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Aires d’extraction et façonnage des stèles de l’âge du Fer de Cornouaille (Finistère, France). Apports de la pétrographie et de l’analyse structurale des roches

Abstract : The Iron Age stelae from Cornouaille (Finistère, France) display a great variety of shapes and lithological types. They are monolithic monuments, partly or entirely carved. Their morphology can result from the natural shaping of rock blocks or they may have an architectural form. In the latter case, the stelae are built around an axis. Their volume can be decomposed into three parts: top, shaft and baseplate. The shaft sides may be decorated. In the context of the inventory initiated by the Centre de l’archéologie du Conseil départemental du Finistère, the purpose of this study is to highlight the specific contributions of both petrography and rock structural analysis to the identification of extraction sites of the Cornouaille stelae, as well as to the understanding of shaping and transportation modes. The methodology is based on the comparative study, on macro- and microscopic scales, of both the monuments and the surrounding geological massifs. The latter are parts of the Variscan crystalline basement of Armorica which recorded strain, metamorphism and granitisation about 300 million years ago. Four criteria have been taken into account: 1) comparative analysis of the natural forms (as either erosional features or tectonic fabric) of the rock blocks in the inferred source massifs and the general shapes of the monuments assumed to have been extracted from them; 2) texture of the rocks (grain size and crystal relations); 3) structure of the rocks (isotropic or strained — schistosity plane and lineation — presence of Z-or S-shaped stretched megacryst); and 4) mineralogy. Two test-zones have been selected: the Cap Sizun and Concarneau areas. Stela density is very high in the first area, where fifty-six uniformly distributed monoliths were identified. No stela has been found in an archaeological context. In the Concarneau area, with a nearly equal surface, only twenty-eight stelae are recorded. The archaeological context of two of them, adorned with fluting, is well-known. A first result is that the stela location is generally either within or near the boundaries of the massifs having a similar granitic or metamorphic lithology. The Cap Sizun stelae are characterized by a greater petrographic homogeneity than in the other area, up to more than four kilometres from the boundary of the source granitic massif. Another original result has been the demonstration that the Iron Age artisans used natural forms and that the latter result from two contrasted mechanisms: 1) various erosional processes for the small granitic stelae and 2) high tectonic strain for the tall cigar-shaped gneissic stelae. It would thus seem that the Iron Age artisans usually used pre-existing natural morphologies of stones with a view to shaping their monoliths with the least possible effort or for reasons of symbolism. Regarding the four specific stelae in Nizon orthogneiss (Concarneau area), two potential extraction zones have been identified fairly precisely, by mapping various types of highly strained orthogneisses, referred to as ‘tectonites’: S/L-tectonites, dominated by both flattening and stretching (corresponding exactly to one of the gneissic stelae), and L-tectonites that no longer display any flattening plane, but instead a prominent stretching lineation (corresponding to the other three gneissic stelae). One of the two potential extraction sites (Moulin du Chef-du-Bois, La Forêt Fouesnant) includes natural cylinders with 3D-dimensions quite comparable to those of the gneissic stelae. Furthermore, the presence of an unfinished stela to the north of Cap Sizun, a few kilometres away from the northern limit of the source massif, indicates than this monument was probably shaped close to the final erection site rather than near its extraction zone. Thus, it would appear that the Cap Sizun artisans moved rough granitic blocks rather than shaped stelae. Finally, the location of two granitic stelae in the Concarneau area near a ria, away from the assumed granitic massif source, suggests that they were moved using maritime transport for a distance of about six kilometres.
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Martial Caroff, Bertrand Grall, Moysan Marine, Bernard Le Gall, Anne-Françoise Cherel. Aires d’extraction et façonnage des stèles de l’âge du Fer de Cornouaille (Finistère, France). Apports de la pétrographie et de l’analyse structurale des roches. Bulletin de la Société préhistorique française, 2016, 113 (4), pp.765-784. ⟨10.3406/bspf.2016.14687⟩. ⟨hal-02315706⟩



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