Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Monday, 8 November 2010

Bluestone Rock Types -- 15 and counting

A short extract from "The Bluestone Enigma":

It was Herbert Thomas who speculated on a Preseli origin for the Stonehenge bluestones in 1908 and then went on to propose, in his famous 1921 lecture, that the bluestones were identical to rocks cropping out within one small area around Carn Meini. These bluestones, located within the bluestone circle and the bluestone horseshoe, comprise 43 stones, give or take a few.  The best estimate is that there are 27 spotted dolerites (including at least two that are broken in half), three unspotted dolerites, five altered volcanic ashes, five rhyolites (of which two are ignimbrites and two are lavas), two micaceous sandstones and a greenish sandstone called the Altar Stone.   The spotted dolerites are more variable than one might think;  some of them have obvious and large spots of whitish or pinkish feldspar, and others have spots that are almost too small to see with the naked eye.  The two micaceous sandstones and five volcanic ashes are just left as stumps, buried beneath the turf.

Most people believe that Herbert Thomas actually sampled the bluestones in the stone settings at Stonehenge.  He did not do that.  Instead, his work was based upon a visual examination of 34 in situ stones and an analysis of fragments and samples from assorted collections made by William Cunnington, Nevil Maskelyne and William Judd. His analytical method was called “standard transmitted light petrography” which involved a detailed examination of thin sections made from his samples.   He also looked at thin sections made from samples taken from the tors in the Preseli Hills, although it is unclear whether any of the samples were his own.  Again he seems to have depended largely upon samples collected by other geologists.   Some of the samples came from bluestone fragments found in the soil during excavations.  

So if we look at the main bluestones at Stonehenge (the standing stones or orthostats, and the stumps below ground) and ask how many localities they have come from, we immediately have problems.  This is partly to do with the variation in the dolerite and rhyolite groups -- we cannot assume that the spotted dolerites have all come from Carn Meini (it seems that Carn Goedog is a better bet if we want to look at a single locality) because there are many degrees of spottiness both within individual outcrops and within the Stonehenge bluestone assemblage.

The OU team that worked on the Stonehenge bluestones in the late 1980's concluded that out of the 39 samples they analysed, there were at least 3 distinct dolerite sources, and 7 different rhyolite sources.  Then there were also 2 micaceous sandstone sources and a number of volcanic ash sources.  That suggests about 15 sources in total -- and we are still counting.  Watch this space....... I'll put up another post shortly.

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