|A GEOLOGICAL SURVEY OF THE REGION...
CAUSSES, CEVENNES, GORGES & MONT LOZERE
region of the Cevennes, Causses and 'Gorges' constitutes the Southern
Central Massif ;
It's a transition-zone in between the "garrigues" of the south Languedoc and the
geology are closely related, both they define the the natural zones.
The steep declivities of the Cévennes
consist of metamorphical bedrock dating from the primary era.
It's a country with deep valleys separated by
sharp crests, almost without flat parts.
Analogous terrain prolongs the Cévennes to
the North-North-East direction by the Vivarais.
South-Westwards there is the giant tabular
plateau of the Grands Causses, carved out by the famous Gorges, vertiginous and beautiful,
like the Gorge du Tarn and those of de la Jonte.
These big limestone tables are formed in the
The Mont Lozère and the Mont Aigoual, two
big granitic massifs which enclose the region respectively in the North and the South.
They also have been formed in the primary
era; they are the frame of the region. The Margeride a bit further to the North their is
In the South and the East the typical
garrigues of the Languedoc are the first echelon of the Cévennes.
The presence in the South
of the coal basin of Alès also is a very remarkable geological caracteristic
THE GEOLOGICAL HISTORY OF THE CEVENNES
In the Precambrien, the middle of the primary
era (between minus fivehundred and threehundred million years) there was only ocean
Enormous layers of clayey mud, marl, some
limstone mire and different sorts of sand settle down at the bottom of the sea.
This proces took hundreds of million years; the actual layer still is several thousands
meters. These sedimentary strata are eroded older mountains.
A lot of fossils attest to the vanished
traces of life.
Long chains of granitic mountains rose
throughout the crust of the earth surface without coming systematically to the surface.
These granitic magmas toil slowly through the
layer of five à six kilometers depth towards the surface, across the other more heavy
This way the birth of the Mont Lozère and
the Mont Aigoual transform the sedimentary rocks into metamorphical rock.
The very high temperatures of the magmatical
rock and the enormous pression raise, distort, fold, crush, break, warm and even bake the
bedrocks which mineral structure changes; they modifie and blister. They become gneiss and
micaschist, the micaschist of the heart of the Cévennes and the cristallin limestone of
This mouvement takes decades of million years
with alternate active and calm periods.
Some calm periods lasts such a long time that erosion has got time again to attack the
new-builded summits to settle down the material in the most hollow zones like the coal
basin near Ales
Now the structure is there:the granitic
massifs Mont Lozère, la Margeride and the Mont Aigoual (called Saint Guiral longtime ago)
and the lower zones: the Cévennes and the Causses.
This ensemble is rigid, the granite don't
fold because it's out of one mass and the later working mouvements to it only cause
modifications in detail like fissures and no folds (deformation).
At the end of the primary era everything
calms down, erosion resume, wearing down the slatestone crests and stripping of the
granitic rock, filling up the lower neighbour-parts like Lodève and Saint-Afrique.
Suddenly a new phenomenon occurs, the sea
which we thought to be gone for ever starts recovering the left terrain, submerging the
region little by little, now twohunderdfifty million years ago.
She deposit new sediments, at first the sands and sandstones which delimit the old
shore, then the marnes with numerous blisters of mica which proves the erosion continues
in the old granitic and slatestone massifs.
In the end a big layer of limestone is
deposited, which orinigally certainly sticked out a lot compairing with the nowadays known
limit, maybe even she covered the totality of the Cevennes.
This explains the rarely found formations of corals and shells which proves it once has
been a deep underwater spot.
About hunderdtwenty million years ago a
slight lift up occurs and the sea quits the region, still staying at the foot: les
garrigues of the Languedoc and the Rhône-valley, leaving there also bit by bit.
Then a new mountain chain is coming up: the
Pyrenees. This movement also lifts up the Alpes. A countermoving lift up our region and at
the same time propulsives from the South crush the sedimentaryires secundary layers
against the granitic chains. This creates a multitude of folds and fissure tectonic
Fissures appear, as well as the first
volcanoes in the North and the West.
From then on only the erosion
determines the actual aspect, she fashions our region in the thousands of valleys and
dales with sharp crests of the Cévennes and wears out the deepness of the Gorges which
divides the big limestone plateau in the Causse Mejean, the divides the big limestone
plateau in the Causse Mejean, the Causse noir, the Causse de Sauveterre and the Causse de