The mound Today, the Barnenez cairn is 72 m long, up to 25 m wide, and over 8 m high. It is built of 13,000 to 14,000 tons of stone. It contains 11 chambers entered by separate passages. The mound has steep facades and a
stepped profile. Several internal walls either represent earlier facades or served the stability of the structure. The cairn 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.
Construction phases The monument is the result of at least two phases of building.
Cairn 1, before 4500 BC In a first phase, a slightly trapezoidal mound of 32 m by 9 to 13 m was erected. It contained 5 chambers and was surrounded by a double kerb. The first phase favoured the use of
dolerite.
Cairn 2, c. 4200 – c. 4000 BC In a second phase, an extension with six further chambers was added in the west. At the same time, Cairn 1 was enveloped in a wider and taller structure; its passages had to be extended. More
granite was used in this phase.
Logistics One cubic metre of the Barnenez cairn contains 1,500 kg of stone. It is estimated that the quarrying, fashioning, transport and construction of such an amount represents about four work days for a single worker (assuming a 10-hour day). The original monument, Cairn 1, had a volume of about 2,000 cubic metres; it is built of 1,000 tons of granite and 3,000 tons of dolerite. It would thus have required 15,000 to 20,000 working days; in other words, it would have taken 200 workers three months to erect Cairn 1 alone. In its final form, the Barnenez mound is nearly three times as big as the first phase.
Engraved symbols Engraved symbols occur in several of the chambers and passages. They depict bows, axes, wave symbols or snakes and a repeated U-shaped sign. One of the carved slabs is in secondary use; it was originally part of a different structure, an interesting parallel to the situation in several other such monuments, including
Gavrinis. The symbols on the engraved blocks resemble those found in other megalithic monuments in Brittany; in broader terms they belong to the cultural phenomenon described as
megalithic art. One of the recurring symbols is sometimes interpreted as an
anthropomorphic depiction (the so-called "Dolmen Goddess").
Finds Material from the original period of use Only Cairn 2, namely chambers A, C and D, contained
Neolithic finds at the time of excavation. They included pottery, polished stone axes (of
dolerite),
flint blades and arrowheads.
Bronze Age pottery Pottery shards found outside the monument indicate that it was reused in the
Bronze Age (3rd millennium BC). A copper dagger and a barbed arrowhead are of
Chalcolithic date. ==Comparable monuments==