Geoscience Reference
In-Depth Information
fraction (sands, silts and precipitated CaCO
3
). These formations are
common in limestone areas but are splendidly developed in France
in the valley of the Charente. Their thickness can exceed ten metres.
In length they often extend over an entire hillside, starting from a
rocky ridge, which acts as the source of the material, up to the line
of the stream course. The size of these formations, their nature and
their location in the landscape suggest that they were put in place in
a periglacial climate. Frost acted to splinter the rock, at least if it were
porous and liable to absorb water.
Where a slope does not exist, on hard limestone substratum, no scree
is formed. But if the region had been subject in the past to a periglacial
climate, the rock has splintered to one to ten decimetres depth and is
finely divided.
In crystalline and crystallophyllitic terrain, arènes result from the
weathering and disintegration of the rocks, which release the crystals
composing them (chiefly quartz, micas and feldspars). If weathering is
weak, the texture remains sandy. With stronger weathering, the micas
and feldspars yield clay. The texture then becomes sandy clay or clayey
sand. In the French Massif Central, the most typical arènes are found
on slopes, and traditionally comprise three levels (Fig. 2.15).
At the bottom is seen an
arène in situ
or
isoaltérite
or saprolite. Above
it, a
bedded arène
, often thin, presents horizontal stratification of the
materials present (sands and gravel). At the top of the cut is a
bouldery
arène
comprising very coarse elements embedded in a gangue of fine
material. It contains A, B and, at times, C horizons.
In the establishment of these deposits, the first event was fragmentation
by frost (bouldery arène) and then mass movement on the slope in
a semi-plastic state linked to frost heaving (bedded arène). Perhaps
several successive glacial and interglacial periods were necessary for
this. The local presence of fine deposits of pure clay within the bedded
arènes could be interpreted as a present-day filling up of discontinuities
(Legros 1975, 1976).
There are other formations on slopes:
boulder trains
and also
rock
glaciers
. Details of their formation are still controversial. But what has
been said about them is enough to show that the arènes, present-day
foundations of many European soils, are the products of an ancient and
complex history.
Arènes
