Both to the east and to the west of this depression the Archean and Palaeozoic rocks which form the greater part of the island are strongly folded, with the exception of the uppermost beds, which belong to the Permian system.
There were, therefore, two principal epochs of folding in the island, one at the close of the Palaeozoic era which affected the whole of the island, and one at the close of the Mesozoic which was felt only in the western region.
The Palaeozoic rocks form two extensive masses, one in the south-east and the other in the south-west.
Outside the northern circle lie on the west the folded Palaeozoic rocks of Brittany.
And on the north the Palaeozoic massif of the Ardennes.