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Saturday 28 October 2017

On the redistribution of bluestone erratics

In the light of my previous post, on the "bluestone erratic train", I have been re-reading this interesting paper by Phil Gibbard et al:

New insights into the Quaternary evolution of the Bristol Channel, UK
ISSN 0267-8179.
DOI: 10.1002/jqs.2951

I covered it briefly in a previous post, here:

It's great to see that things are moving (so to speak) in the Bristol Channel arena at last, and a number of geomorphologists are realising that hereabouts we will find information crucial to our understanding of the Quaternary of south-west Britain.  Sounds obvious -- so it's all the more surprising that the area has been relatively neglected while research has been concentrated in other areas like north Wales, eastern England and the fringes of Scotland. 

For many years it has been known that during the Late Devensian glacial episode, ice from the South Wales outlet glaciers pushed across the coast in three main areas -- Carmarthen Bay (the Tywi Glacier), Swansea Bay (the Tawe and Neath Valley glaciers) and in the Cardiff-Newport area (the (Taf and Rhondda Glaciers).  The Usk Glacier is not thought to have reached the coast.  Most of the work on these features, as described by Prof David Bowen and others, has been land-based, and is summarised on this map:

A lot of work is now becoming available in the literature relating to the Bristol Channel sediments and bedforms, as repeated in other posts on this blog.  This is suggesting that Devensian ice did cross the coast around Cardiff and Newport, and that the above map needs to be corrected in that respect.  And the evidence for the inner and outer moraines in Swansea Bay is truly spectacular, as shown in the map at the head of this post, taken from the Gibbard et al paper.

What this all means is that any bluestone erratic train running between Preseli and Salisbury Plain would have been effectively disrupted or smashed to pieces by these Devensian advances involving ice from the Welsh ice cap, flowing broadly southwards from the Brecon Beacons and the uplands of the South Wales Coalfield. 

Here is another map from Gibbard et al, with my hypothetical bluestone erratics train superimposed upon it.

So if anybody asks me in future why there is no trail or train of large bluestone erratics running all the way from Preseli to Stonehenge, I will show them this map.  It demonstrates that there is NOWHERE in South Wales where the erratic train could have survived intact -- in an unmodified form.  In Carmarthen Bay, Swansea Bay and the Cardiff-Newport area old Irish Sea Glacier erratics might well have been flushed out beyond the present coastline.  In other areas any erratics lying about will have been incorporated and reworked into Devensian morainic deposits.  The erratics will still be there -- somewhere, and probably buried -- but the chances of finding them will be infinitesimally small.  In fact pre-Devensian glacial deposits are also very difficult to find in Glamorgan; we only know of one substantial survival, at Pencoed.

So that leaves two areas where we might find erratics -- immediately to the south of Preseli and in Somerset and Avon.  In the latter area, as we all know, no big bluestone erratics (demonstrably from Pembrokeshire) are known apart from those at Stonehenge.  But in eastern Pembrokeshire, down-glacier from the Preseli Hills, there are abundant bluestone erratics, as noticed by Thomas, Griffiths and other researchers.

By the way, ALL of the glacial limits on the above map are speculative.  I have argued over and again that the Late Devensian limit shown does not make any sense and I don't know why researchers keep on showing it on maps when it defies all the rules of glacier behaviour.  And the Early / Middle Devensian limit is even more controversial --  I'll report soon on a new paper that demonstrates that the assumed glacial episode at that time is just as fanciful as a bluestone quarry. (My words, not theirs.......)


Dave Maynard said...

So you are implying that at the start of the Mesolithic, the only unaltered Pleistocene bluestone spread is to be found beyond the limits of the Devensian glaciation, i.e. South West England.


BRIAN JOHN said...

No -- not South-West England. Much narrower than that -- however broad the erratic train might have been. If it shifted left or right, the train might be 10 -15km wide. That means there might be bluestone erratics in the Vale of Glamorgan....... Somerset too, in the broad embayment where the Levels are. Then again, there might not. Just thinking out loud here and having fun, as one does..........

TonyH said...

You say bluestone erratics may be in an erratic train 10 - 15 kms wide in Somerset and Avon. Does that possibility extend slightly beyond Avon into Wiltshire near Bradford - on Avon, for example? We know about glacial till near Bathampton, downstream from the Wiltshire borderlands. I do not think glaciers had the forethought to obey 1974 Reorganisation of Local Government Boundaries!

Also, what about the glacial till observed by you near Frome in the Holwell quarry near Nunney?

BRIAN JOHN said...

This is all speculation, Tony! But I hope it is intelligent speculation which accords with the facts. We just don't know how wide the erratic train might have been, or what wobbles there were in it. One other factor might be the tendency for ice streams to seek out the lowest and easiest route which allows ice discharge -- so the main streamlines of the Irish Sea Glacier might have been a bit further south than the red arrow shown. But we are, I think, talking here about the edge of this ice stream, so the arrow will not be far off. And yes, if the Somerset Levels were invaded by ice there is no reason at all why the ice could not have reached the Bradford-on-Avon area. Where are the bright young geomorphologists who should be out there, looking for evidence?