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Wednesday, 2 November 2016

Strange things near Devil's Den...


Click to enlarge these photos -- they are very high quality.

Many thanks to Neil and Pete for these three photos -- just three out of a very big sequence from the area around the Vale of Pewsey, where sarsens are still abundant.  Clearly many big blocks have been cleared away, but the thing that interests Neil is the "sarsen drift" as he calls it -- made up of a great concentration of sarsens "sloshed" up against a hillside.  I just love that description........  but the top and middle photos show the context beautifully.  I'm not sure about the "ridgeline" which in some places seems to mark a steepening of the slope, and in others not.  Maybe Neil or Pete can explain further?

Can they also explain the labelling in Photo 3?

Neil says he is at a loss to explain the physical process that might have led to the situation shown particularly well in Photo 2.  I too am a bit mystified.  The accumulation of sarsens does seem to have a ridge-like or embanked form,  and although we know that some sarsen or duricrust formations seem tohave coincided with very ancient stream-bed situations, this does not seem to be the right place for such a block or slab that might later have been broken up over many millions of years.  Neither does the context look right for an accumulation of sarsens that have simply slid downslope, maybe under periglacial conditions during the Ice Age.  If you were to show Photo 2 to me and say it was a photo taken somewhere in Wales, I would immediately think "trim line" and "moraine"..........

This deserves careful thought.  What do others think?

Postscript

More images, courtesy Chris Heaton and Andrew Smith.




 These give a slightly different perspective, suggesting that the dense litter of sarsen stones lies on the base of a long gradual slope, with the lowest point in this asymmetrical valley lying where a slight path can be seen, at the foot of the chalk scarp.  There doesn't seem to be any mound or ridge in these two photos.  Needs to be investigated on the ground......









14 comments:

Neil Wiseman said...

Hi Guys,

Firstly, the shots are above the Pewsey Vale, not in it, in the neighborhood of Devil's Den. Pete will have the precise location. The Pollisoir Stone shows that this area is well recorded.

The yellow diagrammed 'Ridgeline' defines where the rise is in that shot, as foreshortening of the camera makes this less obvious. It's just a rough line used as visual aid.

The thin scatter of stone seen prior to concentrations along this rise seem to show graduated deposition, though I think it's safe to say that these areas have been picked through for bigger stuff over the millennia.
If ice is the reason, the concentrations would be a termination, with the scatter released from suspension as the glacier melted/receded.

The third photo (probably should have been four) shows that the stone also occurs in a different context than in the first two shots. This is where they appear to have accumulated in lower — in this case grooved — areas. Obvious is that the big stuff has been cleared, while some had been pushed aside in the past to allow for agriculture. But enough remains to show the collection of material.

I offer ancient ice for these peculiarities — much further in the past than the more recent ones. But I'm no glaciologist, so will certainly entertain other explanations.

The notation of 'Abandoned Rock' refers to the big stone that's been moved up-slope — probably, though not certainly, by human agency. There are 3 others to be seen embedded in the turf.

Again, I say: Ice, basically because, by all appearances, it looks like this material has been acted on by force. The only thing I come up with which fits the bill is glaciation because some of the rock rests on top of other rock, rather than the chalk base. This tends to negate the original sand/silicate having been deposited by gravity in those low areas.

Neil

BRIAN JOHN said...

Interesting info, Neil. Many thanks. Will re-name the post to get the location right......

BRIAN JOHN said...

Can't find this location on Google Earth -- can you give us a grid ref please?

Neil Wiseman said...

It's in the heart of the Fyfield Down

Grid Reference SU12837150

PeteG said...

start with this
https://www.amazon.co.uk/Landscape-Plotted-Pieced-Archaeology-Antiquaries/dp/0854312765

PeteG said...

the 3rd photo is Lockeridge Dean stone drift Not Fyfield

PeteG said...

the Pollisoir stones is marked wrong, it is closer to the bottom of the photo.

PeteG said...

photo 2 Abandonded Rock is known as the Monster stone. The Old Enclosure is where a pheasant enclosure stood untill a few years ago.

Neil Wiseman said...

Hi Brian,

In both of the new shots you provide, the now-beloved "Abandoned Rock" can be seen on the slope to the right.

Neil

Neil Wiseman said...

Pete is, of course, correct about my mis-labelling the Pollisoir Stone.
My focus in getting the pictures was of the sarsen deposits and didn't know at the time (Sept 2013) what many of the close-order features were called.
To be honest, if the dear boy had walked me through the hundreds of shots he provided there's little doubt we'd still be sitting there! But he shot so many, so frequently, that it's actually possible to track the aircraft's path through the landscape. On the whole, a fantastic documentary.

"Scarp" is a much better word than 'Ridge' in this case.

I knew the 3rd photo wasn't Fyfield, but didn't know it was Lockeridge Dene.

Neil

Timothy Daw said...

Chalk is weirdly English

Chalk forms a distinctive landscape called downland that is the quintessential English green rolling landscape. Chalk is very homogeneous and so is a little like a blank slate on which other processes can act. Southern England was never glaciated, but was near the edge of the ice during glacial periods. It is therefore a good place to recognise periglacial landforms. One example is valleys within downland that don’t contain rivers (called ‘bottoms’) which are sometimes asymmetrical, with a shallow side and a steep side. The shallow side is usual the sunny side (South-facing) where more vigorous freeze-thaw broke-up the chalk and flattened the slope compared with the darker side...

http://all-geo.org/erratics/2011/05/chalk-is-weird/

TonyH said...

Brian, do you not have an old fellow - geomorphologist friend/ colleague who knows a thing or two about Quaternary Studies, based at the relatively nearby University of Reading? I wonder whether he has an opinion on the landform features Neil and Phil are drawing to our attention.

It is also interesting to realise that Mike Pitts, editor of British Archaeology, lives in Marlborough. He was formerly Curator of the Alexander Keiller Museum at Avebury, and we know what a good analytical mind he has.

EMAILS FOR MIKE PITTS:-

editor@archaeologyUK.org

diggingdeeper.co.uk

BRIAN JOHN said...

Tim -- thanks for that link. What a weird web site! Chalk is not particularly English -- but yes, the processes that operate on chalk are interesting. The asymmetrical valleys are intriguing, and are difficult in some cases to understand. In this case the scarp does not face north, but NE. More research needed......

Tony -- ah yes, my old mate Peter Worsley. Not sure how much work he has done on chalk. Might drop him a line......

TonyH said...

EMAILS FOR MIKE PITT

Whoops!

2nd one should be:-

mike@diggingdeeper.co.uk