Geoscience Reference
In-Depth Information
setting in motion tremendous landslides. In Saint-Jean-Vianney in
Quebec, Canada, 31 persons died in 1971. The significance of the name
given to these formations is obvious…
During a flood, it is observed that the current is stronger at the top
level of the river or stream than on the sides of the submerged valley.
In consequence, the fine particles cannot be deposited on the bank once
the speed of the stream exceeds a few metres per second. At the same
time, the coarse particles—gravel and pebbles—cannot be transported
on the sides of the valley, that is, away from the stream, at least if it is
broad. The deposits are thus sorted by particle size from the centre of
the valley to its sides (Fig. 2.18).
Fluvial deposits
Alluvial soils
3
Sands and pebbles
1
Loamy sand soils
2
5
1
Clay loam soil
3
Excessively wet soils
4
2
Soils of all kinds
5
6
Direction
of flow
Humiferous clay soil
6
Limestone in which the
valley has been cut
4
Fig. 2.18
Typical structure of recent alluvial deposits (numbered 1 to 6) in a fi ssured lime-
stone.
Close to the river (1), are found sands and pebbles, reworked
with each flood. This is the weather strip of the river bank (
levee
).
The soils are of ecological interest only if forested.
A little farther from the river (2), the soils are sands or loamy
sands. Almost all over Europe, they overlie coarse gravelly
deposits. Can a better agricultural situation be imagined? A
material that is not too coarse or too fine rests on pebbles that
