The MIS 4 (Early Devensian) glaciation

 

New Quay, Ceredigion.  Some rather interesting exposures....

It's interesting to see the debate about how the MIS 4 (Early Devensian) might have affected the western parts of Britain.  If you put "Early Devensian" into the search box on this blog, you will see that over the years I have revisited this issue over and again, seeking to explain details of the local Quaternary stratigraphy with reference to modelling studies and also field evidence from the south coast of Ireland, north Wales and elsewhere.

I have also highlighted this issue in some recent posts, arguing that it is a mistake to pull in evidence and suppositions about the "giant erratics" of the Bristol Channel and to pretend that they are related to MIS 4 glacial events.  I now think that these boulders are simply the last remnants of ancient glacial deposits, visible by chance on "washed" rock platforms near present sea level and also found at higher altitudes (60m, 80 m and even higher) above the modern cliffline.

But the evidence from southern Ireland (Courtmacsherry and Fethard, for example) is important and persuasive, and we have to accommodate it in whatever narrative we develop.

At the moment, in West Wales, we have not much to go on,  apart from evidence of two (at least) glaciations -- one before the Ipswichian Interglacial and the other later, in the Late Devensian.  Key evidence of a pre-Ipswichian glaciation comes from Black Mixen, near Lydstep, and Ceibwr on the north Pembrokeshire coast.  Look them up using the search box.

The only evidence of two recent (Devensian) tills separated by evidence of a melting interlude comes from New Quay -- and I am sure that one of the tills is the Irish Sea Till and the other is a till derived from the Welsh Ice Cap.  Could one of the tills date from MIS 4 and the other from MIS 2?  I'm not sure about that, since that would require a "non-glacial interval" of around 40,000 years.

Many researchers in the New Quay - Llanon area have been confused by the sequence of deposits. At New Quay I recorded a lower Irish Sea till and a local till above it -- the two separated by evidence of a warmer non-glacial interval.  Others, including Eddie Watson, recorded at Llanon a "basal" Welsh till overlain by Irish Sea till.  There is much discussion in the literature about a "Llansantffraid soil" which might be interglacial or interstadial, and abundant cryoturbation and other features indicative of intense permafrost conditions. During his lifetime I disagreed on many things with Prof Dai Bowen, but I do agree with him that there seems to have been an oscillating ice front on the Welsh coast in the New Quay - Llanon area during the Devensian........ but which till deposit (if any) dates to MIS 4?  And can it be correlated with the MIS 4 glacial deposits of Southern Ireland?

My interpretation (1965) of the stratigraphic sequence on the south 

coast of Ceredigion.

Quote from one of my earlier posts:

It's interesting that Etienne et al (2005) refer to another exposure at Llannon where a shelly Irish Sea till overlies "subglacial tills of Welsh provenance". This site is also described in the "Quaternary of Wales" volume edited by Campbell and Bowen, on p 65. They refer to a Welsh till overlain by an Irish Sea till, and argue for a complex interaction between Welsh Ice and Irish Sea ice along the coast of Cardigan Bay. They do not quite resolve the issue of the intense periglacial disturbance of the surface of the Welsh till, which seems to have taken place prior to the laying down of the Irish Sea till. Was there a prolonged periglacial episode with deep permafrost between the two glacial phases?

I am also very sceptical about the raised beaches of West Wales being interpreted as "cold climate" features just because they have erratics in them.  As far as I can see, all of the raised beaches contain erratics of various sizes, and that is not surprising since they are formed from whatever deposits happen to be available for wave attack and the removal of fines.  Raised beaches are essentially "lag deposits" reworked by the action of tides and waves.  Besides, faunal and microfossil evidence suggests that the bulk of raised beaches are genuine representatives of warm or interglacial climates.

This is all rather interesting.  I am looking at my West Wales stratigraphy to see whether there are sites that will be susceptible to "alternative" explanations as to the chronology and the severity of the glacial / periglacial climate.  One problem is that most of the sections I have examined over the years are in coastal valley locations, where a thick accumulation of pseudo-stratified slope breccia does not necessarily indicate a long period of climate dominated by permafrost and freeze-thaw processes. Rockfalls and scree development continues in such locations even when the climate is relatively mild and one has to be very careful about "climatic indicators".....

Much thinking going on........

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PS.  Let's not forget that there has been a considerable debate in Eastern England about a pre-LGM glaciation (assumed to be from the Early Devensian) that was more extensive than the LGM.   Our late colleague Allan Straw was instrumental in proposing this to other specialists:

http://www.emgs.org.uk/Mercian/Mercian%20-%20Individual%20papers/Mercian%20Geologist%20volume%2019%202016-2019/Mercian%202019%20v19%20p207%20MIS%204%20glaciation%20in%20eastern%20England,%20Straw.pdf

See also Prof Peter Worsley:

Worsley, P., 1991. Possible early Devensian glacial deposits in the British Isles. 47-51 in Ehlers, J., Gibbard, P.L., & Rose, J. (eds.), Glacial Deposits in Great Britain and Ireland. Balkema: Rotterdam.