Integrated GPR and archaeological investigations reveal internal structure of man-made Skiphóll mound in Leiruvogur, Iceland

Abstract During the landnam (period of land-taking, 870–930 CE), people from Scandinavia emigrated to Iceland. Mosfell Valley which is found in the Southwest of Iceland was settled during the early days of the landnam . The region was one of the country's cultural centers. The valley's harbor site was mentioned to be in Leiruvogur Bay. Modern constructions make the prospection and exploration of this harbor site nearly impossible. One of the few places that are left untouched is a mound known in oral tradition as Skipholl (Ship Hill), that could once have been used as landmark for incoming ships. In this paper we present an integrated study comprising a GPR (Ground Penetrating Radar) survey, excavation work and coring that was conducted on the Skipholl. The aims of the study were to date the Skipholl to show that it existed during the Viking Age by imaging specific volcanic tephra layers and to investigate the interior of the mound to understand its use. GPR was used because it has a suitable spatial resolution and depth penetration when investigating targets of the size of the Skipholl. Nevertheless, because of the topography of the mound and the demand to image thin tephra layers, efforts have been done to increase the GPR resolution by topography based imaging and to create an integrated interpretation of all data. A reconstruction of the mound's appearance in 1500 CE was possible through a depth comparison of the specific tephra found in cores with the near-surface reflections in GPR. Excavations showed several mound and ditch features dating to between 871 CE and 1226. The GPR data correlates well with these features. Furthermore, the correlation of the GPR reflections with the excavated portions of the mound allows the projection of the archaeological features beyond the excavation area. The nature of the mound and ditches exposed in excavation and a ship-shaped anomaly reconstructed from the GPR slices, makes it appear most likely that the Skipholl mound had a ritual function beyond just marking the harbor.