Barnenez

The cairn is built of 13,000 to 14,000 tons of stone and has steep facades and a stepped profile. Several internal walls either represent earlier facades or served the stability of the structure. It consists of relatively small blocks of stone, with only the chambers being truly megalithic in character. The monument overlooks the Bay of Morlaix, probably a fertile coastal plain at the time of its erection.

The 11 chambers are of the type known in French archaeological terminology as dolmen à couloir ('passage grave'), built of large slabs of slate and granite. Originally all were entirely enclosed by the mound; that several are now partially exposed is the result of modern quarrying. Each is reached from the southeast via a long narrow passage 7-12 m long, and the passages are arranged parallel to each other. In nine cases, narrow passages lead to corbelled chambers; in one chamber the corbel vault sits on the ground, forming a true tholos. The passages have slab-built or dry stone walls and are covered with slabs. One chamber has a dry-stone vaulted ante-chamber.

The monument is the result of at least two phases of building. In the first phase (Cairn 1, before 4500 BC), a slightly trapezoidal mound of 32 m by 9 to 13 m was erected, containing 5 chambers and surrounded by a double kerb; dolerite was favoured. In the second phase (Cairn 2, c. 4200 to c. 4000 BC), an extension with six further chambers was added in the west, Cairn 1 was enveloped in a wider and taller structure, and its passages were extended. More granite was used in this phase.

Radiocarbon dates indicate that the first phase was erected between 4850 and 4250 BC and the second between 4450 and 4000 BC. Cairn 1 had a volume of about 2,000 cubic metres and is built of 1,000 tons of granite and 3,000 tons of dolerite. It has been estimated that it would have required 15,000 to 20,000 working days, or 200 workers for three months. In its final form the mound is nearly three times as big as the first phase.