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Sunday, 26 April 2015

Stone shapes

Now for a bit of simple sedimentology.  The slight confusion over the description of rhyolite flakes in the Stonehenge debitage as "rounded" or as "more rounded than the less rounded ones" (just joking) reminded me that actually we don't need any confusion at all.  I have done posts on this topic before -- just put in "angular stones" or some such thing into the search box, or take a look at this post:

http://brian-mountainman.blogspot.co.uk/2010/04/more-on-stone-shapes.html

The literature is full of charts and diagrams of particle shapes -- ranging in size from sand grains to mighty boulders -- and the measurement of these shapes is still a favourite field trip exercise for school children and degree students.  In a river valley filled with terrace gravels or pebble beds, for example, you would expect the degree of roundness of the particles to increase steadily from the headwaters towards the sea.  You go to your first sampling site and collect your pebbles systematically and without bias, compare them visually against a chart (or measure much more carefully, using the Cailleux technique), make your records, and then go off and do the same thing at all your other sites.  Simples!

You can make as many particle shape categories as you like.  As a research student, I used just four -- I reckon six to be good, but more than 8 to be more trouble than it's worth.....  The chart below with 9 categories is by Krumbein, and the one with 6 categories is by Power,


You can portray your results in many different ways -- bar charts, pie charts etc.  Below is a typical representation of the results of a river valley pebble survey.  You can of course make things more sophisticated by taking samples of different stone sizes, and measuring the particle shape distributions for each stone size....... that can sometimes tell you something interesting.


So there we are.  No excuse for misunderstandings in the future.....




20 comments:

chris johnson said...

Useful as always, but in the interesting part of this discussion is to identify stones that have been modified by people to serve as tools. There are many classifications here and one learns to see the signs after working with experts in the field (literally).

On the other hand, if a majority of debitage is natural then maybe it was moved there naturally.

Personally I am inclined to accept the view of the experts who have handled and examined this material that the current shapes are largely the result of human interventions.

BRIAN JOHN said...

Yes, there is a whole new field of research into lithics and tools -- Rob will no doubt enlighten us!

Myris of Alexandria said...

By chance Dr Ixer had Dr Vin Davis (chair of the Implement Petrology Group aka the mad axers)
looking at the shape of the 'debitage' yesterday. ((Part of the subversive influence of this blog has made Dr I re-examine the volcanics A))
He (Dr Davis) looked at much of the 2008 assemblage (D and W) initially with Dr Ixer; he (Dr Davis) is an expert in axes both petrologically but as artefacts also in the lab and field and has no axe to grind boom boom.
He alongside all other artefact authorities pronounced the lithics as struck.
Nobody including the blessed Victorians has ever suggested the debitage is natural scatter.
I know Dr Ixer lays awake at night regretting the word rounded.
The pics etc Brian are lovely BUT they are for grains not rocks.
M

BRIAN JOHN said...

Thanks Myris. The Power roundness scale was I believe developed for sand grains, but the others are nothing to do with grains -- they are for pebbles and stones of all sizes. Exactly the same scale and terminology can be used for sand and gravel particles, pebbles, cobbles, boulders, mega-erratics and so on and so forth..... a shape is a shape is a shape.

Myris of Alexandria said...

Can you promise that is also true for Kostas?
M
Then volcanics A are rounded edged, lath-shaped tabular.
The internal fabric controls the final shape.
Think shale/slate litter.

Constantinos Ragazas said...

Ah Brian, you're up to your old tricks of filtering what others can be allowed to think! Let's see if the following revised post gets past your gate keeping!

Myris,

While showing Dr Davis the stones, couldn't you have sneak-snapped a photo or two for us too?

I accept some (many?) stone bits found at Stonehenge may be the result of stone working. Certainly that would be so, if Stonehenge was used as a Roman or Medieval? stone factory! As I believe it was.

But if there are some stone fragments that don't match any of the Stonehenge stones (as is likely the case with the Rhosyfelin rhyolite bits) and have the characteristics of being fluvial-glacial (according to your own description), shouldn't we take such evidence seriously and expand our thinking beyond the current narrative?

Dr Davis you say has characterized the volcanics A as being "struck". Such description does not help resolve the issue for me. Couldn't that happen naturally also? But if the stones were shaped on both sides as tools by people, I agree that would be relevant. So are all the stones Dr Davis examined "tools"? Even the gravel-size bits <10g ? I find that hard to believe.

There are other indications of fluvial activity here, besides the size and shape of these "stone bits". Look at some of the Atkinson photos of vertical sections of the Stonehenge Layer. You will see many bits of chalk and stone fragments uniformly and ubiquitously distributed top to bottom throughout the Layer. This cannot be accounted by any human activity! [...]

Kostas

BRIAN JOHN said...

Kostas -- letting this through the gate, but no apologies for stopping other comments -- and not just from you, if they are too personal or demonstrate irrational obsessions! Myris has NOT described Stonehenge "gravels" or stones as having fluvio-glacial characteristics. Stop putting up Aunt Sallys just in order to try and knock them down. It's a waste of everybody's time -- I can see more deletions of comments coming.

Myris said...

Just read the opening sentence.
The rhyolite flakes have never been described as rounded, it was the volcanics with sub-planar texture now called Volcanics A that are flattened and have softer edges than the rhyolites.
M

Evergreen said...

Myris, apologies if this has come up before and ive missed it, but does the debitage found suggest the stones were worked in a specific area or areas of the monument?

Myris of Alexandria said...

Too few data, much of the material through out the Stonehenge landscape is from disturbed contexts or from open contexts.
Read Pitts in I and B's paper on the Heel stone ditch (on line).
Other people talk about dressing zones north of Stonehenge circle.
I don't know it is a question for archaeologists.
M

Myris of Alexandria said...

The data for half of the circle were destroyed by Hawley and are buried in a number of holes, 1000s of uncontested bluestones in litho-death pits.
M

Evergreen said...

Thanks, I will search out that paper.

Kosta Dean said...

Myris, you write

"The rhyolite flakes have never been described as rounded, it was the volcanics with sub-planar texture now called Volcanics A that are flattened and have softer edges than the rhyolites." Photos please?

My apologies for assuming otherwise. I was mislead by your laconic comments. But won't look back for when years ago this occurred.

Question. Do all of the volcanics A trace to Stonehenge orthostats? And if there are volcanic debris at Stonehenge that do not trace to any orthostat, how did they get to be at Stonehenge? That is the key question for me. And whether these are Rhosyfelin rhyolites or volcanics A or any other "stone bits" that do not match any Stonehenge orthostat is not too relevant.

You write, "I don't know it is a question for archaeologists."

Really? There is no room for geologists and geomorphologists in such deliberation? I worry!

Kostas

Constantinos Ragazas said...

evergreen, you write

" does the debitage found suggest the stones were worked in a specific area or areas of the monument?"

This is a key question! One that I also raised often here.

If indeed the "stone bits" found at Stonehenge are from stone dressing, these should be found in very specific places, likely at the foot of the orthostats being dressed. And in distinct layers in the top soil.

The Atkinson photos of vertical cross-sections of the Stonehenge Layer show something very different! That can only be explained, imho, as fluvial. I like to know what distinguishes the Stonehenge Layer from the surrounding perimeter.

These are important questions that CAN be answered by a serious geomorphological study of the Stonehenge Layer. Why it hasn't continuous to be the greater mystery here!

Atkinson, I believe, did examine the Layer and placed great importance on it. I recall him saying so in a live interview on site with Walter Cronkite back in the 60's. But Atkinson's excavation reports have never been published!

Kostas

Evergreen said...

Kostas, I asked the question to perhaps get an indication of how the Bluestones were viewed by those who worked them. If it was in a very practical sense, and the resulting shaped stone was the point, then, I suppose, you might expect the debitage for individual stones to be found in one location - wherever they were worked. But if the stone itself had 'value', in whatever form (orthostat, chipping) wouldn't you imagine the debitage was perhaps collected up, moved around, even perhaps used for other reasons elsewhere?

BRIAN JOHN said...

Are you suggesting, Evergreen, that the debitage itself wasn't something rejected or thrown away, but had an intrinsic value or currency?

Evergreen said...

Yes, just a thought, something to consider perhaps?

Constantinos Ragazas said...

evergreen,

... and if that "debitage" is found buried deep in the topsoil, what purposeful "value" it would have? No good for even "souvenir collecting"!

Sorry! I don't buy it!

But you are missing the larger and more telling point in my last comment to you. Shouldn't we expect distinct layers of such "debitage", if human agency was involved in making them? That is NOT what the Atkinson photos are showing!

Kostas

Evergreen said...

Kostas, you're not buying, I'm not selling. It's just an idle thought, nothing to get too excited about.

Tell me if I'm wrong, but what you DO buy are suggestions that the struck flakes are not the result of human involvement, but arrived at the site via a natural process? (The site itself being the result of a thermal geological hotspot?) And to complement this, the SH ditch was created by meltwater and any deer antler found in it is due to long dead animals falling off the ice edge into the ditch? The SH avenue was formed by the sun being more intense along the line of the solstice, so there was 'greater melting' along that line?
It's hard to know where to begin with all of that, but as you're keen on retail terminology, my money is staying in my wallet.

No, I don't think we should necessarily expect distinct layers of debitage at SH.



Myris of Alexandria said...

Sometimes Truth and its seekers are stranger than fiction.
M
Evergreen so rustic, and May so near.
But so succinct.
M