February 15th - North Pembrokeshire Volcanics – provenance of the Stonehenge Bluestones by Dr. Richard Bevins, Keeper of Geology, National Museum of Wales, Cardiff
The Stonehenge ancient monument is constructed of Sarcens (locally sourced sedimentary rocks) and the so-called Bluestones, comprising principally intrusive, volcanic and pyroclastic rocks, along with minor sandstones. The origin of the Bluestones fuels emotive debate as to where they came from, how they were transported, and why these particular rocks came to be utilised.
Spotted dolerites form a main part of the Bluestones and the consensus is that they are derived from the Mynydd Preseli region in southwest Wales. However some bluestones are of rhyolitic composition and identifying their possible sources has attracted relatively little attention. This is largely because, unlike the coarser grained doleritic rocks, the rhyolites are ?ne-grained and lack any obvious distinctive characters, especially in hand specimen. However, recent work on the chemistry of the accessory mineral zircon in these rocks suggests that it can be used as a diagnostic provenancing tool. Using this approach, some of the rhyolitic Bluestones have been identified as coming from the Ordovician Fishguard Volcanic Group exposed in the Pont Saeson area of north Pembrokeshire. This identification will without doubt lead to fresh debates about the mechanisms of transport of this component of the bluestones to the Stonehenge site.
Further details on the WEGA website