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Sunday, 29 March 2015

Crescentic gouges at Rhosyfelin




In glacial geomorphology we tend to put a lot of emphasis on roche moutonnees, troughs and cirques on a large scale and on striations at a smaller scale, but we hear much less often about crescentic gouges or chatter-marks.  But they are very widespread, and every summer I marvel at the thousands of crescentic gouges which have been created on the streamlined rock surfaces of the Stockholm Archipelago.  Often striae and crescentic gouges are found on the same rock surfaces -- sometimes suggestive of different episodes of erosion nebeath deep streaming ice.

At Rhosyfelin, I have not seen anything you which I would -- with conviction -- refer to as glacial striations.  But there are some features which do look suspiciously like crescentic gouges (see photos above).  These are different from the percussion fractures that you might get on a rock surface which has been struck with a maul or hammer stone.  The process is similar but different, if that does not sound too much like a contradiction. It is now widely assumed that crescentic gouges are created beneath very thick ice where bedrock surfaces are subjected to heavy pressure and stress, and where sharp-edged tools are frozen into the bed of a glacier and are pressed down onto the rock.  The tool tends to be under compression, which makes it relatively strong, whereas in some circumstances the rock surface might be under tension, which makes it relatively weak.  Thus we might find that even if the tool is of the same rock type as the bedrock, compression damage might occur, and a crescentic fracture might be formed.  In some cases, the tool itself might be damaged or even broken into smaller fragments during this process -- but we seldom find the hard evidence for this, because this process is hardly ever observed in the real world.  (Glacier beds are difficult and dangerous places for scientific observations......)  Where a tool is made of a very hard rock type (for example dolerite) and a bedrock surface is relatively soft (made, for example, of foliated rhyolite) then the creation of crescentic gouges might be commonplace.

Of course, the rock surface being subjected to pressure need not actually be bedrock.  It might be a large flattish slab of bedrock embedded into the ground on the glacier bed.

Two generalisations are made in the literature:

1.  The thicker the ice, the greater the pressure and the longer and deeper the crescentic gouge might be.  (Note that the fracture is always perpendicular to the direction of ice flow.)

2.  The right conditions for the creation of crescentic gouges seem to occur beneath ice which is below the pressure melting point or very close to it.  In contrast, abrasion (and the creation of striations) seems to occur where basal ice conditions are wetter, with ice at or above the pressure melting point and where bed materials are more fluid and hence more mobile.

I need to make more observations on this.....





20 comments:

chris johnson said...

I assumed for some time that glaciers were present on Prescelly at different periods. The glaciers would therefore have been pressing on Rhosyfelin.

Am I missing something incredibly obvious?

BRIAN JOHN said...

This is just another piece of evidence to go with the discovery of till at the site, ice-moulded surfaces, erratics, fluvioglacial sediments and meltwater channels. There is no doubt in my mind that the Devensian ice of the Irish Sea Glacier crossed the site of Rhosyfelin and pressed against the northern face of Preseli. There are lots of other posts on this.

Myris of Alexandria said...

Rhyolite is more siliceous than dolerite hence it's greater quartz content baked it the harder rock, if we be judging by Moh.
M

BRIAN JOHN said...

Hardness seen as resistance to attack is always difficult to define! Maybe it's the dense foliations in the Rhosyfelin rock that makes it feel softer? It certainly rots easier than dolerite. Weathered dolerite surfaces are very hard -- almost encrusted with iron oxide etc. But weathered surfaces of rhyolite at Rhosyfelin are almost chalky or powdery -- and can easily be scratched.

TonyH said...

If the geomorphologists nowadays have coined the phrase "crescentic gouges" to describe some results of rock erosion, I wonder what prehistoric man made of these crescent shapes? I am the last person to be remotely New Age in persuasion/ distortion, but just maybe Neolithic Man onwards would have sometimes noticed these shapes like the crescent moon, and pondered thereon? Am I barking up the wrong rock, or just barking?

TonyH said...

Perhaps we should offer my last suggestion to the archaeologists? Who knows, they might, unwittingly, thereby accept and digest a little glacial geomorphological knowledge? Heaven knows, we might even get some kind of Sharing Of Knowledge between Geomorphology and Archaeology! "Pass me that Peace Pipe, could you Old Man? Splendid!"

Constantinos Ragazas said...

Chris, you write

" Am I missing something incredibly obvious?"

Yea! Glacier erratics!

Kostas

Constantinos Ragazas said...

Tony,

"Crescent moon"? Nah!

These are Neolithic proto-type hatchet heads etched on rock! At Stonehenge we have daggers (cloak optional).

Kostadinos

TonyH said...

Kostas

"Neolithic proto - type hatchet heads etched in rocks"

Who knows, maybe the Neolithic Preseli tribes had a "Hatchet Man". He would be something like the opposite of a Shaman, being not terribly spiritual, i.e. he would make Thatcherite hachet - man decisions, very unpopular with most of the Tribe, perhaps rather in the style of Mr MacGregor, who Maggie Thatcher brought over from the U.S.A. to devastate the U.K. coalmining industry.

Myris of Alexandria said...

Tony your red knickers are showing.
The deep mined coal industry was long
passed its usebydate.
For decades the correct price of coal was opencast coal prices.
Scargill was correct when he said British deep mined coal was the cheapest in Europe what he did not say was all deep mined coal was totally uneconomic.
McGregor did what needed to be done.
At a significant social cost.
M

TonyH said...

Myris, you misjudge me, I was concentrating upon the very real possibility that Neolithic folk DID have hatchet men, as Kostas has rightly put forward. Mr Mac, or Mc, - Gregor was merely a comparison figure. But, agreed, he was no hero of mine, and was a foreigner to boot. Of course, we were told, "There's no such thing as Society" by the lady whose hero was an ex - Hollywood Actor,in charge of a country to which we almost became the 52nd or whatever State, so we had to put up or shut up in the '80's.

I am, however, still waiting for South Wales' Sir Tom "The Voice" Jones to find Some One (it could be me, or it could be YOU! - especially with your specialist geological knowledge!) to write him a song, comparable to "The Green Green Grass of Home", about the decimation of the Welsh Coal Mining Industry (and beyond).

Myris of Alexandria said...

No we are both amateurs, it is uncle Phil who has direct knowledge.
I have voted commie in general elections, stood behind a barricade, of course not for decades now, not with these knees and back.
Ah the Belgrano Bitch she probably was a a good thing, I wrote some of my Ph.D by candle light due to those Welsh coal miners. Under the BB it would have been full uninterrupted electricity.
M

Phil Morgan said...

Just a thought on the killing of the 'deep mine' British coal mining industry which left millions of tonnes of recoverable coal untouched.
An acceptable and cheap method of sealing-off a mine was to place concrete plugs or 'caps' at the top of the shafts. It would then be left to nature, which doesn't like voids below ground, to crush the shafts and complete the closure of the mine.
However, in many cases the more expensive alternative was used where the shafts were filled with re-cycled material from the colliery waste tips. At first sight this may seem an unnecessary, time consuming and expensive operation.
However, if it was planned to recover the in situ coal at some future date, and if the delightful Thatcher had so decreed, then it is far easier, quicker, and cheaper, to empty the rubbbish from an existing shaft than it is to sink a new shaft.
I know not if this was the plan but I'll probably be long gone before it happens. I wonder what European country will supply the personnel to operate the reopened mines for there'll be no one left in the UK with the necessary education, qualifications or experience to comply with current mining legislation. Perhaps the mining laws will be repealed.

Back to sleep now.
Phil M.

Myris of Alexandria said...

I said uncle Phil would know.
I did not know about the back filling of the shafts.
Interesting thought, was there a long-term view on energy requirements, there must have been. Cannot think that green was seriously thought of then, before the Ozone layer loss was known and global warming was recognised.
Of course oil shale will save us for the next couple of decades but again not seriously considered then.
Of course the Turner of the Ixer and Turner Altar Stone paper is the number one British oil shale/fracking man. Has been banging on about it for decades and is finally listened to.
Nuclear Winter was all the rage in the BB times.
M

TonyH said...

Wow! We really livened up this Post on glaciological crescentic gouges from 30th March (the day Dave unnecessarily flew his toy helicopter from Buckingham Palace to Number Ten as a blatant Tory Publicity Spin), didn't we?

I'd still like someone to come up with a highly "coaltroversial" song for Tom The Voice which he can release during this Election period. Wonder if Neil or Glennis are up to the task?

BRIAN JOHN said...

Not sure how we got onto the politics and economics of deep mining. Suffice to say that the Tory assessments of what was economic and what was not were fraudulent to say the least. The value of economic assessments is dependent upon the parameters you feed into your formula -- and governments are very practised in the arts of quietly forgetting about the real costs of peripheral social damage and environmental damage, for example. As for Thatcher, the only good thing she did for Britain was to invent raspberry ripple ice cream when she was an industrial chemist.

Alex Gee said...

As we're on the subject of politics, I suppose the most appalling legacy she left us was the corruption of science and certain scientists.

I live in the Mendip karst area! It is quite obvious that fracking in this environment will result in the pollution of groundwater and Karstic aquifers.

Unfortunately the Fracking companies, In common with corrupt pro Thatcherite mining concerns, are in the fortunate position of being financially able, to meet the financial demands of a band of corrupt earth scientists whose scientific opinion is available to purchase.

Witness the appalling environmental destruction by mining concerns in Africa.

The only reason this occurred is because the mining concerns were able to find enough corrupt earth scientists to provide the scientific opinion they required for a price.

Alex Gee said...

As we're on the subject of politics, I suppose the most appalling legacy she left us was the corruption of science and certain scientists.

I live in the Mendip karst area! It is quite obvious that fracking in this environment will result in the pollution of groundwater and Karstic aquifers.

Unfortunately the Fracking companies, In common with corrupt pro Thatcherite mining concerns, are in the fortunate position of being financially able, to meet the financial demands of a band of corrupt earth scientists whose scientific opinion is available to purchase.

Witness the appalling environmental destruction by mining concerns in Africa.

The only reason this occurred is because the mining concerns were able to find enough corrupt earth scientists to provide the scientific opinion they required for a price.

Myris of Alexandria said...

The strata needed to extracting oil are Upper Carboniferous and younger,
The Mendips are Devonian and Lower Carb with a little P-T cover.
There is as much chance of oil exploration in the Mendips as "corrupt earth scientists" wielding power.
Love the concept too too Bondian, off to stroke my cat.
I worked for a short time at Rossing in Namibia, one of the largest holes in southern Africa,
Everyone benefited.
Gee take a deep breath read your words and blush at the extent of the crassness.
Outsiders really have no idea how science or the Groves work.
M

J. Keats Esq. said...

Alex Gee it's plain to see,
Your reasoning is lacking.
Don't be a fool, return to school,
And gain some facts on fracking.