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Schematic evolution of the sea level in Brittany since the end of the last glaciation.

Schematic curve of the evolution of temperatures in Western Europe at the Postglacial.

Once the height of the last glacial period was over (around 18-16000 BC), the climate began to get warmer. The process first started slowly and gradually, then accelerated after 8000 BC. Climatic conditions became optimal between 5000 and 3000 BC, with temperatures even slightly warmer than today.
After that, the planet experienced only minor variations, like the "Little Ice Age" (from the end of the Middle Ages to the 19th century), or the slight warming of the past decades.
As regards Brittany and the surrounding areas, around 16000 BC, the sea level was some 120 metres lower than it is today. The English Channel was then completely dry and the Atlantic shoreline a hundred kilometres further west than its present location. The sea level rose most rapidly between 10000 and 5000 BC. The rise then gradually slowed down, with the sea reaching its present level at the beginning of the Christian era.
It is very difficult to imagine the shoreline at that time by studying the seabed as it is today, because of erosion in the exposed areas and sedimentation in the unexposed areas.
However, it is safe to say that even during the Neolithic age Brittany was surrounded by coastal plains and that the present-day islands and reefs are its last visible remnants (among all the Breton islands, only Ushant and Belle-Ile were already isolated from the continent at the beginning of the Neolithic age).

The rise of the sea since the end of the ice age.

A number of megalithic monuments are partly or totally submerged around the coast of Brittany, illustrating sea-level changes. Er-Lannic double enclosure on the Morbihan Gulf is a spectacular example of these shoreline changes: some menhirs now stand 1.5 metres below today's lowest tides.

Given tidal levels, this means that the sea level was at least 6 metres lower than today when they were erected.

Foreground: The upper enclosure of Er-Lannic.
Background: Gavrinis cairn