Caldey Island till — and this blog becomes respectable

The current version of my map showing the proposed Devensian ice edge in Pembrokeshire.  This was published last year, and I still think it is more or less correct......

The Ballums Bay till exposure which has been examined by John Hiemstra and his colleagues

Erratics from the Ballums Bay till

In the latest peer-reviewed issue of Quaternary Newsletter (June 2019) there is an article published by  assorted craggy academics whom I greatly respect:

John F. Hiemstra, Richard A. Shakesby, Geraint Owen and Simon J Carr, 2019.  CALDEY (‘KALD EY’ IN OLD NORSE) WAS LITERALLY A ‘COLD ISLAND’, BUT WAS IT UNDER DEVENSIAN ICE?

Quaternary Newsletter Vol. 148 June 2019, pp 21-31

It’s behind a wall at the moment — available only to QRA members, but I hope one or another of the authors will put it onto Researchgate or Academia.edu.

They were responding to a call I made some time ago for some “bright young things” (in other words,  enthusiastic young glacial geomorphologists) to take a look at South Pembrokeshire in general and Caldey Island in particular, with a view to sorting out the glacial sequence there.  Since they have done a lot of work on the Gower Peninsula — not so very far away — they were understandably intrigued by my identification of a Devensian till on Caldey, since to accept it as such would imply a major realignment of glacial limits in the Bristol Channel region.  It’s a detailed and highly professional paper which is also good-humoured, and that’s something one always likes to see.  But one very interesting feature of it is the citation of this blog on a number of occasions in the text.  The authors have clearly read many of my entries relating to Caldey Island and the Bristol Channel, and they have taken them seriously.  So I thank them for that.  Finally, an acceptance that a blog like this can be competent and capable of contributing to academic debate......

The paper starts with a summary of the various views on ice limits in the Bristol Channel and the SW approaches; my latest proposed Devensian limit is shown, but it is erroneously labelled as the “Cardigan Bay ice lobe”.  I have never called it that, and have referred to it on a number of occasions as the Bristol Channel lobe or the Carmarthen Bay lobe. That’s a minor point.  But the authors do correctly refer to my view that the Devensian ice cannot, according to glacial theory and ice mechanics, have reached far to the south of the Scilly Isles without also pushing far to the east in the Bristol Channel and also impinging on the coasts of Devon and Cornwall.

The bulk of the paper deals with the small exposure of reddish till (which I refer to as the ORS facies) in Ballums Bay towards the eastern end of the island.  Following a field visit in 2016 the authors subjected the till (or diamicton, if you want to call it that) to a host of analytical procedures, after which they conclude that it is indeed a till.  But what sort of till, and of what age?  They consider two alternatives: Anglian or Devensian?  They argue quite convincingly that the till is probably Devensian, then they argue less convincingly that it might be Anglian, and then come down with the conclusion that the latter (older) age is to be preferred.

Much as I respect John Hiemstra and his colleagues, I do not agree with them. They have omitted two very important lines of evidence in their discussion:
1.  The regional context from Ballums Bay, other parts of Caldey, and the nearby mainland;
2.  The presence of cemented deposits in the neighbourhood, including raised beach and brecciated slope deposits and ancient till which are vastly different in appearance from the unconsolidated sandy till at Ballums Bay.

On the first point, there are other exposures of fresh till on the island, including one near the island landing stage. There are also abundant other exposures of fresh and unconsolidated till on the cliff tops of south Pembrokeshire and in coastal embayments, as I have shown in a series of posts on this blog.  (In fairness to John and his colleagues, most of these posts were published last year — maybe after this article was written.). These other deposits have not been analysed in detail, but they lie in the same stratigraphic position as the Ballums Bay till, and share many of the same physical characteristics.  They are not clay-rich lodgment tills but flow tills and ablation till subjected to varying degrees of "rearrangement" or redeposition following initial emplacement.  If these deposits are of Anglian age, why are they not overlain by Ipswichian interglacial deposits and periglacial and other deposits dating from the Devensian cold period?
See these posts:

https://brian-mountainman.blogspot.com/2011/08/devensian-till-at-bullums-bay-caldey.html  (this is the post which is cited in the new QN article)

https://brian-mountainman.blogspot.com/2011/08/glaciation-of-caldey.html

https://brian-mountainman.blogspot.com/2018/08/south-pembrokeshire-devensian-till.html

https://brian-mountainman.blogspot.com/2018/04/the-devensian-ice-limit-in.html

https://brian-mountainman.blogspot.com/2018/04/strange-till-near-stackpole-head.html

https://brian-mountainman.blogspot.com/2018/08/more-south-pembrokeshire-till-locations.html

https://brian-mountainman.blogspot.com/2018/04/even-more-south-pembrokeshire-till.html

https://brian-mountainman.blogspot.com/2017/03/the-south-pembrokeshire-lgm-again.html

The exposure of reddish Caldey Island till near the landing stage.  This has similar colouring and other characteristics as the Ballums Bay exposure

On the second point, it’s a pity that the authors have not visited Lydstep and Black Mixen.  If they had, I venture to suggest that they would have come away convinced that there are TWO tills in South Pembrokeshire, one cemented with calcium carbonate or iron oxide / manganese oxide cement, and the other unconsolidated.  Both tills are visible at Lydstep.  If the Ballums Bay till had been laid down during the Anglian Glaciation, I am convinced that it would by now have been solidly cemented, given that it rests on Carboniferous Limestone.  Only 50m away, on the south side of the bay, there are solidly cemented brecciated slope deposits which appear — by correlation with other S Pembrokeshire exposures — to be of Ipswichian or Early Devensian age.  If the Ballums Bay till is even older, and deemed to be of Anglian age, why is it not also cemented, located as it is in a gully where water flow -- and thence carbonate precipitation -- would have been considerable?

Solidly cemented Anglian (?) till at Black Mixen, Lydstep.

See these posts:

https://brian-mountainman.blogspot.com/2018/03/lydstep-headland-key-quaternary-site.html

https://brian-mountainman.blogspot.com/2018/05/lydstep-ancient-till-site-confirmed.html

https://brian-mountainman.blogspot.com/2018/05/devensian-till-at-lydstep.html

https://brian-mountainman.blogspot.com/2018/11/on-significance-of-cementation.html

So this is a detailed and welcome paper, which draws attention to an important site — but some of the lines of reasoning are questionable, and I think the authors have got their tentative conclusion all wrong!

Suggested relationships of ice masses during the LGM in the Bristol Channel area (my map, not that of Hiemstra et al)

PS.  In 1905 EL Dixon wrote of Bullum's Bay: "..... the glacial deposit appears to overlie the raised beach, although the exposure is obscure, and the evidence of superposition is not so conclusive as in Gower." (Summary of Progress for 1905, Mem Geol Surv, p 70).  This is my impression too -- but excavation will be needed to establish where the till lies with respect to the cemented limestone breccia just around the corner towards the south.  Is there a cemented raised beach too?  I think so, but I must look back at my field notes for reassurance.  I'm surprised that the authors of the new paper make no mention of the local sedimentary stratigraphy -- which in itself suggests a Devensian age for the till:

6. Modern soil
5. Sandy loess or brickearth (c 1m thick)
4. Reddish till with ORS and other erratics (c 3m thick).  This is the till now analysed.
3. Blocky broken limestone head (up to 2m thick)
2. Concreted raised beach and included angular limestone fragments (up to 50 cms thick)
1. eroded raised beach platform up to 10m above MSL.

There is nowhere where you can see this full sequence, but bits of it (including other patches of till) can also be seen in the cliffs to the south of the main exposure.