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Monday, 25 January 2010

The Great Bluestone Hunt



This is the area where, I suspect, real progress is likely to be made in the coming years. The area shown in yellow is the approx area of the "Fishguard Volcanics" on the northern flank of Mynydd Preseli -- here, as I have indicated, there are complex extrusive rocks thrown out during Ordovician volcanic eruptions -- and some intrusives too. Typical rocks are rhyolites, breccias, ignimbrites, tuffs and welded tuffs -- and because some of the eruptions were in the sea, there are layers of detritus and pebble beds that look more like sedimentary rocks than igneous ones. The map is not very accurate -- there are certainly some ashes and other volcanics exposed in the coastal cliffs between Cwm-yr-Eglwys and Newport.

I wouldn't mind betting that some of the igneous fragments from the Stonehenge area will be matched up with rocks from these Fishguard Volcanic Series outcrops. Secrets out there, waiting to be discovered........

The latest geological revision by Ixer and Bevins




Photos: Top: Carnedd Meibion Owen. Middle: Pont Saeson ford. Lower: Pentre Ifan cromlech.


The interesting thing about this work is that it shifts attention to a large area of North Pembrokeshire, to the north of the Presely Hills, where there are many outcrops of flinty rhyolites and related igneous rocks. Some of the rhyolites are quite clean and coloured with beautiful blues and greens, with slate-like cleavage, and others are more massive, with very complex crystal structures. I have seen such rocks around Tycanol Woods, on Carnedd Meibion Owen, at Brynberian and Crosswell, and around the famous cromlech at Pentre Ifan. I wonder what the geologists will find next?


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From British Archaeology, Nov / Dec 2009

Important revision to Stonehenge bluestone theory

In the News pages of the Nov/Dec 2009 issue of British Archaeology, it is reported that new petrographical work by Rob Ixer (University of Leicester, Department of Geology) and Richard Bevins (National Museum of Wales) had suggested that some of the Stonehenge bluestones had not come from Pembrokeshire, but (in Ixer's words) from "a far wider and, as yet, unrecognised area or more likely areas". As the magazine was being printed, however, Bevins was out in the field, and found an apparent source for the rocks in question north of the Preselis. Ixer and Bevins have kindly written this interim note on this latest development.
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Stilpnomelane-bearing rhyolites/rhyolitic tuffs at Stonehenge are most probably from the Preseli Hills region
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Field and petrographical work continues on new Stonehenge lithics and on in situ material from areas around the Preseli Hills. This includes excavated material from the Avenue at Stonehenge, and rocks from undistinguished outcrops in the low ground north of Mynydd Preseli, close to Pont Saeson.

The former, as expected, conformed to the range of lithologies seen throughout Stonehenge. But the latter had surprising results, and has led to our radically modifying our proposal that many of the bluestones do not have a Preseli Hill origin, but have an unknown and possibly non-southern Welsh origin.

In thin section the Pont Saeson fine-grained acidic rocks show most of the features of our class of Stonehenge rocks, informally called "rhyolite with fabric", including a lensoidal fabric and the presence of stilpnomelane. Despite nearly a century of collecting and analysis, this is the first record of this mineral in rhyolitic rocks in south Wales. The only previous recorded occurrences of stilpnomelane in acidic rocks in Wales are from the Cregenen granophyre in the Cadair Idris area of southern Snowdonia, and in granophyric rocks of the St David’s Head Intrusion, in north-west Pembrokeshire.

Although not an exact match for the Stonehenge rocks, the Pont Saeson lithics strongly suggest that the "flinty rhyolite/rhyolite with fabric" found in the excavations at Stonehenge has an origin in the Preseli region, and that there is no longer a need to look further north in Wales for this important class of Stonehenge debitage.

The other and more abundant unusual rock-type (carrying distinctive titanite-albite inter-growths) from the Great Cursus area (but not so far identified at Stonehenge) is still unprovenanced, and its petrography has still yet to be matched with rocks from south Wales, or indeed from the rest of Wales.

An interim summary of where we now believe the Stonehenge bluestones come from, and incorporating these new data, is:

* Spotted and unspotted dolerites, the flinty rhyolite/rhyolite tuffs and possibly the basaltic tuffs have a Preseli origin, but a search for their associated source rocks must no longer be restricted to the prominent outcrops on the Preseli Hills
* The Altar stone Devonian sandstone – the largest bluestone – cannot be from the Preseli region
* The rare other sandstone orthostats comprising a Palaeozoic sandstone are also not from the Preseli Hills, but may be southern Welsh in origin
* The titanite-albite-bearing rhyolitic rocks have yet to be sourced, but it is now anticipated that they too will have come from the Preseli region; only detailed and dedicated collecting and petrography will be able to prove that.

Rob Ixer & Richard Bevins

The Pont Saeson connection




Been to have a look at Pont Saeson, between Brynberian and Crosswell in North Pembrokeshire -- whence Richard Bevins took his rock samples which have now been matched up with rock fragments from the Stonehenge area. The interesting thing about this area is that there are no upstanding tors or craggy hills at all -- the landscape is gently undulating, with a couple of deep river valleys which have within them smallish outcrops of rhyolite on the slopes. In one place, pictured above, there is a prominent "spur" or crag of rhyolite, not far from the Pontsaeson ford, which owes its prominence to stream action -- and maybe to subglacial fluvial action during one of the glacial episodes that affected the area. But even this spur is within the valley -- it does not project above the overall land surface.

It is clear that the entrainment of erratics into the base of an overriding glacier could have occurred here -- but I can see no reason whatsoever why any Neolithic tribesmen would ever wish to take a stone, or a collection of stones, from an innocuous and undistinguished location such as this. Here, it just does not make sense -- even though there may some attraction in the idea (put forward with great enthusiasm by HH Thomas and Richard Atkinson) that the Stonehenge bluestones were taken from "sacred hills" or prominent high points in the landscape -- like Carn Meini -- which might have been treated with reverence by passing traders.

Another thumbs-down for the crazy human transport theory........