III. PHYSICAL AND VEGETATIONAL ASPECTS OF THE SAHARA
123
1963), as they seem to represent the biologic realities quite well. Aridity
is represented by the ratio of rainfall to evaporation. The index chart
drawn from it superimposes that of the pluviometry.
To sum up, the principal climatic characteristics of the Sahara are (a)
low amount of precipitation received annually (mean annual rainfall is
less than 10 cm) (see Fig. 1 and Table I I ) , (b) extreme irregularity of
rainfall frequency, (c) air dryness, and (d) wind amplitude. In the
Sahara, seasons are not well delineated. It is generally possible, however,
to distinguish a warm and a cold season.
D. DESERT R E L I E F
1. General Considerations
The Sahara is a peneplain with a few mountainous massifs jutting out,
known as jebels, such as the mounts of Ougarta, the Adrar of Moras,
the Aïr, the Hoggar, and the Tibesti.
The relief of the Sahara has been greatly affected by the climatic variations of the Quaternary (rainfall, fluvial, and aeolian erosion), entirely reshaping the landscape and creating its distinctive features (Chavaillon,
1964; Planhol and Rognon, 1970). Only in a desert can one find this kind
of erosion and transportation of the surface materials by the wind that
reaches such an amplitude and violence (Figs. 4 and 5 ) .
2 Rock Formations
When original rock formations are composed of large, homogeneous
tabular beds, the effects of erosion result in the formation of rocky plateaus—the "hamada" or "tassili." These hamadas show traces of ancient
fluvial erosion, as evidenced by remaining buttes capped most of the time
with a hard, rock, tablelike feature, and known as "gour," (sing.: "gara")
(Fig. 6) as well as by gorges dug into the plateaus (the Mouydir and the
Ajjer Tassili). On the other hand, when the original rock formations
are of the heterogeneous type, the wind disaggregates the less resistant
components leaving only the hardest particles to form a gravelly surface,
the "reg." If we consider the reg genesis, we can identify two major types
of ergs: autochthonal, resulting from the dissociation of elements; and allochthonal, originating in the transport and accumulation of various elements, and which is also called alluvial reg. However, hard rock is often
covered with some kind of a patina having the appearance of a varnished
luster; this patina is, in fact, "a crust of chemicals which have been brought
to the surface by capillarity and fixed by evaporation; it is of a dark red
123
1963), as they seem to represent the biologic realities quite well. Aridity
is represented by the ratio of rainfall to evaporation. The index chart
drawn from it superimposes that of the pluviometry.
To sum up, the principal climatic characteristics of the Sahara are (a)
low amount of precipitation received annually (mean annual rainfall is
less than 10 cm) (see Fig. 1 and Table I I ) , (b) extreme irregularity of
rainfall frequency, (c) air dryness, and (d) wind amplitude. In the
Sahara, seasons are not well delineated. It is generally possible, however,
to distinguish a warm and a cold season.
D. DESERT R E L I E F
1. General Considerations
The Sahara is a peneplain with a few mountainous massifs jutting out,
known as jebels, such as the mounts of Ougarta, the Adrar of Moras,
the Aïr, the Hoggar, and the Tibesti.
The relief of the Sahara has been greatly affected by the climatic variations of the Quaternary (rainfall, fluvial, and aeolian erosion), entirely reshaping the landscape and creating its distinctive features (Chavaillon,
1964; Planhol and Rognon, 1970). Only in a desert can one find this kind
of erosion and transportation of the surface materials by the wind that
reaches such an amplitude and violence (Figs. 4 and 5 ) .
2 Rock Formations
When original rock formations are composed of large, homogeneous
tabular beds, the effects of erosion result in the formation of rocky plateaus—the "hamada" or "tassili." These hamadas show traces of ancient
fluvial erosion, as evidenced by remaining buttes capped most of the time
with a hard, rock, tablelike feature, and known as "gour," (sing.: "gara")
(Fig. 6) as well as by gorges dug into the plateaus (the Mouydir and the
Ajjer Tassili). On the other hand, when the original rock formations
are of the heterogeneous type, the wind disaggregates the less resistant
components leaving only the hardest particles to form a gravelly surface,
the "reg." If we consider the reg genesis, we can identify two major types
of ergs: autochthonal, resulting from the dissociation of elements; and allochthonal, originating in the transport and accumulation of various elements, and which is also called alluvial reg. However, hard rock is often
covered with some kind of a patina having the appearance of a varnished
luster; this patina is, in fact, "a crust of chemicals which have been brought
to the surface by capillarity and fixed by evaporation; it is of a dark red