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Tuesday, 13 March 2012

The Magic Cave of Abermawr


This cave at the northern end of Abermawr beach in North Pembrokeshire is rather mysterious, since it wasn't there 50 years ago, when I was a research student studying the glacial deposits in the bay.  In fact, it was there -- it's just that it was hidden.  In 1962 the storm beach was about 5 m higher than it is today,  and the glacial and periglacial deposits masked virtually all of the bedrock that we can see in this photo.  The "drift cliff" has retreated about 20m over the space of 50 years -- a very dramatic rate of change.

So this cave is very old, and has nothing to do with present-day coastal processes.  At the very least, it must be from the last interglacial episode, around 100,000 years ago.  Because of its position, around HWMST, the sea-level at the time of its formation must have been considerably higher than it is today, and there must have been a deep embayment running well inland, along an ancient valley which is nowadays protected by a big storm beach.   Later on, as the climate cooled and as sea-level started to drop with the onset of the last glacial episode, the cave was left high and dry, and was gradually choked or blocked up with debris falling down the cliff face and slope -- we would call this material rockfall scree or periglacial "head"  -- since there is evidence that much of the accumulated slope deposits was laid down at a time a very cold climatic conditions.

The cave might indeed be several interglacials old, periodically exposed and covered up as the climate has warmed and cooled over the last half million years or more.

Human artifacts would have no chance of surviving here, but this illustrates how caves higher up on cliff faces would have been attractive settlement sites in the Palaeolithic and Mesolithic periods, and how they could have been periodically "sealed" by slope and glacial deposits and then exposed again by the processes of coastal erosion.  This is precisely what seems to have happened to many of the limestone caves of South Pembrokeshire and the Gower Peninsula -- perhaps explaining why very significant deposits and artifacts (and indeed human remains such as those of the "Red Lady") have been well preserved.

3 comments:

Tony H said...

Mention of caves such as this Abermawr one, higher up on cliff faces, and thus being attractive settlement sites in the Mesolithic as well as Paleololithic, reminded me that, at the photogenic Carreg Sampson chambered tomb, only 3 miles away westwards and also coastal, Mesolithic flintwork [now in Scolton Museum] was found.

alexgee said...

Hi Brian
This is ery interesting, could you confirm a couple of thoughts.
Is it formedin Carboniferous Limestone?.
When you refer to Medium High water Spring Tides,does this equate to the top of the beach platform?

Regards Alex

BRIAN JOHN said...

No -- Ordovician quartzites, sandstones and shales, with some quartz layers. This is not a solution cave -- caused bu mechanical processes on the cliffline. I mean "mean high water spring tides" -- much higher than the mean tidal position halfway between HW and LW, which is the altitude around which most coastal erosion processes operate. Some bits of the raised beach platform are higher than this cave -- some bits are at about the same altitude.