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Origin and history

The tumulus of Château Bû, in Saint-Just en Ille-et-Vilaine, is an exceptional megalithic building, unique in Europe. Dated from Neolithic (circa 3500 B.C.), it consists of an original dolmen covered, at the age of bronze (circa 1500 B.C.), by a tumulus surmounted by four menhirs in white quartz and blue schist. This complex site combines a central corridor, two side cells and a terminal chamber, all included in a cairn with concentric walls. The excavations of 1990-1991 revealed a superimposition of funeral structures, demonstrating ritual reuse over several millennia.

The monument was described in the 19th century by authors such as Bézier and Banéat, who mention a local legend of annual sacrifices. Ranked a Historic Monument in 1975, he suffered damage during the fire of the Lande de Cojoux in 1989, resulting in excavation campaigns led by Jacques Briard. This research revealed a corbelled pyramid at the top of the tumulus (destructed in 1990), as well as three individual bronze-age graves, dug at the foot of the menhirs. Archaeological furniture, including vases, blades and pendants, confirms an ongoing occupation of the site.

The architecture of the tumulus suggests a three-stage construction: an initial neolithic dolmen, later covered by a monumental terre with menhirs, then rearranged to accommodate individual burials. Menhirs, some of which could be "indicators" of graves, reinforce the sacred character of the place. The presence of cupulas on slabs and collapsed wooden structures in tombs reveals evolutionary funeral practices. This site illustrates the transition between megalithic neolithic traditions and individual bronze age cults.

The excavations also revealed a small characteristic funerary furniture, such as a five-angled biconic vase dated 1500 B.C., linked to the culture of the Armo Rican tumulus. Ceramics and flint tools of the dolmen (circa 3500 BC) contrast with the absence of furniture in some tombs, suggesting differentiated social status. The tumulus, owned by the department, remains a major testimony of the religious and funeral practices of prehistoric Brittany, combining architectural innovation and symbolic reappropriation.

The interpretation of archaeologists highlights the possible re-use of an earlier dolmen, integrated into a wider set. The quartz menhirs, aligned or erect, could mark graves of important characters, while their layout accentuates the monumentality of the site. Although partially destroyed, the tumulus retains a complex structure, with concentric walls and carefully equipped slabs. His study continues to shed light on the cultural dynamics between Neolithic and Bronze Age in Armoric.

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