Geoscience Reference
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bottom to the top, thus beginning with what pertains to the C and
ending with the surface horizons.
Residues of B left within the E
A and E horizons in which deferrification
is general: clay minerals are destroyed,
particularly in the tongues where water
circulates preferentially (4);Si and
Al are eliminated laterally (5)
E
4
5
Descent of the
degradation
front
B horizon where clay-weathering
phenomena (1), translocation over
short distances of fine ferric clay (2)
and deferrification followed by
formation of iron concretions (3)
coexist
3
B
2
1
1
Descent of
the
weathering
front
BC or C horizon, more or less
unaffected by pedogenetic
transformation with the exception
of filling up of cracks by brown
ferric clay come from the B (2)
2
C
Fig. 8.6
Scheme of formation of an Albeluvisol on gneiss in the framework of the weather-
ing-front model.
The following are the differences compared to the classic model:
Clay enrichment in the B is largely linked to weathering
in situ
of the minerals of the parent material. Thus, this is essentially a B
of weathering. The phenomenon of clay illuviation could affect it
spectacularly but it involves a small quantity of material and often
occurs internally without noticeable export out of the horizon. Studies
by various authors (Chap. 9) also show that the movement of clay is
most visible at the base of the B and not at its top as the classic theory
of argilluviation will require. At the extreme, working to the benefit of
the BC or C, it serves to deplete the B of clay (as in arènes on granite
or gneiss)!
The formation of the E results chiefly from the attack of clays in the
upper part of the B (separation from the iron and partial destruction).
Secondary argilluviation is necessarily limited by mechanical factors
(rapid blocking of downward drainage).
The two forms of argilluviation, in our view, play only a small
quantitative role in the textural differentiation of horizons in Luvisols.
The E horizon is a well for the clay, not a production site.
