No dayas were seen after leaving Tilrempt, and the aspect of
the country changed markedly and suddenly as the drainage became
better defined. The hills were more abrupt and in systems, and
the valleys became broad and continuous. At first the valleys were
wide and shallow, the hills being low and with flat summits, but
as the distance from the daya region increased, the valleys became
deeper, until at Ghardaia the effect was that of low, flat-topped
mountains with broad valleys between, thus remotely suggesting
the topography of southern Arizona. However, in southern Algeria
the mountains are not so high nor is the “mesa” (hamada) so
extensive as in Arizona. The general level of the daya region is
prolonged as the summits of the mountains of this the chebka region,
while the valleys are eroded to a new level, that of the plain of
the M’Zab. A similar condition is seen as one goes from Ghardaia
to Ouargla, so that in fact there are several immense terraces,
reminders of that remote period when there was more rainfall in
this portion of the Sahara than at present.
Although the drainage to the south of the daya region is well
developed, the valleys and mountains run in a rather confused way,
so as to give to the fancifully inclined Arab the idea of a net,
from which the name “chebka” is said to be derived.
The country from Tilrempt to Ghardaia is characterized by a
continuously decreasing amount of vegetation. In place of the
country as a whole having a covering, however sparse, as in the
daya region, one is apt to consider the chebka a barren desert,
absolutely devoid of plant life; but closer inspection dispels this
illusion and reveals the presence, in the more favorable situations,
of not a little vegetation.
In the northern portion of the chebka region one sees here and
there, on the bottoms, specimens of the jujube and the betoum,
as well as _Zilla macroptera_, _Retama sphærocarpa_, and _Coronilla
juncea_. On the rocks at Settafa, Massart reports finding lichens,
the first he had seen after leaving Biskra. However, crustaceous
lichens occur on the flat tops of the low mountains by Ghardaia.
Massart suggests that the absence of lichens in the Sahara
(possibly they are not to be found south of Ghardaia) is because
of the intense dryness and the great heat, the temperature of
the rocks becoming from 60 to 70° C.[17]
At Berriane, one of the M’Zab cities, there are over 30,000
palms of a superior sort, watered from over 400 wells. The
surroundings of this oasis are extremely desertic and a casual
survey of the route between this place and Ghardaia, 44 kilometers
distant, reveals almost no vegetation. Here the calcareous plain
of Cretaceous origin, the Chebka, is even more eroded than in the
portion farther to the north, and the valleys are wider. The soil
is a fine clay without an admixture of sand. It is only in the most
favorable places, along the washes, that plants are to be found, and
here are _Deverra chlorantha_, _Anabasis articulata_, _Gymnocarpon
fruticosum_, _Artemisia herba-alba_, _Ononis angustissima_, _Linaria
fruticosa_, _Antirrhinum ramosissima_, and _Haloxylon articulatum_
(Massart, _loc. cit._). _Peganum harmala_ occurs very sparingly
by the roadside. The habits and the habitats of certain of the
above-mentioned species will be described in greater detail later
in this paper.
=GHARDAIA.=
The Ghardaia region can be characterized as a vast plain, broken to
the north by low, irregularly disposed mountains, and stretching for
a great distance to the south and southeast with a fairly monotonous
surface, diversified only by oueds, chotts, or occasional dunes,
which may be the size of small mountains. Thus on the one hand one
finds the fairly diversified and stony Chebka and on the other the
hamada, which has been aptly described by Brunhes[18] as “le désert
par excellence, la vrai désert . . . les grandes plaques pierreuses
indéfinies des hamadas!”
The leading oued of this region is the M’Zab, which extends for
about 270 kilometers in a direction south of east across the southern
part of the Chebka. It takes its origin about 80 kilometers west of
Ghardaia and extends to the vicinity of Ouargla, where it debouches
on the Ouargla plain. At Ghardaia the oued lies in a valley, with
abrupt sides, which is sunk about 60 meters below the surrounding
plain and which at this place is about 3 kilometers in width. (See
fig. 16.) There are four main tributaries of this oued, all of which
join it from the north or the Chebka side. The valley of the M’Zab
becomes more and more shallow as one proceeds eastward and at last
lies but little below the general level of the country. Like the
other deeper valleys of the Chebka, the M’Zab Valley represents
the work of erosion by water at an earlier geological epoch, when
the great terraces were formed. The filling of the eroded valleys
has perhaps taken place during the long arid period since that time
and has probably proceeded very slowly.
It appears to be uncertain how long the M’Zab has been inhabited
by man, or, more accurately, by the race now dwelling there;[19]
but it has probably been not less than nine centuries.[20]
At Ghardaia are several well-defined plant habitats, which may or
may not be distinct topographical areas, and which differ from one
another in exposure, soil conditions, and water relations. These are
the plain of the Chebka (hamada), the low and flat-topped mountains
resting on the plain of the Chebka, the walls of the M’Zab Valley,
and the valley floor with its gardens, cemeteries, and waste lands.
The soil conditions of the areas mentioned are very diverse. On the
valley walls and the mountains there are bold rock outcrops with soil
in the interstices only, and here the most intensely arid conditions
prevail. The soil on the hamada also is exceedingly meager. Rocks of
various sizes strew the surface. It is only between them, as well
as in the washes of gentle gradient, that the best soil conditions
of the plain are to be found. Here a cursory examination shows a
large admixture of small stones to the fine clay, the prevailing
soil type, and that at a depth less than 50 cm. A white hardpan,
similar in appearance to the caliche of the southwestern United
States, may usually be encountered. In the drainage depressions the
soil is relatively more coarse than on the more level portions of
the hamada. There is also great variation in the character of the
soils of the valley. Above the upper palm gardens, which are about
2 kilometers above the town of Ghardaia, will be found much sand
and fairly large stationary dunes, while smaller dunes, shifted by
the winds, are to be found at various places in the valley. About
10 kilometers down the valley, toward the east, the sand is blown
against the valley sides, and in certain places where the walls are
low it has been sifted in a thin layer over the plain itself. At
the sister city of El Ateuf the drifting sand is a continuous menace
to the gardens.
Between Ghardaia and the upper palm gardens, and also between this
city and Beni Isguen and Melika, are bare areas, free from sand
or clay, where the soil is so hard as to be used for threshing
floors and where the small amount of grain grown in the valley is
threshed and winnowed. The hardpan is similar in appearance to the
caliche of the southwestern United States and may be essentially the
same. It is of wide extent in the valley and probably underlies the
largest portion of it. Near the threshing floors the upper portion
of the hardpan stratum is from 2 to 3 meters above the bed of the
oued M’Zab; the stratum is about 30 centimeters in thickness
and is of fairly uniform structure throughout. Beneath this is
another stratum, less well defined perhaps, of approximately the
same thickness, and with nearly the same character, but carrying
a noticeably large percentage of sand. The lower stratum is less
hard than the upper one. Underneath the second stratum is soil,
largely sand, containing rocks of various sizes. Where erosion of
the oued banks has occurred the soft lower hardpan stratum and the
yet more soft underlying soil are both removed, leaving the upper
stratum projecting as a shelf, sometimes as large as 2 by 4 meters
in extent. When the shelving banks break they remain practically
intact, partly buried by the sandy floor of the oued. (See fig. 17.)
Along the sides of the valley, and at a distance more remote
from the oued, there are occasional washed-out areas, really box
cañons, where the banks show a slightly different condition of the
hardpan from that just described. Here there may be three strata of
hardpan. The uppermost is of the same stratum as the top stratum
by the oued, and the second stratum also resembles the lower one
just described. There is also a third hardpan stratum of a much
different character, in that it has a very large admixture of sand
and gravel, and large as well as small stones, making it more easily
eroded than either of the upper strata. The soil in which the roots
studied were found varied from a fine sand, with waterworn pebbles,
near the oued, to a clay mixed with sand nearer the sides of the
valley. In places the sand is cemented so as to resemble one of the
hardpan strata above described, but it is less hard and apparently
is penetrated by water without great difficulty.
The Oued M’Zab, whose channel is 15 meters more or less in width,
is dry most of the year, containing water for only a few hours
following the rare storms. Wells are very numerous in the valley and
furnish a good supply of water. At the time of my visit to Ghardaia
the water lay from 10 to 25 meters from the surface, depending on
the position of the wells. The depth to water in a single well is
said to vary from 1 or 2 meters to 15 meters; in other words, the
water-table of the valley varies 13 to 15 meters between the dry and
the moist seasons. No analysis of the water is available, but it is
reputed to be noticeably saline near and below the town of Ghardaia,
while above the town this quality is not apparent to the taste. The
water relations of the plain are much less favorable for plants
than those of the M’Zab Valley. In addition to the fact that the
soil of this area receives only such water as falls directly on it,
there is so little depth that the water escapes shortly after it
falls, leaving only the most favorably situated soils, for example,
those beneath shallowly placed rocks, or between rocks, or in deeply
penetrating cracks or the depressions, with sufficient moisture for
long use by plants. The depth to water on the hamada is so great
that successful wells have never been dug.