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Sunday, 13 December 2015

Quaternary stratigraphy in Pembrokeshire

Correlated stratigraphic columns for the Quaternary sequence in North Pembrokeshire.  These are for five key coastal sites. Key:  1: bedrock. 2: erratic traces of earlier glaciations. 3: raised beach. 4: stratified slope deposits / head. 5: Irish Sea till and equivalent local till. 6: fluvioglacial sediments incorporating flow till. 7: upper head / stratified slope deposits. 8: blown sand / brickearth / colluvium capped with modern soil.

Since it is our pleasure on this blog site to spread darkness into dark places by providing information that people want (and sometimes don't want, but let that pass.....) we had better explain the Pembrokeshire Quaternary stratigraphy.   We have done this before, but here we go again, just for our esteemed friend Myris.

Feel free to take a look at these:

The essentials of this stratigraphy (which we can pick up in scores of sites) were established by TJ Jehu in 1904.  Other field workers confirmed it, and in 1963-66 I devoted three years of my valuable time to investigating and elaborating it and working out what it all meant.  It is backed up, in my doctorate thesis and in published papers, by scores of sediment analyses, stone shape analyses, provenancing exercises, stone orientation analyses, fossil studies and radiocarbon dates.  I am rather sure that I know what I am looking at, as are all the other geomorphologists who have worked on the West Wales Quaternary. There are areas of disagreement, of course, and these are summarised in the two books called "The Glaciations of Wales" and in the GCS review volume called "The Quaternary of Wales."  Prof DQ Bowen has also published many papers in which he discusses the relationships between the stratigraphies seen at assorted sites across South Wales.  On the stratigraphy, we are mostly in agreement, even though we might disagree on Stonehenge.......

Essentially we have traces of an ancient glaciation in erratics on rock platforms and incorporated into later sediments.  Then a raised beach, indicating interglacial (Ipswichian) conditions with a sea level higher than that of today.  Then a long period of oscillating cold climate with accumulating "lower head" -- assumed to represent the Early and Middle Devensian.  Then Irish Sea till and equivalent local tills in inland sites -- representing the Late Devensian glacial phase.  In the Cardigan area this till is underlain by lacustrine deposits associated with Lake Teifi -- formed as a result of advancing ice blocking off the exit of the Teifi valley and river system.  The wastage of the ice is represented by widespread fluvioglacial deposits, sometimes with flowtills and rockfall debris incorporated.  Then there is evidence of another cold phase in the upper head -- with signs of the Upper Dryas cold snap showing up in some sediments.  After that, we see assorted relatively recent deposits which vary according to location and slope characteristics -- in some places blown sands or brickearth, in others stratified slope deposits, and in others colluvium.  Then at ground level we have modern soil.

At Rhosyfelin the archaeologists have recognized the glacial till, but have totally ignored the fluvioglacial sediments and the rockfall accumulations for reasons which I do not understand.  At any rate, the only layers they are really interested in are layers 7 and 8 in the stratigraphic sequence, since they coincide with the "archaeological horizons" and occupation traces.  Bits and pieces of those layers are now also dated by radiocarbon and OSL methods.

For further enlightenment, as we are often exhorted to do, go to the primary literature!  A lot of it is there, ready to look at, on my Researchgate pages. 

Some typical deposits:

 Raised beach platform at Poppit, overlain by Ipswichian raised beach and Lower Devensian rockfalls and head

 Late Devensian Irish Sea till at Aber Mawr

 Stratified fluvioglacial deposits (Late Devensian) at Trefigin

 Late Devensian / Holocene upper head, rockfall debris and brickearth at Aber Mawr South

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