Le douzième dolmen de Barnenez : destructions et reconstructions au sein d’une nécropole mégalithique

International audienceThe southern tumulus of Barnenez, situated on the north coast of Finistère, is one of the most famous Breton megaliths. It was excavated during the 1950s by P.-R. Giot, and the results published in 1987. Since then, data concerning this well-preserved monument have not been updated with the new knowledge regarding megaliths gained over the last three decades. In 2010, an international research program began on this tumulus, integrating an ongoing PhD at Rennes 1 University. The first part of the study concerns the ornamentation inside the funerary areas, with the discovery of paintings and the redefining of the engravings. The second section is an architectural study through interpretation of the elevations thanks to the use of archaeological building methodology. Both revealed multiple phases inside the tumulus, unveiling a complex history. This article will focus on two structures of the tumulus which contain discreet evidence of that substantial history, before the monument attained its imposing final form, seventy metres long.The first structure is passage grave H. Two registers of ornamentation have been found, revealing two distinct and successive phases inside the chamber. In the passage, the architectural study showed some ruptures in the constructional mode, revealing extensions of the passage. Passage grave H seems to have had a previous state, destroyed by the Neolithic builders themselves. We decided to do a survey to test if the previous monument had left marks on the ground of the current passage grave. The results exceeded our expectations, with conservation of the plan and of part of the cairn with, in several places, two courses in elevation. Not all the monument was levelled off, the orthostats of the end of the chamber remained in place, integrated into the current chamber, which explains the differences of ornamentation. The survey proves that a monument was dismantled and partially reused in the current dolmen H. It is one of the first times we can prove the reuse of stones taken from a dolmen to be integrated within another. We needed all the megalithic architectural elements, marks on the ground, on the elevations and on the ornamentation, to partially understand the architectural history of dolmen H.The second part studied is the western façade of the tumulus. It contains a row of many raised stones, whose wider sides face the bay of Morlaix. This alignment is a kind of buttress used to block the external mass of the tumulus against the slope on which it was built. Furthermore, resemblances have been found between the stones of the façade and alignments of raised stones in the open air. The main result, however, lies in the similarity between the organisation of these stones and those inside the passage graves, with shared rhythms in the forms and the geology of the blocks as criteria. This western façade shows, as it were, a flat version or representation of the walls of an orthostatic passage grave. This alignment of raised stones is an architectural manual to explain the internal space of a dolmen and how to build it. The corresponding dolmens inside the tumulus bore marks of extensions of the passage with the addition of new slabs, matching the phases of the tumulus. The alignment includes these extensions but with differences in rhythm. It allows us to suggest that the raised stones came from the dismantling of another dolmen, with its own architectural history, different from the tumulus, but "embedded" or recorded in the western façade.The southern tumulus of Barnenez provides evidence that the Neolithic builders did not avoid the destruction of a previous monument in order to build a new architectural project, where the stones could be reused. To restrict this process only to the large broken menhirs, initially raised in open areas, was probably a misinterpretation.A la suite des travaux de P.-R. Giot, réalisés à partir du milieu des années 1950, le tumulus sud de Barnenez, situé sur la côte nord du Finistère, est l'un des plus emblématiques parmi les mégalithes bretons. Depuis lors, les connaissances concernant ce monument de soixante-dix mètres de long n'avaient guère été renouvelées, si ce n'est par la publication de la monographie correspondant à ces travaux, à la fin des années 1980. Trente ans plus tard, ce fut notamment l'objet d'un programme de recherche international et d'une thèse à l'université Rennes 1. Il mêle l'étude de l'ornementation des espaces internes et celle de l'architecture. Parmi les résultats obtenus, la chambre H et son couloir montraient des évidences d'état antérieur à celui que l'on observe aujourd'hui. Nous avons donc décidé de réaliser un sondage pour rechercher des traces éventuelles de l'état antérieur au sol. Les résultats ont dépassé nos attentes par la bonne conservation du plan du monument précédent qui a été en grande partie volontairement démonté. Son couloir débouchait directement dans une chambre mégalithique dont certains éléments sont encore en place. L'antichambre que nous connaissons aujourd'hui pour le dolmen H n'existait pas. L'étude de la façade occidentale a également révélé une histoire architecturale complexe. Elle présente de nombreuses pierres dressées. Il s'agit d'un renfort du tumulus contre la pente sur laquelle est construite la masse tumulaire. Mais, il y a également une réelle mise en scène proche de celle d'un alignement de pierres dressées à l'air libre. De plus, des rapprochements ont été faits avec le rythme des orthostates d'autres dolmens du tumulus, nous permettant de dire que les pierres dressées de la façade occidentale pourraient également provenir du démantèlement d'un autre dolmen. Réserver un tel processus aux seuls cas particuliers du remploi de fragments de grandes « stèles » brisées, initialement dressées à l'air libre, fut probablement un contresens