The Chaussée de Sein is composed of a barrier of rocks jutting out to sea off the Ile de Sein which did not have the benefit of any marine signalisation at the beginning of the 19th Century. After the sinking of the frigate Sané on 24th September 1859, the Commission des Phares et Balises (Lighthouse and Beacons Commission) decided to build a lighthouse on the open sea, at the end of the Chaussée. From the three rocks suggested, Ar Men, the “stone” rock measuring 15 metres by 7 metres at low tide, but completely submerged at high tide, was chosen on 24th September 1859.

The engineer Paul Joly decided to “drill holes over the whole surface of the rock at 1 metre spaces on the site of the building and a few others outside these limits, both to allow the fixing of anchor rings for berthing and also to connect with the scaffolding shrouds to come”.

Galvanised iron studs 6cm square in one metre lengths, embedded into 30 cm deep holes fixed the masonry to the rock, integrating the masonry with the block and its fissures. This masonry, made up of granite rubble aggregate and Médina-Parker cement mixed with seawater, would be also be sunk into a circular footing dug into the rock.

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