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Thursday 11 October 2018

Abermawr -- the new Quaternary stratigraphy

This is a gorgeous photo showing the raised beach sitting on its rock bench and sealed beneath pseudo-stratified brecciated slope deposits (we used to call that  layer "the lower head"........  courtesy Pembs Coastal Photography, whose pictures are far better than mine!  The bit in the box is the enlarged section of the upper part of the parent photo.

As we can see, when we examine this image carefully, there is a wide range of rock types in the raised beach.  That almost certainly signals the incorporation into the beach of pre-Ipswichian glacially derived materials.  How else would they have got here?  From the Anglian glaciation?

So let's examine the rest of the modern stratigraphy. The full sequence looks like this:

8.  Blown sand, loess and colluvium, incorporating modern soil (c 1m)
7.  Upper brecciated slope deposits (c 2m)
6.  Reddish stained colluvium -- a pro-glacial redeposited layer (c 2m)
5.  Glaciofluvial sands and gravels incorporating flowtills (c 2m)
4  Mobile meltout till and flowtill (c 3m)
3.  Irish Sea till -- massive, with shells and carbonized wood (c 4m)
2.  Brecciated slope deposits with rockfall detritus close to broken-up cliff face (c 3m)
1.  Raised storm beach on rock bench (c 1m)

Thicknesses are approximate -- no part of the cliff shows the full stratigraphic sequence.  In general, the lower part of the sequence is in the north and the  upper (younger) deposits are best preserved in the south.  The upper brecciated slope deposits are, however, best preserved near the top of the highest cliffs, close to a convenient bedrock source.  They appear to be approximately equivalent to the reddish stained colluvium.  Together, deposits 6 and 7 used to be referred to as "the Abermawr rubble drift".

The raised beach (1) appears to be around 1m thick, but exposures are intermittent and sometimes masked by debris falls from above.  It's in a very risky position.  I tried to climb up to it once, and had to give it up, fearing for life and limb.

The lower brecciated slope deposit (2)  is clearly just a jumble of rockfall debris close to the rock outcrops, but further away there are signs of rough stratification, as seen in the exposures 50 years ago.  Now, as then, we can assume that these rough stratifications within the unit do represent some climatic oscillations, maybe over a very long period of time.

Broken rockfall debris within layer 2 -- note the heavily weathered igneous erratic boulder embedded within it.

Close-up of another heavily weathered igneous erratic in the lower slope breccia.

Lower slope breccia above the bedrock contact.  Here there is a rough stratification and the deposit is matrix-supported.

The Irish Sea till (3) has been well described in earlier posts and in the literature.  It is now known to be a lodgment till consisting for the most part of old sea-floor material from Cardigan Bay, dredged up and mixed with far-travelled materials as the Irish Sea Glacier came in from the north-west.  There are complex thrusting, folding and other internal structures.

The base of the Irish Sea till layer -- stained reddish by oxidation associated with water penetration from the  brecciated layer below. The upper part of the breccia has been churned by overriding ice and now incorporates some erratics carried by the glacier in its basal layer.

Massive Irish Sea till with an oxidised layer above, grading into flowtill (?) and mixed sediments with a lower clay content.

Above the lodgment till layer, which is grey or blue in colour when fresh, there is a complex layer with a foxy red or brown colour (the result of iron-staining) in which patches of flowtill and some detached masses of lodgment till exist in a complex relationship (4).  This can only represent the ice watage phase of the Irish Sea Glacier.

Above this, layer (5) consists mostly of water-lain materials, although some masses of flowtill are incorporated.  Collapse features and other structures indicate the existence of wasting ice masses at the time of deposition.

The junctions between layers 3, 4 and 5 are in places difficult to discern.  They must be very closely related in age.

Glaciofluvial deposits incorporating masses of assumed flowtill -- stratigraphically above the Irish Sea till layer.  Above the lighter coloured sandy layer in the centre of the photo, we can see the junction with the overlying foxy-red colluvium and redeposited till layer.

Redeposited glacial and glaciofluvial sediments towards the southern end of the exposure.  The greater the distance from the Irish Sea till exposure, the smaller the number of erratic clasts contained in the reddish silty and sandy matrix 

Colluvium with very few large erratics, near the southernmost part of the cliff exposure.  Note the rough stratification.

Recent exposures near the southern end of the cliff section reveal that the reddish stained colluvium (6)  is made for the most part from the material originating in layers 4 and 5.  The clay content is low, and this indicates that water has been of great importance in the deposition of the layer.  Sands, gravels and blown sand may all be incorporated; but the greater the distance from the Irish Sea exposure, the lower the content of large boulders and cobbles.  As one approaches the rocky crag which has recently emerged in the storm beach, the material exposed is best interpreted as a colluvial layer incorporating redeposited till and glaciofluvial materials.  As we have remarked in another post, this is remarkably similar in appearance to the "redeposited till" seen on the south Pembrokeshire cliffs, on Caldey Island, and in numerous exposures on the coasts of the Isles of Scilly.  At Westdale Bay there is a great thickness of this material, with marked pseudo-stratification.

Layer 7 (upper brecciated slope deposit) is only obvious to the untrained eye near the top of the highest part of the cliff, with signs of incorporated material from the glacial deposits below.  Most of the angular bedrock fragments have come from exposures upslope.  On the southern part of the exposure there was no adequate supply of frost-shattered or rockfall material, and so the slope breccia is not present at all.  It seems to be equivalent in age to the "redeposited till" and colluvium allocated to layer 6.  Together, these two layers comprise the "rubble drift" of earlier studies.

Layer 8, made up of blown sand, loess and colluvium, appears to be rather modern, and the modern soil has developed within it.

Blown sand and organic-rich soil horizon at the top of the cliff section.

In terms of the age of the deposits seen here, no evidence has emerged which casts doubt on the sequence which I proposed in 1965.  The raised beach is assumed to be Ipswichian or Eemian in age. The lower rockfall and slope breccia layer must span the early and middle Devensian, with subtle changes in stratification, texture and colour maybe representing environmental or climatic oscillations.  The flowtills and glaciofluvial deposits must represent the ice wastage phase, maybe over just a few centuries.  A cold episode following that (and maybe involving permafrost conditions) was responsible for the formation of the uppermost slope breccia.  This appears to be approximately equivalent in age to the redeposited till and colluvium.  The blown sand must have accumulated during the Holocene And the surface soil layer is still forming today.  

Where does the submerged forest come in this sequence?  It must rest upon the redeposited till and colluvium, and it must be approximately equivalent in age to the blown sand; maybe the sand dunes and the forest co-existed in close proximity.

Remnant of the submerged forest exposed in the sandy beach at low tide.

Sorry this is a rather protracted description!  I wanted to get it recorded while I still have the energy to do it!  It is, after all, one of the top 50 Quaternary sites in the UK, and possibly the most important in Wales, in that it holds a record of a complete glacial - interglacial cycle over a period of well over 100,000 years.

1 comment:

TonyH said...

Was there in May during splendid weather.