Why the "West Woods" Sarsens couldn't come from Salisbury Plain

 According to the document "Earliest Movement of Sarsen Into the Stonehenge Landscape: New Insights from Geochemical and Visibility Analysis of the Cuckoo Stone and Tor Stone", the sarsens from West Woods, located on the Marlborough Downs, could not have originated from Salisbury Plain due to a combination of geological, geochemical, and observational evidence. Below is an abstracted and comprehensive explanation:

Geological Conditions

The formation of large sarsen stones, like those found at West Woods and used at Stonehenge, requires specific geological conditions that are not sufficiently met on Salisbury Plain. The document outlines five key factors necessary for sarsens with West Woods' characteristics to form locally:

1. Presence of Paleogene Sediments:
   • Salisbury Plain likely had Paleogene sediments of mid-Eocene age or earlier, making this condition feasible. However, this alone is insufficient without the other factors aligning.
2. Thick Sandy Horizons:
   • The sediments on Salisbury Plain are dominated by clays, silts, and thin sand units rather than the thick, sandy horizons (1-2 meters or more) needed to form large sarsens. In contrast, such thick sands are present in the Marlborough Downs, where West Woods is located.
3. 'Clean' Sands:
   • The sandy horizons must be free of clay minerals that inhibit quartz overgrowth cementation, a process critical to sarsen formation. Salisbury Plain's sediments contain clay, making them less suitable, whereas West Woods’ sands are relatively 'clean'.
4. Geological Structure:
   • A structural context, such as a synform, is required to promote sustained silica-bearing groundwater flow for cementing large sarsens. The Marlborough Downs feature such synformal structures, but Salisbury Plain exhibits only gentle dips and lacks comparable formations, reducing the likelihood of significant sarsen development.
5. Geochemical Similarity:
   • For sarsens to match West Woods geochemically, the sands must have a similar mix and abundance of non-quartz minerals. The document notes that sediment composition varies over short distances (e.g., within the Marlborough Downs), and Salisbury Plain’s depositional environment differs, making an identical geochemical profile unlikely.

Geochemical Evidence

• Geochemical analysis using portable X-ray fluorescence spectrometry (pXRF) and Bayesian Principal Component Analysis (BPCA) shows that the Cuckoo Stone, Tor Stone, and most Stonehenge sarsens (e.g., Stone 58) share a statistically similar chemical composition, pointing to a common origin at West Woods. This similarity is improbable if the stones formed on Salisbury Plain, given the differing sediment sources and conditions.

Physical Distribution and Size

• Sarsens on Salisbury Plain are smaller (largest recorded less than 3 meters) and less numerous compared to the Marlborough Downs, where large boulders predominate. The sarsens at Stonehenge, the Cuckoo Stone (2 meters long, 6.5 tonnes), and the Tor Stone (2.8 meters long, 4 tonnes) are significantly larger than those typically found on Salisbury Plain, supporting the conclusion that they were transported from West Woods rather than sourced locally.

Conclusion

The sarsens from West Woods could not have come from Salisbury Plain because:

• The geological conditions on Salisbury Plain—lacking thick, clean sandy horizons and appropriate structural contexts—do not support the formation of large sarsens like those at West Woods.
• Geochemical evidence links the Stonehenge sarsens, Cuckoo Stone, and Tor Stone to West Woods, not Salisbury Plain.
• The size and scarcity of sarsens on Salisbury Plain further indicate that the large stones used in the Stonehenge landscape were imported from the Marlborough Downs.

Thus, the document concludes that these sarsens were likely transported from West Woods rather than formed locally on Salisbury Plain.