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Tuesday, 11 December 2018

Poppit raised beach platform and overlying sediments

I visited Poppit (at the mouth of the Teifi estuary) the other day, just to check out what might have changed since my last visit.

The raised beach platform is as prominent as ever,  just round the corner from the main beach and most easily accessible at low tide.  But a lot of storm waves have been breaking onto the RB platform surface, and the overlying deposits are in something of a mess.  There has been a lot of slumping, and the sequence is in many places difficult to discern.

Here are some images:

Broken rock platform some metres above HWM but still affected by storm waves.  Much slumping here in the drift cliff, with some traces of the raised beach and much mixing of periglacial (brecciated bedrock) deposits and glacial deposits related to the Irish sea Glacier.

At the base, c 1m of brecciated bedrock debris (head) representative of Early and Middle Devensian (?) periglacial conditions but also containing raised beach cobbles.  There is a clay-rich matrix, suggesting that this is a reworked deposit incorporated into the base of the overriding Irish Sea Glacier.  In the top part of the photo we see 1.2m of clay-rich Irish Sea till with apparent shear structures.  Above that, a 1m thick glacial deposit with a greater sand and gravel content, suggestive of flowtill deposition during ice wastage.  At the top of the section, a darker sandy deposit which might be sandloess.

This section is a bit of a puzzle.  Bottom left -- the exposed surface of the RB platform, here composed of thin-bedded soft dark shales.Above that (at the base of the section) a 30 cm layer of convoluted brecciated  bedrock fragments incorporating larger local sandstone clasts and erratics.  Above that, up to 50 cms of brecciated sandstone and shale rubble held in a matrix of silt and clay.  Well-rounded RB cobbles are incorporated.  This is probably a basal glacial deposit incorporating periglacial and interglacial deposits.  Above that, up to 1m of relatively stone-free flow till (?) with more sandy materials towards the ground surface.  

A good exposure of the raised beach platform overlain by the sequence of deposits as described above.


I am puzzled by the layer of fine broken shale fragments at the base of the sequence.  Surely it cannot predate the raised beach?  At the moment I am inclined to think that it post-dates the beach sediments, which were protected near the innermost edge of the platform while the platform itself was subjected to severe periglacial conditions at a time of relatively low sea-level -- during the Early and Middle Devensian.

More work needed here.......














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