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Tuesday 3 May 2022

Porthmelgan Irish Sea till exposure

 

The discontinuous exposures of glacial and other deposits at the head of Porthmelgan bay.  They are difficult to interpret because of slumping.



Porthmelgan is a small bay between Whitesands and St David's Head.  I spent a lot of time here in the course of my doctorate research in 1963 and 1964, and I was intrigued by the somewhat unusual "boulder bed" interbedded in the Irish Sea till.  Others might have interpreted it as representing an interval between two glacial phases, but I cannot see any weathering horizon or other unconformity, and conclude that it is simply made of a broken mass of bedrock (from the gabbro exposures on the headland) which has been incorporated into the basal layers of the ice and hence into the sticky clay till layers accumulated by lodgement.  The boulders are mostly sub-angular, so they appear not to be related to the boulder bed above the raised beach in Whitesands Bay.  I would describe them as typical glacial erratics that have not travelled very far.

There are some far-travelled erratics in the till, claimed by Cox to have come from Scotland and Antrim.

I plan to go back and take another look.......

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