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Saturday, 21 January 2012

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.

10 comments:

chris johnson said...

I am curious about this remark. Maybe it is well covered in the pay-for article, maybe not.

"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"

My own impression is that large erratics were left intact by stone-age people, in general.

Any other opinion?

Geo Cur said...

Chris , what I thought was interesting was the use of "alternative " rather than say "as well as " . At least there is some provenancing (something missing from the “Myth “ article ) , if they would only do the same for the RSC area . Just as we all know that there was long distance transport of rocks outwith glaciated areas , we also know that axes were transported over long distances too . Erratics may have been used as a source of material but simply because they were on the route of ice movement doesn’t necessarily means they were and is insufficient evidence alone .
As I have mentioned before erratics were sometimes engraved ,usually with simple cup marks but there are rare examples where the markings are much more ornate e.g . http://www.stone-circles.org.uk/stone/copthowe.htm

Dan said...

On the topic of erratics from Whin Sill, we have just had delivered to Bedfords Park Visitor Centre near Romford, Essex, an erratic traced to Whin Sill found in a quarry in Havering, just by the Thames, so quite some way further south than Stevenage.

I found your blog whilst researching info for the interpretation.

BRIAN JOHN said...

Very interesting, Dan. Within the glacial limit? Have you got a grid ref for the location of the find?

BRIAN JOHN said...

... and what are its dimensions? I'll do a post about it, if you'd like to send me some info and a pic...

Dan said...

Glad it's of interest Brian. The boulder was found at the Brett Lafarge Marks Warren quarry, approximate grid ref. TQ488896. It's approximately 0.9 tonnes. I believe it was south of the glacial limit, probably carried further by the Thames whe it flowed further north of its current course.

It's now at the Essex Wildlife Trust's Bedfords Park Visitor Centre at TQ520922. I'm awaiting further information from GeoEssex to finish the interpretation, and I don't have a photo of it yet, but I'll forward relevant info to you when I have it, a blog post about it would be great.

Don Aldiss said...

The Marks Warren quarry is only about 3 km south of the nearest remnant of the Anglian till, and about 10 m lower in altitude. Also, at Hornchurch, just 6.5 km to the east, the till occurs south of Marks Warren. So this Whin sill erratic might mark the local glacial limit

BRIAN JOHN said...

Thank you Don -- that's very interesting. Are you doing the interpretation for Geo Essex? It would be good to get a pic of this particular erratic...

Dan said...

Hi Brian,

Sorry, I've got some photos now, I'll email a couple through to you if you let me know your email address.

I'm doing the interpretation for Esex Wildlife Trust, but obviously depending on GeoEssex for the text, which hopefully will be forthcoming relatively soon, as we're getting lots of questions in the visitor centre about the boulder sat outside. Obviously realise there's a lot to investigate before anything goes to print though.

All being well we'll have some photos of the crystal structure too.

BRIAN JOHN said...

Great! Thanks Dan. You can reach me at brianjohn4atmac.com -- don't advertise it generally so as to avoid spam. OK if I stick it on the blog for others to see?