Dunes are not present in large enough numbers, or of large enough
size, to figure very prominently in the topography of the environs
of Laghouat; but they occur both to the east and to the west of the
town, those to the east being the larger. Sand is found facing the
south side of the Rocher des Chiens, a rocky hill on the western
edge of the oasis, and the south side of a portion of the Nomad
Mountains, to the north. There is also a succession of low dunes
between the Nomad Mountains and the oasis. The Rocher des Chiens
dune is moving from west to east, but the dune along the face of
the Nomad Mountains is probably stationary. The series of dunes on
the plain are moving toward the northeast.
An inspection of the dunes shows that the number of individuals,
as well as the number of species, is very limited. In fact there
are apparently fewer plants here than on the dunes of large size
subsequently encountered between Ouargla and Touggourt. The most
characteristic plant is “drinn” (_Aristida pungens_), but it
is by no means common; there are also a few specimens of _Tamarix_
growing near the Nomad Mountains. As frequently happens where
there are moving dunes, the passage of the dune greatly changes the
character of the flora. Although this feature was not especially
studied, it was noted that the number of grasses where the dune
had swept was greater than it was before this. _Acanthyllis
tragacanthoides_, a plant typical of the plain, was found to
survive the passage of the sand, although changed in appearance in
a characteristic manner. It will be recalled that the species has
a top consisting of several shoots, springing from the root-crown
at the level of the ground, and a root-system in which the laterals
and the main root are alike well developed. In order that a plant,
already established, can maintain this general relation during the
heaping up of the sand, the main root must grow at the crown as
fast as the sand encroaches. This is exactly what happens, so that,
when the dune has passed on, the shoot of the species is elevated
for a space equaling the depth of the sand, which is half a meter
or more. From the probability that _Acanthyllis_ is of very slow
growth and that the dunes are low, it follows that the rate of dune
movement must be slow.