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Wednesday, 25 May 2011

On the trail of Bluestone Erratics (1) Principles

A huge erratic on the plains of Patagonia.  Where did it come from?  How did it get here?


The question I am most frequently asked about the Stonehenge bluestones is this:  "If the bluestones really were carried from West Wales to Salisbury Plain by a glacier, where is the trail of erratics that we should find in the landscape between Carn Meini and Stonehenge?"  Another form of the question is this:  "Where is the moraine that should be near Stonehenge, and where are the till formations and other glacial deposits that should be well mapped by now, given the many years of intensive research in the area?"

These questions are fair enough if they are put by laymen, or even by professional archaeologists who cannot be expected to know much about glacial geomorphology.  But it does tend to irritate me when they are put by senior academics who teach and research in various fields of geomorphology, especially when they then conclude, having considered the matter, that glacial transport from the Mynydd Preseli area to Salisbury Plain was "impossible."  To the best of my knowledge, that word has actually been used in this context in discussions and in print by James Scourse, Chris Green and David Bowen.  As I have said before, they should know better;  anybody who says that something is "impossible" is asking for trouble......... especially when there is abundant evidence on the table that glacier ice has indeed crossed the Pembrokeshire Peninsula and extended into the Somerset lowlands, on at least one occasion (and possibly several) during the Quaternary.  These three learned gentlemen know that, but they have still chosen to turn a blind eye to the evidence presented long ago by Judd and later by Geoffrey Kellaway -- and to sign up with precipitate enthusiasm to the human transport myth peddled by the archaeology establishment.  Why?  Better ask them that......

Interestingly enough, the reasoning of these three gentlemen appears to run as follows:  "if the route followed by the Irish Sea Glacier up the Bristol Channel was indeed as suggested, there should be a continuous trail or fan of spotted dolerite and rhyolite boulders roughly the same size as those of Stonehenge stretching from the eastern Preseli Hills and onto the chalklands of Salisbury Plain.  There isn't, and therefore the glacial transport didn't happen."

And it's also quite intriguing that since these senior academics have been selected as "THE authorities" on glacial transport, their pronouncements have also been taken on board by geologists who should know better.  Why?  Better ask them that too........

Part of the problem is that none of those who have been chosen as "expert geologists" or "expert glaciologists" is actually a glacial geomorphologist.  I am (or was, a long time ago!) and I think I can claim to know what I am talking about.


The other part of the problem is that "the experts" have never seriously considered the factors that are involved in getting large stones from A to B.  They tend, in a very simplistic way, to look at an erratic on the ground, and say "Ah yes, this came from location A -- therefore the ice crossed that location and then dumped it here, at location B."  But that may be only a very small part of the story, and it is now my mission to educate the world about glacial erratics.

In even the simplest glacial erratic story there are three (at least!) factors to be considered: entrainment, transport, and emplacement.  In other words, the picking up of the stone, the transport of it, and finally its dumping in a location where it is out of place, and therefore "erratic."  It seldom happens that a stone (or a collection of stones) has one simple glacial journey over a measurable amount of time;  I suspect that most erratics in the UK have been moved several times and have followed wildly erratic courses.    I'll come back to this in a later post.

Watch this space.........

4 comments:

Constantinos Ragazas said...

Brian,

The lack of glacier erratics, moraine, till formation and trail of spotted dolerite from Preseli Hills to Salisbury Plain seems to be the “Achilles heel” of your glacier transport theory. It is a serious argument that I am sure you will address in the future.

But to argue that glacier erratics were 'moved around a lot' over long stretches of geological time will not satisfy your detractors, I don't think. The counter argument that I anticipate they will make is that though stones can be moved about from one glacier episode to another, they don't disappear from the landscape. And once a trail of spotted dolerite reached Salisbury Plain by glacier flow, what subsequent glacier flow 'backward' could have removed them from the area?

As a glacier advances, it carries such debris of stones. But as a glacier retreats, it leaves behind that debris. A retreating glacier will not carry erratic stones back. While a subsequent glacier advance beyond that point will carry these same stones only farther along in the same general direction.

But there is a way of explaining the lack of such 'glaciation evidence' in Salisbury Plain while still maintaining your basic premise of 'glacier transport'.

Glaciers can make such 'glacier deposits' either on land or on vast solidly frozen bodies of water, like lakes or rivers or seas. If these glacier deposits were made on land, they will surely stay on land. But if these glacier deposits were made on an ice surface of a river or lake, they would be washed away from the area as the ice sheet melts and the melt water drains into the sea. I suspect there are many glacier erratics submerged along the coast. What you have suggested in the past yourself!

This explanation for the lack of glaciation evidence at Salisbury Plain will also help explain many other geological differences between east and west England that you brought up in previous posts --including why the ground water levels are so much lower in the western part than in the eastern part (since most of the water over an ice cover would drain into the sea and not be absorbed in the ground).

Kostas

BRIAN JOHN said...

Kostas -- what I am suggesting is that the distribution of glacial deposits and erratics across SW Britain is not an Achilled Heel for the glacial transport theory at all, but something which is quite in tune with what we know about glacial processes. Much has already been covered in previous posts -- please use the search facility, and type "erratics" into that little box!

The only situations in which glaciers dump debris onto sheets of frozen water are those in which an advancing ice front meets a frozen lake or a frozen sea -- as in parts of Antarctica. Most debris is still dropped onto the lake or sea bed -- but some may be carried away on ice floes and icebergs as the summer melt sets in. I've covered this before, with respect to the giant erratics of Southern England.

Anonymous said...

http://www.megalithic.co.uk/modules.php?op=modload&name=Forum&file=viewtopic&topic=4583&forum=1&start=0

Seen this?
I'm not sure how to comment on this idea!
PeteG

BRIAN JOHN said...

Well, I think that idea is quite fun! There appears to be an endless stream of ideas as to how Neolithic man might have moved big stones from A to B. I suppose this might work in somebody's back garden, or maybe even on the chalky grasslands of Salisbury Plain, but it wouldn't have a cat in hell's chance of working on the rough, stony and boggy terrain of west Wales.....