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Tuesday, 15 January 2013

Those old valleys

I found this map in an old paper by Eyles and McCabe (1989) and have tidied it up a bit for clarity.  But it's rather interesting, showing the "tunnel valleys" in and around the Irish Sea, Cardigan Bay and St George's Channel.  These are the old bedrock channels, often deeply cut to well beneath sea level  -- Milford Haven, Wexford Harbour and Waterford Harbour could also have been shown with the aid of arrows going "with the trend".

What we see here is a pattern of drainage which is essentially dendritic, and which has its origins far back into the Tertiary and maybe even earlier.  The key components of the uplands and lowlands have thus been in place for many millions of years, so that when the Pleistocene glaciations occurred the "grain" of the country, and the alignment of these old drainage routes, have strong influences on the patterns of both glacial action and fluvio-glacial channel cutting.  Much meltwater was directed into these old valleys -- as we can see in the case of the Gwaun-Jordanston valley system, where subglacial channels have been deeply cut into the floors of older shallower valleys.  The remnants of these old valleys can still be seen as "shoulders" high above valley floors.  In some cases the positions of interfluves have been moved, and in other cases (as with Cwnm Gwaun) they have simply been lowered, so that we now have humped long profiles on channel floors. (This is a good diagnostic feature for a sub-glacial meltwater channel.....)

As I argued many years ago, and as now seems to be acceptd by the new generation of Geomorphologists, these channels are MUCH older than the Devensian.  they have been used over and again, someimes by meltwater flowing subglacially and sometimes by water flowing subaerially -- in spillways from the glacial lakes of the area.

Is all of this relevant for our understanding of Stonehenge?  Not really, but there are implications here for our understanding of landscape evolution at Rhosyfelin.

Some of the subglacial meltwater channels near Fishguard -- the big one near the base of the photo is the Nant-y-Bugail Channel, so deep that it was used in WW2 for the creation of long tunnels cut into the valley sides for the storage of mines and other ordnance.  These valleys have been superimposed onto a landscape of much older valleys, orientated in more or less the same direction.


12 comments:

TonyH said...

Changing the geographical subject rather dramatically, but staying on the subject of Welsh glaciation, archaeologist Francis Pryor got himself into an argument with his chief geophysicist on Time Team's 2nd episode of the new Series, shown this Sunday/ Monday.

Beneath the turf at the Caerau hillfort near Ely, Cardiff,Pryor was disputing with his colleague whether they had reached signs of human habitation.

Francis insisted what had been revealed were the scrapes through the clay of glacial movement, and ordered the digger-driver to go deeper with his machine. Francis Pryor is, of course, frequently ridiculed for his alacrity in pronouncing that dicoveries "could well be ritual".

The Caerau hillfort has been mooted as the site of the long-lost capital of South Wales, within the territory of the Iron Age Silures tribe.

Constantinos Ragazas said...

Brian,

I find these 'sub-glacial meltwater channels' very intriguing. Here we have meltwater streams under the glacial surface running possibly in the opposite direction to the flow of surface meltwater streams, or even the glacier flow.

Of course, such a phenomenon can occur at any time when there are ice sheets!

Can we postulate that the most pronounced superficial landscape features (like earth mounts, channels and embankments) occurred during the most recent glaciation?

Kostas

Constantinos Ragazas said...

Tony,

I am a little confused here. Was Francis Pryor (the archeologist) arguing “what had been revealed were the scrapes through the clay of glacial movement” or was his geophysicist colleague arguing this?

I have argued for a long time the human remains often found buried in such landscapes as Salisbury Plain were carried and deposited there from elsewhere by natural agency (meltwater streams, glaciers, etc.) rather then "could well be ritual". That would explain the skeletal dismemberment, broken bones and sculls, jumbled buried positions, random group burials along with animal bones, etc.

Kostas

TonyH said...

Francis Pryor, the lead archaeologist, was the one making the claims on behalf of glacial movement through heavy clay. The geophysicist (another "old-timer" on the Show) was expecting to find the proof of human activity that his equipment appeared to be showing on the print-outs; Pryor was refuting his expectation.

Perhaps there are some south Walians reading this blog who have a contribution on his TV show?

Personally, I thought it was nice to see an old-school, Old Etonian educated archaeologist like Francis recognising glacial features in his trench! Perhaps the tide, metaphorically, is turning.......Whatever next?!

Constantinos Ragazas said...

Tony,

An archeologist arguing with a geophysicist in favor of glaciers? I cynically doubt. In the totally scripted scenario of a TV show nothing happens unscripted.

What we have here is a sinister ploy to gain credibility with the wider audience. Easier to argue against glaciers if you can first argue for them. Doing this with a geophysicist seemingly adds 'intellectual courage'. Sorely lacking among archeologists (and others).

If Francis Pryor really thinks what he argues, why doesn't he submit a paper to a peer-reviewed journal?

Kostas

TonyH said...

Kostas

I agree with you to a point, particularly as regards TV shows not being unscripted. In this Show we have an ex - actor, Tony Robinson, being the link man between the audience and the professionals on-site.

For what it's worth, I think Honorary Professor, Francis Pryor, probably possesses a lot of intellectual integrity. He said he recognised the scrapings inflicted by glaciation from other excavations he's led. And he's no youngster, out to make a name for himself on the issues surrounding our particilar Blogsite. This excavation was quite a distance from Preseli, being on the edge of Cardiff in south-EAST Wales, where glacial events were, of course, not quite the same as those Brian relates further west.

Francis is genial and avuncular, and has written one of the best popular books on British Archaeology, "Britain B.C."(2003).He has spent well over 30 years investigating the prehistory of the fens of Eastern England.

BRIAN JOHN said...

Thanks for the comments, guys. Scrapings on clay? Evidence of glaccial action? That all sounds a bit strange to me. You don't normally get striae or grooves on the surface of till -- I would like to see the evidence!

Anonymous said...

Yes, indeed, Brian, this all Sounded a trite glacially unique to me too!!

They do seem to like to have these pre-arranged (?) professional disagreements between the 'experts' on Time Team these days. Of course, they've also just re-jigged their format, and Prof. Mick Aston is no longer the Grand Old Hairy Man of proceedings (more's the pity).

For those who want to know more, I suggest they contact Time Team, they have lots of publicity and contacts. They also visited the National Museum of Wales in the Show.

BRIAN JOHN said...

Yes, I do have a few problems with archaeologists trotting out things like "periglacial stripes" or "glacial scrapes" without ever expaining what the processes involved might have been. Sounds very learned, I suppose, from the point of view of a TV audience .... and that appears to matter more than scientific accuracy. Why don't these guys occasionally go off and consult a proper geomorphologist?

Constantinos Ragazas said...

Tony,

Not knowing Francis Pryor myself, I accept all that you know and say about him. I was only speaking in generalities. As I often do in this blog. But often, details hide obvious truths.

My apologies if I caused offense.

Kostas

TonyH said...

No problems, Kostas. I was surprised as you and Brian are when I heard Francis trot this out about evidence of glaciation in the thick clay. And his programme sidekick, the geophysicist, seemed equally non-plussed. I will see if there is a down-load available of the programme from Channel 4/ Time Team, as I,ve "wiped" my recording off my Library. Is there no-one from Cardiff/ South Wales area who also heard this programme?

My main point in mentioning this was, at least they're thinking about glaciation on this TV show, however incorrectly!

TonyH said...

Here are a couple of websites/ blogsites for those who are intrepid enough to want to Know More......

http://www.channel4.com/programmes/time-team/articles/caerau-hillfort

The above one claims to give access to the programme, but I hit a snag (I suppose you may well be able to get the programme via Channel 4's i-player for a week.).

Then I came across:-

pryorfrancis/wordpress.com/2013/01/12/time-team-the-20th-series-second-episode-rural-cardiff/

Social Geographers may find this interesting, as he writes about the hill fort's position adjacent and above closely-packed housing, and compares it to Arthur's Seat in Edinburgh.