The Limeslade erratic boulder -- probably not from Mynydd Preseli

Thanks are due to Dr Katie Preece and Prof Peter Kokelaar for their initial professional assessments of the Limeslade erratic boulder. Thanks are now also due to to Prof Tim Darvill and his colleague Dr Steve Parry, who offered their services back in the spring of 2022 when the Limeslade boulder was discovered by Phil Holden.  We now have the pXRF and thin section analyses from the two samples collected by Phil.  Better late than never -- and one should never turn down kind offers of assistance, especially if one has no access to academic research funding.

According to Prof Kokelaar, cited by Rob Ixer on the Megalithic portal discussion forum, the Limeslade "giant erratic" is "......... a metamorphosed coarse dolerite (not quite gabbro), sparsely porphyritic with oscillatory-zoned euhedral and subhedral plagioclase phenocrysts mostly ~0.5 cm and up to 1 cm; dark patches could be (altered) ophitic pyroxenes. The rock shows sub-parallel feldspathic banding roughly perpendicular to the long axis (2.2m) of the boulder. ...... I suggest that from my limited experience the boulder is an erratic and would be consistent with derivation from the Lower Palaeozoic of north Pembs. The banding perpendicular to the ‘columnar’ length is fairly typical of some coarsely jointed sills I have mapped between Fishguard and St David’s Head.  Ironically the only thing that can be said with certainty now, firmly established by the extraction scar, is that the erratic is NOT preselite from the Preseli Hills -- the most abundant rock type of the Stonehenge bluestones -- as there is not a spot on site or in sight, so it is not a spotted dolerite, hence eliminating it from the established source area for the Stonehenge orthostats and suggested quarries."

We will shortly publish the geochemistry (pXRF) results and the petrological analysis, but we can say now that the work confirms the initial visual assessments by Dr Katie Preece, Prof Peter Kokelaar and others who concluded that it is a greenish unspotted dolerite or microgabbro with crystals of feldspar and pyroxene similar in some respects  to some of the unspotted dolerites of North Pembrokeshire.  According to Dr Steve Parry the rock is best described as an ophitic microgabbro.  He states: “In terms of its state of alteration, the constituent feldspar is argillized and some of the pyroxene appears to be uralitized."  Whether these characteristics, together with the plots of trace elements, will provide an accurate provenance for the boulder remains to be seen.

Once the results are published, we will of course welcome further research and comments which might help to narrow down the range of possible sources for the erratic.

Weirdly, in the discussion blogs we have been accused by Rob Ixer and Tim Daw some sort of cover-up with regard to the analytical results, although they have never explained what advantage we might have obtained from keeping information out of the public domain. As far as I am concerned, I have no "preference" regarding the origin of the boulder.  I really do not care where it has come from, although it is self-evidently a glacial erratic carried from somewhere to the west or north-west.  I doubt that even Ixer and Daw would argue that it is a bluestone monolith destined for Stonehenge but dumped on the coast as a result of a pre-historic shipwreck..........

To conclude -- the new evidence supports the suggestion that the erratic has not come from Mynydd Preseli, and that it is most likely from one of the Ordovician sills or dykes in NW Pembrokeshire.  The source might be a prominent outcrop such as Penbiri -- or the rock might have been entrained by overriding ice from one of the less significant exposures in the landscape.  As suggested in earlier posts, the entrainment "event" might have occurred during the Late Devensian glaciation -- but it is more likely to have been during the Anglian glaciation around 450,000 years ago.

Whatever its erratic history may be, the boulder demonstrates that the Irish Sea Glacier impinged upon the Gower coast, carrying erratics from the west and displacing local Welsh ice on at least one occasion.  This confirms the conclusions drawn by Prof Peter Kokelaar in his 2021 book on Gower.