Phil's Giant Erratic

 

Erratic hunter Phil Holden on the very rough and rocky foreshore close to the site of the Mumbles giant erratic. He's a photographer by profession, and he lives not far away from the find site.  He wanders along this coast very frequently, and says that he looked at the erratic several times, on a number of visits, before he realised that it was very different indeed from the Carboniferous Limestone bedrock.  He was attracted by both the colouring of the rock, and its texture,  and realised that it was very similar to a dolerite rock sample he collected many years ago from Foel Eryr, near the western end of Mynydd Preseli.  He took some high definition photos, and asked me to take a look.  I was fairly convinced that he was right in assuming the greenish rock to be unspotted dolerite.  He also asked Dr Katie Preece, an igneous rock specialist working at the Swansea University Geography Dept, to take a look, and on the basis of visual inspection she confirmed the rock as dolerite or micro-gabbro.  It was similar to the Foel Eryr sample, but the crystal sizes were different.  Whether that proves significant, time will tell.    Further work is in the pipeline, so watch this space.......

Last Sunday afternoon there was a low tide, and the weather was dry and calm -- so I hopped into the car and drove over to The Mumbles at high speed.  Phil and I took a good look at the giant erratic and at some of the nearby coastal exposures.  I became even more convinced that the erratic was made of unspotted greenish dolerite, and we compared the boulder surface with a number of unspotted dolerite samples that I just happened to have in my rucksack.  In the photo below the one on the right (from a boulder in my own garden!) looks remarkably similar.  In close-up (sorry about the lack of definition) my garden sample looks a bit greener, and the giant erratic looks a bit bluer -- but that may be a trick of the light.

But I'm not getting over-excited.  This is just the start of a long journey, and clearly samples from this boulder will need to be subjected to a whole range of analytical techniques before we can be sure what the provenance really is.

From the unspotted dolerite studies published thus far by Bevins, Ixer and Pearce, it seems that there are at least four different types of unspotted dolerite in eastern Preseli alone -- almost certainly far more if we include western Preseli and the extensive area between the upland ridge and the north coast.  Every intrusion, sill or dyke might have its own geological signature.  There's a lot of it about -- see my posts on the Preseli tors for further info.

https://brian-mountainman.blogspot.com/2021/01/the-tors-of-carningli-and-mynydd-dinas.html

https://brian-mountainman.blogspot.com/2016/12/the-preseli-tors.html

Bear in mind, however, that in the hunt for entrainment sites we should not just concentrate on the tors.  As my friend Dave Sugden and others have shown, tors are sometimes "preferentially protected" during glacial episodes, with erosion and entrainment processes concentrated instead on fractured and damaged rock outcrops in valleys and on hillsides.

Key papers:

"Carn Goedog is the likely major source of Stonehenge doleritic bluestones: evidence based on compatible element geochemistry and Principal Component Analysis"
Journal of Archaeological Science
Richard E. Bevins, Rob A. Ixer, Nick J.G. Pearce
Journal of Archaeological Science, 19 November 2013
http://www.sciencedirect.com/science/article/pii/S0305440313003956#FCANote

Revisiting the provenance of the Stonehenge bluestones: Refining the provenance of the Group 2 non-spotted dolerites using rare earth element geochemistry
Bevins, RE, Pearce, NJG and Ixer, RA
Jnl of Archaeological Science: Reports Vol 38, Aug 2021, No 103083.
https://www.sciencedirect.com/science/article/pii/S2352409X21002959?via%3Dihub

Unfortunately both of these papers are behind paywalls, and the authors have not so far made them available on Researchgate or any other "sharing" website. 

The most accessible paper that has a bearing on the possible origins of  the Mumbles boulder is this one:

http://publications.thebritishacademy.ac.uk/pubs/proc/files/77p133.pdf?fbclid=IwAR2Z9gMUTDgt-nM-4TBFvGpqF8aApC5WZ8AJUv3HWkGNLaikWV4_XDR3y0E

Geochemistry, sources and transport of the Stonehenge bluestones, by Olwen Williams-Thorpe and Richard Thorpe.  Proc British Academy 77 (1992), pp 133-161

They present chemical analyses of the spotted and unspotted dolerites sampled at Stonehenge, and also the Boles Barrow bluestone.