I have been reminded several times in the last few days about the erratic boulders (some weighing several tonnes -- which makes them as big as the Stonehenge bluestones) in the area around Kenn and Court Hill in western Somerset. At Kenn Pier David Gilbertson found till with a large striated boulder of Carboniferous Limestone (origin unknown) and also other striated boulders, including one which was over half a tonne in weight. In the body of the till at Court Hill there are boulders (up to .5m in diameter) which have been identified as Carboniferous Limestone, Pennant Sandstone, Mercia Mudstone, Old Red Sandstone, Greensand chert and flint, and Carboniferous chert. It's difficult to know where some of these have come from, but the authors who have described these deposits infer that they have come from the west.
Now then -- how on earth did an erratic of undoubted Ulster White Limestone (a variety of chalk) get from Ulster to Somerset? If ever you wanted evidence of ice crossing Pembrokeshire and reaching this Somerset coast, this is it. From everything we know about ice streams and erratic transport, there is no way that glacier ice can have flowed from Ulster across the uplands of mid-Wales, because these uplands were occupied by the Welsh ice cap -- so if you reconstruct flowlines the ice has to have crossed Pembrokeshire. This does not mean to say that there will be Pembrokeshirec erratics here -- the entrainment of erratics, and their onward transport, depends upon so many factors that the ice that transported this erratic (and there must be other erratics like it, as yet undiscovered) might have flowed across Pembrokeshire far above the present land surface, underlain by a great thickness of either stagnant or sluggish ice. We still have to work all of that out.....
8 comments:
Steep Holm might be worth a look at .
There was a lot of discussion about Steep Holm and Flat Holm a few years ago -- and some wild goose chases re erratics. I'm not sure what the truth is -- I would quite like to have a look for myself.....
Brian you write,
“… the ice that transported this erratic ... might have flowed across Pembrokeshire far above the present land surface, underlain by a great thickness of either stagnant or sluggish ice.”
Sounds like a 'local ice sheet', Brian! My assumption that many of these areas were covered by a 'local ice sheet' ('stagnant' as you describe) helps answer many of the questions concerning the lack or very sparse evidence of glacier advance/retreat in these areas, including Salisbury Plain.
Many of these erratics would have been 'flushed into the sea' by meltwater or glacier flow over the hard surface of a local ice cover. My sense is that this 'local ice cover' may have been formed by 'frozen waterways' created when the glaciers melted; or perhaps this ice cover was what remained on the ground when the earlier glaciers melted but got 'trapped in time' by the Deventian from melting further.
This would have resulted in a very hard and smooth ice surface over which new glacier advances may have flowed flushing into the sea whatever debris was on the ice surface.
Everywhere we look, ICE IS THE ANSWER!
Kostas
Sorry Kostas -- none of this makes sense. You do not get flushing of large quantities of debris over and across frozen waterways. This is to do with the temperature gradient in ice bodies, the fact that water will always find its way to the bed, and other factors related to ice physics. As usual your problem is that observations do not match your hypothesis -- and we therefore have to conclude that the hypothesis is wrong.
Brian,
My hypothesis is that a 'local ice sheet' covered the area. I make no other claim as to how that 'local ice sheet' was formed; but only suggest that it may have formed by frozen bodies of water.
Please explain how your quote,
“… the ice that transported this erratic ... might have flowed across Pembrokeshire far above the present land surface, underlain by a great thickness of either stagnant or sluggish ice.”
differs from the mechanism of 'glaciers over local ice' I am arguing … Seems like a difference without distinction!
Is your “ great thickness of either stagnant or sluggish ice” different from the 'local ice cover' I hypothesize? If I drop the suggestion that this 'local ice cover' was formed by 'frozen waterways', will we then be in closer agreement?
Let me guess! NO! NO! A thousand times NO!
Kostas
The 'Stonehenge' erratics of Steepholme were described by Ixer years ago and shown to be Precambrian South Wales rocks (but from where??). Discussed in 'Waiting by the River' and a letter to one of the Archy magazines where the possibility that the rocks had as much involvement with Stonehenge as the Hanging Gardens of Babylon
A view I believe he still holds.
Doubting Thomas
Yes, I recall the debate. I think Prof GW wanted the "bluestones" to be bluestones (as at Stonehenge) because he had a theory that the transport expeditions stopped off there on their way through, and maybe did a bit of shaping while they waited for a storm to subside (or whatever.) I recall he was more than a little disappointed when they turned out not to be "proper" bluestones at all -- ie they had not come from Preseli.
As far as I'm concerned, that's no big deal. We know that the erratics turning up on the Vale of Glamorgan and around the shores of the Bristol Channel are very varied -- I'll do a post on the Storrie Collection of erratics from Pencoed......
Kostas, I do have a problem with frozen waterways and great sheets of ice accumulating on the land surface. I do not have a problem with coalescing snowfields on pleateaux and other uplands gradually combining, as a glacial episode sets in, to form an extensive set of neve fields and small glaciers which gradually thicken and become incorporated into the Irish Sea Glacier or into the expanding Welsh ice cap. That's exactly what Henry Patton is showing in his animations. I'll do a post on this before long...
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