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Tuesday, 6 April 2010

Stonehenge: Local Bluestones from the west

I am far from being an expert on the geology of Somerset and Wilts, but on looking at the geology maps I am encouraged to do a bit of speculation. Let's refer to everything at Stonehenge that is not sarsen stone as BLUESTONE, since that's what people have been doing for years. There are hundreds if not thousands of bluestones of all shapes and sizes at Stonehenge that could well have come from the west and north-west, within 80 km of Stonehenge, and which could well have been carried by ice, if it did indeed come in from the Bristol Channel in the direction(s) discussed in earlier posts. So which "local" rock types might we expect to be represented in the Stonehenge bluestone assemblage? Look at the map and key above.

For a start, we might expect erratics of Upper Greensand, Corallian, Portland beds (Upper Oolite), Middle and Lowe Oolite beds (including some limestones and some sandstones), Upper Triassic beds (marls, sandstones, conglomerates), Carboniferous Limestone (from the Mendips) and even Coal Measures (mostly sandstones and shales, with interspersed coal-bearing beds). There are also a few outcrops of Old Red Sandstone on the flanks of the Mendips, to the west of Bristol and along the North Devon Coast -- and a few small exposures of igneous rocks too. I have forgotten here about the clay beds, on the assumption that they would not have contributed large blocks and monoliths to an erratic load or a till cover (they would have been broken down for the most part into silt and clay fractions). In short, sandstones, gritstones, and limestones galore COULD have been entrained, transported and finally dumped by overriding ice from the west.

There are lots of all of these rocks at Stonehenge, but archaeologists have tended to assume (a) that they have been imported manually by the Stonehenge builders, and (b) that they have therefore come from quite close at hand -- ie from the Vale of Wardour or from the Eocene outcrops to the south of Salisbury Plain's chalk downlands.

Here is a question for the geologists: Have any of these "local bluestones" found at Stonehenge been definitely fixed to the rock outcrops to the south or south-west of Stonehenge, or could they equally well have come from the west or north-west? Chilmark, in the Vale of Wardour, is often mentioned as an Upper Greensand source for "Chilmark stone" -- but is that just wishful thinking on the part of geologists and archaeologists who had no great desire to look for sources further afield, or in another direction?


Ed said...

I really must take issue with the statement:

"Let's refer to everything at Stonehenge that is not sarsen stone as BLUESTONE"

When what we need is a clear scientific definition of what actually constitutes a "bluestone".

At this rate we'll be saying the new visitor centre is going to be built out of bluestone because it's not sarsen.


Brian said...

Just being devil's advocate, Ed! But there is a serious point too -- as I mention over and again in the book, the term "bluestone" is a curse placed upon Stonehenge studies, since it is used without thinking by almost everybody to flag up the "special" nature of the non-sarsens and to bolster the idea that they were selected and brought to Stonehenge as part of a grand project. The term itself has become a part of the spin. Profs Darvill and Wainwright -- and many others -- have made their careers on it. Wouldn't it be nice if we could just say that Stonehenge is made of lots of stones of various sizes and shapes, from many different sources, some near at hand and others further away? But we are stuck with this ridiculous separation between the sarsens and the others -- largely because of Atkinson.

How would we define "bluestone"? Anything that comes from Pembrokeshire? That's no good, since some of the bluestones appear to have come from somewhere else...... Stones that look blue? That's no good either, since very few of them look remotely blue, even when fresh...... Igneous rocks rather than sedimentaries? No good either, since sedimentaries (and maybe metamorphics too) seem to be part of the assemblage.....

Maybe we should ask the geologists. But I doubt we'll get anywhere, since Rob and others seem to be showing now just how complex and varied the "foreign" rocks on Salisbury Plain actually are.