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Friday, 7 January 2011

The Glaciation of Devon and Cornwall

There has been an interesting debate on this topic over the years, with some support in the 1940's and 1950's for glacial activity on the higher parts of the SW Peninsula, then a period in which glaciation went out of fashion, with most Pleistocene landscape change put down to periglacial and fluvial processes, and then within the last few years a growing recognition of the role played in many parts of the world by thin, cold-based glaciers in protecting the landscape at times when there may have been intense glacial streaming and erosion in the lowlands and in glaciated troughs.  Interestingly enough, the authors of the Geological Conservation Review for SW England (1998) dismissed the idea of glaciation in the SW out of hand, while getting quite tangled up in the debates about the giant erratics on the coast, and the glacial deposits found in pockets along the Bristol Channel coasts of Devon, Somerset and Cornwall.  Not entirely logical, you might say -- and I might agree.

Stephan Harrison, in the book called "The Glaciations of Wales and Adjacent Areas" (2005) takes a much more nuanced approach, and argues for intermittent small, thin ice caps over Dartmoor, Exmoor and other uplands -- and he cites a good deal of evidence which is difficult to interpret except by reference to glacier ice.  More recently the modelling exercises undertaken as part of the BRITICE project has also led the research team to the view that there WERE thin ice caps over the uplands of Dartmoor, Exmoor, Preseli and the Yorkshire Moors at the peak of the Devensian Glaciation, around 20,000 years ago.

If you look at the maps above you will see that there is --  and was -- considerable high ground across the SW Peninsula, capable of supporting extensive caps of snow and firn for long periods, and even true glacier ice during the coldest and snowiest parts of the Devensian.  For the most part, the ice will have been largely immobile and frozen to its bed, but there may have been some movement (and the creation of till and even morainic features) in the deeper valleys on E- and NE-facing slopes.

I postulate that c 20,000 years there were thin ice caps on all of the "high plateax" and "high hills" areas shown on the top map.  Lower plateax and hills might well have had a more or less continuous cover of snow and firn patches, which occasionally melted out during warmer episodes.  In the lowlands and valleys there would have been less extensive snow cover, but still permafrost -- so periglacial processes would have been active.

Wherever the edge of the Devensian Irish Sea Glacier might have been (somewhere in the Bristol Channel,  maybe quite close to the coastline) the intensity of glaciation in the SW Peninsula might not have been great enough to depress the land surface sufficiently for iceberg transport of erratics in the manner seen during earlier glacial episodes.

27 comments:

Robert Langdon said...

Don't get me wrong Brian, I do enjoy seeing all this lovely icey water spread over my 'ground zero'.

But every time I see maps (yours and other geologists)about the Devensian Glaciation, it makes me smile, because like my archaeological colleagues, you use maps of today.

In 20000 BC at the peak of the last Ice Age water levels (as you keep telling me!) was 150m lower than today - The map of Britain would look completely different!

With all my years of research not one DETAILED map has been produced shown the true contours of the pre-British continent.

Which 'begs the question':

How can you accurately trace the ice cap movement over the landscape that you have no model or knowledge?

Yes the peaks are as now (but 150m higher)- but during the time we are considering, we were joined to France as far down as the Channel Islands (a part from a large river), scilly Isles existed as a mountain and Ireland was a high plateau of Britain that went way into the Atlantic.

Show me the maps!!! - prove to me that Geologists are true scientists and not a rock collecting club.

RJL

BRIAN JOHN said...

Maps are aids to understanding -- and if we are to understand where we are and what we are talking about, it's actually rather useful to put in the position of the present coast and to use that as an aid to understanding. Everybody knows that the coastline has been in many different positions over time, depending on the relative movements of land and sea.

You cannot expect any geologist or geomorphologist to produce an accurate map of past shorelines, since most of the evidence you crave is actually bebneath present sea-level. that doesn't prove that geologists are stupid -- just prudent!

Of course we all know that the Channel was largely dry at certain times, and that there was just a narrow gulf running up across the Celtic Sea towards St George's Channel. There have been plenty of reconstructions of those low sea-level shorelines. There is no reason at all why ice movements cannot be reconstructed or modelled across large areas now above sea-level and large areas now beneath sea-level. If "ground truthing" works for the models, then they are probably OK -- if the field evidence does NOT match, you have to re-run your models and adapt some of your parameters. Perfectly normal, sound science.

Constantinos Ragazas said...

Robert and Brian,

You both seem to agree that the shoreline was some 150m lower 20,000 year ago than now. What I have hard time accepting is that the sea level rose by 150m mainly through the melting of the glacier ice. Wont a rise of sea level by 150m in SW UK also result in a rise of 150m of the sea level all around the world? So I tend to favor the idea that such rise of the seacoast is probably due to isostatic rebound, where the NE coastline of the UK rose while the SW coastline sunk.

Robert, how can the water inundation of Salisbury Plain remain 'watery' during 1500 years of freezing temperatures? Everyway we look at the evidence, I see ICE!

Constantinos Ragazas

BRIAN JOHN said...

Kostas -- have a look at my post for 7 June 2010:
http://brian-mountainman.blogspot.com/2010/06/recent-sea-level-changes.html

You will see there that the eustatic sea-level curve (with a lowest Devensian sea level of around -135m at c 20,000 BP) is based not on European or North American data but from tropical and sub-tropical areas with stable coasts, where there was no glacial isostasy going on which might confuse the picture.

I have a number of posts in which I try to unravel the isostatic effects around the British coasts in particular. What we have to do is try and unravel the absolute (ie global) sea-levels from those that are RELATIVE -- which is why many authors use the term RSL (relative sea-level) when discussing coastal phenomena.

Robert's vast inundation is a wild fantasy unsupported by any field evidence like sediments, shorelines, vegetation sequences, or radiocarbon dated organic materials.

Constantinos Ragazas said...

Thanks, Brian! Your June 7 post is excellent. That does help explain that mystery! I am intrigued with the idea, however, that the land in the north rose while that in the south sunk. I assume this is relative to the British Isles. Would this create an overall gradient to the geomorphology of the land tilting it southwesterly ever so slightly?

Kostas

BRIAN JOHN said...

Not sure of what you are getting at here, Kostas. the isostatic uplift of northern Britain was a consequence of recovery following deep loading or depression. In the south, as I explained in a number of posts, some people think there has been a sinking in compensation -- but others think that the gradual sinking of the English Channel region is a longer-term tectonic effect related to crustal movements linked to the building of the Alps. As I have explained, the apparent sinking of the south of the UK may owe more to theory than to observable fact.....

Robert Langdon said...

Your Quote:

'Robert's vast inundation is a wild fantasy unsupported by any field evidence like sediments, shorelines, vegetation sequences, or radiocarbon dated organic materials.'

Is incorrect as:

Field evidence from 'Crustal uplift in southern England; evidence from a river terrace records' Geomorthology 33 (2000) 167-181. D. Maddy et al.

Radio carbon dating from HAR-455: 9130+-180 BP) Pinus charcoal from Posthole A at height of T7 from above.

Supports my hypothesis.

RJL

BRIAN JOHN said...

Those articles do nothing whatsoever to support your hypothesis, Robert. Maddy's work on river terrace development relates to a long period of river terrace development and river downcutting. The C14 dates from the car park post holes simply show that there was Mesolithic occupation of the landscape near Stonehenge -- that is no surprise to anybody. What has any of that got to do with deep inundation of the landscape by the sea? Show me some shoreline deposits please .......

Constantinos Ragazas said...

Brian/Robert,

I have been puzzling about these post holes you have been referencing for awhile. Are these cut into the chalk bedrock? If so, do they have a definite shape? Are there pictures of these? How big are they? Are these post holes bowel-like, or cylindrical in shape? Do they align linearly or in a circle? And what size posts are they structurally able to support, and for what purpose? Also, what organic material found in them has been C14 dated? And could this material (though Mesolithic) been deposited there naturally from elsewhere, like from higher elevations like Preseli?

There are many questions concerning these, and the data I feel is open to interpretation, depending on what theory you support. What are your thoughts?

Constantinos

BRIAN JOHN said...

Re Mesolithic post holes -- all I know is that there were at least 4 pine posts, presumably locally cut, placed vertically in "deep pits". Not sure whether these pits were just in the regolith / soil layer, or whether they penetrated into the underlying chalk. There are at least 5 radiocarbon dates -- all from charcoal (which means that the occupants were making fire) and which range from 9130 BP to 8090 BP. As with all C14 dates, there may be some calibration problems, but the ballpark figures will not be too far off -- everybody seems to agree that they are all from the early Mesolithic.

The downland at the time was covered with open hazel and pine woodland with some man-made clearings -- probably cleared by burning. There is nothing to link either the post-holes or the posts -- or indeed the inhabitants -- with the Preseli Hills or anywhere else.

Constantinos Ragazas said...

Thanks for your thoughts on this, Brian.

There are certainly many questions and many ways of looking at the data. We should keep an open mind about all these, and I do!

Robert Langdon said...

Your Shoreline deposits can be found on bore-holes:

SU14SW25, SU14SW79, SU14SW76, SU14SW56, SU14SW71

which are on the other side of the road from Stonehenge bottom, parallel to the A303 - all holes report in various thicknesses - "Soft, gravely, slightly sandy clay. Sand is fine to course; gravel is angular to sub-round, fine to course weak consisting of chalk and flint, sand is fine with occasional roots"

This was found at 0.8m - 1.2m from the surface - ie recent history !!!

Remind you of anything Brian?

Give you a clue from the BGS glossary: "Sandy gravel, fine grained sand and peat or silty clay"

Is called 'Alluvium' and its found all over Stonehenge Bottom, its produced in streams and rivers, and from the bore-hole data we have found that this river was over 500m wide.

Game set and match young Brian - publish that if you dare!

RJL

BRIAN JOHN said...

Game, set and match, Robert? Oh, come off it.

Since when is alluvium a shoreline deposit? Alluviation is well known all over the Downs and in every river valley you can think of. The alluvial layers in the Downland valleys relate to episodes of quiet river flow, when smaller rather than larger particles were being transported. Sometimes, yes, there were flooding episodes -- as in any river valley. The dates of alluviation phases are highly variable, and the evidence points to land clearance and increased runoff and sediment transport as playing key roles in the process. The silty loams / alluviums contain pretty good pollen and molluscan evidence consistent with Neolithic / Bronze Age woodland / clearance / grazing / agricultural activities.

Not a trace of an inundated landscape anywhere.

Anonymous said...

Neolithic pottery at Durrington Walls is partially tempered with freshwater bivalve debris!! Delta C13 values.
(Not certain this is common knowledge yet)
GCU In two minds

Constantinos Ragazas said...

Brian,

This is an interesting debate going on between you, Robert, me and occasionally others. It will make your blog 'the place' to go to have a meaningful discussion on Stonehenge.

In a nutshell:

1) Robert argues that Salisbury Plain was inundated with meltwater when Stonehenge was made.

2) I argue that Salisbury Plain was a frozen meltwater lake when Stonehenge was made.

3) While you argue that Salisbury Plain remained for the most part dry, much as it is today.

Of the three theories, mine alone is able to explain all the 'facts on the ground' and all the enigmas of Stonehenge.

Constantinos

BRIAN JOHN said...

Ah, your confidence does you credit, Kostas!! as you might expect, I don't agree with you. Your theory doesn't explain "all the facts on the ground" at all. In fact, it is directly contradicted by the evidence collected by archaeologists, geologists, palaeobotanists and many other workers over many years. The evidence tells a pretty consistent story, and you can't just wish it away, and substitute for it a hypothesis that has no evidence to support it.

As I have said before, I am not going to devote large amounts of time on this blog to a detailed analysis of your ideas.

Constantinos Ragazas said...

Brian,

I need to clarify what I mean by 'facts on the ground'. You misunderstood me! By these I mean what can be observed on the ground at Stonehenge: the sarsens lasting perfectly erect for many millennia, the concentric circular design, the transporting of huge stones over hundreds of miles, the circular outer ditch, the embankment higher on the outside, the Aubrey holes, the Stonehenge Layer, the empty pits, the position of the Heel Stone, the orientation of The Avenue, all the linear stone alignments, the conical mounts and long burrows, etc. I can go on and on. We don't need archeologists or paleontologists or theologists to tell us what we see!

By nature, Brian, I am a meek and unassuming person. What gives me confidence is Truth. My conviction comes from the strength of my arguments. I am ready, whenever you decide, to air our differences in public. It may even make your blog 'historic'!

Constantinos

BRIAN JOHN said...

Not on my blog, thanks. Kostas -- none of the "facts" you cite does anything to support your theory. Show me an analogy from either the ancient or the modern world, and it might be worth thinking about -- but I don´t know of anything from many decades of research in geology, geomorphology or glaciology that suggests that the processes you talk about might have happened. Like Robert's theory, yours is a hypothesis in search of some evidence.

Time to move on to other things...

Robert Langdon said...

Bravo Brian!

"The silty loams / alluviums contain pretty good pollen and molluscan evidence consistent with Neolithic / Bronze Age woodland / clearance / grazing / agricultural activities"

Showing that the area was still waterlogged some 7,000 after the ice had melted - consequently indicating through reverse engineering (as the rivers have gone now) that these waterways were larger and around during Mesolithic period.

You just added yet another proof to my hypothesis - your on a roll!

RJL

BRIAN JOHN said...

Sorry Robert -- but I don't have a clue what you are talking about. Your geomorphology doesn't belong in any textbook I have ever read, nor has it figured in any of the debates and field excursions in which expert gatherings have pondered over field evidence.

" ..... the area was still waterlogged some 7,000 after the ice had melted..." Alluvial deposits and the organic remains contained in them do NOT indicate waterlogged landscapes -- they indicate short-term periods or episodes of alluvial deposition in rivers or lakes. To jump from that to a conclusion that the whole landscape -- or even the Salisbury Plain river system -- was deeply inundated beneath some great lake or arm of the sea is frankly absurd.

Constantinos Ragazas said...

Bravo Brian!

Certainly we can all agree that with the rapid melting of the glacier ice some of that meltwater had to collect in basins. Even if these were relatively temporary lakes, geologically speaking. If the rate at which the meltwater collects is greater than the rate at which the meltwater drains into the sea, meltwater WILL collect (a simple law of mathematics!). And surely there was lots of meltwater around, if the ocean levels all over the world rose by 200m. Since some of this glacial ice covered the land, it is reasonable to conclude that some of the meltwater from such ice collected in lakes, at Salisbury Plain and elsewhere (just as we currently have the lakes in the Lake District further north).

The problem with Robert's theory is that it forces him to hypothesize the inundation of Salisbury Plain at a very specific time period. Namely when Stonehenge is said to have been build by Neolithic men around 3,000BC and later. That is Robert's theory Achilles heel and you have in every reply to him pierced that fatal flaw in his hypothesis.

My theory does not suffer from the same flaw (regardless of your protestations to the contrary – if we have more time and space on your blog I feel I could convince you). Why? Because all I am hypothesizing is that Salisbury Plain was a frozen meltwater lake. It does not put any particular time period to this. The scientific evidence and common sense allows that if during a hot period the glacier ice melted quickly, meltwater lakes would form. And if this episode was followed by a deep freeze lasting some 2,000 years from 14,000 to 12,000BP, these meltwater lakes will freeze. Simple, natural, common sense reasoning!

Once we have ice on the ground, all other enigmas of Stonehenge can be naturally and logically solved.

Constantinos Ragazas

BRIAN JOHN said...

Kostas -- you keep on coming back to simplicity and common sense. Salisbury Plain is an undulating downland plateau with lower land beyond its edges, which are marked by chalk escarpments. There is no way that territory like that will support large lakes, let alone a single large water body. Lakes form in enclosed depressions -- water escape has to be blocked by moraines, or landslides, or whatever. A simple look at a map of Salisbury Plain will show you that these conditions do not exist, whereas they DO exist in the Lake District, Snowdonia and other parts of the UK. Common sense, please......

Constantinos Ragazas said...

Brian, you write,

“Salisbury Plain is an undulating downland plateau with lower land beyond its edges, which are marked by chalk escarpments.”

What lies beyond the 'chalk escarpments' at the edges of Salisbury Plain? What I saw along the southern coastline at Portland Bill are high hills and tall mature trees of the type that you don't find at Salisbury Plain. What I saw in the photos you posted of the western coastline along Bristol Channel are also hills separating the sea from the lowlands. I am using the term Salisbury Plain as a general term for that region. And perhaps technically speaking you may be right in saying that Salisbury Plain ALONE cannot be a lake basin. But together with the larger surrounding area it is still possible for such area to have been a meltwater containment basin with limited drainage capabilities to the sea, relative to the amount of meltwater purring in.

Our disagreement comes down to the 'basin' nature of Salisbury Plain and the surrounding area of SW England. I take that as a vast improvement from where we were just weeks ago.

Constantinos

BRIAN JOHN said...

Don't misrepresent me, Kostas! I disagree with your ideas on many levels -- not just on the matter of the supposedly enclosed basin.

Please look at any map of Salisbury Plain and Southern England -- just go to Google maps and check it out. There are river systems in all directions from Salisbury Plain -- there is no way you should even begin to think of "containing hills." Yes, there are hilly areas, as some of the maps on my blog have shown, but they are scattered about here and there, and there are far more gaps and intervening lowlands. The fact that there are some hills near the south coast is quite irrelevant -- there is no large basin that could have held a body of water, either under your preferred scenario or Robert's.

Constantinos Ragazas said...

Sorry Brian! I didn't intent to misrepresent you! I know how that feels and I apologize for it.

This three-dimensional jig saw puzzle you, Robert and I are seeking to piece together is becoming very interesting. How our respective ideas coalesce or diverge at various times seems to have a dynamic of its own driven by some underlying truth none of us entirely possess yet. Sometimes my ideas align more with Robert's. Other times they align with yours. Still other times Robert and you align against my position.

Generally I feel that Robert is correct in claiming that Salisbury Plain and the surrounding area was inundated by meltwater. My difference with him is the time frame. I think this water inundation happened much earlier than 3,000BC when Stonehenge is said to have been built. My theory is not restricted by time. Only by ice!

Constantinos

Robert Langdon said...

Actually Costa if you read my hypothesis the archaeological carbon dating shows your 'melt water' which I call a river at Stonehenge bottom was dated at 8000BC to 7500BC - which coincides with my Phase I (moat and Bluestone)construction date of Stonehenge, NOT 3000BC which is the incorrect traditional interpretation.

But i'm very happy that Brian does now conceive that water DID exist at that period hence the alluvium - what needs to be looked at is the period it was there - so your reference to 'short-term' rivers - are you talking weeks, months, years, decades, centuries or millenniums?

Geological speak is hard to define - quite deliberately i'm sure!

RJL

BRIAN JOHN said...

Sorry you guys -- there is no meeting of minds on this supposed inundation by meltwater. I see no evidence for it whatsoever, either 10,000 years ago or 5,000 years ago. Neither do I see any evidence for this supposed mass of water being transformed into extensive sheets of ice with a convenient surface gradient from A to B.

With ref to me conceiving (presumably you mean "conceding"?) that water did exist at whatever period you want to talk about -- that's not a concession, Robert -- it's plain common sense! Water exists on the surface even in periods of continuous permafrost -- and for the most part it runs away, except where land surface gradients are very low indeed. By short-term I mean seasonal -- for some weeks every summer.

Now I'm fed up with this discussion. Believe what you want, you guys -- but this thread ends here.