Geoscience Reference
In-Depth Information
9
and Dry Tropical Zones
Between the humid tropical zones and the temperate zones we see
regions with relatively dry climates where the soils are red or at least
have a red B horizon. The ancients used to call them:
Terra rossa
or
sometimes
Ferretto
(Italy). The process leading to their differentiation
is termed
rubefaction
(
rubification
or
reddening
). They belong to different
taxa in classifications but they have dynamics of iron in common (Yaalon
1997). All categories together they cover at least 200 million ha in the
world.
9.1
SOIL SEQUENCES IN THE RHÔNE VALLEY
We shall present an example to establish our thinking.
9.1.1 Soils and Landscapes
We have already indicated in Chapter 2 the importance of alluvial
terraces for studying the evolution of soils with time. In France, in
the valley of the middle Rhône, on the left bank (Fig. 9.1), we see
many stepped surfaces between 200 m and 700 m in altitude (Bornand
1978).
These terraces are composed of the same material—a gravelly deposit
of siliceous pebbles, igneous rock fragments and limestone fragments
mixed with calcareous and micaceous sands. No silt, no clay. The
siliceous pebbles are composed of quartz and quartzites. The igneous
rock fragments are essentially granitic.
