A Whin Sill erratic cluster in the Stevenage area?

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

http://onlinelibrary.wiley.com/doi/10.1002/%28SICI%291520-6548%28199903%2914:3%3C209::AID-GEA1%3E3.0.CO;2-7/abstract

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.


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Abstract

Sixteen basic and intermediate composition igneous glacial erratics from Anglian (pre-423,000 years) deposits in Hertfordshire and Buckinghamshire, southern Britain, were selected for chemical and petrographic analysis in order to determine their original source outcrops. Major and trace element compositions suggest that seven samples (plus two uncertain) originated in the Lower Carboniferous volcanics of the Scottish Midland Valley (SMV), four came from the Upper Carboniferous quartz dolerite association which crops out in Scotland, northern England (Whin Sill) and extends to Norway, and one came from the northern England Cleveland Dyke. One sample of altered dolerite is ambiguous but has some similarity to the Old Red Sandstone (Devonian) age lavas of the SMV, and one meta-basalt sample may be from southwest Scotland or Scandinavia. These results identify specific outcrops which provided glacial erratics within currently accepted ice trails in the United Kingdom, and provide the first supporting evidence based on geochemistry, rather than petrography, for these ice movements. The distribution and provenance of glacial erratics are of importance in archaeological studies, because erratics provided a potential source of raw material for stone implement production. There is a marked geographical correlation between the distribution of prehistoric stone implements of quartz dolerite in the United Kingdom, and directions of ice movements from quartz dolerite outcrops within Britain. This correlation lends support to the hypothesis that prehistoric man made extensive use of glacial erratics for implement manufacture, as an alternative to quarrying at outcrops and subsequent long-distance trade.