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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.....




Saturday, 25 April 2015

On pingos and circular enclosures


 The slightly raised embankment of the mysterious feature on the Preseli north slope.  Outside the circular feature there is dry heath, and inside, as we can see, is a soggy area with rushes.

Thanks to Hugh and Geo, we have homed in on a rather interesting circular embanked feature not far from Carn Goedog.  According to Dyfed Archaeological trust, its origins are mysterious, because there does not seem to be a ditch and bank arrangement (either outside in or inside out) and because the central enclosure is sunken and soggy.  When I read the site description I immediately thought "Pingo?" -- and this possibility is worth considering.

Here's the description of the Nant Cledlyn SSSI in Ceredigion:  "Nant Cledlyn Pingos SSSI is of national importance because it provides some of the best- preserved pingos in Wales. Each individual pingo comprises an elevated rampart that surrounds an internal basin that in some cases contains a significant thickness of peat. It is thought that the pingos were formed during very cold climate (permafrost) conditions that prevailed approximately 10,000 years ago, and that sediment deposition commenced at approximately the same time."

There are quite a few fossil or relict pingos in Wales, and they were properly described for the first time by Edward and Sybil Watson from Aberystwyth Univ. Geography Department -- a somewhat eccentric and wonderful couple who were inveterate field geomorphologists.  Here is their map from a 1974 paper in Geografiska Annaler:


The Ceredigion examples are highly degraded, and sometimes all we can see today are slightly curved sections of embankments or ramparts.  Here is another example from Thompson Common in Norfolk:


These are very subtle features in the landscape, dating for the most part from the Younger Dryas around 10,000 years ago, and it's very easy to confuse them with ring cairns or other man-made features.  But geomorphologists have now found them all over the British Isles, and they are indicative of very severe permafrost conditions where peculiar hydrological conditions apply.  When I was in East Greenland in 1962 some of my colleagues were studying a group of pingos in Pingodal which were in various stages of disrepair.  They are ephemeral, and may last for a few decades or centuries.  They have a core of ice -- an ice lens -- which tends to grow each year as more water arrives by subsurface flow (beneath the permafrost layer) and then freezes onto the lens.  They can grow to 30m in height, and in Siberia and Alaska they are very spectacular features of the tundra landscape,  as shown in the photos below:



They are also called "hydrolaccoliths" and "pseudo-volcanoes" -- the latter name is quite descriptive, because when the ice lens expands to the point where the surface sediment is too thin to provide effective insulation, it all starts to melt.  The surface subsides, bits of the lens are exposed, and a "crater" begins to develop. This fills with meltwater, and that accelerated the melting of the lens until it is entirely wasted away, leaving a circular depression with an irregular rampart around its edge.

We should use the term "pingo" for something which is active, as in the Arctic examples above, and the term "fossil pingo" or "relict pingo" for the sort of thing we see in Wales.

There are two types of pingo -- open system and closed system -- but we won't go into the technicalities here.  The most spectacular examples tend to be located on wide open plains or gravelly river valley floors, but they are also found in undulating or sloping terrain.

So could the Preseli example be a pingo?  Quite possibly --  there are some small relict pingos on Trerhos Common, to the west of Wolfscastle, and others on the south side of Preseli.  More to the point, the SSSI citation for Mynydd Preseli mentions relict pingos on Brynberian Moor, Gors Fawr and Waun Isaf -- and this is the type of territory we are talking about here.

Watch this space....... I suspect that the more one looks for ancient pingos, the more one will find.

Wednesday, 22 April 2015

Llangolman gravel pit -- Anglian fluvio-glacial gravels?


Prof Danny McCarroll examining one of the erratics in the Llangolman gravel pit, adjacent to St Colman's Church


It was good to take another look at this site today, in the company of farmer Huw Absalom and Prof Danny McCarroll and his wife Louise.   It was almost too hot!  And the faces of the gravel pit were so dry that it was quite difficult to pick out the stratigraphy.

But in our discussions Danny agreed that we were probably looking at Anglian fluvio-glacial materials, maybe 450,000 years old.  He agreed with me that the degree of staining by manganese and iron oxide is indicative of great age, as is the extent of pebble "rotting" which is far greater than in the Devensian gravels seen in other pits in the Cardigan - Moylgrove area.

We examined the fossil frost cracks in the upper gravel horizons, which suggested the presence of permafrost during and maybe after a long period of gravel deposition.  Another thing Danny noticed is that one of these frost cracks seems to have a sand layer running across its top edge -- suggesting that this sand layer might have been deposited in a subaerial environment following an episode of ground freezing -- making it a suitable candidate for OSL dating. That's something we need to work on.

On my previous visit I thought that the gravel spread in the vicinity of St Colman's Church was a remnant of a previously much more extensive cover.  But  now I think I agree with Danny that this is probably a mound of fluvio-glacial material which we might call a kame.  It has been eroded away in some places by spring sapping, but it is clearly in a hilltop position, and it is probable that it owes its origin to an accumulation of meltwater debris carried into an open pit in a dead-ice environment.

More Rhosyfelin geomorphology


Two developments today:

1.  I received a letter from a geologist with whom I exchange occasional correspondence -- and he tells me that he has been in touch with the National Museum of Wales on the subject of Rhosyfelin and the archaeological interpretations of the features there.  He says he was surprised, at a recent lecture by Richard Bevins, to hear such a senior geologist  "indulging in propagating the MPP meta-narrative, riddled as it is with speculative interpretations."  He also complained about the manner in which the archaeologists have "progressively destroyed" the geological / sedimentary evidence that might have given solid information to anybody with geomorphological training about the Holocene history of this site.  He made some very cogent points about the fracture plane -- which I look forward to discussing with him -- and finally expressed his great surprise that "MPP has been allowed to wreak havoc in a section of the National Park, effectively reconfiguring the topography by what can only be regarded as an engineering activity."  So there we are then.......  I am clearly not alone.

2.  I spent a pleasant couple of hours at the site today in the company of Prof Danny McCarroll of Swansea University and his wife Louise, giving the place a good going over.  They are both geomorphologists with great field experience, and suffice to say that like other geomorphologists who have visited the site they could see NOTHING indicative of any human intervention either on the rock face or in the area dug over and left in such a mess.  We looked at the so-called railway tracks, wedges and props, scratches made by heavy stones being dragged along, shattered slabs damaged by the transit of orthostats, and so on.  Their response was the same as mine:  "Is this some sort of a joke?"  Enough said.  If Danny and Louise read this and want to add anything, they will be very welcome.

One thing Danny did suggest -- and I am in agreement with him on this -- is a visit from the specialists who belong to the Quaternary Research Association if the dig site can be left open for inspection at the end of the Sept 2015 excavation season.  The Association has a very big membership of Quaternary specialists from the UK and further afield, and if 15 - 20 of them arguing things out on site cannot sort out the truth, nothing will!  So I will get a request into the system.

Wednesday, 15 April 2015

The Floss Stone at Garn Turne


This photo shows the Floss Stone in the foreground, slap in the middle of the forecourt or portal area as designated by earlier archaeologists.  The green grassy area between the Floss Stone and the "giant capstone" is where most of the digging was concentrated in 2011 and 2012.  Naturally enough, not much attention has been paid to the Floss Stone in the past, because the "giant capstone" is the one featured in all the books!  We would probably agree that the huge "capstone" has not been moved, but there has been some excavation beneath it.  As I understand it, Vicky Cummings and Colin Richards think that the Floss Stone may have originally been closer to its massive relative at the "focal point" of the forecourt, where the horns come together, but that it slid or fell over before landing in its present position.

Hope I've got that right.......

Tuesday, 14 April 2015

Back to Garn Turne



 Above: one of the natural rock outcrops of Garn Turne, about 50m from the earthfast dolmen.  Below:  View directly towards the SW, with Lion Rock (Poll Carn) in the distance, beyond Trefgarn Gorge.  The "entrance" to the burial chamber faces the camera -- away from the impressive view across the landscape.  The builders of the tomb probably did not move this gigantic stone at all -- it was used exactly as it was found.

Been to have a look at Garn Turne today.  It's about 2 km from Wolfscastle and just over 1 km from Sealyham Bridge, in the middle part of Pembrokeshire.  There is some doubt about whether the Devensian ice reached this far, but the current feeling is that the ice limit lay a little distance to the west, near Wolfscastle and the northern edge of the Trefgarn Gorge.

The site is a very beautiful one -- with a small series of crags in a rough paddock which is surrounded by green cultivated fields.  The outcrop -- or series of outcrops -- called Garn Turne is made of the volcanic rocks of the Sealyham Volcanic Formation, intruded into or resting above dark grey silty shales and mudstones of Arenig (Ordovician) age.  The rocks are older than the Fishguard Volcanic Formation -- green-grey in colour and with a rough knobbly surface texture.  The rocks are classified as andesite and dacite lava and hyaloclastite, with some interbedded tuffaceous mudstones.

The crags consist of a series of vast detached and tumbledown blocks resting on bedrock outcrops with small cliff faces.  Right across the paddock there are hundreds of other boulders and detached blocks, many of which are in approximately their "original" positions and others which have been moved downslope by slope processes over hundreds of thousands of years and possibly by Anglian ice action as well.

The famous earthfast dolmen excavated by Colin Richards and others a couple of years ago is just a huge slab of bedrock weighing more than 60 tonnes with other stones moved into position around it.  Colin thought originally that the big "capstone" (if that is the right word) was quarried from the rock outcrops 50 - 100m upslope and moved to its lower position by the cromlech builders; but he seems now to have abandoned that idea.  I agree that it was used where it was found, with a burial "chamber" simply excavated beneath it.  The forecourt faces NE -- for the simple reason that that was the natural direction in which the stone "faced" when it was embedded in the ground.

The smaller stones do not appear to have been shaped at all by the men who placed them into position-- they have sub-rounded edges but rough and irregular outlines.  Some of them have probably been moved a short distance by ice -- so they have again been used more or less where found.

Geologists Unearth Fully Intact Rock



Thanks to Geo for the heads-up on this one.  Great report!   Should be read by all archaeologists who do not already subscribe to THE ONION........


Geologists Unearth Fully Intact Rock
News in Brief • Nature • environment • Science & Technology • science • ISSUE 51•13 • Apr 3, 2015

FORT COLLINS, CO—Describing the discovery as the most flawless specimen ever unearthed, a team of geologists working in northern Colorado announced Friday they had excavated a fully intact rock. “This stunning find provides an illuminating glimpse into what rocks may have looked like in their complete form millions of years ago,” said lead geologist James Powell, adding that the extremely well-preserved rock offers unprecedented insight into the physical structure, shape, and characteristics of early rocks in ways that incomplete stone fragments and shards never could. “Previously, reconstructing a whole rock from small remnant pieces was difficult, and we never had a complete specimen. We’d invariably end up with several missing parts, and we could only speculate about what might have filled them. But now, we can safely say that these empty spaces were most likely filled with other bits of rock. It looks nothing like we could possibly imagine.” Powell added that further research was necessary to determine if a geological link existed between rocks, pebbles, and boulders.