Geoscience Reference
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whereas the latter correspond to surface erosion. The values obtained
were of the same order of magnitude: what the profile gained yearly
through progressive penetration was almost equal to what was lost from
the surface. But this is not always true.
It is shown in Figure 3.4 that the horizons seen today are not exactly
derived from each other. They are derived from and are the descendants
of horizons that have disappeared (black rectangles). In this evolutionary
model, the present-day profile is the image of all the changes undergone
by the horizons. It forms a kind of spatial indicator of the evolution over
time.
The main thing is to see that each horizon of the profile 'eats or
gnaws at' its lower boundary (which is also its front edge) that precedes
it downwards. At the same time, it is 'eaten' at its upper boundary
(which is also its rear boundary) by what overlies it. This is the concept
of the
queue of cannibal horizons
(Legros 2008).
The weathering-front model has been introduced piecemeal by
various authors, our role here consisting of combining the elements to
make from them a coherent system of presentation of pedogenesis. Many
precursors are French (Collinet 1969; Boulet 1974; Fritz 1975; Nahon
1976; Bornand 1979; Millot 1979a; Lucas 1989; Tardy 1993). But others
are Anglophone and can be quoted exactly in their words:
'
Evidence that simultaneous advance of the interfaces has occurred and these
interfaces are functionally related to each other is evident in the character of
the pathways
' (Brimhall
et al.
1991).
'
The dynamic nature of the weathering profile (is) characterized by advancing
reaction fronts
' (Soler and Lasaga 2000).
We shall now make the weathering-front model an explanatory system
for understanding pedology. We will return to it often in this topic. Like
all models, it aids in looking at things differently:
Potential of the model
The soil must be studied, horizon by horizon, in the appropriate
direction, that is in the direction of weathering from the begin-
ning, that is from the bottom to the top. A proper understanding
of pedogenesis is the reward.
If we postulate that the weathering fronts follow each other by
descending at their own speeds, we must expect to find all the
possibilities, that is thick E horizons with thin B horizons or
the inverse. We shall show that Nature effectively offers such a
variety of profiles!
