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Sunday, 3 September 2017

Devensian sediments at Aber Bach (Hescwm)

We actually went in for a swim the other evening,  at Aber Bach (Hescwm), between Pwllgwaelod and Fishguard on the Cardigan Bay coast.  Freezing cold, it was......

Anyway, some good came of it, because I discovered that there is a rather fine exposure there at the moment, at the head of the bay, where the footpath comes down to the pebble beach.  We can see about a metre thickness of beautiful pseudo-bedded slope deposits (made of mudstone fragments from the adjacent cliffs).  On top of that is a layer of "churned" slope deposits, almost certainly attributable to the movement of ice across the old shoreline, pressing south-eastwards.  This is about 50 cms thick.  And above that is classic Irish Sea till, fine-grained, calcareous and containing a scatter of small erratics cobbles and pebbles.  There are some larger clasts as well, but the exposure is obscured by slumping and vegetation, so the thickness is difficult to determine.  It seems to be at least 2 m thick.

So the early part of the glacial cycle is represented here -- and of course the section ties in very nicely with that at Aber-mawr.  Here is the sequence, in its correct stratigraphic position:

Dessicated surface of the exposed Irish Sea till.

"Churned" slope deposits underlying Irish Sea till.  This represents the arrival of Irish Sea Glacier ice coming in from Cardigan Bay

 Pseudo-stratified slope deposits, possibly accumulated during the Early or Middle Devensian. At the base the sediments are contorted.

Without excavation it is not possible to see what happened here during the Late Glacial and post-glacial period.


Myris of Alexandria said...

What is going on in the bottom left of the last photo?
There seems to be a change in orientation.

BRIAN JOHN said...

Quite correct, Myris. The exposure is not very good, but I think that is the "bending over" of the mudstone strata, which are here almost vertical. This is in the broken rock zone, between the solid rock and the mobile surface layers or regolith. We see it on the top of most of the coastal cliffs in Pembs.

Myris of Alexandria said...

Yes that can be a bugger in highly tectonised areas with poor exposure, as it can be very misleading.

Rocks are so much simpler in thin section. Warmer and drier too.

Anything more on the erratic HH Thomas comment? Don't know it or the reference.

Argillaceous foundations?