Heritage Cases

Site and Approximate Age of Materials: Grassridge Rockshelter, Eastern Cape The sediments all come from layers at or below where we have received calibrated radiocarbon dates of ~45,000 – 35,000 years ago, which is at the practical limit of radiocarbon dating. We anticipate the ages of the OSL samples to date to the Late Pleistocene, with the upper samples dating to ~40,000 years ago. We are unsure of the age of the lower samples, and are hoping that the OSL dating will clarify chronology of the site. Aim/Rationale: With support from ECPRHA, the Albany Museum, SAHRA, and the Leakey and Wenner-Gren Foundations, we have compiled a series of radiocarbon age estimates for the upper sediments from Grassridge Rockshelter. These estimates indicate a middle Holocene occupation (~7,000 years ago), an early Holocene occupation (~11,500 years ago), and a late Pleistocene occupation (~40,000 years ago) (Collins et al. 2017). However, these lower radiocarbon age estimates are at the limit of radiocarbon dating, and sit roughly 60+ cm above bedrock, limiting our chronological resolution of the occupational sequence and human behaviours represented in these lower deposits. To refine our chronological resolution, we propose to take 14 OSL samples from these lower deposits. These samples will be analysed by Dr. Luke Gliganic, an expert in OSL dating (Gliganic et al. 2012; 2014; 2016), and will provide a high resolution chronology for the site that will inform on site formation processes, the nature and tempo of human occupations at the site during the Late Pleistocene, and how the Late Pleistocene occupational and cultural sequences from Grassridge relate to other sites in the region and across Southern Africa. Methodology: OSL and infra-red stimulated luminescence (IRSL) dating techniques can be used to determine the amount of time since quartz or feldspar grains were last exposed to sunlight, thus providing an estimate of when a sedimentary layer was buried as far back as ~300 ka, and possibly older (Wintle 2008). The measurement of OSL signals from individual quartz grains has become commonplace. This technique allows grains with inappropriate intrinsic OSL characteristics to be rejected and enables the generation of large equivalent dose datasets that can be statistically modeled. This approach has become a critical tool for identifying and resolving post-depositional mixing and stratigraphic integrity, and has been used extensively to develop chronologies for Pleistocene southern African archaeological sites (Jacobs et al. 2008). We will collect and analyse a series of 14 OSL samples from Grassridge. These samples will be taken using a 3 cm diameter x 10 cm length PVC pipe that will extract sediments from within the deposit that have not been exposed to sunlight, as per the requirements for OSL dating. We have chosen a small diameter sample to maximise the resolution of the dates and to disturb the profile and deposits as little as possible. Eight of these samples will be taken from the B2-C2 North profile that was originally excavated by Dr. Hermanus Opperman. This profile has been sampled for the majority of our radiocarbon age estimates, as well as for geological and micromorphological analyses, which are currently underway. OSL samples from this profile will provide a chronology for the lower deposits, which we can relate to Opperman’s original findings, as well as our ongoing excavation 1 m north of this profile. Six samples will be collected from the Late Pleistocene sediments in our ongoing excavation. These samples will be taken from the west wall, which is ~20 – 40 cm from the wall of the shelter, and so in an area that will cause minimal disturbance to the deposits in the ongoing excavation. These samples will help to refine the high density human occupations from this area, which include a rich lithic sequence, as well as preserved plant remains. Damage/Destructive Analysis: The OSL analysis will damage the sediments. However, we will only sample sediments that do not contain cultural material. References: Collins, B., Wilkins, J. and Ames, C. (2017). Revisiting the Holocene Occupations at Grassridge Rockshelter, Eastern Cape, South Africa. South Afr. Archaeol. Bull. 72: 162–170. Gliganic, L. A., Cohen, T. J., May, J.-H., Jansen, J. D., Nanson, G. C., Dosseto, A., Larsen, J. R. and Aubert, M. (2014). Late-Holocene climatic variability indicated by three natural archives in arid southern Australia. The Holocene 24: 104–117. Gliganic, L. A., Cohen, T. J., Slack, M. and Feathers, J. K. (2016). Sediment mixing in aeolian sandsheets identified and quantified using single-grain optically stimulated luminescence. Quat. Geochronol. 32: 53–66. Gliganic, L. A., Jacobs, Z., Roberts, R. G., Domínguez-Rodrigo, M. and Mabulla, A. Z. (2012). New ages for Middle and Later Stone Age deposits at Mumba rockshelter, Tanzania: Optically stimulated luminescence dating of quartz and feldspar grains. J. Hum. Evol. 62: 533–547. Jacobs, Z., Roberts, R. G., Galbraith, R. F., Deacon, H. J., Grün, R., Mackay, A., Mitchell, P., Vogelsang, R. and Wadley, L. (2008). Ages for the Middle Stone Age of southern Africa: Implications for human behavior and dispersal. Science 322: 733–735. Wintle, A. G. (2008). Luminescence dating: where it has been and where it is going. Boreas 37: 471–482.