128
CLAUDE J. GRENOT
manent under-seepage (Dubief, 1953; Roche, 1968; Conrad, 1969). It
can drain extra Saharan waters through the desert as far as the Touat with
its meridional effluent, the Wadi Messaoud. In large wadis, there is, in
fact, a seeping of the water, called "underflow," through the alluvia.
2. Subsurface Water
Underground hydrography maintains a ribbon of vegetation, discontinuous but persistent, at the surface of the ground. These valleys or shallow
basins are mostly used as grazing areas. On the other hand, in regions
where there is no underground water, only the scarce Saharan showers can
initiate the growth of vegetation, which is then ephemeral and disappears
rapidly and is known as acheb or rbia.
Under most of the great dune massifs exists an aquifer located at varied
depths, whose waters for the most part originate out of heavy, local precipitations (soft waters without leaching of salts) but which had occurred
during the course of the moister periods in the past. The aquifer of the
Great Occidental Erg was replenished roughly 5000-7000 years ago during the neolithic humid era (Conrad and Fontes, 1970). It should be noted
that the gradient of salinity of this aquifer runs from the north to the south
which does correspond to the inclination of this water-bearing stratum
(Roche, 1968). Cornet (1952) believes that the origin of such subsurface
reservoir would be essentially from waters provided by the meridional
slope of the Saharan Atlas. These waters would also supply, to the south,
not only the erg but also the Gourara and Touat regions (Conrad and
Roche, 1965). Away from the valleys, these phreatic reservoirs are accessible through some rare wells generally marking the crossings of various
camel trails.
3. Salt Deposits
In certain porous formations where water is close to the surface, it
evaporates after having carried with it to the surface various soluble salts
from the soil. These are then deposited on the surface to form a compact
crust of "évaporites," composed of gypsum, calcium, and chlorides (Conrad, 1969). Certain flat-floored depressions, perfect zones of evaporation
for the running water, or underground waters, are characterized by the
presence of saline deposits (gypsum) (Figs. 10 and 11). These have their
origin in the "intercalary continental" beds, and are known under the name
of sebkha, in which the flora is rather poor. Such flora is composed of
well-adapted halophytic species such as the Chenopodiaceae.
Depending on the regions, the water can be quite pure (as in BéniAbbès), or it may contain a great number of salts in solution, (Reggan)
CLAUDE J. GRENOT
manent under-seepage (Dubief, 1953; Roche, 1968; Conrad, 1969). It
can drain extra Saharan waters through the desert as far as the Touat with
its meridional effluent, the Wadi Messaoud. In large wadis, there is, in
fact, a seeping of the water, called "underflow," through the alluvia.
2. Subsurface Water
Underground hydrography maintains a ribbon of vegetation, discontinuous but persistent, at the surface of the ground. These valleys or shallow
basins are mostly used as grazing areas. On the other hand, in regions
where there is no underground water, only the scarce Saharan showers can
initiate the growth of vegetation, which is then ephemeral and disappears
rapidly and is known as acheb or rbia.
Under most of the great dune massifs exists an aquifer located at varied
depths, whose waters for the most part originate out of heavy, local precipitations (soft waters without leaching of salts) but which had occurred
during the course of the moister periods in the past. The aquifer of the
Great Occidental Erg was replenished roughly 5000-7000 years ago during the neolithic humid era (Conrad and Fontes, 1970). It should be noted
that the gradient of salinity of this aquifer runs from the north to the south
which does correspond to the inclination of this water-bearing stratum
(Roche, 1968). Cornet (1952) believes that the origin of such subsurface
reservoir would be essentially from waters provided by the meridional
slope of the Saharan Atlas. These waters would also supply, to the south,
not only the erg but also the Gourara and Touat regions (Conrad and
Roche, 1965). Away from the valleys, these phreatic reservoirs are accessible through some rare wells generally marking the crossings of various
camel trails.
3. Salt Deposits
In certain porous formations where water is close to the surface, it
evaporates after having carried with it to the surface various soluble salts
from the soil. These are then deposited on the surface to form a compact
crust of "évaporites," composed of gypsum, calcium, and chlorides (Conrad, 1969). Certain flat-floored depressions, perfect zones of evaporation
for the running water, or underground waters, are characterized by the
presence of saline deposits (gypsum) (Figs. 10 and 11). These have their
origin in the "intercalary continental" beds, and are known under the name
of sebkha, in which the flora is rather poor. Such flora is composed of
well-adapted halophytic species such as the Chenopodiaceae.
Depending on the regions, the water can be quite pure (as in BéniAbbès), or it may contain a great number of salts in solution, (Reggan)