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Monday, 8 November 2010

The Stonehenge bluestone sources -- 24 and still counting?


Igneous outcrop at Pontsaeson near Brynberian, one of the recently identified bluestone sources

To continue:

After counting the 15 or so bluestones and fragments so far mentioned, we have abundant packing stones and mauls that are often conveniently forgotten about -- but there are massive quantities of them, some as large as 60 kg in weight.  There isn't much geology on these -- but we know that there are small sarsens, and also Jurassic oolitic limestones and glauconitic sandstone, probably from the fringes of Salisbury plain.

At least 18 rock types, and still counting.

Then we have the famous Altar Stone, probably from the Senni Beds of Carmarthenshire or Powys, and other fragments of volcanic ashes (two distinct types), Preselite (a term which is almost as vague as "bluestone")  and sandstones.

What we still don't know, until this is sorted out by the geologists, is how much repetition of rock types there is in the listing above.  The provenancing of finds from the Stonehenge Layer at Stonehenge and from the other sites near the Cursus and at "Bluestonehenge" may ADD to the list above, or some identifications may replace earlier crude identifications made without the benefit of modern techniques.  Rob Ixer, Richard Bevins and others are working on this right now, and their discovery of rocks from Pont Saeson and other sites north of the Preseli Hills is highly intriguing.

My guess is that we are now up to 24 different rock types, and still counting.  I wouldn't mind a small bet that the total number of sources will reach 30 before long.  There is no reason at all, in an assemblage of glacial erratics or in an ancient and degraded till deposit, to find even more rock types from the north and west.

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