Agriculture Reference
In-Depth Information
Soils of the Paris Basin
Different geological beds show up in a number of types, belonging to Secondary formations
made up of the Lias, the Jurassic, and the chalky Cretaceous. However, the most extensive cover
is that of the Quatenary formations, comprising numerous sediments. The Pleistocene is essentially
made up of Loess. Several types of loess of different ages are present, the most recent cover dating
from the end of the last glaciation (Wrm III), with quite signiÝcant original carbonate contents.
The presence of loess from Wrm II and I has been noted. During the Quaternary, alternations of
glacial periods and thaws permitted erosive phenomena, typical of these climates, which included
gelifraction, cryoturbation, and soliÞuction, signiÝcantly affecting the surface layers. It appeared
that a loess sheet covered the whole landscape during the Upper Pleistocene. The late glacial erosive
phenomena at the end of the Pleistocene left only the present day cover of the plateau, as well as
outliers located on the Þats and terraces, or maintained on local topographic units.
Soils of Loamy Landscapes
These materials, most often of loess origin, are favorable to a rapid differentiation by clay
leaching. However, a number of them were initially calcareous and have previously undergone
decarbonation, successively giving rise to
calcareous brown soils
then
calcic
(
Calcaric or Eutric
Cambisols
[WRB];
Typic or Rendollic Eutrudepts
[Soil Taxonomy])
.
The continuation of this
evolution leads to
[Soil Taxonomy]),
which differentiates itself to a well-structured and colored horizon at shallow depth. The progressive
desaturation of the soil by leaching then creates conditions favorable to eluviation. We then witness
the differentiation of
brown soils
(
Haplic Cambisols
[WRB];
Typic Eutrudepts
leached brown soils
(
Leptic Luvisols
[WRB]);
Inceptic Hapludalfs
[Soil
Taxonomy]), and then of
[Soil
Taxonomy]), typiÝed by a horizon impoverished in clay and iron overlying a horizon enriched in
these same elements. Mobilized clay distributes itself by concentrating in the pores and on faces
of the structural elements, in characteristic coatings.
leached soils
(
Eutric or Haplic Luvisols
[WRB]
; Typic Hapludalfs
Soils of Calcareous Landscapes
Soils of calcareous landscapes consist of numerous areas in chalky, marly regions, and on hard
limestones. The majority of the parent materials are made up of the products of chalk alteration,
most often resulting from mechanical or physical phenomena, due to freezing and to the different
modes of displacement closely linked to the climatic Þuctuations. The soils developed from chalk
are
typical rendzinas
or
brown rendzinas
(
Rendzic, Calcaric,
or
Mollic
Leptosols
[WRB];
Lithic
or
Typic Udorthents
[Soil Taxonomy]),
calcareous brown soils
and mostly
calcic brown soils
(
Calcaric
or
Eutric Cambisols
[WRB];
Typic
or
Rendollic Eutrudepts
[Soil
Taxonomy]). A progressive decarbonation may occur, which results in the differentiation of a
structural horizon within the insoluble residual products that form in the upper part of the solum.
On these deposits of chalky origin, we Ýnd allochtonous Pleistocene material of eolian, niveo-
eolian or land Ýll origin, having a mostly silty texture.
Typic Haplrendolls
,
Example from a Hilly Loessic Region of the Paris Basin
The example shown comes from a silty-chalky landscape in Picardy (Figure 13.2), in the
northwest of the Paris Basin, and corresponds with one of the most representative Soil-systems in
this region. A loessic cover dating from the recent Quaternary (Wrm III) overlies a Cretaceous
chalky substrate strongly marked by phenomena of cryoturbation dating from the last glacial period.
This substrate is exposed at the slight break of slope, while the depression is characterized by the
