Geoscience Reference
In-Depth Information
Period of rhexistasy
with
translocation of solid materials
Period of biostasy
Coarse
deposits
Forest vegetation
Plant cover removed
and soils eroded
Translocation
of ions
Deep
soils
Coal deposits representing the residue of
fallen trees and transported into the basin
Precipitated
fine deposits
Lake — free water
Fig. 3.20
Biostasy and rhexistasy (Erhart 1956) observed in the Bourbonnais region (Favrot
et al
. 1973).
Lignite deposits are rather rare. As for the rest, this diagram is
general for Western Europe and is applicable to kaolinitic saprolites of
the end of the Cretaceous and the beginning of the Tertiary (Thiry
et al
.
1999).
3.5.3 Soils at the Landscape Scale
This question has already been tackled in Chapter 2 (§ 2.4.1): movement
of water, ions and suspended materials takes place in soils on slopes.
But understanding of the processes of desilication and neoformation
(see above) enables a better evaluation of the considerable impact slope
can have on the differentiation of soils at the scale of the landscape. Of
course, there is interaction with the climate (quantity of water available)
and the general relief (water saturation at the bottom slope versus
drainage towards a stream).
3.5.4 Variability at the Profile Scale
The pattern of progressive desilication (Fig. 3.9) shows that we must
expect soils that will display the phenomenon from the bottom upwards.
Therefore, in hot and humid country, we are bound to find the two
profiles represented in Fig. 3.21. This was predicted long ago through
simulation of the reactions between minerals and solutions (Fritz
1975).
