Tinghert Hamada, and the Mzab) (Bernard,
1939; Joly et al., 1954; Lavocat, 1955).
The alpine movements (Atlasic) causing the development of folded
chains in Barbary, have only resulted, on the platform, in the formation
of vast alterations with moderate degree of curvature.
3. Paleoclimate and Prehistory
The Quaternary is a particularly interesting period owing to the climatic
variations sustained at that time by the Sahara (Balout, 1952; Chavaillon,
1964;Alimen, 1963).
According to Conrad (1971) : "The evolution of the Sahara is governed
by a succession of climatic pulsations which apparently occurred at a
rather rapid rhythm, since, following the Neogene sedimentary formation
of the hamadas, no less than five different cHmatosedimentary periods have
been established over a 2-3 millions of years time."
"These climatic changes have affected regions located in the desertic
belt centered on the Tropic of Cancer. The direction, duration and interactions of these variations which have determined the climate of the