Luminescence dating of ditch fills from the Headland Archaeology Ltd. excavation of Newry Ring Fort, Northern Ireland

This study supports a new investigation into the construction, occupation and utilisation history of a Mediaeval ring fort near Newry, southwest Northern Ireland (section 2). Optically stimulated luminescence (OSL) profiling and age determinations have been made for two sequences of sediments accumulated in the ring ditch surrounding the fort, and potential has been assessed for TL dating of a Souterrain-Ware sherd from a pit feature within the site complex (section 3). The archaeological significance of the age determinations has been reviewed in the light of the luminescence results and the samples’ depositional contexts, to constrain the deposition/formation dates of the sampled sediments (section 6). A total of 31 profiling (sections 5.1, 5.2) and 12 age (section 5.5) determinations were made. Profiling measurements were made using simplified equivalent dose determination procedures on polymineral coarse and hydrofluoric etched sand-sized mineral grains (sections 4.2.2). Dose rate determinations were made using thick source beta counting, high-resolution gamma spectrometry, field gamma spectrometry, measured water contents and calculated cosmic dose rates (sections 4.2.1 and 5.3). Equivalent dose determinations were made (sections 4.2.2, 5.4) using the OSL signals from sand sized grains of quartz separated from each sample. The luminescence behaviour of the Newry Ringfort samples was generally very good. Profiling indicated variable levels of residual luminescence signal through the sections (sections 5.1, 5.2), but OSL on the etched fraction was found to be least affected, and measurements on fully prepared quartz for dating appeared even less so (sections 6.1, 6.2). Dose rates ranged from 2.6 to 3.9 mGy/a, De values from the dating samples ranged from 0.7 to 5.0 Gy. Estimates of sediment accumulation date ranged from 410AD to 1750AD (section 5.5). Uncertainties on the age estimates were commonly around 3%, but young samples with scattered equivalent dose distributions had estimated age uncertainties of up to 11%. The external dose rate to the sherd was estimated to be 1.33 mGy/a ± 0.12 (sections 6.3, 7). Precision was limited by uncertainties in average water content during burial rather than heterogeneity in the gamma radiation field: providing the range of sediment radioactivity at a site can be assessed, and the average burial water contents of sherds excavated from it can be well constrained, then it is likely that sherds from around a site could be dated with sufficient precision to establish a broad chronology for Souterrain-Ware. The earliest sediments in the ditch of the ringfort indicated that its construction predates the end of the 6th Century AD (sections 6.4, 7). These and other OSL age estimates indicated continued occupation until the mid 11th Century, or phases of occupation in the 7th, 9th and 11th Centuries. Results from the base of a colluvial soil sealing these layers indicate that the site was set to cultivation at or around the time of the Norman invasion of Ireland (1169AD), rather than in the post Mediaeval period. Abandonment of the ringfort must have occurred at the time of the invasion or in the century before it. Samples from throughout the colluvial soil also indicated that it continued to accumulate until at least the 18th Century, and probably into the 20th Century