In the middle of our recent discussions about erratic clusters, I was reminded of this rather interesting paper by Olwen Williams-Thorpe and others from 1999:
"Geochemical provenancing of igneous glacial erratics from Southern Britain, and implications for prehistoric stone implement distributions" by Olwen Williams-Thorpe, Don Aldiss, Ian J. Rigby, Richard S. Thorpe, 22 FEB 1999, Geoarchaeology, Volume 14, Issue 3, pages 209–246, March 1999
The main point of the paper was the link between erratics and stone implement distributions, on the assumption that Neolithic and even later people would instinctively have made use of whatever handy erratic material they found lying around in their neighbourhoods. Leaving that on one side, this is another excellent example of the increasingly accurate provenancing that can now be done -- this time using geochemical methods. The "cluster" of four Whil Sill erratics found near Stevenage, about 360 km from their source area and close to the southern limit of glaciation, is intriguing. (The box on the map shows the area examined by the authors -- and from which they collected 16 erratics for detailed examination.)
Of course, far-travelled erratics are not at all uncommon -- one of the most common and distinctive erratic types found on Pembrokeshire beaches is Ailsa Craig microgranite, which has come from the Firth of Clyde, near the original source area of the Irish Sea Glacier.
In the article, the authors discuss the possibility of "selective" erosion of the Whin Sill by overriding ice and the possibility of a zig-zag transport route over several glacial episodes. It's an interesting discussion, which obviously has a bearing on our debate with respect to Craig Rhosyfelin and Stonehenge.