Geoscience Reference
In-Depth Information
In China, the aeolian silts can exceed 300 m in thickness. Rates of
deposition of up to 20-40 cm per thousand years have been measured
there (Lu and Sun 2000). But the rates are very variable and diminish
farther away from the sources of the materials.
In Iowa, USA, aeolian silts reorganized after their deposition form
hills 70-m high.
Actually, these fragile dusts are preserved mostly in areas that escape
erosion by water. They are often found on alluvial terraces, the stability
of which has already been emphasized. There, they are easy to identify
because they form a silt cover on the underlying fluviatile pebbles.
The mineralogical composition of these deposits is variable and
depends on the source materials. They are generally rich in quartz,
micas, feldspars, iron, clay minerals and calcium carbonate. Table 2.5
gives the particle-size distribution of a typical loess. The data presented
are mean values pertaining to the Rhône valley in the Vienne region in
France (Trouche 1971).
Table 2.5
Particle-size distribution of the loesses (C horizons of the soils) of
the Rhône valley, south of Lyons (Trouche 1971).
Coarse sands
Fine sands
Coarse silts
Fine silts
Clay
9.6%
14.8%
24.8%
32.5%
18.2%
The loesses, deep, fine-textured, relatively permeable and rich in
weatherable minerals, yield very fertile soils. In Central Europe, the
best soils—the Chernozems—mainly correspond to the loesses, having
developed in cold, rather dry continental climate. In Atlantic climate,
loesses gradually lose their carbonates. They take on a brown colour
and are argillized to give Luvisols (
lehms
of old geologists).
The glaciers of the Far North manufactured large quantities of glacial
flour (see above), which were deposited in marine environment, at the
mouth of the present-day rivers. The muds, exposed by continental uplift
and then dried, are stable to begin with because the salts they contain
develop ionic forces bonding the fine particles of the material. But
rainwater leaches out the salts and destabilization occurs (see Chap. 13),
Sensitive clays
