The late Nordic Iron Age and Viking Age royal burial site of Borre in Norway: ALS- and GPR-based landscape reconstruction and harbour location at an uplifting coastal area

Abstract

The Nordic Iron Age and Viking Age royal burial site of Borre on the western coast of the Oslofjord in Norway is an exceptional archaeological site in Northern Europe. The burial mounds, associated archaeological structures as well as geomorphological features have been analysed by a 1 × 1 m digital terrain model derived from airborne laser scanning. The interpretation of this data used different derivates of the digital elevation model including hillshade, slope map, local relief model and their combination. Additionally, ground penetrating radar profiles have been measured to investigate the internal structure of selected micro-topographic features. Based on the high-resolution topographic data, four smaller burial mounds were added to those previously known. Scandinavia is strongly affected by ongoing post-glacial isostatic recovery and, consequently, a sequence of elevated beach ridges were documented within the burial site down to the present shoreline. Local sea-level reconstructions in the Oslofjord indicate that the burial site of Borre was located close to the shoreline in the period of its use when the local sea-level was 3.5–5 m higher than today. Two prominent ridges between 4.5 and 0 m above present day sea-level are interpreted as Viking Age jetties facilitating safe landing on an otherwise unprotected coast.

Introduction

Airborne laser scanning (ALS) has found widespread application in archaeological research (Crutchley and Crow, 2010), providing high-resolution digital terrain models (DTM) with a ground sampling distance of typically 0.5–0.1 m and a vertical accuracy which is usually better than 0.1 m. In open areas, these DTMs are of high value as they display the detailed topography (Challis, 2006). ALS-based DTMs are, therefore, of great importance in detecting and documenting archaeological and palaeoenvironmental features in vegetated areas (Doneus and Briese, 2011, Doneus et al., 2013). Here, sophisticated visualisation techniques (Hesse, 2010, Kokalj et al., 2011, Doneus, 2013) of the microtopography provide a new understanding of the layout and function of archaeological remains, which are commonly better preserved there compared to areas with less vegetation, where archaeological remains are exposed to erosion and/or ploughing.

Any archaeological interpretation of prospection data has to include mapping in 3D, describing and explaining both the relevant archaeological structures and (palaeo-) environmental features. Reading and extracting archaeological information from prospection data, therefore, require an understanding of the dynamic interaction between humans and environment. This is especially important when working with ALS-derived data, as Earth's topography is a combination of manmade structures and geomorphological processes including erosion, sedimentation and tectonics (Crutchley and Crow, 2010). Consequently, the interpretation of these high-resolution DTMs clearly demand an integration of archaeological and geoarchaeological interpretation.

This is especially true for Scandinavia where remains of the last glaciation, such as drumlins, moraines, periglacial processes, but also the isostatic rebound of the Scandinavian crust, shape the landscape. These features are also omnipresent in excavation and prospection data and therefore have a pronounced impact on the archaeological interpretation. This will be demonstrated using the prehistoric site of Borre as a case-study. The investigated site is situated on the western shore of the Oslofjord (Fig. 1) and is well-known for its outstanding Iron Age royal burial mounds (Brøgger, 1916, Roesdahl, 1991, Myhre, 1992a, Myhre, 2003a, Myhre, 2003b, Myhre and Levanger, 2013) (Fig. 2). Borre is mentioned in the Old Norse literature (Krag, 1991) and the myths connected to the place were already established before the archaeologist Nicolaysen excavated a Viking ship in one of the large burial mounds in 1852 (Nicolaysen, 1854). Despite huge interest for the monumental mounds, there have been relatively few archaeological investigations in the 20th century. Later, questions concerning possibly related settlement and harbour have been raised and after 2007 geophysical prospection and airborne laser scanning have produced data that might change the understanding of the Borre site. This article is the first step to address these questions and to show the applicability and benefit of integrated remote sensing, geophysical and geoarchaeological investigations for archaeological interpretations.