Geological sources and chronological change in ground-edged artefacts of the Hawkesbury region, the Sydney Basin: A Macdonald River case study

Evaluating temporal changes in exchange systems and social networks in pre-contact Aboriginal societies remains a major challenge for Australian archaeology. As part of a larger research program identifying exchange systems and social networks in the Sydney Basin, several excavated sites were selected to identify changes over time in the relationship between ground-edged artefacts and sources of rock used. Portable X-Ray Fluorescence (pXRF) instrumentation was used to non-destructively elementally characterise and compare excavated stone artefacts and source rocks in a geological reference collection.
For one of the sites on the NSW Central Coast - Macdonald River rockshelter (MR/1), in the lower reaches of the Macdonald River close to its junction with the Hawkesbury River - 86 stone artefacts derive from the production and/or repair of ground-edged artefacts. A large proportion of these artefacts match basalts from a local source at Peats Ridge–Popran Creek in the adjacent Mangrove Creek Valley. Other artefacts match hornfels and quartzite cobbles from the Nepean–Hawkesbury or Hunter Rivers. Peats Ridge–Popran Creek basalt was present throughout the final 3000 years of the MR/1 archaeological record, though it decreased relative to hornfels from between 1700 and 1150 years ago until contact. The sequence at MR/1 shows historically-reported exchange routes may have a time-depth of at least 3000 years.