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Sunday, 23 January 2011

On frozen gravels

Superficial deposits around Mendip

In trying to find out what the glacial and non-glacial deposits are around the Mendip Hills - Salisbury Plain area, I came across this old map drawn by Alan Fells of Bristol University about 50 years ago!  It's actually quite useful.  Click to enlarge. 

There are strange gravels all over the place, many of them not yet properly analysed and classified.  These gravels occur both within and beyond the assumed limit of glaciation in Southern Britain.  The term "Taele Gravels" used to be used for gravels of uncertain origin that were at one time frozen.  It's a bit misleading to refer to them as "periglacial gravels" -- because sorted gravels are not formed by freeze-thaw processes as such, but laid down by by flowing water and then frozen.  (That having been said, I have seen some head deposits in West Wales that are remarkably fine-grained, and which appear to have been laid down by trickling water, bit by bit, on a frozen surface........) 

In some periglacial river systems with perennial flow, gravels can be laid down each summer and frozen each winter, building up gradually over time and maybe incorporating involutions, ice wedges or other diagnostic features.  Erratics may be found in these gravels, if glacial or fluvioglacial deposits are to be found in the source areas or catchments of the rivers.  Landslide, slump and solifluction deposits (often called "head" or "rubble drift" can also be incorporated into the taele gravels, providing layers or lenses of coarser material.  Similar conditions must have occurred right across Southern England.

From a survey of the geology of Surrey:
"Whilst no true glacial (moraine) deposits such as chalky boulder clay are to be found on the North Downs or further south, the furthest extent of the earlier ice sheets over southern Britain during the period of glaciation (intermittent between 500,000 and 15,000 years ago) remains somewhat ambiguous. There is however much better evidence for glacial outwash gravels (Plateau Gravels such as cover Epsom Common, Great Bookham Common, Chobham, and at Newdigate) and permafrost chalk soils and rubble (Coombe Rock) and frozen gravels (Taele Gravels as found at Great Bookham, Ashtead, Epsom, Brockham and Betchworth).
River gravels such as occur in the Thames River Terraces (Flood Plain, Taplow, and Boyn Hill Terraces) can occupy large areas. The earlier terraces which may have been formed more than 250,000 years ago were deposited at a higher level before the downcutting of the bed of the river. Most of the surface geology within the Bos. of Runnymede, Spelthorne, and parts of Elmbridge relate to these terrace gravels of the Thames and this has also become an important economic resource in terms of sand and ballast gravel for the construction industry. Gravel extraction, both old and modern, has resulted in the discovery of Pleistocene mammal remains including many examples of cold-living fauna such as woolly mammoth, woolly rhinoceras, red deer, reindeer, wild horse, aurochs etc. Most of these finds have been of disarticulated remains since these would often (though not neccesarily) have been washed downstream by the river. Although modern methods of gravel extraction through dredging does not favour the preservation and discovery of such fossils, remains of mammoth teeth and tusks, bones etc. are still deposited with museums and form a useful if partial record of animal distribution. River Terraces and gravel deposits, though much reduced in comparison, occur on most of the large Surrey rivers - in particular the Mole, the Wey, the Blackwater and others. Excavations for gravel or construction have produced similar finds of Pleistocene mammals, mammoth teeth being amongst the commonest finds deposited in nearby museums. Not all such finds have been from river terrace deposits and locality details in these cases can prove extremely important."

15 comments:

Constantinos Ragazas said...

Thanks for this post, Brian!

Though we may disagree on how and why Stonehenge was made, we do agree that the more scientific evidence we have on this subject the better. I appreciate your efforts in this regard.

In you capacity as a scientist I ask you. Does any of this evidence mentioned in your post rule out my theory on Stonehenge?

Constantinos

BRIAN JOHN said...

I fear it does, Kostas. What I am trying to do is work out what the climate and environment were like around 20,000 years ago -- which might then give us some guidance as to what went on during the earlier glaciations when (I think!!) the erratics from west Wales were transported eastwards.

My thoughts on the snowblitz theory and on small ice caps and snowcover in the SW of England have to take account of the evidence from many fields. I have no reason to doubt that the peak of the last glacial was around 20,000 years ago, or that the most recent cold phase (the Younger Dryas) occurred between approximately 12,800 and 11,500 years ago (between 10,800 and 9500 BC). That lasted for 1,300 ± 70 years.

In terms of human habitation, this was still within the Palaeolithic period, with very small and scattered tribal groups and no evidence of a megalithic culture. If you want human beings to be involved in your stone sliding theory, I fear that the timing is all wrong.

Constantinos Ragazas said...

Brian,

I know you do not agree with my theory, but how does the evidence itself that you present in your post about "Taele Gravels" conflict with my hypothesis that Salisbury Plain and the surrounding area was covered by a local ice sheet as recently as 9500 BC and at a time when there were 'people around'? Certainly nothing in the scientific evidence you present contradicts that claim. But just as certain, however, your conclusions of what the science tells us are framed to conflict with this.

You write,

“In terms of human habitation, this was still within the Palaeolithic period, with very small and scattered tribal groups and no evidence of a megalithic culture.”

If we assume that such 'megalithic stone movement' was done by men on the ground, than we are forced to date such movement latter than the Palaeolithic to have enough human development for such undertakings.

The argument is cyclic! We are assuming our conclusion in our premise! Our premise must be independent of our conclusion, if our conclusion is to be logically valid!

What “evidence of a megalithic culture”, Brian, is independent of the megalithic sites? Are there written records of Neolithic people moving stones, or we presume without questioning that the huge stones we see must have been moved by men? Please don't mention Carbon dating of organic material found at Salisbury Plain! Such dates pinpoint perhaps when organic material start growing again at Salisbury Plain after a long period of freezing conditions or were deposited there from elsewhere higher. But they cannot date when Stonehenge was made! The dating of Stonehenge continuous to be an open question.

Constantinos

BRIAN JOHN said...

We do indeed go round in circles. It was certainly cold in the Younger Dryas, with frozen ground, snowpatches and solifluxion going on in Wiltshire and the West Country. But there is NO evidence for a"local ice sheet" stretching all the way from West Wales to Wiltshire and also covering the Bristol Channel. Sheer fantasy, Kostas.

And the megalithic phase is not just dated by reference to the standing stones themselves. That phase was part of a continuum -- which started with causewayed enclosures, simple burial chambers, long barrows, passage graves, gallery graves, grave clusters etc; then earth and timber structures; then Stonehenge as a rather strange "extension" or flight of fancy developed from henge monuments and ring cairns; then simpler burials, round barrows, henges and cairns, standing stones and stone alignments in the Bronze Age. OK -- there are overlaps and difficulties, but the overall story is confirmed in thousands of dates including C14 dates and published in the literature. Why would you want to challenge the Salisbury Plain "timeline"anyway? It's a pointless and futile exercise.

Take a look at some of the megalithic web sites, and you will find that there are megaliths in almost every country -- not all of the same age, but most certainly not all put into position with the assistance of convenient local ice sheets!

Constantinos Ragazas said...

Brian,

I am not interested in going over the same arguments and rejections and blocks to my posts by you. All the mounts and long burrows and grave clusters and stone alignments you and others claim are evidence of a “megalithic culture” I claim are evidence of nature. I have my reasons but you consistently choose to block my arguments. So lets just leave it at that but instead examine the scientific facts.

My question to you was: Does the scientific evidence you have presented in your posts rule out my claim that Salisbury Plain and the surrounding area was covered by a local ice sheet around 9,000BC and even latter.

Constantinos

BRIAN JOHN said...

Q: Does the scientific evidence you have presented in your posts rule out my claim that Salisbury Plain and the surrounding area was covered by a local ice sheet around 9,000BC and even latter.

A: Yes, it does. There is nothing, in any of the evidence I have presented, or in any of the other evidence in the literature, to support the idea of an ice sheet over Salisbury Plain (and to the west) --let alone the peculiar sort of ice sheet that you talk about -- at the time you want.

Robert Langdon said...

Interesting debate boys!

Just one question getting back to the blog and maybe answering Kostas doubts - is there evidence for ALL past glaciations which are shown various gravels or sub-soils?

We know from written history that the Thames frozen over in 250, 1536, 1564, 1608, 1683, 1688, 1788, 1814. Which no doubt effected the whole of the UK including the River Avon.

Consequently, do we have evidence of all of these 'mini-ice ages' in the geological soil and if not, can concluded that this evidence is ONLY found in 'long term' Arctic conditions?

This would prove both of you correct and I will be happy with my observation that the geological evidence is 'incomplete' and open to reinterpretation.

RJL

Constantinos Ragazas said...

Robert,

I agree with you that the geological evidence is open to reinterpretation, as is the archeological evidence.

Brian argues that none of the geological evidence conclusively confirms my hypothesis. Therefore my hypothesis must be false. I argue that none of the geological evidence conclusively contradicts my hypothesis. Therefore my hypothesis could be true.

I like Brian to admit in public that there is a logical difference between evidence that 'rules out' (contradicts) a hypothesis and evidence that 'rules in' (confirms) a hypothesis. I know as a scientist he knows this basic principle of evidence. I simply want Brian to state this obvious logical fact, rather than framing his responses in a way that leaves the impression that I am just plainly wrong!

Maybe he will address this important issue of logic (along with Occam's Razor and Popper) in responding to you! This should make all of us more 'open minded' about Stonehenge.

Constantinos

BRIAN JOHN said...

Re Robert's points:

His question: "is there evidence for ALL past glaciations which are shown various gravels or sub-soils?" I don't know what this question means, Robert -- but of course if there has been a glaciation in a particular area you should be able to find some trace of it, either in erratics in later layers of gravels or alluvium or head, or in surviving layers of till or fluvio-glacial deposits. But deposition is always patchy, and later events (whether related to temperate climate or glacial climate) will tend to erode or destroy very old deposits. For example, in South Pembrokeshire there are plenty of erratics scattered around which show there has been a early glaciation -- but I know of only one small deposit of something I would refer to as till. So survival of ancient deposits is a matter of chance as much as anything! The older the deposit, the greater the chance that it will be virtually removed by later events.

Short term winter freezes have happened many many times -- particularly in the Little Ice Age -- and they still do! A single winter freeze is very unlikely to be recorded in the sediment record -- but you would certainly pick up the Little Ice Age (which lasted for several centuries) in the pollen record or in deep sea cores.

You cannot take that as confirmation that the geological record is "incomplete" and that therefore your deep inundation might have happened without leaving any trace.

Show me some evidence, and I might be more inclined to believe in Noah's Flood.... since something on the scale you propose must have allowed sediment accumulation and shoreline development over centuries or millennia.

BRIAN JOHN said...

Kostas -- this is getting wearisome! There is LOTS of evidence that directly contradicts your hypothesis. If your sheet of ice extending from West Wales to Salisbury Plain had actually existed in the Palaeolithic or Neolithic, evidence of it would exist towards the top of the stratigraphic record. There is no such evidence in any of the pollen sequences or stratigraphic sequences that I have seen -- indeed, the evidence of coleoptera etc shows that in Southern England during the Younger Dryas the average summer temperature was around +10 deg C. What we have at the top of sediment strata is often brickearth (blown sediments, showing strong winds and cold arid conditions) grading up into a thick soil horizon which brings us up to the present day. There are NO glacial, fluviglacial or even marine or lacustrine deposits in these sequences, which means that there was no deep inundation by the sea or a "mini glaciation." Woodland gave way to tundra conditions for a while, and then trees and other warm-loving plants moved back in to create the "boreal forest" very shortly after 10,000 yrs BP. Pollen records, lake cores, deep-sea sediment cores, oxygen isotope studies all show the same thing -- NO return to glacial conditions except in the highest mountain areas of the UK where a short glacier advance is called the Loch Lomond Stadial.

Look this all up on the web (Wikipedia is a good place to start) or in scores of textbooks or in hundreds if not thousands of scientific papers. There is no gigantic conspiracy out there by a scientific community trying to hide the truth!

I say it again, Kostas -- your hypothesis is DIRECTLY contradicted by the evidence on the ground.

Constantinos Ragazas said...

Brian,

Nothing about Stonehenge is written in stone! As long as there are open questions there must also be open questioning.

Rough details to a theory do not in themselves invalidate the theory. Only help polish it.

You argue against an ice sheet extending from West Wales all the way to Salisbury Plain. But how is that different from the glacier ice sheet you hypothesize brought the bluestones from West Wales to Salisbury Plain? How far apart are we if your glaciers transported the bluestones not on topsoil but on top of a solidly frozen lake or sea (which later incrementally melted and drained into the Irish Sea leaving no trail behind other than some frozen pebbles)?

Brian, can you explain to me how a 50 ton sarsen stone be raised from the ground up and stay balanced and erect for several millenia just by being buried and packed with small stones at its base? Some of these huge stones are very uneven. Hard to know where their center of gravity is, even if you should have them up for awhile. Gravity and geology would act over time taking advantage of such faults to topple the stones. It's an engineering problem that can be tested.

Popper is happy, while Occam's Razor cuts sharply into such human erection!

Constantinos

BRIAN JOHN said...

Kostas, the difference ifs that my ice sheet has masses of independent evidence to support it -- and your has none. And timing is another problem with your theory -- you simply cannot argue that something happened and conveniently left no evidence behind!!

Big standing stones are not a problem as far as I am concerned. The Stonehenge monoliths were put up using fairly simple engineering techniques. Some were more stable than others -- many have keeled over a bit, or fallen right over. The Stonehenge sarsen standing stones are small by comparison with those in Brittany -- they are there, whether you like it or not, in areas never glaciated during the Pleistocene.

Constantinos Ragazas said...

Brian, you write

“...the difference is that my ice sheet has masses of independent evidence to support it -- and your has none.”

Can I borrow then YOUR ice sheet in my theory? Thus, YOUR glaciers brought the bluestones, not to dry land but to some solidly frozen body of water. All I really need is a local ice sheet around time uncertain when Stonehenge was made. The rest than follows.

Constantinos

BRIAN JOHN said...

Sure, you can borrow my ice sheet! But not at the time you want. There is NO evidence in support of your frozen body of water at the time you want it -- or at any other time, for that matter, in or near Salisbury Plain. "The rest then follows"? Oh no it doesn't.

Constantinos Ragazas said...

Ah! Brian, nothing will satisfy you short of a photo! Common sense reasoning is not enough. I'll try to get you the photos you need to see my theory!