Geoscience Reference
In-Depth Information
The true justification of a new model is its ability to explain better what,
till then, had been incomprehensible. The new model, in temperate and
tropical zones (Chap. 6, ยง 6.7) enables us to show that the uniqueness
of the pedogenetic mechanisms is exhibited through an infinite variety
of soil profiles seen in nature. For us the front of clay formation (F1)
and that of degradation (F2) are independent and have their own rates
of advance. It is therefore of no use to examine if the losses from the E
are equal to the presumed gains in the B. It is a misinterpretation. Let
us examine a few typical cases (Fig. 8.7).
Dynamics of the profiles
A and E
A
and
(B)
A and E
F2
F2
B
F1
B
F1
C
C
F2
1: Dystric
Cambisol
4: Clay-depleted
residual loam
2:Luvisol
3: Planosol on deeply
weathered material
Fig. 8.7
Development of different types of profi les in temperate environment, as function of
the rate of movement of the clay-formation front (
F1
) and the clay-degradation front (
F2
).
Case 1 is common: the weathering front has descended, and has
produced clay, but the degradation front has still not appeared. This
is the Cambisol stage. Case 2 corresponds to the typical Luvisol: clay
appears at the bottom and is destroyed at the top. Case 3 is also common.
It corresponds to the alluvial materials argillified throughout their
thickness. Then there is no visible F1 front and no identifiable clay bulge
(= more than C!), but only surface degradation over a few decimetres.
This is what we have observed in le Bourbonnais in 30 percent of the
profiles (Favrot
et al
. 1973). We should note that the classic theory of
vertical clay translocation is unable to account for this phenomenon: thin
E horizon and a B horizon several metres thick! Case 4 is illustrated
by some surface formations that geologists called 'plateau silts . It is
not always a matter of aeolian additions but often of residual material
resulting from weathering
in situ
. Depletion seems to have been effective
up to the base of the horizons argillified earlier. This is a case observed
on the limestone plateaus of France, in Lorraine for example (J.P. Party,
pers. comm.). We have also found it on schist in the Vivarais.
