Upon arriving at Ghardaia in November, it was surprising to find
several species of perennials, under strictly desert conditions,
putting on new growth, forming fresh leaves, or coming into
flower. Especially was this unexpected in view of the fact that
no rain had fallen in the region for twelve months. A study of the
roots in relation to the depth to water showed also that most of the
native plants, during the dry season, could not have penetrated to
a depth anywhere near that of the water-table of the valley, and the
water relations of the plants growing on the plain above the M’Zab
Valley were even more severe. Further, most of the species do not
possess water-storage faculty. It should be noted that the only
apparent difference in the water relation between November and in
early autumn, or summer, was that of raising the relative humidity
of the air through the lower temperature. In a preceding section it
has been stated that little or no evaporation took place at night
in November at Ghardaia, and possibly less in the daytime than
would have been supposed. The leading environmental changes were,
of course, the lower daily temperature and the really cool nights.
It is not uncommon in the Arizona desert for a species to form
flowers or leaves, following a change in temperature, the moisture
conditions being not otherwise changed, but, so far as I know,
the temperature changes bringing about this result are always from
a cooler to a warmer condition and not the reverse. It would not
be expected, consequently, that in the present instance a renewal
of vegetative activities would follow as a _direct_ result of a
lower temperature, although analogous changes are necessary before
certain species, after rest, will start development. Whatever may
be the immediate cause of the renewal of activity on the part of the
plants at Ghardaia as noted, the following species were seen to have
formed new leaves: _Henophyton deserti_ and _Zilla macroptera_;
also the following fresh flowers with or without shoot growth:
_Fagonia bruguieri_, _Haloxylon articulatum_, _Henophyton deserti_,
_Ononis polyclada_, and _Zollikoferia resedifolia_, and three other
species not determined.
=GHARDAIA TO TOUGGOURT.=
From Ghardaia the route, consisting of camel trails only, pursued
a course south of east to Ouargla, and from Ouargla a direction
east of north to Touggourt, over 400 kilometers, Ouargla being about
half-way. There are no villages between Ghardaia and Ouargla and none
between this place and a point 20 kilometers south of Touggourt,
so that in accounts of the vegetation or the topography, lacking
convenient points around which to center descriptions, the device
will be adopted of using distance estimations to or from the three
chief towns.
The trail followed the valley of the Oued M’Zab, or kept close to
it, for 63 to 73 kilometers before finally leaving it. It passed
the sister towns of Ghardaia and crossed short intervals of plain,
descending occasionally to the oued. The walls of the M’Zab
Valley, 60 meters more or less at Ghardaia, become lower and
less precipitous as one goes down the drainage, until at length
they become little more than rounded banks. The low, flat-topped
mountains, which are a feature of the topography about Ghardaia,
were soon left behind, and nothing similar was encountered until
the vicinity of Ouargla was reached. Between Ghardaia and El Ateuf
are small dunes in the valley and sand is drifted along the base of
the walls at various places. Near and immediately east of this town
the sand is especially abundant and, being shifted by the winds,
constitutes an ever-present menace to the small gardens belonging
to the inhabitants of the place; in order to control its drifting,
fences of palm leaves are made or the sand is removed when it becomes
too abundant. Often the gardens are abandoned, leaving the palm
fences to mark their sites after the sand has gone beyond. Where
the air-currents are most powerful or most consistent, or the walls
are broken down, the sand may be carried in small quantities onto
the plain, where it constitutes a mulch, influencing in a striking
manner the character of the vegetation (fig. 42).
Finally leaving the valley of the M’Zab about 73 kilometers from
Ghardaia, the trails wound upwards through low rounded hills to the
hamada. This is the northern edge of the region of the Gantara, 100
by 150 kilometers or more in extent, reaching from the valley of the
M’Zab on the north to the region of the dunes to the south. It
slopes towards the Oued Igharghar, or the drainage depression
connected with this great oued. The Gantara has a few chotts and
is crossed by three oueds in the southern portion. It is probably
the most arid part of southern Algeria. About 60 kilometers of the
plain were crossed and here it was gently rolling and stretched
without a break to the horizon. The surface resembles that of the
hamada at Ghardaia, that is, stones of various sizes, usually small,
lie on its surface, but never forming a continuous cover, as in some
portions of the Arizona desert. The soil is brown, of fine grain,
and with little or no addition of sand. In the innumerable little
hollows the soil is deeper than on the slight rounded ridges. Wind
is apparently the most potent erosive agent.
Two chotts were encountered between Ghardaia and Ouargla, one
unimportant, the other large and with many features of interest. The
latter, the Chott Mellala, is about 10 by 15 kilometers in size. The
trail descends from the plain, winding through a zone of rounded,
cone-shaped hills or mamelons, to the floor of the chott, which lies
about 60 meters lower than the general level of the plain. The chott
was quite dry in November when we visited it, but at rare intervals
water is said to flood the central portion. Toward the outer edges
the floor is thrown into waves, where the heavy incrustation of
salts is broken. In the center the salt crust forms an unbroken
and level surface. Gypsum (calcium sulphate) is the predominant
salt. On the eastern side a long and high ridge of sand rears its
uneven summits. The height of this ridge was estimated to be 250
meters, and was said by Massart to be the largest seen by him in
the Sahara. This dune we had seen lying on the eastern horizon for
one or two days before reaching the chott.
The relation of the Chott Mellala to the country to the north or
the south was not seen, but between it and Ouargla there lies a
succession of smaller and more irregular chotts, which together
form a fairly well-connected chain. These chotts are separated
by low passes and flat-topped hills whose summits are on a level
with the neighboring plain. Many of the hills are cone-shaped
and in other topographic features the region shows the eroding
action of wind. About 7 kilometers from Ouargla an opening in the
mamelons gives a view of a plain extending on a lower level to the
horizon. This is the reg, or fluvial desert. The Ouargla plain, or
reg, is connected with the drainage of the great Oued Ighaghar and
has a character which in many ways is different from the Gantara,
over which we had just passed.
With the descent to the reg desert a more monotonous region is
encountered. In the vicinity of Ouargla and for some kilometers to
the north the topography is quite flat and gives the impression of
a flood-plain. To the east it stretches unbroken to the horizon,
but to the west it is bounded by a fairly abrupt wall, the Gantara
escarpment. At intervals of several kilometers low sand ridges cross
the route over the plain, and on the second day somewhat higher
ground was traversed and a sand ridge about 4 kilometers across
was encountered. The country then becomes somewhat more broken and
presents the appearance of being the remains of an ancient and more
elevated plain. About 56 kilometers from Ouargla are the largest
dunes crossed; where traversed, these were 10 kilometers from north
to south and extended beyond our vision both to the east and the
west. This is apparently the edge of extensive dune regions which
lie mainly east of Touggourt and of the Oued Rirh. For possibly the
last 30 kilometers of the journey to Touggourt there are dunes and
chotts in alternation.
A word should be said regarding the hydrography of the region whose
surface features have been sketched above. Between the cities of Beni
M’Zab and Ouargla two wells were passed, although a route could
have been taken which could have included three wells. The wells are
125 meters or less in depth and are maintained by the government for
the benefit of the caravans, as well as to provide water for the
large number of goats and sheep pastured in the neighborhood. The
situations of the wells are always in depressions, either along the
Oued M’Zab or in similar although smaller drainage areas, and none
are on the Gantara. At Ouargla and on the reg desert to the north of
the town the water lies very close to the surface of the ground. It
can be dipped with buckets and the roots of the palms reach to the
water-table. The water from the shallow wells is strongly impregnated
with salts. Before reaching Touggourt standing water was seen where
the trail crossed certain chotts. Numerous artesian wells have been
made by the government which penetrate the ground several hundred
meters and give a large and continuous supply of sweet water.