Geoscience Reference
In-Depth Information
because of lack of liquid water, downstream evacuation of debris does
not occur in valleys. This results partly in
valley flats
and partly in
alveoles
that are the rounded heads of talwegs (like glacial cirques), but
of small size: a few tens of metres deep and a few hundreds of metres
in diameter. After postglacial warming, small streams are established
on these forms in some way too large for them. They wind across the
accumulated rubble and struggle to find an outlet. Such a situation is
favourable for stagnation of water and for preservation of marshes and
peatlands. The latter, at the end of the last glaciation, became '
veritable
islets of Lapland in France
' as a geographer said. In rather dry climate, the
wet lands stretch along streams and rivers: gallery forests of the tropical
zones and riparian forests (ripisylves) in temperate zones. In Amazonia,
peatlands and swamps occupy wide bands (many tens of km) along the
streams and rivers: Orinoco, Rio Negro, Amazon, Madeira, etc. This is
related to the very flat configuration of the alluvial basins and to the
heights attained by floods, which at times exceeds several metres for
many weeks!
Climatic wetlands
correspond to areas where evapotranspiration
does not compensate for rainfall. In Indonesia (Sumatra, Kalimantan
and Irian-Jaya), peatlands cover large areas with a continuous
mantle and are not specifically limited to depressions (Sieffermann
1988). Of course, they are thicker in the low areas, as much as 15
metres. But on the interfluves they can still be 5-m thick, which
shows that they are primarily related to the perhumid climate of the
region. In the very cold zone, the glaciers of northern Europe, while
receding at the end of the Würm, left behind bottom moraines (the
tills
of Anglophones); these deposits show many closed depressions
in which the substratum is impermeable for various reasons. Firstly,
the materials of the tills are often fine. Then, in very cold climate, the
subsoil is frozen. Lastly, in high-latitude regions, the generally gneissic
or granitic substratum is naturally impermeable. This results in Histosols
covering large areas (northern Canada and the former USSR). But these
soils are discontinuous (Richardson
et al
.
1994).
Acid peat is largely composed of sphagnum moss, a genus that
comprises many species. But Cyperaceae (
Carex
) also occur and all
kinds of other plants that vary with region. The most unusual species
are undoubtedly
Drosera
spp., butterworts and some others that have
the faculty of being carnivorous, that is, of trapping and digesting small
Vegetation
