Geoscience Reference
In-Depth Information
Paleosols related to basalt flows are seen in many locations of the
French Massif Central (Escandorgue, Cantal, etc.). However, it is always
difficult to define which pedological group they belong to. Indeed, the
basalt flow bakes the materials under it and transforms them to a sort
of red brick. It totally eliminates the organic substances. It compacts
the soils considerably under its enormous weight and also fissures
them according to the pressure exerted. Also the flow buries not
only soils. It can also cover volcanic ejecta and transform them. Thus
there can be confusion between the Paleosols and some fine-textured
volcanic materials (Moinereau
et al
. 1972). To find out, it is necessary to
first turn to the chemical and mineralogical properties: which are the
secondary minerals present, what is the distribution of Si, Al and Fe in
the profile?
Fig. 2.10
Paleosol of Paulaguet-Langeac (Massif Central, France) buried under a basalt fl ow.
Embankment on Highway A75. Below the basalt prisms are recognized, from top to bottom:
light-coloured A horizon, dark B horizon with prismatic structure, lighter-coloured C horizon
with platy structure.
Photo
: author.
2.4 GEOMORPHOLOGY AND SURFACE FORMATIONS
We shall now examine the role of relief and also the role of surface
formations that result from ancient weathering of rocks on the surface
of the Earth, often under the influence of slope and of gravity.
