Geoscience Reference
In-Depth Information
soil occurs (up to 200 t
-1
ha
-1
y
-1
) and there is danger of flood-
ing downstream.
(2) In addition, these rains represent large quantities of water at a
time when PET is low; thus there can be saturation of the soil
and mass movements such as liquefaction, solifluction, gully
erosion (Bou-Kheir
et al
. 2001).
(3) The Mediterranean regions comprise many mountains and
sloping zones with increased danger of runoff.
(4) Last but not least, in the uncultivated zones, the mode of
exploitation of the environment has long been unfavourable for
sustaining a dense plant cover. On the scale of history, man has
cut forests for providing fuel for the furnaces of potters and
glassmakers; man has burned down inaccessible slopes to make
pastures for sheep. It is often reported in the media that the
vegetation, after a fire, takes 30 or 40 years to re-establish. In
fact, the environment never returns to exactly the same state as
before. The land, bare for several months after each burning, is
stripped by the first rains. The soil gradually gets thinner and the
vegetation changes to adapt to the corresponding reduction in
water-holding capacity. In the shrubby formations of Languedoc,
the beautiful strips of the original forests with holm oak (
Quercus
ilex
) are gradually replaced by small thorny shrubs: Kermes oak
(
Quercus coccifera
). The name comes from the cochineal insects
they harbour and which, when ground up, give the much-sought-
for colour 'bright red'.
In these areas subject to erosion, the soils are progressively planed down
from the top. A typical sequence formed is shown in Fig. 9.11. In France
for example, it is seen in limestone country in the Charente region
(Callot 1972). Under forest cover on the plateaus, the soils are deep (one
metre or several metres), decalcified, probably very old, of A/E/B/R
type. Chestnut trees, which dread CaCO
3
, have been able to establish.
On the slopes, the soils are gradually thinned down by erosion. Finally,
we get a soil of A/R type, called '
terre de groie
'. Actually, ploughshares
bring up splinters of limestone and reintroduce them into the fine earth
of the B carried to the surface.
This toposequence can also be viewed as a chronosequence. With
time, in an environment characterized by erosion, the soils of the left-
hand portion of the diagram transform into those of the right-hand
portion.
Resultant organization of the soils
