France has been affected by two main geological events, both were
periods of mountain building - the first occurred at the end of the
Primary era, when the block Hercynian mountains were formed, the other
occurred in the middle of the Tertiary period and was responsible
for the uplift of the Alps and Pyrenees, then the folding of the Jura
mountains and the faulting of part of the Hercynian mountains and
a volcanic outbreak to the centre and east of the Massif Central.
Between these major forming periods, the land was eroded and deposition
of materials took place in the seas and lakes formed between mountain
areas.
Today, there are four main Hercynian massifs, composed of granite,
sandstones or shale. They are blocks of hard rocks, worn smooth by
many cycles of erosion. Between these blocks, the sedimentary basins
are linked to one another by lowland corridors. To the south-east
and south-west are the high fold mountain ranges of the Jura, Alps
and Pyrenees.
The Alps form a great arc of folded and overthrust mountains, sharpened
by frost and eroded by glaciation, rising to over 4000m. They are
separated from the Hercynian Massifs by the faulted trough of the
Rhone valley.
The French Alps include the highest peak in Europe, Mont Blanc 4107
m (15,872 feet). The Alpine mountain building event took place in
Miocene times. The French section was pushed up against the reasonably
stable Massif Central block to the west.
The lower pre-Alps are calcareous, showing glacial and karstic features.
The High Alps are very complex in structure, especially in the southeast,
with folding, overthrusts and nappe formation. This is a high energy
environment with active periglacial processes produces large amounts
of debris transported by mass movement and erosion. In the Alps the
crystalline massifs form the highest peaks.
There are two main types of rock structure in the Alps, the Palaezoic
rocks found in the ancient crystalline uplands found in the central
core of the High Alps and the highly folded and metamorphosed secondary
rocks located around the fringes of this central area.
