Optically Stimulated Luminescence Dating of Ditch and Pit Fills From NAA Ltd Excavations at Scorton Quarry, North Yorkshire: Field Report and Initial Luminescence Assessments

This study supports an investigation into the construction, occupation and utilisation history of the Scorton Cursus site, undergoing archaeological investigations by Northern Archaeological Associates Ltd (NAA) in advance of gravel extraction at the Scorton Quarry, Scorton, North Yorkshire. The work part fulfils a condition attached to planning permission granted for the development (Permission ref C1/39/34G), and the written scheme of investigation (WSI) prepared by NAA for Tarmac Ltd in accordance with the relevant standards and guidance published by English Heritage and the Institute for Archaeologists. The WSI provides for investigations of the archaeological features within the gravel extraction area, using a combination of excavation and recording methodologies. It also provides for collection of samples for dating the monument features, and in particular for assessment of the suitability for optically stimulated luminescence (OSL) dating. This summary report describes the fieldwork undertaken by the Scottish Universities Environmental Research Centre (SUERC) in September and October 2011 to retrieve sediment samples from key features to be used in assessing the suitability of the material for OSL dating, and to provide material for OSL dating analyses during post-excavation stages. Site visits took place on the 8th - 9th September and the 6th October 2011, over which period six sections were sampled for luminescence profiling and additional samples were collected for full dating analysis. Initial OSL screening was performed on site using portable OSL equipment, and in-situ gamma spectrometry was used to record environmental dose rates in the positions of the dating tube samples. Rapid calibrated luminescence measurements, using Risø D-15 equipment, were made on paired aliquots of HF-etched quartz and polymineral concentrates, from each of the profiling sample locations, to provide an assessment on luminescence sensitivities and stored dose values through each profile. During the field trip gamma dose rate measurements were made at the positions of all 11 full dating samples. Profile 1 was taken through the main ditch (section 327); 14 profiling samples covering the primary substrate, the lower coarse fills, and two finer fills representing natural sedimentation after the primary use and infilling of the ditches. A set of five dating samples was collected through this section, which represent the sequence of early and later fills, with the aim of dating the infilling chronology. In a section (section 221) between the southern and northern ditches a preserved soil was exposed which lies on material presumed to represent the gravel substrate, and is in turn covered by material attributed to part of the bank associated with the southern cursus ditch. A series of eight profiling samples was collected through these deposits to assess the suitability of the material for OSL dating, and five dating tubes were collected with associated gamma spectrometry data to represent the depositional sequence. The significance of these samples is that the lower units will define the environmental history of the substrate and early soil formations prior to construction of the cursus monument. Assuming that the lower bank deposits which overlie the palaeosol are in their original setting, the upper preserved soil layer is expected to provide a terminus post quem for the construction of the cursus monument itself. The upper bank deposits provide a measure of control over the redepositional history of the bank, which may be associated with later re-shaping of the landscape after the initial period associated with the cursus monument in its original form. The third area investigated during the fieldtrip was the ditch fill associated with the ring ditch to the south of the cursus (section 352). Here six profiling samples were collected covering lower and upper fills. A single tube sample for OSL dating was collected from the lower fill, having examined the field profile. This should provide an early infill date for the ring ditch, and determine the chronological relationship between this feature and the cursus. The fourth profile was taken through the fill of the main ditch, 130 m northwest of section 327 (8 samples), to examine whether there are spatial variations in the luminescence properties of the fill and identify any differences in the infill chronology. A sand lense is present in the lower fill of the ditch, potentially providing a means to constrain the age of this lower unit. A short profiling sequence (4 samples) was taken through the substrate at the base of the ditch, and on the northeastern flank of the ditch, to provide a means to directly compare the luminescence properties of the different units. The fifth feature investigated was the ditch that runs outside of, and sub-parallel to the main western cursus ditch. The feature was examined in two sections, at either end of the excavation (separated by 4.3m). A single sample for OSL dating was taken from the main fill of the ditch, context 1786. A series of six profiling samples were taken through the fill of this ditch, covering the lower and upper fills, and bracketing the full dating sample. The sixth feature was a ditch that runs at an angle to (approximately perpendicular) the main eastern cursus ditch. This feature was also examined in two sections, at either end of the excavation (separated by 6.9m). 3 profiling samples were taken through the fill of this ditch, to assess its luminescence properties. A full dating sample was taken from one of the ‘better bleached’ horizons. The full dating samples collected from the latter two features, should provide early infill dates for the smaller features, and establish the chronological relationships between these features, the cursus and the ring ditch. Field and laboratory luminescence screening measurements have provided an assessment of luminescence sensitivities and stored dose values through each of the sampled sections. The following are our preliminary conclusions: (i) the lower fill of the western cursus ditch was not fully re-set during the period of archaeological interest; it has luminescence sensitivities and stored dose values comparable to those measured in the substrate; (ii) the upper fill of the western cursus ditch has sensitivities and stored dose values which may be consistent with an archaeological age; (iii) field and laboratory profiling results from section 211 suggest that a re-interpretation of the ‘cursus bank’ is required; stored dose values decrease throughout the profile, without a return to stored dose values comparable to the substrate, as one would expect if material was re-deposited from the bank of the western cursus ditch; (iv) the ring-ditch has sensitivities and stored dose values which may be consistent with an archaeological age, possibly younger than the main cursus ditches; (v) the smaller features, outside the main cursus ditches (both sub-parallel and -perpendicular to the main ditches), have luminescence properties which should be amendable to dating; and interestingly, the preliminary findings suggest a relatively young archaeological age; (v) the OSL sensitivity in all upper layers is significantly larger than that of the substrate and lower layers; explanations may include: an additional input of sediment into the depositional system from a different source, with higher sensitivities; heating of the quartz in the archaeological sediments through the use of fire in landscape clearance, or seasonal burning