THE COMTESSE BEACH
A family beach
At the foot of the island, Plage de la Comtesse has been a popular beach since the second half of the 19th century, when the virtues of ‘sea bathing’ were first recognised.
Families have been holidaying here for generations. Rolling cabins, fixed cabins and tents have followed one another up to the present day.
Berthe Morisot, her daughter Julie Manet and Paul Signac loved to come and paint from the roofs of the cabins above the Comtesse beach.
Why black sand? A little geology
Black streaks often appear in the sand on the beach. Sometimes the sand is even completely black… How can this be explained?
Brittany’s geological bedrock is extremely old, dating back 2 billion years. Some of the oldest rocks in France are found here. Three mountain ranges have succeeded one another. The Armorican Massif, the last of these, was formed between 600 million and 300 million years ago. A cordillera chain similar to that of the Andes or the Himalayas saw major volcanic eruptions.
In Saint-Quay-Portrieux, the geological massif is made up of very ancient magmatic rocks, diorites and gabbros.
The black sand on La Comtesse beach comes from the erosion of dioritic rock, which is rich in ferrous elements. These heavy minerals are concentrated in what is known as a “geological placer”, i.e. a deposit of heavy sediment. The placer on Comtesse beach has been studied since the 18th century. It contains mainly ilmenite and magnetite. These heavy elements form the deep layer of the sand, while the lighter light-coloured grains come and go with the tides.
A combination of high tidal coefficients and strong easterly winds removes the surface layer of sand. The black sand is then very present. But if the wind blows from the west during high tides, the sea brings back the light-coloured sand from the open sea…
In Saint-Quay, the same phenomenon occurs on the black beach to the north of the Casino beach.
