Altar Stone Provenancing Study Target

 

The Lybster-Clyth Coast as an Altar Stone Provenancing Study Target

 

Clyth shore harbour, 3.2 km. east of Lybster, Caithness 1910 - https://www.trove.scot/image/2779309

 

The provenance of the Stonehenge Altar Stone is now constrained by a clear logical split in the evidence. Detrital mineral indicators (zircon age spectra and apatite) strongly support a source within the Orcadian Basin of northeast Scotland, while the distinctive diagenetic signature,  including tosudite and kaolinite in the clay fraction, early baryte cement, and very low K-feldspar, has not yet been matched in the samples tested to date. This leaves two main possibilities: either the required combination of facies and burial conditions exists somewhere within the Orcadian Basin, or the source lies further south in a region that shares a closely similar detrital mineral signature.

 A convergence of national geochemical datasets, recent zircon provenance studies, local stratigraphy, and structural evidence from nearby megalithic monuments isolates a 15 km stretch of the East Caithness coast as the primary field target for any Stonehenge Altar Stone Sourcing Enquiry.

 Here is the consolidated evidence for targeting the Lybster-Clyth coastal belt:

• The Geochemical Engine (The Ba/Rb Proxy): The search targets "Cluster 18," a 42.5 km² stream-sediment anomaly that ranks as the strongest bedrock-verified Old Red Sandstone (ORS) cluster in the country (98.2% genuine ORS; the mapped formations are Middle ORS Caithness Flagstone Group — a fact about the bedrock, not the Altar Stone’s own stratigraphic assignment, which is unpublished). Because Rubidium (Rb) substitutes for Potassium (K), a basin-relative Ba/Rb ratio threshold (>13.76) was used to successfully isolate the Altar Stone’s highly specific diagenetic signature: high barium (baryte cement) coupled with a near-total deficit of K-feldspar.  Daw (2026) - https://www.academia.edu/169688751/The_Stonehenge_Altar_Stone_Screening_the_Orcadian_Basin

• Resolving the Clarke et al. Zircon Data: Clarke et al. (2024, 2026) definitively locked the Altar Stone's detrital zircon age spectrum to the Orcadian Basin. However, their strongest specific sample sites (Sarclet and Braemore) are Lower ORS red arkoses — the wrong facies for the grey-green, K-feldspar-depleted rock the Altar Stone requires. (The Altar Stone itself has no published Lower/Middle assignment; zircon spectra constrain the source terrane, not the depositional tier, so a Lower ORS zircon match is not in itself a mismatch — but neither can it localise the source.) The Lybster-Clyth coast offers the grey flagstone-facies expression — mapped as Middle ORS — of this exact same zircon source terrane.

• The Facies Match: The locally mapped Lybster Flagstone Formation directly matches the Altar Stone's descriptive requirements: it is a very fine-to-fine, well-sorted, grey-green sandstone featuring unidirectional ripple cross-lamination.

• Structural Proof from Local Monuments: The Altar Stone is a thinly layered rock (~10 cm layers) held together as a ~0.5 m block by pervasive calcite, baryte, and kaolinite cement. The critical mechanical hurdle is finding a local bed capable of yielding a 0.5 m thick monolith without delaminating. Two nearby monuments prove this rock exists locally. The Bronze Age horseshoe at Achavanich features local grey Middle ORS stones up to 0.51 m thick. Furthermore, excavations at the Grey Cairns of Camster (just ~5.5–6.3 km from the anomaly’s centre) uncovered foundation blocks, also local grey Middle ORS, in the south-west forecourt and structural tail stones measuring up to 0.5 m thick and weighing up to 2 tonnes.

• The Coast First. The outcrops the monument-builders used inland are likely still there under post-Bronze-Age peat, hidden, not gone, and worth checking at ridge crests and burn cuts. But at the coast the bedrock is still exposed and loose blocks are present. And the coast is structurally distinct ground, not just convenient ground: The clay mineral assemblage of the Altar Stone (tosudite + kaolinite with expandable illite-smectite) is a strong and legitimate provenance filter. Most published datasets from the Caithness and Orkney flagstone sequences record illite-dominated or low-expandability assemblages consistent with relatively high thermal maturity and therefore fail this filter. However, almost no published sub 2 µm XRD or expandability data exist from the specific coastal East Caithness strip.  BGS land-gravity data place the coastal target on a positive Bouguer high, consistent with a structurally thinned basin margin, shallow dense basement and no concealed granite thermal overprint; stream-sediment U is correspondingly quiet.  Inland Caithness lies on Bouguer lows with elevated U, indicating thicker Devonian fill and different basement/fluid regime. These structural and geochemical contrasts support the possibility of distinct diagenetic histories (and therefore clay mineral assemblages) between the coastal facies and more internal parts of the basin. The coastal margin therefore remains an untested but viable candidate; a single well-chosen sample from the geochemical hotspot for full clay mineralogy would provide a simple, low-cost, and decisive test — with both outcomes stated in advance: kaolinite and tosudite with higher I/S expandability would strongly support the target; an ordinary illite–chlorite assemblage with no tosudite would exclude it. (Tosudite has never been reported from the Caithness flagstones, so its presence in the coastal facies is a prediction of this framework, not a datum.)

 

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Main Draft Paper:

https://www.researchgate.net/publication/408461829_The_Stonehenge_Altar_Stone_Screening_the_Orcadian_Basin_A_Multi-Element_Geochemical_Screen_Verified_Against_Bedrock_Geology_for_the_Source_of_the_Stonehenge_Altar_Stone_Within_the_Orcadian_Basin

https://www.academia.edu/169688751/The_Stonehenge_Altar_Stone_Screening_the_Orcadian_Basin