Discussions on the complementarity of the geomagnetic method and aerial imagery for the study of Neolithic enclosures in West Central France

A total of 312 Neolithic causewayed enclosures were referenced in 2020 between the rivers Loire and Dordogne in Central-Western France. Most of these enclosures were discovered using aerial photography, which highlight the cropmarks of the enclosure ditches, taken by prospectors or by State services such as the IGN. Since the 2010s, in addition to aerial survey, geomagnetic surveys are increasingly used to map these enclosed sites. Geomagnetic surveying offers a complementary approach, which enables the detection of small entrance features, palisades and postholes rarely visible in the cropmarks seen on aerial photos. The number of geomagnetic surveys of Neolithic enclosures has considerably increased thanks to the ANR MONUMEN with the survey of more than eighteen sites between the end of 2018 and the beginning of 2020. These new surveys have led us to qualify and quantify how geomagnetic survey has contributed to the study of these enclosures and to determine its complementarity in relation to aerial survey. The local geology influences the quality of aerial photos, as the appearance of cropmarks is conditioned by water stress. The latter is more or less important depending on the water retention capacity of the soil and thus depends on the nature of the soil and the bedrock. Cropmark visibility varies depending on the local bedrock and the restitution of the archi tecture of the enclosure is more or less reliable. It was possible to quantify the loss of information by comparing aerial photos to the geomagnetic survey. Aerial images of enclosures located on a limestone substrate have an information loss of less than 33% compared to magnetic anomaly maps. On this type of bedrock, aerial photography is relevant and the cropmarks of the ditches can be superimposed on the geomagnetic survey. On the other hand, aerial photographs of enclosures located on a marly bedrock lose about 66 % of information compared to the magnetic anomaly maps. On this type of substratum, aerial photography is less relevant and part of the enclosure is not visible on the pictures. A comparison of the enclosure architecture seen on the aerial photographs with that seen on the magnetic anomaly maps is also relevant. The enclosure of the Coteaux de Coursac at Balzac (Charente) is a site where the cropmarks of the ditches are visible only under certain conditions (hydric stress). The quality of the aerial images is excellent and highlights numerous causewayed structures (discontinuous ditches, fences, postholes). However, the aerial imagery does not match the magnetic anomaly map, which is more precise in its mapping of smaller causewayed features (palisades, postholes). Moreover, the variations of magnetization contrasts within the ditches are an indication of the variations of their fill. Aerial imagery is certainly relevant for mapping the architecture of an enclosure but is not has detailed as the geomagnetic survey. When comparing the quality and quantity of the detail visible between the magnetic anomaly maps and the aerial images, magnetic anomaly maps are able to detect small causewayed features such as entrances, palisades and postholes. Several examples are presented here including the sites of le Pontet at Saint-Nazaire-sur-Charente (Charente Maritime), la Vieille Garenne at Courcome (Charente), le Peu at Charme (Charente) and le Perradet at Saint-Groux (Charente). These small features are rarely visible on aerial photos. Some features have thermo-remanent magnetization from fire action, which can be detected by geomagnetic survey. This is the case at the site of le Peu at Charme (Charente), where the postholes inside the enclosure mark to several burnt buildings. The fills of the postholes are thermo-remanent magnetized and are visible on the magnetic anomaly map. These postholes are however not visible on the aerial photographs. The dynamics of the magnetic signal or the magnetization contrast of the ditches of the Neolithic enclosures is not homogeneous and these variations reflect material heterogeneities in the ditch fill. These variations and their link to the magnetic signal were studied in the paper by Ard et al (2015) on the Bellevue enclosure at Chenommet (Charente). They are not generally visible on aerial photographs, but can in rare cases be detected on aerial photographs, notably at the site of Bellevue at Chenommet (Charente). To conclude, aerial and geomagnetic survey are complementary methods for the study of the architecture of Neolithic causewayed enclosures. Most sites are discovered by aerial survey, which is the primary source of information for mapping the ditches. These plans are based on the survey of ditch cropmarks, the appearance of which is governed by numerous factors making them difficult to predict from one year to another. Moreover, depending on the geological bedrock, the cropmarks are not always clearly identifiable and do not allow the entire architecture of the enclosure to be perceived. Finally, aerial photos are mostly shot at an oblique angle, which distorts distances and lengths. Orthorectification can alleviate this problem but is rarely used by volunteer prospectors. Magnetic prospecting allows the causewayed structures of these sites to be mapped with better resolution and accuracy.