Dating earthworks with luminescence : Insights from the medieval ringfort of Den Burg, Texel (the Netherlands)

Ancient human-made earthworks such as mounds, middens, levees, raised or terraced fields, enclosures and forts are omnipresent in many areas across the globe. They may offer rich and important information on past societies. However, harvesting such data is significantly hampered by the general lack of robust earthwork chronologies. Dating earthworks is notoriously difficult due to a variety of factors, including a scarcity of closely datable archaeological finds, the frequent absence of suitable and relevant material for radiocarbon dating, and lacking or ambiguous historical sources. Luminescence dating may provide a solution for these problems because it enables direct dating of sediment, a ubiquitous and relevant material in earthwork construction. In this paper we focus on ringforts: circular or semi-circular defensive structures surrounded by earthen banks and ditches, which originated in the first millennium CE and appear to reflect major transformations in early historical societies. We present the results of a detailed luminescence study of a ringfort underlying the modern town centre of Den Burg, on the Wadden Isle of Texel in the Netherlands. We dated samples from different sedimentary contexts (bank and ditch infills) using both quartz optically stimulated luminescence (OSL) and feldspar single-grain post-infrared infrared stimulated luminescence (pIRIR). We analyze the results in the context of age interpretations based on other proxy data (radiocarbon dates, archaeological finds, historical evidence). This yields a new, robust dating chronology that changes the biography of the ringfort. The initial construction dates between 730 and 840 CE, making Den Burg the oldest known ringfort in the Low Countries. We reflect on the best practices for luminescence dating of earthworks and on archaeological implications for other ringforts in the Netherlands and beyond.