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Wednesday, 14 February 2018

Abermawr Holocene sediments

Over all the years I have been studying the sediment sequence at Abermawr (on the North Pembrokeshire coast) I have never before seen so much of the storm beach stripped away.  And it has been a big surprise.

I had assumed that the storm beach - which is quite spectacular -- was very thick indeed -- maybe 3m or 4m thick, and that it was resting on a deep notch cut into the underlying Devensian and Holocene sediments.

However, we can see in the above photo (showing the northern drift cliff)  that there is no deep erosional notch, and that the pebbles rest on a steeply sloping surface of till and underlying periglacial slope deposits (the lower head).  The volume of the pebble beach is maybe just 50% of what I had assumed.........

If we assume a similar situation for the middle part of the bay, this is realistic:

In other words, the storm beach in the middle of the bay does not rest on a deeply cut notch but on a slope of organic silts and clays dating from the Holocene.  I am not sure whether there is actually a stratigraphic break between the submerged forest layer and the Holocene organic layers -- maybe they are both part of a continuum.

I now think that the front face of the stormbeach is not very thick at all -- I shall keep it under observation.

The sandy beach rises and falls according to the severity of winter storms -- so sometimes the submerged forest is visible, and sometimes not.


Abermawr is one of the "top 50" Pleistocene sites in the UK -- details here:


Myris of Alexandria said...

I am not certain of the significance of the observation, can you explain a little more.

BRIAN JOHN said...

I had assumed that the storm beach ridge was very deep, and that it was tucked into a notch cut out of the sediments behind the ridge. That would have meant no continuity between the sediments outside and inside the ridge. What can currently be observed is that the beach ridge is not very thick at all, and that it rests on the same set of sediments that we see in the boggy area on the inland side. So the ridge must be relatively recent...... To sort out the sedimentological / time sequence properly somebody needs to do some sampling and pollen analyses. I suspect that this site may be extremely important for sorting out the details of the Devensian and Holocene in West Wales. We (sort of) knew this already, but now it's even more intriguing.