The Altar Stone Source - The Algorithm Result

 

"A transparent, multi-proxy desk screen—barium–rubidium stream-sediment geochemistry (a proxy for the Altar Stone’s diagnostic baryte cement and K-feldspar deficit), thermal-maturity mapping, clay mineralogy, sedimentary facies, and detrital-zircon provenance—has been applied iteratively across the UK Old Red Sandstone (ORS). It has narrowed a national-scale problem, step by step and at the appropriate level of evidence, to a single field-accessible target. The screen and Clarke et al.’s (2024, 2026) detrital-zircon data agree that the source lies on the East Caithness coast; but the prime barium cluster (Sarclet–Lybster–Clyth) sits in the high-maturity zone mapped by Hillier & Marshall (1992), where vitrinite reflectance of 3–6% R₀ has driven the delicate expandable and aluminous clays of the Altar Stone past preservation.
Two independent lines then refine, rather than defeat, the result. The clay evidence (Hillier & Clayton 1989; Hillier et al. 2006) shows the Altar Stone’s tosudite–kaolinite–dioctahedral-chlorite assemblage to be an aluminous sandstone diagenetic pathway. While classically expressed in the UK Lower ORS, work on offshore Middle Devonian blocks (e.g., the Clair Group) demonstrates that this signature is strictly facies- and fluid-controlled rather than stratigraphically restricted, meaning it can be seamlessly accommodated within porous sandstone bodies encased in lower-maturity segments of the Caithness Flagstone Group.
One place is identified where such a rock could occur:"

I've included a photo of a megalithic monument that is there, but I need to just check it again before revealing the site.

The main problem is that the lack of data means that unsampled gaps in the record don't get filtered out, and that small areas are filtered out by being smeared in with the surrounding geology. Other data may highlight such areas, and suggest better screening. So such a desktop screening exercise can only suggest places worth further investigation, with a rock hammer.