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Geological Regions
The French Alps can be divided into 4 parallel north-south regions
i) The High Alps
This belt is about 100km wide and lies along the Franco-Italian border. It has a great
variety of sedimentary rocks in great overfolds and nappes. They incorporate rocks such
as granite, gneiss and schist. The main mountains are the Mont Blanc, Mont Pelvoux and
Mercatour (all 3000-4500 m).
ii) The Longitudinal Trench
This includes the vale of Chamonix and the valley of the Isere between Grenoble and
Albertville. It is only up to 5km wide at its greatest and is several thousand metres lower
than the surrounding mountains. The trench is formed by a band of soft Liassic shale, that
has been rapidly eroded by fast-flowing rivers.
iii) The Pre-Alps (to the West)
From Lake Geneva to the River Durance, they are mainly composed of Limestones and are only about half the height
of the High Alps. They are crossed by 4 important 'cluses' (steep sided valleys), which once held large rivers
and are today occupied by large lakes (e.g. Annecy).
iv) The Alpine Foreland (Dauphine)
This lies between the Rivers Isere and Rhone at the western foot of the pre-Alps. It is mainly covered by fluvioglacial
deposits, consisting of pebbles embedded in clay, swept down from the Alps in the Quarternary Ice Age. The area
also includes moraines, rock basins and sheets of outwash material which have since been terraced by river action.
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