Comment on the Dalle de Saint-Bélec as a Bronze Age map

Nicolas et al. (2021) recently published a very interesting interpretation of the Bronze Age Dalle (slab) de Saint-Bélec as a Bronze Age map of a region in Brittany. The interpretation is rooted in the intuition felt by some of the early discoverers of the slab, in the early 20^th century, that it might be a map. We all have many and varied intuitions about archaeological material. What matters in the end, is whether and how we can test the hypotheses that emerge from those intuitions. Nicolas et al. make a concerted and systematic effort at testing the hypothesis that the slab is a map.

The slab itself is a 4 by 2 meter flat rock covered in carvings (Figure 1), that was part of a Bronze Age burial structure in Brittany, France. The claim is that the carvings are a topographic map of an approximately 400 square km region of Brittany.

Figure 1: Correspondence between the main topographic features of the Dalle de Saint-Bélec and the River Odet, from Nicolas et al. 2021.

The article is well worth reading if only for the extremely careful and complete documentation of the slab, which includes 3D mapping and scanning of the surface as well as a detailed chronology of the carvings, and for the interesting review of literature on cartographic representations in the archaeological and ethnographic records. Even more interesting though, is the attempt to evaluate the claim that the carvings are a map of the region.

The authors find good agreement between main physical features of the slab and main topographical features of the region. This agreement is anchored around the correspondence of the slab’s topography with the line of the river Odet and the Black Mountains to the north (Figure 2). Their attempts to show correspondence between more detailed features of the slab and the landscape are somewhat less intuitive, but overall, they can show that quantitatively, the correspondences are similar to those achieved by ethnographic examples of cartographic representations.

Figure 2: Correspondence between rivers represented on the slab and on the landscape, from Nicolas et al. 2021

I would say the spatial statistics the authors perform on the slab and the landscape definitely do not refute the hypothesis that the slab is a map of the region. The results, especially looking at the most general features, are highly suggestive.

The more detailed tests, however, pose some problems, perhaps not surprisingly. For example, when looking at rivers other than the Odet, the authors divide the river-like carvings on the slab into two categories: Rivers on the landscape that are represented on the slab, and rivers on the landscape that are not represented on the slab. There is a third, very important category that they should consider: Rivers represented on the slab that are not on the landscape.

Several of the carvings on the slab are difficult to distinguish visually from what Nicolas et al. consider representations of rivers, especially if we consider the representation of the river Aulne, which is meandering in this region, and claimed to be represented by a looping shape on the slab. There are a number of such looping carvings elsewhere on the slab that are not classified as representing rivers.

More generally, it is always difficult in this kind of work to determine the direction of relationships: To what extent did the features of the slab drive the selection of landscape features to be included in the analysis, and vice-versa? By selecting the right features, it is possible to find close correspondences between maps and regions that are not necessarily connected in reality. Anyone who has gotten lost, even momentarily, while navigating with a topo map will relate to this.

A useful next step, which I encourage the authors to pursue, would be to systematically overlay the slab’s possible map on topographic maps of a large number of randomly selected regions, perhaps even of different sizes, and performs the same analyses. They would have to limit their analysis to a small number of features that can be automatically identified for comparison. I would suggest major rivers and crest lines, since this is the heart of their case for identifying the Dalle de Saint-Bélec as a map of the river Odet region. A straight raster elevation difference between map and region might also be informative.

Another possible avenue, would be to overlay the slab on the other hypothesized Bronze Age political territories they identify on the basis of Thiessen polygons drawn from “princely sites” in Brittany. Doing both would actually be best, and I would love to see the results. Should the correspondence of rivers and crest lines be much better for the river Odet than for any other maps from the set of Thiessen polygons and randomly selected regions, we would really have something, and I think this result is not out of reach.

Whether using random regions or the hypothesized Bronze Age territories of Brittany, I would suggest that the authors overlay the slab over the real topography of each region (including the Odet river), rotate it through 360 degrees, and pick the best fit for comparison.

This more systematic data set would give us a basis from which to evaluate just how surprising are the correspondences they identify here.

References

Nicolas C, Pailler Y, Stéphan P, Pierson J, Aubry L, Le Gall B, Lacombe V, Rolet J 2021. La carte et le territoire : la dalle gravée du Bronze ancien de Saint-Bélec (Leuhan, Finistère), Bulletin de la Société préhistorique française 118 : 99-146.