Identification of materials used for earthwork construction: the semi-circular fortification rampart of Hedeby, Northern Germany

The protected status of archaeological sites requires using minimally invasive methods of material investigation such as coring. In contrast to excavations, the coring method does not present a complete view of the inner structure, and the limited amount of material in cores represents a small portion of the study object. This complicates the interpretation of material genesis. This problem is particularly relevant for complex structures such as earthworks. Nonetheless, the proper interpretation of the construction materials in earthworks is crucial for geoarchaeological investigations of the anthropogenic transformation of landscapes. We proposed a method for classifying and identifying the earthwork materials, sampled by the coring technique. It was developed using an example of the semi-circular fortification rampart around the former Viking settlement Hedeby, which was an important early medieval international trading center and today is a UNESCO world heritage site. Materials from the coring transect across the semi-circular rampart were described. The physicochemical properties of 139 samples from three cores were determined: grain size distribution, weight percentages of gravel, artefacts, bones, and charcoal, loss on ignition, magnetic susceptibility, and element concentrations. The statistical methods (normalization, correlation, standardization, principal component analysis, cluster analysis) were applied to distinguish and group materials according to their physicochemical properties. The resulting clusters were used as a basis for material classification. Some clusters were mildly transformed according to the morphological properties of the material. Consequently, 20 groups were distinguished based on the physicochemical and morphological properties of the material. The rampart was constructed from soil horizons, cultural deposits, and Pleistocene material, which were used in different proportions along the investigated transect. In addition, the constituents of mixed layers were identified. Overall, the presented method for the statistical classification of the material considerably facilitates and objectifies the identification of material genesis. This is particularly valuable in tackling the challenges of coring-based investigations of earthworks.