Geoscience Reference
In-Depth Information
2.4.1
Effects of Slope
The soils on slopes are often arranged in a typical manner from the top to
the bottom and are genetically linked. Those on the upper slope supply
to those of the bottom: (i) water, (ii) ions and/or (iii) solid materials.
Then we have a
soil chain
or
catena
.
Let us take an example. Granite is an impermeable rock, but the soils
it gives rise to in temperate environments are sandy and permeable.
In this situation, the lateral circulation of water and other constituents
is facilitated. The soils are thus strongly differentiated laterally, a
phenomenon observed as early as in 1600 by Olivier de Serres (Legros
1973).
From top to bottom of the slope (Fig. 2.11) the soils get deeper
and deeper, more and more moist, and also less and less acid because
of lateral movement of basic cations (calcium, sodium, etc.). But the
increasing thickness of the soil is not only because of transport of
materials on the slope. It also results from an especially vigorous
weathering in the lower slopes where the environment is wetter.
Differentiation of catenas
Scree
Wall of stones
Outcrops
Isolated
Pines
Sequence on the Haut
Vivarais plateaus
;
altitude 400-500 m;
rainfall 800 mm
boulders
A
R
Broom
Poplars
Various crops
Willows
Shallow
soils of
A-R type, stony,
dotted with
outcrops, hard
to cultivate,
abandoned to
wasteland or poor
pine groves
A
Meadows
(
B)
Soils of the slopes,
of A-(B)-C type, neither
too wet nor too dry,
adequately deep, suitable
for crops or orchards
Excessively wet soils of depressions (basins)
characteristic of granite; A-B-C type; used for
permanent grassland; dotted with poplars, unless drained
A
C
R
B
C
R
200-400 m
Fig. 2.11
Toposequence of soils of Haut Vivarais on the edge of the French Massif Central,
up to 400-500 m altitude (Legros 1973).
