Breaking news: another igneous erratic (or several) on the Wiltshire chalklands

One of the lumps of granidiorite found in the West Kennet excavation.  From Josh Pollard's talk.

https://www.wiltshiremuseum.org.uk/?event=online-event-connections-avebury-and-orkney&event_date=2021-11-26&platform=hootsuite&utm_campaign=Castle&fbclid=IwAR0tfR0Wtlt_dIA086d9VTMkan7bHy9lgRCt-RbuT1c5ug3kD6t1SrZHFaw

According to some posts on Facebook, quite a few people have known about this for quite a while, but now Josh Pollard has formally announced the finding of erratic material (a lot of it) from "Structure Five" at West Kennet, not far from Avebury.  The material appears to be a very distinctive form of granidiorite from the eastern edge of Cheviot.  According to Josh, the geologists (Ixer and Bevins) have provenanced the rock type to Cunyan Crags, near Dunmore.  It's very crumbly and heavily weathered, and from the lumps of rock collected from various parts of the excavation, there are at least 130 kg of it -- and probably a lot more.  Did the lumps of rock all come from a single erratic, or could there have been several in the vicinity?   Josh -- of course -- assumes that the boulder or boulders might have been "direct or indirect" imports, but then he's an archaeologist who has an established preference for the human transport of large lumps of rock, and a reluctance to believe that ice is capable of carrying large stones over great distances and dumping them anywhere near Salisbury Plain or elsewhere in Wiltshire.   The suggestion in the talk is that these lumps of rock (found  mostly in post holes) are from a destroyed standing stone -- but there is currently no evidence to support that.  He does note that no lumps of the granidiorite have been found on the surface -- and he assumes that any that did exist have simply been weathered away.  Even the lumps that have been examined are more or less reduced to "grus" (the crumbly residue left when granitic rocks rot away) -- and that all suggests great age.

But we can rest assured that the southward transport of igneous material from the far north was not just possible but probable, during the Anglian and earlier glacial episodes.  In 1999 Olwen Williams-Thorpe and others described the occurrence of Whin sill quartz dolerite and many other far-travelled northern erratics in the glacial deposits of Hertfordshire and Buckinghamshire:

Note that this map from 1999 is now somewhat dated.  The outer ice limit as shown is very inaccurate; ice extended at least as far to the SW as the letter (a) on the map.

"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

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.

In other papers Olwen and her colleagues have also noted the presence of other "inconvenient" erratics in archaeological contexts:

The Geological Sources and Transport of the Bluestones of Stonehenge, Wiltshire, UK
Proceedings of the Prehistoric Society , Volume 57 , Issue 2 , 1991, pp. 103 - 157
DOI: https://doi.org/10.1017/S0079497X00004527

Published online by Cambridge University Press: 18 February 2014
Richard S. Thorpe , Olwen Williams-Thorpe , D. Graham Jenkins , J. S. Watson , R. A. Ixer and R. G. Thomas

From the Abstract:

‘Bluestone’ fragments are frequently reported on and near Salisbury Plain in archaeological literature, and include a wide range of rock types from monuments of widely differing types and dates, and pieces not directly associated with archaeological structures. Examination of prehistoric stone monuments in south Wales shows no preference for bluestones in this area. The monoliths at Stonehenge include some structurally poor rock types, now completely eroded above ground. We conclude that the builders of the bluestone structures at Stonehenge utilized a heterogeneous deposit of glacial boulders readily available on Salisbury Plain. Remaining erratics are now seen as small fragments sometimes incorporated in a variety of archaeological sites, while others were destroyed and removed in the 18th century. The bluestones were transported to Salisbury Plain from varied sources in south Wales by a glacier rather than human activity.

This map shows Cheviot and the directions of ice movement during the Late Devensian glacial episode:

Was it possible that the erratic(s) from Cheviot were carried by ice all the way (c 480 km) from Cheviot to West Kennet and Avebury?  I wouldn't rule anything out.  The Anglian ice edge is most frequently shown as lying some distance north of Salisbury Plain,  but clearly much more research is needed, particularly on some of the deposits conveniently lumped into the "clay-with-flints" category -- and the discovery of this new erratic occurrence keeps the debate bubbling along nicely.