A recent study published in "Communications Earth and Environment" has presented evidence challenging the long-standing glacial transport theory for the origin of Stonehenge's bluestones and the Altar Stone. The research utilized modern geological techniques to analyze mineral grains in river sands near the monument, concluding that the stones were likely transported by humans rather than by natural ice movement.
Background on the Glacial Transport Theory
The glacial transport theory proposed that large ice sheets carried the bluestones from Wales and the Altar Stone from Scotland to Salisbury Plain in southern England. Under this theory, these stones would have been deposited as "glacial erratics," meaning rocks moved from their original location by glacial action.
Research Methodology
To investigate this theory, researchers employed mineral fingerprinting, specifically using uranium-lead (U-Pb) dating. This technique measures the decay of radioactive uranium into lead to determine the age of individual mineral grains. The principle is that different regions in Britain have distinct rock ages, allowing a mineral's age to indicate its geological source.
The study involved collecting sand samples from rivers surrounding Stonehenge. Researchers then analyzed over 700 microscopic mineral grains, including zircon and apatite. The methodology operated on the premise that if glaciers had transported the Stonehenge stones, they would have also left behind microscopic mineral grains from those source regions, which should be detectable in the river sands of Salisbury Plain.
Key Findings
The analysis of the mineral grains yielded results inconsistent with the glacial transport theory:
- Absence of Source Matches: The study found virtually no mineral ages that matched the geological signatures of the bluestone sources in Wales or the Altar Stone's reported source in Scotland.
- Zircon Grains: The majority of zircon crystals identified in the Salisbury River sands ranged in age between 1.7 and 1.1 billion years old. These ages align with those found in the local Thanet Formation, a loose sand blanket that historically covered much of southern England. This indicates that the zircon grains are local, derived from ancient sedimentary rocks within the region, rather than from recent glacial deposits originating from distant sources.
- Apatite Grains: All apatite grains analyzed were approximately 60 million years old. This age does not correspond to any potential source rocks in Britain and is attributed to geological processes such as heating and chemical changes caused by distant mountain-building in the European Alps, which effectively reset the apatite's U-Pb clock. The findings suggest that these apatite grains are also local to Salisbury Plain, having been present in the area for tens of millions of years.
Conclusion
The absence of a mineral fingerprint matching the bluestone and Altar Stone source regions in the local sands suggests that glacial material from these distant locations did not reach the Salisbury Plain area. This research strengthens the conclusion that the monument's exotic stones were deliberately selected and transported to the site through human effort, rather than arriving by natural ice movement.