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Friday 13 January 2017

Time to bring the Wolstonian in from the cold?

 Suggested ice cover for the Wolstonian / Riss / Saalian glaciation of Wales and adjoining areas -- showing relations between Welsh ice and ice flowing from other source areas.  The location of the Gwaun-Jordanston meltwater channel system is also shown.  The limits of the ice cap are taken from one of the models run by Henry Patton and colleagues, assuming a steep precipitation 
gradient from west to east.

 The subglacial meltwater channel complex to the south of Fishguard and the Carningli upland.  I have now come to the view that these channels cannot have been formed during the wastage of an Irish Sea Glacier that came in from the NW, flowing towards SE.  The orientations and characteristics of the channels are all wrong.  The channels must have been formed beneath ice flowing broadly from east to west -- in other words, under a segment of an expanded Welsh ice cap.

When  I was a research student working towards my doctorate, the prevailing view was theat there was a rather complex glacial history (involving many glacial phases) on the eastern side of Britain, but that there had been just two discernible glacial phases in the west.  These were called the "Older Drift" and "Newer Drift" glaciations, and most people in the 1960s and 1970s equated them with the Riss/Saalian / Wolstonian on the one hand and the Weichselian / Vistulian / Wisconsin / Wurm / Devensian on the other.  The assumption was that glacial activity had been much more concentrated and therefore vigorous in the west than in the east -- and this was an explanation for the lack of any coherent glacial deposits beneath the raised beach dated almost everywhere to the last interglacial (Ipswichian).  Older glacial deposits had simply been eroded away by succeeding advances (we thought), whereas in eastern England they has simply piled up like layers of a cake. Then things started to get a bit more complicated, as field workers started to find things that did not quite fit and as more and more dating evidence started to pour in from all over the UK and Ireland.  Eventually the most recent big glaciation (the LGM) was assigned to the Late Devensian (at least I got that bit right, in spite of much opposition) and the Older Drift Glaciation was reassigned to the Anglian glacial episode.  That pushed it back from c 250,000 years ago to around 500,000 years ago.  And that left a very big gap between 500,000 BP and 20,000 BP.........

  The Wolstonian glaciation was not abandoned, because there is clear evidence of it in East Anglia and south of Birmingham, for example -- but because many of its deposits were overridden by Devensian ice and because its outer limit lay within the greatest extent of Devensian ice here and there, researchers were very reluctant to portray its maximum extent on a map.  This is one suggestion, from Gibbard and Clark:

The dashed line on the map shows the tentative Wolstonian limit, but as I have suggested before, it is not sensible across south Wales.  There is no sound reason for assuming glaciation across mid Wales but not across the Brecon Beacons and the Coalfield uplands -- so the southern limit in Wales must have been quite close to that shown for the Devensian.  What about Pembrokeshire, the Bristol Channel and the Celtic Sea?  Difficulties galore........

There are several features in Pembrokeshire and Ceredigion which cause me now to think that there was a greatly expanded Welsh ice cap during Wolstonian times, around 250,000 years ago.

First of all, there is the matter of the orientation of the Gwaun-Jordanston meltwater channel system, as explained at the top of this post.  I think these channels might actually have been cut during the Wolstonian Glaciation, and later modified during the Devensian.  It's possible that some of the other coastal meltwater channels, and the spectacular channel in Ramsey Sound, are also of Wolstonian age.

Second, there are apparent double and triple till sequences in northern Cardigan Bay with -- in some cases -- weathering horizons between them, suggesting that interstadial or maybe interglacial conditions prevailed between lower and upper tills.

Third, I am still mystified by what went on in the Pleistocene on the coasts of Ceredigion, where thick diamictons are variously interpreted as till laid down under ice moving westwards from the Cambrian Mountains, as periglacial slope deposits (by Eddie and Sybil Watson) or as redeposited tills.  The glacial / periglacial sequence at New Quay is also a puzzle, as I explained in an earler post.  There is a reasonable chance that some of these deposits are Wolstonian in age, laid down by ice flowing from an expanded Welsh Ice Cap.

Fourth, I now think that the frequency of Silurian gritstone erratics in the Devensian till at Newport  must have come from previously deposited Welsh till far out in Cardigan Bay.  That till is most likely either Early Devensian or Wolstonian.

Fifth, we have the matter of the till patches of central and south Pembrokeshire.  They look old, but are they old enough to have been associated with the Anglian glacial episode, when almost all other coherent till deposits associated with it have been worn away?

 Extract from the BGS geology viewer, showing fluvioglacial sands and gravels (pink) and till (light blue) outside the supposed Devensian limit in SE Pembrokeshire

Sixth, the map of rock troughs and tunnel valleys (which we have discussed earlier) suggests formation at least in part beneath a big Welsh ice cap -- much bigger than the LGM ice cap modelled by Henry Patton and colleagues.

So here is a proposal.  During the Wolstonian Glaciation Wales was completely inundated by ice from the Welsh Ice Cap, which was powerful enough to flow right across Cardigan Bay and into St George's Channel. It may be that during this glaciation the Irish Sea Glacier was less powerful and less extensive than it was during the Anglian and Devensian glacial episodes.

There we are -- a working hypothesis........ comments please!  I would be rather interested to know how the evidence from coastal sections in North Wales and South Wales fits -- or does not fit.......

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