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Monday, 5 April 2010

The Great Erratic Hunt

Wiltshire geology map and cross-section. Courtesy Wilts Geology Group. With a bit of luck, if you click on either image you should get an enlargement on your screen. If there are Jurassic or Lower Cretaceous erratics in the vicinity of Stonehenge, my theory is that they should have come from the west or north-west.

If glacier ice approached Stonehenge from the west, how far eastwards did it extend? One theory that I have put forward with Lionel Jackson (in EARTH magazine) is that the ice actually came quite close to Stonehenge, with a line of "bluestone" boulders extending eastwards from the Chalk Escarpment across the gently undulating chalk downs -- marking the position of the junction between Welsh ice (on the north side of the junction) and Irish Sea ice (on the south side). We argued that this line or "train" of erratics, like the Foothills Erratic Train in North America, would have been quite easy to follow -- and would have been known to the Neolithic tribes inhabiting the chalk downs. So they collected them up for their building project, at first close to Stonehenge and then successively further and further away, until either the stone supply ran out or they ran out of energy. I still think that is a reasonable theory, in terms of glacial theory -- especially if we think in terms of the modelled ice extent described in earlier posts on this blog.

But how likely is it that the stone collectors will have collected ALL of these erratics? Would there not have been some, deeply embedded in the ground, or maybe hidden in thick copses of trees or undergrowth, that they will have missed? Quite so -- I freely admit that it would be good to find some more erratics still in position to the west of Stonehenge. Well, we have Boles Barrow bluestone boulder, for a start -- but that was a "collected boulder" too -- and not in its original position.

Are we too preoccupied with spotted dolerite? I wonder what other stones there might be on the Plain, or in buildings or stone walls, to the west of Stonehenge? And maybe we should think much more about erratics derived from the rocks which outcrop to the west -- for example the limestones and sandstones of west Wiltshire, Somerset and the Mendip area?

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