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Tuesday, 9 June 2009

The Erratic Train

One of the key hypotheses which I have put forward, with various colleagues, is that the erratic boulders and pillars used at Stonehenge were carried from West and South Wales in a narrow band (not much more than 1 km wide) between two streams of ice, probably at the time of the GBG (Greatest British Glaciation). We are still not sure when this occurred. But it's pretty certain that when the ice of the Irish Sea Glacier was crossing Pembrokeshire and flowing broadly SE and then E up the Bristol Channel, it was flanked to the north by a stream of Welsh Ice which came from the Welsh uplands. So there were two ice streams running in parallel -- with the erratics entrained and carried along between them. The "train"of erratics is unlikely to have been very long -- since the erosion and entrainment may only have taken place when a particular set of glaciological conditions were met -- maybe for a couple of hundred years during a glacial episode that lasted many thousands of years. When the ice began to melt, the train of erratics was let down onto the land surface as a "line" rather than as a fan. This is just what happened with the Foothills Erratic Train in North America. We still do not know how close to Stonehenge the nearest erratics from this train were dumped -- they may have been some miles to the west. But once the builders of Stonehenge had discovered them, it was easy to find more, simply by following the train westwards. When the distance became too great, or maybe when the stones had all been collected up, that was the end of the enterprise. I still think that there were no more than 43 stones suitably large for using as monoliths (standing stones or lintels). Other smaller erratics might well have been used for the manufacture of hand axes, or as packing materials in the pits.

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