Geoscience Reference
In-Depth Information
which could have been the last glacial epoch or 18,000 years BP
(Podwojewski 1994). This seems to be confirmed by the isotopic
content of
34
S.
We have seen in Chapter 1 and in Chapter 5 that it is sometimes
possible to estimate the time required for reaching the degree of
weathering found. According to Quantin (1992), 15-30 thousand
years are necessary to form a Vertisol one metre deep from vol-
canic ash.
To conclude, formation of a Vertisol demands between 20,000 and
60,000 years (Podwojewski 1994). Anglophone authors give comparable
figures: 40,000 years for obtaining a soil with maximum expression of
'vertic' features. This is a short time considering the pedological time
scale.
6.5 CLASSIFICATION
Vertisols are so peculiar that they have been identified in many countries
and regions by specific vernacular terms:
Tropical Black Clays
(Tercinier
in 1953),
Tirs
(Morocco),
Regurs
(India),
Smolnitza
(Eastern Europe)…
But they have points in common with several other soils they are often
associated with.
They can be considered as a very clayey and peculiar variant of
calcareous or calcium-saturated soils (Chap. 7).
Vertisols in which the organic matter is highly stabilized, at depth
as in the surface layer, can be placed next to the Chernozems
(Chap. 14), for example in the Caucasus (Kovda
et al.
1996).
Considering their frequent occurrence in basins, they are often
close to Gleysols or Stagnosols (Chap. 12), in the Caucasus (Kovda
et al.
1996) and in France in the Allier valley.
Many soils that are not true Vertisols may be termed 'Vertic' in the
WRB if they have one of their features: more than 30 per cent clay,
cracks, slickensides or even COLE > 0.06. In fact, Vertisols are mainly
distinguished by their texture and their functioning (swelling-shrinkage).
It is thus a view oblique to a purely genetic approach.
Connections with other soils
In the Vertisols, beyond the typical case (Haplic), the WRB introduces
distinctions according to major features than can appear:
The WRB
