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Friday, 27 October 2017

The Bluestones Erratic Train


Over the years we have discussed erratic trains and trails on many occasions, and I have just prepared a new map showing the main ice streams affecting the Bristol Channel area in the Anglian Glaciation.  (Or maybe it was the Wolstonian?  Nothing is certain in this life......)

I'm fairly happy with the ice stream arrows for both the Irish Sea Glacier and the Welsh outlet glaciers affecting South Wales.  The red arrow is based on the paper I did with Lionel Jackson in 2009 in "Earth" magazine, suggesting a contact zone along which the two ice masses ran side by side at more or less the same speed, without much mingling.  (Ice acts in some ways like a fluid, but we must not carry that analogy too far.....)

Along the red line we might expect to find a train of bluestone erratics, but only if there was a continuous process of erosion and entrainment at the Presell end of the line, in the source area. As I have explained, I think the entrainment of Preseli erratics (spotted and unspotted dolerites, rhyolites, dacites, volcanic ashes and sandstones) might only have occurred on a substantial scale at the beginning of the glaciation concerned, with the supply cut off as the ice thickened.  So the route might be approximately OK, but the "erratic train" might just be a pipe dream.

There would also have been wobbles in the route, and in reality the "red route" would have had a lot of kinks in it, in response to waxing and waning ice pressure both from the southern flank and the northern one.

Then there comes the last complication -- the wastage of the Irish Sea Glacier, which would probably have been catastrophic and very rapid.  It appears to be normal towards the end of a glacial episode for "pulses" or readvances to occur around the fringes of smaller ice masses such as the Welsh ice cap -- and advances of the South Wales glaciers could well have pushed the debris associated with the Irish Sea Glacier (including quantities of erratics) southwards, beyond the present coast line and into the area now submerged beneath the sea.  There are signs of just such terminal and lateral moraines both in Swansea Bay and in Cardigan Bay, associated with Devonian glacier advances following Irish sea ice wastage.

More to be discovered -- of that I have no doubt.

6 comments:

TonyH said...

Any updates on, I think it was a Cambridge University study into what lies beneath the Bristol Channel in the vicinity of Bridgewater Bay? You reported on this a couple of years ago. Sorry to be so vague....

BRIAN JOHN said...

Tony -- the latest is in this 16 July post:

https://brian-mountainman.blogspot.co.uk/2017/07/the-bristol-channel-glaciations.html

Is this what you mean? Phil is a Cambridge -- and the Bristol Channel work is, I think, ongoing......

TonyH said...

No, there was an earlier one, I'm sure, on the Blog........ will endeavour to retrieve it - Inspector Morse - style! - in due course.

BRIAN JOHN said...

Yes, I did mention earlier on that Cambridge Univ had advertised for a research student to work with Phil Gibbard in the Bristol Channel area -- this research is now starting to produce results.

TonyH said...

The Post on Cambridge University's advertising for a research student was "Bristol Channel Glaciation - Are Things Stirring? 15th January 2015.

TonyH said...

In passing, I noted that for the 15th January 2015 Bristol Channel Glaciation Post, I then commented there had been 618,474 views of your Blog.

To date, this has more than doubled in less than 3 years, to over 1,240,000 views.