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.
The Central Plateau has probably been a land mass ever since this period, but the rest of the country was flooded by the Palaeozoic sea.
Towards the close of the Palaeozoic era France had become a part of a great continent; in the north the Coal Measures of the Boulonnais and the Nord were laid down in direct connection with those of Belgium and England, while in the Central Plateau the Coal Measures were deposited in isolated and scattered basins.