Geoscience Reference
In-Depth Information
que Phénomène Géologique
considered that this type of vegetation works
as a perfect filter, allowing the loss of dissolved substances but not of
solid matter. This is a theoretical view, because measurements show
that suspended materials in streams represent a mass at least equal to
that of dissolved substances. But, of course, the ratio is much wider if
land-clearing is done.
Very long periods of time are necessary for very mature soils to be
formed, which means that relatively stable old shields (Africa, South
America…) are a requirement. This has been stressed earlier.
Role of time
5.4.2 Types of Ferralsols
We shall now examine the types of Ferralsols that differ from the one
taken as reference (Fig. 5.1).
This soil belongs to dense ombrophilic forests, the
rain forests
, with
annual rainfall of 1600-3500 mm (Africa, Madagascar). The mean
temperature is of the order of 28 °C.
There is in the profiles no layer hardened by iron. It has either not
been formed or has been completely dismantled and eliminated. The
thickness of the rubefied (L.
ruber
= red) surface horizon is more than 2
m and sometimes 5 m or more. Its fine microstructure is characteristic.
In Central America, it is colloquially called 'coffee powder'. The clay is
dominated by kaolinite-type minerals. It varies widely in quantity from
5 to 80 per cent and is associated with iron, which gives it its colour.
Deeper still is the saprolite.
Here again, the relief is very characteristic. On crystalline rocks it
often consists of convex interfluves defining a landscape of hemispheres.
The hillocks are 40-50 m high and a few hundred metres in diameter.
They are separated by lowlands, the topographic plane of which is
slightly dissected by the drainage axis. But their borders are very steeply
sloping (Fig. 5.8).
Rhodic Ferralsol (red, non-cuirassed)
On the hills, between the saprolite that has retained the structure of
the original rock and the soil that has been homogenized by
pedoturbation
,
is often found a stone line, composed of quartz pebbles (§ 5.5.2).
