The strange case of the West Kennet granite debris

The glacial map published by Ixer, Bevins and Pirrie in their article.

Two articles of interest.

(1)  Pebbles in the pond: huge posts and some very odd rocks at West Kennet

by Mark Gillings, Ben Chan, Ros Clkeal, Stu Eve and Josh Pollard

British Archaeology, March/April 2022, pp 34-38

This is an article about the excavations at the West Kennet palisaded enclosures, including some information and speculation about the grus (decomposed granite) found at the site.  Quote:

"..........we began encountering lumps of friable stone that was clearly not local. Rob Ixer, Richard Bevins and their team were to tell us that this was “grus” – as they explain below, entirely out of place in north Wiltshire! Altogether, we found over 77 pieces of the grus (much had decayed to sand) weighing around 22kg – about the weight of a bag of cement. Most of the rock had been placed in the tops of former postholes, in post pipes or small recuts. A ring of 14 small, rounded fragments was set around a Beaker-period double burial cut into the top of one of the larger postholes; one came from the outer palisade. We have just a sample, and considerably greater quantities of the rock may be spread across the rest of the excavated monument. Not one has been found outside Structure 5.  Even if the grus was collected from glacial tills, bringing it closer to Wessex as Ixer and Bevins say is a possibility, people will have had to carry the stones over a significant distance during the latest Neolithic/ Chalcolithic."

Of more interest to us is the following article:

(2)  The West Kennet grus: how far?

by Rob Ixer, Richard Bevins and Duncan Pirrie

British Archaeology, March/April 2022, pp 39-41

This is a strange article, which I knew was coming, since I have provided quite a bit of information to the authors over the past few months on ice movement directions and ice limits.  They didn't show me the text before going to print, but that doesn't surprise me.  I expected a certain "spin" on glacial transport matters, and here it is.

I have no problems with the basic geology and the attempts to provenance the rock on the basis of its recorded characteristics. Techniques included quantitative petrography, whole-rock and individual mineral geochemistry, and radiometric dating.  Excellent work, as far as I can see.  The conclusion is that the grus, including small rotten cobbles and pebbles, has probably come from the Cheviot Granite, somewhere in an area of around 60 sq km.  They guess at Cunyan Crags as a possibility, largely because there are upstanding rocky outcrops there.  So far so good.  More work will follow. 

Then the authors get round to the question of how a very rotten erratic -- or maybe several -- travelled from Cheviot to West Kennet.  As they say, the material is unsuitable for standing stones, querns or axeheads and has not been recognised as filler added to clay in pots.  So could the material have been carried by ice?  They say:  "..... there is very little, if any, evidence to support direct transport by glaciers. West Kennet is 75km south of the generally accepted maximum Pleistocene glaciation limit. Indeed, no glacial erratics of any kind have been recorded from the Avebury area (Stonehenge is even further south and also lacks credible glacial erratics)."

This is where we part company.  There is in fact abundant evidence to support direct transport by glaciers. The "credible glacial erratics" which the authors choose to ignore are all over the Stonehenge landscape, in the form of fragments and stones of many lithologies, shapes and sizes, as identified long since by Olwen Williams-Thorpe and her colleagues. They are not abundant, but remember that only a very small percentage of the land surface has ever been investigated.  And the most obvious glacial erratics of all are staring the authors of this piece in the face at Stonehenge -- heavily weathered and abraded boulders of "bluestone" (again of different types and from different locations) in the bluestone circle which would not look out of place at the snout of any modern glacier.  These are NOT quarried and carefully selected monoliths, whatever Mike Parker Pearson may think.  This constitutes evidence -- whereas there is no evidence of any sort in support of the long-distance transport of a rotten granite boulder -- or maybe several -- over a distance of 75 kms, let alone 450 kms.  Nor is there any reason to think that a rotten boulder might have been extracted from glacial till on the Yorkshire coast -- or somewhere else -- and then carried to West Kennet. That's just another flight of fancy.

On the matter of ice directions and ice limits, the map reproduced by the authors of this article is just about right as a representation of recent thinking from Gibbard, Clark and others, but adjustments are being made all the time, on the Bristol Channel coasts and elsewhere.  There is still much debate about the extent of glaciation in the Mendips, Cotswolds and Thames Valley (in the latter the river terrace gravels have masked older enigmatic deposits that are only occasionally exposed through gravel working).  There are too many enigmatic boulders and "remanie deposits" to rule out glaciation well to the south of the line shown on the map, during the many glacial episodes that preceded the Anglian Glaciation which we are currently trying to understand.  

As the authors say, the rotten granite or grus is indeed an extraordinary and puzzling occurrence; but it defies all logic to assume that it was somehow "given a value" and collected up by our Neolithic ancestors and carried to West Kennet only to be scattered about and treated like inconvenient waste once it arrived.  That material has been there maybe for millions of years, and it must have been carried in by ice.  My money is on the finding of other "alien erratics" on the chalklands in the future, turning up when and where we least expect them.

 A rotten lump of granite from West Kennet.  Some of the debris is much more rotten than this......

Here is something on the Cheviot granite.  Cunyan Crags is on the SE flank of the intrusion, and is referred to as "marginal granidiorite"

https://www.geologynorth.uk/cheviot-hills/origin-and-structure-of-the-cheviot-igneous-complex/

=======================

PS.  We should explain that while most of the grus described and analysed in the literature is "granite grus", the word is also used for the weathering products of other coarse-grained crystalline rocks as well.  See this article:

Grus weathering mantles—problems of interpretation, 2002,  Piotr Migon , Michael F. Thomas

Catena 49 (2002) pp 5–24.

The authors describe grus associated with many different rock types, with regard to chemical composition, physical characteristics, downward migration of the "weathering front", occurrence of corestones and many other matters.