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Sunday, 22 April 2018

The raised beach at Broad Haven South


The northern section of the raised beach platform in Broad Haven South bay.  This section -- in a more sheltered part of the bay -- is much influenced by the fracture patterns in the Cartboniferous Limestone bedrock, so the rock platform is discontinuous, with fragments broken up by deep gullies. 


The southern section of the rock platform, out towards Saddle Point.  Here, because exposure is greater and wave action more concentrated, the platform is more coherent and easy to examine.  The cemented raised beach is so thick and resistant to erosion that in places it roofs over gullies and caverns that appear to have formed more recently.

Apart from Poppit on the Teifi estuary, this is the easiest raised beach exposure to examine, at least in West Wales. The rounded cobbles in the beach are almost all made of locally derived limestone, but there is a lot of broken bedrock debris mixed in, which suggests a cold climate with cliff falls and periglacial slope accumulations in the period immediately following beach formation. In places there are shells in the raised beach gravels and sands, but because everything is cemented these are difficult to extract, let alone identify.   The assumption is that the beach itself probably dates from the last (Ipswichian) interglacial, and that the raised beach rock platform is quite probably older -- maybe it is very old indeed, freshened up over several interglacials when sea-levels seem to have returned to similar high "interglacial" levels.

Here are a few images of the raised beach exposures -- transformed into concrete because of the calcium-rich environment.





What of the stratigraphy?  It's difficult to discern, but in places we can see up to 2m of sandrock (probably blown sand either partly of fully cemented) above the raised beach, with fresh (uncemented) head or slope deposits above, and then with fresh blown sand and sandy loam at the surface.  There appears to be no till or other glaciation-related material at this site.  



There may be three head deposits in this sequence -- one (cemented) mixed in with the raised beach or closely associated with it; another (also cemented) interbedded in the sandrock; and another (uncemented) higher in the sequence, beneath the more modern blown sands and sandy loam.

If this sequence represents the full Devensian cold episode of around 80,000 years, the uppermost head probably represents the late-glacial episode of cold climate incorporating the Older Dryas and Younger Dryas (around 10,000 years ago) and the uppermost blown sands and sandy loam were probably laid down in the Holocene or "post-glacial period".  That would match the situation elsewhere in Wales and in South-west England and the Isles of Scilly.  But where in the sequence does the Late Devesian glacial episode fit?  Somewhere in the "sandrock" period?  I's a bit of a puzzle, but somebody will no doubt sort it out.......

PS.  I found some of my old records from many years ago, and this was the sequence I picked up from some of the exposures in the 1960's:

8.  Sandy loam and blown sand
7. Upper head (uncemented)
6.  (Missing)  Fluvioglacial sands and gravels
5. (Missing)  Till from Dewisland (Devensian) glaciation
4c.  Lower head (cemented)
4b. Cemented sands (sandrock)
4a. Head incorporating raised beach cobbles (cemented)
3.  Cemented raised beach
2.  (Missing) Older glacial deposits -- mostly destroyed
1.  Raised beach platform (complex modifications over several interglacials?)

Note that both the older and younger glacial sequences are missing here.  I was fairly convinced in 1970 that the cemented deposits were all pre-Devensian Glaciation, and that the uncemented deposits are Late Devensian and Holocene in age.






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