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Wednesday, 9 February 2011

More on Stonehenge Geology

The Fishguard Volcanics are shown with the inverted V symbol on the map

Grateful thanks to Rob Ixer for the info about this recently published paper, which again increases our knowledge of the Stonehenge monoliths (orthostats) and their places of origin. Material from six of the stones has been analysed in great detail with regard to their petrography (crystal structure etc). There are two groups of rock types -- which the authors refer to as "preselite" (including spotted and unspotted dolerites) and as "volcanic" (often referred to in the past as ashes, tuffs or rhyolites).

The 2 spotted dolerites are again confirmed as coming from the Carn Meini area -- which is pretty extensive, incorporating at least six tors or craggy outcrops). The other rocks seem to have come from the Pont Saeson area near Brynberian and from other locations (as yet undetermined) within the Fishguard Volcanic Group -- which is exposed at the surface across a large area north of the Preseli Hills, between Strumble head and Crymych.

So there are still many questions to be resolved -- and many "bluestone sources" to be determined. Remember that there are still another 30 or more bluestones at Stonehenge requiring detailed analysis.......

From this work and earlier papers from Rob Ixer and Richard Bevins it appears that there is not much of a link between the standing stones and the "debitage" (accumulation of flakes and fragments in the regolith) at Stonehenge. This might lead one to conclude that the preselite and volcanic monoliths have NOT been shaped or reduced in size to any great extent by the builders of Stonehenge -- and that the debris in the Stonehenge Layer and elsewhere in the soil might have come from other rocks which were either too small or too flaky and soft to use as monoliths, leading to their destruction or fragmentation. This is an interesting question...... on which the authors of this paper might have views!


Wilts Archaeology and nat Hist Mag 104 (2011), pp 1-14

The detailed petrography of six orthostats from the bluestone circle, Stonehenge

by Rob A. Ixer and Richard E. Bevins

Samples from six bluestone orthostats (SH34, 35a, 38, 40, 46 and 48) from the outer bluestone circle at Stonehenge are petrographically described. SH34 and 35a are spotted dolerites (so-called ‘preselite’) and belong to the most abundant class of Stonehenge preselites known as the SH33 group; their petrography is very similar to rock samples collected from Carn Menyn.

The other four orthostats are more siliceous and of volcanic origin with distinctive petrographies. SH38 and 40 are two, different, dacitic crystal-vitric-lithic ash-flow tuffs. SH46 and 48 are rhyolitic crystal-vitric-lithic ash-flow tuffs and, although petrographically similar, are probably not from the same source. The geographical origins for the four lithologies remain unknown, but outcrops within the Fishguard Volcanic Group exposed in the north Pembrokeshire area, between Strumble Head in the west and Crymyrch in the east, are the most likely.

Comparing the detailed petrographies of the four volcanic orthostats with the abundant ‘debitage’ from Stonehenge and its environs demonstrates that SH40, 46 and 48 lithologies do not correspond with any loose material and that only SH38 can be matched to a volumetrically very minor, but distinctive, class of ‘debitage’. Buried orthostat 32e, macroscopically described by Atkinson but not sampled by him, is recorded as being a ‘rhyolite’ and classed alongside the four standing siliceous orthostats.

Macroscopically it appears to share many characteristics with a major class of Stonehenge debitage, the so-called ‘rhyolite with fabric’, some, or all, of which may originate in the Pont Saeson area just north of the Preseli Hills.

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