Geoscience Reference
In-Depth Information
Presence of primary minerals, particularly feldspars, indicat-
ing that weathering has not reached the ultimate stage (in con-
trast to what happens in the yellow soils of the oldest stages in
the sequence).
9.1.3
Genesis of the Soils of the Rhône Valley
The initial material being essentially gravelly, we cannot comprehend
the pedogenesis by limiting our study to the development of the fine
earth. It is necessary to consider the transformation of the coarse
material too. These elements weather and disappear while adding
considerable quantities of material to the fine fraction. Figure 9.4 presents
a diagrammatic summary of the observations of Bornand (1978) on this
subject.
Evolution of the coarse material
Limestone
pebbles
Igneous rock
pebbles
Siliceous pebbles
and quartz
PARENT MATERIAL
(stones)
Export of carbonates
CAMBISOL RHODIC
(red clay + pebbles)
Resistant skeleton grains
Pores
Iron
Rubefaction
=
Concentration of
Fe-III bound to the
clay or not
Siliceous
'ghosts'
Clay
ACRISOL GLEYIC
:
acidification, release
and reduction of iron,
degradation of clay
minerals
Derubefaction
Concretions
Sands
(gravelly grey clay)
Pulverization of
quartz
Æ
gravels
Export of Fe-II
Export of silica
Fig. 9.4
Mechanism of development of the soils of the Rhône valley according to
Bornand (1978).
The limestone pebbles disappear first, through dissolution. They
leave voids that ensure drainage for a long time and prevent the
soils from getting plugged up.
Later, the igneous rock pebbles weather and provide the material
for the fine earth. Thus, from the C horizons to the B horizons,
pebbles of all kinds, sprinkled with yellow calcareous sand, go
to a red clay stuffed with a few uniquely siliceous gravels!
