Armorican Massif

The Armorican Massif (French: Massif armoricain) is a geologic massif that covers a large area in the northwest of France, including Brittany, the western part of Normandy and the Pays de la Loire. Its name comes from the old Armorica, a Gaul area between the Loire and the Seine rivers. The massif is composed of metamorphic and magmatic rocks that were metamorphosed and/or deformed during the Hercynian or Variscan orogeny (400 to 280 million years ago) and the earlier Cadomian orogeny (650 to 550 million years ago). The region was uplifted when the Bay of Biscay opened during the Cretaceous period. The Cantabrian Mountains and the Armorican Massif were then rift shoulders of the Bay of Biscay.

The competent old rocks of the Armorican Massif have been eroded to a plateaulike peneplain. The highest summit, the Mont des Avaloirs (Mayenne département), is just 417 meters above sea level. The western part of the Armorican Massif (which covers Brittany) are the Monts d'Arrée.