Geological Analysis Challenges Scottish Origins of Stonehenge Altar Stone
New research narrows the search for the monument's mysterious six-ton sandstone slab origin.
1Monumental Geological Mystery
The Altar Stone, a 4.9-meter slab weighing six tons, remains a central enigma at Stonehenge. While previous theories suggested a Welsh origin, recent studies shifted focus toward Scotland. However, a comprehensive new investigation by Professor Duncan Pirrie now challenges the prevailing belief that the stone originated in the Orcadian Basin.
2Methodology and Sample Constraints
Because the Altar Stone is largely buried beneath fallen sarsen stones, researchers could not sample it directly. Instead, the team analyzed eight museum fragments confidently linked to the slab. They supplemented this with portable X-ray measurements taken from three small, exposed patches of the stone currently visible at the site.
3Mineralogical Evidence Discrepancies
The stone contains chrome spinel, a heavy mineral rarely found in the Old Red Sandstone of the Orcadian Basin. Furthermore, the stone's mixed-layer illite/smectite clay shows 20% expandable layers, whereas rocks in Caithness and Orkney consistently show values below 10%. These specific geochemical markers effectively exclude those regions as potential sources.
4Excluding Potential Source Areas
The study systematically ruled out several regions, including the Midland Valley of Scotland, due to its abundance of rock fragments. Additionally, the Southern Uplands were excluded because the stone lacks the low-grade metamorphism characteristic of that area. Researchers also confirmed that West Wales and the Anglo-Welsh Basin remain unlikely candidates.
5Barium and Chemical Signatures
A distinctive scatter of baryte, a barium mineral, cements the sandstone grains, resulting in unusually high barium readings. While the stone's ripple-laminated structure suggests deposition by flowing water, the isotopic match for Carboniferous sandstones remains poor. The lack of complete strontium data currently limits definitive isotopic comparisons for these candidate sandstones.
6Refining the Search Parameters
Professor Nick Pearce emphasizes that eliminating potential source areas is as critical as identifying new ones. By establishing the specific geological characteristics required, the team has narrowed the search field significantly. This rigorous process of elimination brings researchers closer to understanding the stone's remarkable journey to the Salisbury Plain.
7Historical Context of Construction
Stonehenge was constructed in multiple phases between 3000 and 1600 BC. While the smaller bluestones were transported approximately 225 km from West Wales, the Altar Stone's unique composition sets it apart. Tracing its true origin would provide vital insights into the logistical capabilities and cultural motivations of ancient Neolithic builders.
8Future Investigative Directions
Although the Scottish theory is now contested, the research team maintains that they are better equipped to focus on likely areas. The study, published in the Journal of the Geological Society, provides a clearer composition profile. Future efforts will target locations that match the stone's unique mineralogical and geochemical constraints.