Geoscience Reference
In-Depth Information
Progress of mechanized farming, with its consequences (crushing
of pebbles, removal of stones with bulldozers, clearing and drainage)
gradually extends the cultivated area and destroys a centuries-old
structure that made possible a superb adaptation of agriculture to the
natural capabilities of the environment. Nonetheless, give or take a few
details, the pattern in Figure 2.11 is still seen on all the plateaus of the
French Massif Central.
It is not necessary to restrict the role of slope to transport of just a few of
the elements that constitute soils. In some cases, the movement can affect
the soil layer in its entirety. First, the soil can slowly slide downslope
(
soil creep
). In the mountains, curved trees reveal this phenomenon
(Fig. 2.12a). They are seen even in regions where there is no snow (the
lower Cévennes, for example).
Mass movement phenomena
Types of creep
Erect
young
tree
Soil
Embankment
Pressure
exerted by
the soil
Adult tree that
curves its trunk
right itself
New road
to
Pit
With time, slow creep of the
soil until it conceals the
embankment
Anchorage of roots in the rock
preventing lateral sliding of the tree
along with soil and providing a fulcrum
for the trunk to tilt downhill
a
b
Solifluction
Detachment knick point of soil with
theoretical profile of a semicircle (Mohr circle)
Transport of material in the form of
a sludge
Accumulation of displaced
material (solifluction lobe)
c
Old profile of the hillside
Fig 2.12
Three examples illustrating the mass movement of soil on a slope: (a) curved tree,
(b) sealing of an embankment, (c) solifl uction.
Another manifestation of creep is the slow advance of the soil into
the pit left by opening up a road cutting, a phenomenon everyone would
have observed (Fig. 2.12b). Lastly, the water-saturated soil (melting of
