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Friday, 20 January 2012

The Age of the Landscape

Top map:  the area of North Pembrokeshire which has been most affected by glacial processes (thanks to Prof David Bowen)
Middle photo:  Part of the area of thick fluvuioglacial sands and gravels, between Newport and Cardigan (the two gravel pits are Trefigin and Pantmaenog)
Lower photo:  Part of the Gwaun-Jordanston meltwater channel system, at the western end of the Gwaun Valley.  The big channel at the bottom of the photo is the Nantybugail Channel -- it can also be picked out on the map.

There have been several comments recently on this blog about the age of the landscape and the age of land surfaces, with some contributors apparently rather keen on the idea that a few thousand years ago there are valleys where there were once hills, and that some low-lying areas have suddenly been uplifted.  Well, let's get one thing clear -- the landscape of Pembrokeshire (and this goes for most of the British landscape) is for the most part very old indeed.  I'm talking about a landscape which has not changed much over millions of years, apart from superficial vegetation change and the introduction of new features created by massive civil engineering projects (motorways, water supply reservoirs, ports and harbours, airports etc).

The only places where we have had dramatic changes are around the coast, where cliff falls, landslides etc triggered off by marine erosion (especially on high-energy coasts) can occur, and where high rates of sedimentation can change the outlines of embayments and estuaries.  In Pembrokeshire, for example, we can see how some Iron Age promontory forts have been largely whittled away by coastal erosion over the course of 2,000 years or less.  But right next door there may be a bit of coast that has incredible stability and longevity, where there is less exposure to wave attack.

We know from coastal  and valley sediment sequences that most of inland Pembrokeshire hasn't changed much for more than 2 million years.  In other words, the basic pattern of hills, valleys and plateau areas is the same now as it was then.  But in the north of the county (see map at top) glaciation has wrought major changes in both surface altitudes and in the arrangement of rivers and valleys.  The big meltwater channels to the south of Fishguard -- and including the Gwaun Valley -- appear to be NEW features created initially underneath the ice -- maybe during the Anglian Glaciation around 450,000 years ago.  The valleys (which are in places more than 100m deep)  have been modified during subsequent glaciations.  There have also been major changes in the drainage pattern in the Teifi and Nevern Valleys, and in the interfluves where meltwater channels have been created subglacially and by overflowing lakes.  Some old valleys have been abandoned, and filled with glacial deposits, and other new valleys have replaced them.  Glacial, fluvio-glacial and lake deposits are is some places hundreds of metres thick.  There are hummocky moraine deposits, kame terraces and eskers -- two of which are currently exploited for sands and gravels for the building industry.

How old are these landscapes of sedimentation?  For the most part, Late Devensian, which means c 20,000 years old.  So these are Pembrokeshire's "young" landscapes.  Further to the south, in South Pembrokeshire, we seem to be looking at landscapes which may be well over ten million years old.....

20 comments:

Robert John Langdon said...

Oh Brian are you sure!

"Well, let's get one thing clear -- the landscape of Pembrokeshire (and this goes for most of the British landscape) is for the most part very old indeed. I'm talking about a landscape which has not changed much over millions of years"

I think your observations are as much set in stone as your rock collection. This must be the most absurd statement you have ever uttered (a part from Stonehenge not being finished, of course).

The landscape is CONSTANTLY changing and evolving, surely this is the basis of the Science, Geology. If the landscape never changes - why bother studying it?

In the past million years you have had floods, ice, forests, more floods and more ice - of course this will change the landscape through isostaty, deposition, erosion and plate tectonics - but yet the landscape has not changed - please!!

You criticised Geo the other day for not being able to see that ice covered the RSC landscape in Scotland - you also seem unable to see the constant changes the landscape goes through in time - which may indicate why you have such difficulties with the progressive theories like mine;-)

RJL

BRIAN JOHN said...

Robert -- I'm a geomorphologist by training, and I'm used to seeing both very old and very new features in the landscape. I am not denying that in the areas affected by Pleistocene glaciations (including Pembrokeshire) there have been some quite dramatic changes -- some in the uplands (Cairngorms, Lake District, Snowdonia etc) and some in the Lowlands (East Anglia, East Midlands etc) where there are thick and complex sheets of till. That's not what I'm talking about. I'm talking about Pembrokeshire, and whatever the wonders of plate tectonics, isostatic and eustatic adustments etc may be, I stick to my point that for the most part the landscape is very old. The coastal platforms of South Pembrokeshire, for example, appear to be several million years old. Do you have any evidence that suggests otherwise?

Anonymous said...

Robert John Langdon

There's ONLY one thing wrong with your nameplate and your coloured photo which proclaims your "contributions" here: I completely endorse that we should know your FULL name...... but that fine, coloured photo should have VERTICAL BARS running down it, and your prison number should appear below.

Why? Because of your disservices to Sensible Science, that's all!!

BRIAN JOHN said...

I think, Robert, that those who read this blog will come to their own views as to which of us is entrapped or imprisoned within a fantasy world dressed up as "science".

Geo Cur said...

RJL , the mistake I made was a misreading of the Clark map I assumed that the wide open spaces without any legend meant that there was no movemnet of ice in that area 'I never suggested that there was no glaciation in the NE but "no marked glaciation " in the area around Old Keig .

Robert John Langdon said...

Brian

Do Wessex archaeology deal with so called 'fantasy worlds' as well?

www.wessexarch.co.uk/projects/marine/alsf/seabed_prehistory/computermodels.html

You will find the River Arun which currently lays beneath the English Channel - it shows it in the Mesolithic just 10,000 years ago - looks a lot different, probably like Pembrokeshire at the same time.

You will find Mesolithic boats as well - I wonder were they got that idea from?

RJL

Anonymous said...

Anon writes,

“ I completely endorse that we should know your FULL name”


Anon!

Anonymous said...

Brian you write,

“We know from coastal and valley sediment sequences that most of inland Pembrokeshire hasn't changed much for more than 2 million years.”


Can you please say more about how the age of a landscape is determined? Is it the age of the bedrock strata that is used to determine the age of the landscape? What “sediment sequences” show the age of the inland Pembrokeshire landscape? Tried to look this up on the internet but with no success!

Kostas

chris johnson said...

I wonder if we mean the same by "landscape"? I can readily accept brian's assertion that the geological landscape has not changed much, but one aspect of this debate is to understand the size and level of human activity in prescelly after the glaciers left and people arrived.

Had there been a major tool using culture around prescelly at that time we might expect there to be more traces. I am not concerned so much about the big changes in sea level, but more about the extent of,peat build up and depth of buried layers.

BRIAN JOHN said...

Anon -- I'm intrigued that my FULL name should suddenly become an important issue. No state secrets on this site -- it's Brian STEPHEN John. There now -- my breast swells with pride, and I feel just as important as Robert John Langdon.

BRIAN JOHN said...

Robert -- I don't have the slightest problem, and never have, with the idea that submerged landscapes exist off our coasts and have been modified substantially over time, as sea level has risen and fallen. As you know full well, there are plenty of posts on this blog dealing with exactly that subject.

BRIAN JOHN said...

Chris -- I agree that we might have different perceptions and scales here. Of course there have been changes in surface levels -- and in the uplands there are areas where several metres of peat have accumulated, raising the surface in the process. Also, soliflucion processes do inexorably move debris down steep slopes and tend to fill in valleys. Soils with organic matter in them will always accumulate over time -- even at Stonehenge -- cf the big earthworm debate!! But by and large, megalithic monuments in Pembrokeshire are set in very similar landscapes today to those in which they were created maybe 5,000 years ago. (But farming practices have actually changed a lot of the cosmetic detail....)

BRIAN JOHN said...

Kostas -- it's all about stratigraphy and about a picture that has been developed over centuries as more and more sections have been examined. If an estuary has a raised beach platform within it, with erratics on the platform and then a raised beach overlain by thick periglacial materials, with an Irish Sea till above that, and with fluvio-glacial materials above the till, we have a sediment sequence which tells us that the estuary has been there for a very long time. That's the situation in Pembrokeshire. Milford Haven and the Teifi Estuary have to be AT LEAST two glaciations and two interglacials old.

In South Pembrokeshire, if an old erosion platform has Oligocene clays and pebbles sitting on it, the platform has to be more than 20 million years old.

And so on and so on.....

See this one:
The Strata do not lie (27 Jan 2011)

Anonymous said...

Brian earlier mentioned either himself or RJL being 'entrapped or imprisoned within a fantasy world dressed up as "science"'

It's great that we are constantly reminded of the narcissistic Robert John Langdon's FULL name - he is obviously very important(to himself), and, as stated at 20.36 hrs on 20 January, his full name is essential knowledge to his Prison keepers.

We just lack his Prison number to complete all his useful details when he makes his "contributions" here.

Robert John Langdon said...

Brian

You missed the point.

The fact that the landscape that was once land and a river is now at the bottom of the sea just 10,000 years ago shows that the landscape is in a constant 'flux' and changes radically over time.

If I placed you a million years ago outside your house in Pembrokeshire you would be completely lost.

This lack of perception is shared by many other academics who can't adjust their visual senses to pick up clues of the lands historic past (its a well known fact that academics lack spatial awareness and that's why their IQ scores are lower than one would expect)

As an example - when your in a car on an old B-road and it takes a sharp corner, do you look around and wonder why the road leads the way it does? - and what influence the landscape has on the road builders (which was originally a cart track or foot path) or do you just accept the road was always here and going the most efficient way possible? i.e what was here once that made them have to turn the road or has the physical topology changed over the years?

RJL

BRIAN JOHN said...

!!!!! I couldn't possibly comment.... your progressive science is now so progressive that only you can possibly understand it.

Anonymous said...

Brian speaks of RJL's progressive science as being now "so progressive that only he [RJL] can possibly understand it".

I beg to differ! I have in my vinyl collection Rick Wakeman's {middle name not known] Concept Album, entitled "Criminal Record". Since Rick was a progressive rock keyboardist, perhaps Rick and Robert John have got a lot to teach one another science and rock.

Brian Cox

Anonymous said...

Thanks Brian, I appreciate your knowledge.

Of course all this discussion about the landscape at Pembrokeshire goes back to discussions we had on whether it was possible erratics from Preseli to have fallen from outcrops and transported on the surface of an ice sheet. Your unequivocal position was and is the terrain would not permit such isolated occurrences.

I have been looking at some photos of rock outcrops at Preseli in the web. Please check the following links to these and for the record tell the world what your expert opinion is for the entrainment of erratics from these Preseli outcrops in the manner I have suggested.

http://en.wikipedia.org/wiki/File:Carn_Menyn_bluestones_-_geograph.org.uk_-_1451509.jpg

http://www.themodernantiquarian.com/site/507/carn_meini.html

Kostas

BRIAN JOHN said...

Kostas -- of course there are little cliff-like features on these tors and crags -- but for large stones to fall onto an ice surface from these crags, the ice surface would have to be at about 340m. The land surface at Stonehenge is at 100m altitude -- so we have to look at a gradient of 240m over a distance (following the streamflow of the ice) of over 200 km. That's not far off a surface drop of 1m per km. That is so shallow that I cannot imagine a glacier being dynamic enough to maintain any forward momentum over the distance required. Even the shallowest surface profiles we know about (in Alberta) were a good deal steeper than that -- and a surface with this small a gradient would have been far too flat for any stones to have slid along on top of it!

BRIAN JOHN said...

Kostas -- comment dumped. Fed up with you trying to reinvent the laws of physics.