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Monday, 2 December 2024

Age of raised beach deposits of South-Western Britain



Raised beach localities in SW Wales

The recent discussions about the Courtmacsherry and Fethard raised beaches, and the suggestion that they might be of early or Mid Devensian age, should not blind us to the fact that over many years evidence has been assembled to indicate that the bulk of raised beaches around the Celtic Sea and Bristol Channel coasts are are truly interglacial -- and probably largely of Ipswichian age. I assembled some of the evidence in my "Nature" article of 1968:

John, BS.  1968. Age of the raised beach deposits of south-western Britain.  Nature, 218 (5142), pp 665-667.

Here is the link:

 https://www.researchgate.net/publication/232783499_Age_of_Raised_Beach_Deposits_of_Southwestern_Britain

The evidence assembled from West Angle and elsewhere still stands -- and of course the evidence relating to the temperate climate of "Patella Beach" times goes back much further, to Strahan (1908) and others.  



Patella and other shell fragments in the cemented Patella raised beach, Gower.  Photo:  Jessica Winder

That having been said, attempts to date the "Patella Beach" exposures on the Gower coasts have been beset by difficulties, and as reported by Jenkins and others in 1985, amino acid dates for Patella shells suggested that they were over 200,000 years old........... Jenkins et al also reminded us that TN George, whose opinions were somewhat mobile, argued at one time that the presence of Lundy granite erratics in the raised beach indicated that it was formed at a time of cold climate, with glacier ice in the vicinity.  Quote: 

"The first modern work on the Quaternary beaches of Gower was by T. N. George (1932) who argued that this Patella beach was deposited in glacial conditions as evidenced by its erratic pebbles which he believed had been rafted on ice-floes. He argued that it predated both Older and Newer Drift Glaciations of Gower....."

It can also be argued that the close proximity of periglacial (?) head or slope breccia to the cemented raised beach exposures might also point to the presence of permafrost or at least a cold climate at the time of beach formation.  As pointed out by Kokelaar (2021) the Patella Beach exposures frequently contain sharp edged and angular rock fragments derived from the immediate vicinity.  But in my view these are most likely to be cliff face rockfall materials which have not been present in the beach for long enough for clast smoothing or rounding processes to have been effective.  (For comparison, many of the present day beaches of Pembrokeshire incorporate recent rockfall materials.)



A cone or fan of rockfall debris dropped onto the pebble beach at Newport in 2019.  Most of this material has now (2024) been dispersed by wave action and incorporated into the beach.

McCarroll (2015) argued that the amino acid dates that have bedevilled the Patella raised beach debate are mostly incorrect, and that the raised beaches date (with rare exceptions) from the Ipswichian Interglacial.

We must keep an open mind and see where the evidence takes us......


Sunday, 1 December 2024

The Whitesands boulder bed

 


Two smoothed and rounded giant erratics resting on the interglacial rock platform at Whitesands South in Pembrokeshire.  Like many other boulders resting on the rock platform at this location, they appear to have been in position, affected by wave washing, prior to the deposition of the materials that lie around and on top of them.  In this respect they bear direct comparison with the famous Saunton pink granite erratic and the giant erratic at Baggy Point, which both appear to have been sealed beneath sandrock and slope breccia before being exposed by coastal processes in the current interglacial.

The most logical explanation of these boulders is that they are "lag" features derived from pre-Ipswichian glacial deposits -- isolated following the removal of finer matrix materials.  Following the Ipswichian interglacial, they were covered by, and incorporated into Early and Mid Devensian slope breccias (sometimes cemented) and sandrock, and then later overridden by the Irish Sea ice which laid down the Irish Sea till and its related ice wastage products.   While these sediments accumulated, the position of the coastline was far away, to the west.

The "free" erratic boulders on the rock platforms around the Bristol Channel coasts could be of many different ages, but I see no evidence which might lead to them being attributed to low sea-level stillstands during MIS 3 or MIS 4.

Devon and Cornwall -- coastal erratics

 


The Giant's Rock at Porethleven. This is of a type of garnetiferous gneiss which is not found anywhere else in the UK .


Twin erratics on the rock platform near Godrevy, near Hayle.  Raised beach, sandrock and slope breccia exposed in the cliff. 


No identifiable erratics here, but this section at Portheras (about 4 km from Lands End in Cornwall) shows a beautiful exposure of the raised beach resting on the rock platform and capped by blocky slope breccia suggestive of a periglacial climate.

Thanks to David Evans and his great website for these images which he has made freely available.




Basalt boulder on the rock platform at Trebetherick (photo courtesy Jenny Bennett).  This rock may have come from far away, or maybe from a local source.  There is a dolerite sill nearby.





Ancient cataclysmic floods -- erratics from Lake Missoula drainage events

 






The floods (maybe as many as 100 flood "events" during the Quaternary) were all cataclysmic in the best sense of the word, but naturally we know most about the latest series of meltwater outbursts at thge end of the last (Wisconsin) glacial episode.    Many of the giant erratics are well over 100 km from their source areas.  Many were incorporated in glacier ice and swept away in icebergs following the collapse of each successive ice dam -- these have many of the diagnostic characteristics of typical glacial erratics.  They are often sub-rounded or sub-angular.  But others are angular or sharp-edged, suggesting they they were literally ripped away from the bedrock by the force of the water, armed as it was with abundant fragments of glacier ice.