Mynydd Preseli 'Bluestones'

A trip to Pembrokeshire presented the opportunity to revisit the Preseli hills and given the weather forecast a trip to Ty Canol wood would also feature.  Surprisingly the Preseli Hills are poorly represented in UK landscape photography imagery, their remoteness from major population centres and lack of car park views may be contributory factors.  There's a lot that can typred about the Preseli's; The geology and geomorphology are represented in landforms that illustrate a landscape evolving in geological time, plentiful archaeology and folklore, photographically the proximity to the Atlantic usually guarantees some photographic interest in the sky too. Failing all else the Preseli's are a Special Area of Conservation with some rare flora and that flora suggests a macro lens should be taken along.

Mynydd Preseli - April 2010

The Preseli hills have long been associated with the bluestones at Stonehenge, which is at least 135 miles as the crow flies distant and inevitably raises the questions Who, What, Where, When, How and Why, for the presence of bluestones at Stonehenge.  I'd lost touch with recent developments at Stonehenge and the bluestones, so a look at the English Heritage website, recent research papers and blogs brought me up to date with the current consensus on all things Stonehenge. What follows is an overview of recent developments for the bluestones at Stonehenge.

Who decided to use bluestones at Stonehenge is still unknown.

What the bluestones are, is a collective name for any orthostat or debitage at Stonehenge that isn't local to the Salisbury Plain i.e not a Sarsen stone or chalk.  The bluestones are not known to outcrop on or anywhere near to Salisbury Plain and are an assemblage of igneous, volcanic and sedimentary stones.  Geologists have categorised the bluestones (orthostats and debitage) into the following types

Devonian sandstone
Lower Paleozoic sandstone
Volcanic Tuff
Rhyolite
Dolerite

Some of the bluestone types have been further defined into sub groups based on their petrology, which for brevity I've omitted.

There are currently 43 known bluestone orthostats and a considerable spread of bluestone debitage at Stonehenge. The debitage is primarily attributed to human activities in working and destruction of the stones. The actual number of bluestones that once stood at Stonehenge remains unknown, but the consensus is that in the past there were more than 43.

Where the bluestones originated from, is currently the subject of an ongoing investigation by geologists, using petrographic and chemical analysis to correlate bluestone orthostats and debitage at Stonehenge with sampled outcrops in west Wales. This line of investigation has had some success and proved the provenance of some bluestones with a reported high degree of certainty to named outcrops on the Preseli hills and on the northern slope of the Preseli hills in the Afon Neyfer valley. Geologists pinpointed one specific location to an outcrop at Craig Rhos-y-felin and archaeological excavation revealed evidence for quarrying activity of an actual Stonehenge sized bluestone orthostat. However, there are a number of other bluestones that remain unsampled, so their provenance currently remains unknown, if not suspected.  Surprisingly no bluestones are currently provenanced to two of the more prominent landscape features on Mynydd Preseli - Carn Menyn and Foel Drygran. There are also a small number of stones which have no known provenance in the Preseli area viz the largest bluestone at ~6 Tonnes (Altar stone) and its attribution to Lower Devonian sandstone of the Senni formation, places it well outside the Preseli area.

Northern slopes of Mynydd Preseli - October 2011

When the bluestones were first used at Stonehenge is not definitively known, as archaeologists  suspect on the basis of excavation evidence, that some or all of the bluestones have been used in earlier phases of Stonehenge viz Q, R and the Aubrey Holes and in at least one other monument elsewhere in the wider landscape around Stonehenge, viz, the bluestone henge beside the River Avon. The latest date for bluestones incorporation into the present stone arrangement is ~ 2,200 BC.  Archaeologists have good reasons to suspect that bluestones were sited in the Aubrey holes, which have been dated to around 3,000 BC.

How the bluestones arrived at Stonehenge is a cause célèbre and tends to polarise opinions. There's a steady stream of alternative theories involving extra terrestrial aliens, UFO's, magic etc and more conventional theories. The theories tend to reflect modern perceptions and prejudices of Neolithic peoples; inventiveness; skills; technology; intellectual capacity; society; beliefs etc etc. Two of the more conventional theories promoted by Geologists and Archaeologists are; human agency and/or glacial ice.

Glacier terminus and debris

The random assemblage of diverse rock types at Stonehenge would be representative of material quarried over a wide region by glacial ice and subsequently transported. The geology of West Wales has suitably diverse bedrock with igneous, volcanic and sedimentary rocks, spanning from the late Precambrian to the Carboniferous, many of those rock types would be expected to be entrained in glacial ice, flowing across West Wales before finally melting out at the glacier terminus.  Glacial erratics deposited on Salisbury Plain would also conveniently explain their use at Stonehenge, i.e an exotic material and minimal effort needed to move. The theory for glacial ice transport to elsewhere in SW England, would mirror the aforementioned theory, albeit with a nod to increased effort and logistics needed in transporting the glacial bluestone erratics over a greater distance. Disapointingly there is zero geomorphological evidence on Salisbury Plain or elsewhere in the SW England for any other bluestone erratics and no known ingress of Irish Sea Ice across Salisbury plain or indeed large areas of the SW England. Geologists have surveyed and analysed fluvial gravels draining Salisbury Plain and have reported an absence of bluestone erratics, which is a tad worrying as glacial ice can carry a lot of material, from huge boulders to the smallest rock grains. The only known presence of bluestones is in the landscape proximal to Stonehenge.

A seeming absence of evidence for bluestone erratics, even pebble sized erratics in SW England might dampen speculation, alas no, and Brian John, www.brian-mountainman.blogspot.co.uk/ author and prolific blogger on the bluestones and a resident of Pembrokeshire, draws on his own insights and knowledge of glaciers and the Pembrokeshire landscape to promote the transportation of bluestones by glacial ice. A few years ago in Pembrokeshire, I picked up an almost mint condition used copy (never a good sign) in a second hand bookshop of John's vanity publication "The Bluestone Enigma" it made for enteraining reading, for all the wrong reasons. A critical review of John's work and the words; hyperbole; flawed logic, fallacious argument and speculative conjecture would be some of the kinder adjectives used.  I do suspect that the prospect of John's having to 'eat crow' on the glacier transport, will raise the stakes with more snarky comments on disparaging a human agency for bluestone transport and increasingly desperate blog posts on fantastical glacial ice streams.

Why bluestones were preferred by Neolithic people for use in various monument phases at Stonehenge and the surrounding landscape remains unknown. The logistics and effort expended to transport stone from the Preseli area, might imply there was a special significance of the Preseli area in Neolithic peoples psyche.

Mynydd Preseli tors - December 2006

My visit to Mynydd Preseli coincided with a forecast vigorous jet stream and leaden skies, rain and upland gales, so ambitions for a walk and photography were lowered to a short walk around Carn Siân examining the outcrops and I made sure the macro lens was in the rucksack.

Heather and Purple moor grass moorland,  Carn Arthur and a distant Carn Menyn - March 2015

An investigation of the damper bits of moorland revealed a single flower head of Spring Sedge (Carex caryophyllea) Bog Cotton (Eriophorum vaginatum) that was the sum total of floral interest, the novelty of traversing purple moor grass soon wore off and it was time to concentrate on the shelter of rock outcrops.

Eriophorum vaginatum flower head

Some horizontal rock surfaces were relatively free of colonising lichens/moss, a legacy of a recent wildfire perhaps and the phaneritic texture of the dolerite (micro gabbro) was revealed, not quite on the same scale as the the texture of gabbro, but characteristic nonetheless.

Weathered dolerite

Detail phaneritic texture

An interesting moss and lichen community; the lichen Graewe (Stereocaulon evolutum) and Dusky Rock Moss (Andreaea rothii) was growing on one outcrop on Craig Talfynnyd and confirmed the presence of acid rock, possibly volcanic rhyolitic tuffs.

Lichen and Moss on acidic rock outcrop

The remainder of the walk in deteriorating weather provided less inspiring photography opportunities and thoughts turned to the following days excursion to Coed Tŷ Canol.