Cliff Retreat at Abermawr

 

The old cliffline, a century ago, was to the west (on the seawards side) of the lane shown by the yellow line. The edge of the pebble ridge was also to the west of the line.

A comment from Tony prompted me to do this post on the rate of cliff retreat at Abermawr, where one of the most important Quaternary sites in Wales is gradually being eaten away by the sea.  I hesitate to blame this on global warming -- this is in my view a natural process triggered by storm waves in a bay choked with soft sediments.  Maybe the increasing frequency of northerly gales has something to do with it?  But I don't have any reliable info on how that might have changed over my lifetime.  It seems to me that we are getting much more frequent northerly, easterly and southerly winds and fewer windy spells coming from the west and south-west -- but I need the data........

Anyway, around 1920 an old roadway (perfectly suitable for wheeled traffic) ran down the slope from near the Telegraph Station, across the valley behind the storm beach ridge, and up the other side of the valley near the old quarry.   We can see this on the old OS maps at the 1-25,000 and six-inch scales.  When I started working in this bay on my doctorate research in 1963, the cliff-line was very close to the yellow line shown on the satellite image above.  A stretch of the old roadway between its hedges was still in existence in the NE corner of the bay, ending in a sheer drop down onto the beach.  The rocky outcrops around the marked cave were hidden from view -- deeply buried beneath Quaternary sediments.  Similarly, there was no sign at all of Halfway Rock (my name) near the foot of the northern slope.  Since then, the cliffline has retreated in the north by about 50m, and more than that in the centre of the valley.  The storm beach is being thrown back (eastwards) across the sediments in the bog, and these sediments are nowadays exposed as "submerged forest" peat beds on the seaward side of the pebble ridge.  The southern sediment cliff has retreated about 30m.  Some of these exposed peats (which appear very intermittently between HWM and LWM, generally in the winter) may be less than a thousand years old.  

So the interpretation of the submerged forest at Abermawr is a three-dimensional exercise, taking into account the Holocene rise of sea-level and the overwhelming of old peat beds and forested areas that once thrived far below present MSL, and also the lateral shift in the position of the storm beach and the sediment cliff in response to coastal erosion.

As the cliffs have retreated over the sixty years of my intermittent research here, the sediment sequence, and the appearance of the cliff face, have changed enormously -- but luckily the Devensian story is still the same.  The only "new" sediment to have appeared is the interglacial raised beach which has been exposed on remnants of a high rock platform not far from the marked cave.  We await further developments........

Bear in mind that this site is one of the type localities for the Quaternary in Wales, since there has been continuous sedimentation here through many changes of climate (including a full glacial episode) over maybe 150,000 years.

https://brian-mountainman.blogspot.com/2018/10/abermawr-then-and-now.html

https://brian-mountainman.blogspot.com/2012/03/magic-cave-of-abermawr.html

Rock outcrops beneath sediments in the NE corner of the bay.  In 1963 these rock cliffs were completely covered with sediments, and I had no idea that they existed.........

As these old rock cliffs have gradually been stripped of the sediments that buried them, we have seen exposures of fragments of an old raised beach platform, with pebble beds resting on them.  Unfortunately they are still inaccessible except at risk to life and limb......

This cave, and its surrounding rock exposures, were all deeply buried by Quaternary sediments until it was exposed about ten years ago.

Fresh rockfalls at the southern end of the bay.  Here both the sediment cliff and the rock cliff (made of soft shales) are retreating inexorably under attack from the waves.

Halfway Rock, a prominent ridge of sandstone and quartzite projecting through the storm ridge.  Sixty years ago this ridge was completely buried beneath sediments -- slope deposits and glacial deposits dating from the LGM..