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Saturday 29 February 2020

Devensian till at Amroth -- an Eureka moment

In this photo we can see a stony blue-grey matrix-supported diamicton beneath a broken cap of peaty material.  The capping material belongs to the submerged forest,  and wave action has already removed most of it from the vicinity.  Normally it would all be buried beneath a beach sand layer.

In this exposure the peaty / silty layer is thicker and more continuous, but beneath it the diamicton is clearly from the same horizon as that shown in the top photo.  Gary's description:  "top layer of woody peat lays on top of a blue organic clay in the lower section of the clay are pieces of limestone which match the limestone of the Ludchurch area".

Here we see several isolated fragments of grey clay layer (belonging to the submerged forest sequence?) with signs of stratification beneath.  There is one quite broad gravelly layer.  Is this the top of the till layer? The sediments are resting on a broken bedrock platform with a litter of broken stones and boulders of local origin.   With a few more storms these patches of clay will be entirely obliterated........

It's always a pleasure to report on observations / discoveries made by others, and today there is some big news from Amroth, courtesy of Gary Davies.  In the context of a discussion about the fresh exposures of the submerged forest around the Pembrokeshire coast, Gary posted some 2014 photos on the Pembrokeshire Geology Facebook page, and I was duly astounded.  Anyway, Gary has kindly given me permission to reproduce them here...... in his original post he says: ........"there's a layer of blue limestone pebbles under the peat and clay layers. Probably brought down from Luchurch as the ice melted ....... prior to the vegetation starting to establish itself."

In the top caption I refer to the clay-rich blue-grey deposit as a "diamicton" -- but there is really not much doubt as to what it is.  If I was to see this in Greenland or Iceland it would be accepted instantly as a clay-rich till laid down at the base of a glacier where lodgement processes were at work.  It is certainly not a river or shoreline deposit, and it is not a brecciated slope deposit or a turbidite.  It has all of the characteristics of a true till -- highly varied stone shapes and sizes, many different lithologies, faceted faces, and no sign of layering or other arrangement.  Are there striations on any of the pebbles?  That could be a clincher......... (There may of course be signs of layering at the top of this bed, where the material becomes richer in organic materials and grades up into the peaty layers of the submerged forest.)

At the moment I see no reason why this deposit should not be interpreted as Irish Sea Till, laid down by the Irish Sea Glacier as it pressed eastwards into Carmarthen Bay.  There are clear similarities with the deposits exposed in Freshwater West and West Angle.......  It is uncemented and fresh-looking, and so my provisional interpretation is that it is of Late Devensian age, like all of the other unconsolidated tills of West Wales.

Grey-blue clay-rich Irish Sea till at West Angle -- interpreted as having been derived from pre-existing sea-floor deposits.

A section from the beach at Freshwater West, interpreted as showing clay-rich Irish Sea till overlain by organic deposits from the submerged forest.  This is remarkably similar to the exposures at Amroth.

Gary thinks that the limestone fragments in the Amroth exposures look as if they might have come from Ludchurch, a few miles to the north.  I'm not very convinced about that, as they could equally well have come from the limestone outcrops of the South Pembrokeshire coast.  The local rocks in Amroth all belong to the Coal Measures -- sandstones, shales and mudstones, and coal bands as well.  That having been said, erratics can be moved about in different directions in different glaciations, and it is known that many erratics rom North and mid Pembrokeshire were indeed transported southwards and eastwards during the Anglian glaciation.  Erratics from Ludchurch could have been introduced to the Amroth area 450,000 years ago and then redistributed during the Late Devensian glaciation.

What does all this mean?

This is all rather provisional, until the "Devensian till" and other deposits have been properly examined, but all of our assumptions about the extent of Devensian ice now have to be subjected to intense scrutiny.  Readers of this blog will know that I have wrestled endlessly with the concept of the "South Pembrokeshire enclave" -- hemmed in by ice from the east, north and west.  But now we are seeing increasingly convincing evidence of ice coming from the south as well, to the point where it becomes ludicrous (ice does not like flowing in a direction opposite to that from whence it came.......).

My latest reconstruction of the Late Devensian maximum ice extent in West Wales.  The large "enclave" shown is problematic, and needs revisiting........  Amroth is located close to the Pembs / Carms border, north of the word "Unaffected" on the map. 

Readers of this blog will also know that I am in a jolly dispute with certain other senior glacial geomorphologists about the nature and age of "diamictons" (which I interpret as Devensian till exposures) all the way along the South Pembrokeshire coast, culminating on the east coast of Caldey Island.  That's the basis for the line shown on the above map.  I have done many posts describing these exposures, which occur mostly on clifftops.

But now are we needing to extend this line right up into Saundersfoot Bay and suggest that the ice of the Irish Sea Glacier travelled NORTHWARDS to reach the coast at Amroth?  What other evidence is there for glacial action on this stretch of coast?  I have to admit that it is the part of Pembrokeshire that I know least well.

But the idea of an enclave pretty well encircled by active glacier ice looks increasingly untenable, especially in a lowland location like this.  This is no upland nunatak.  So did the ice of the Devensian Irish Sea Glacier affect the WHOLE of Pembrokeshire?

Watch this space ..........

This is a nice image (courtesy Pembs Coastal Photography) showing just how extensive the Amroth submerged forest is, when exposed:

And here are some more, gleaned from the web:

In this surface exposure the peat is very thin, and we can see that it grades down into 
a sticky blue clay

It's also worth remembering that the Amroth foreshore is one of the most important Mesolithic sites in Wales:

A paper entitled "Flint-working sites on the submerged forest bordering the Pembrokeshire coast, by Mr. A.L. Leach, F.G.S., will be found in the Proceedings of the Geologists' Association for 1918 (vol. 29, part 2), from which the following remarks are taken.

"Amroth, Site B2. -- Below the western end of this village evidence of flint-working abounds on a site first noted in August, 1912, and examined each summer and winter since. The sea washes away the soft blue silt, leaving the flakes projecting more or less noticeably. On each occasion I removed all visible flints and by the time of the next visit a fresh crop had become exposed. In August, 1917, for the first time in my experience, the whole site lay buried under several inches of sand. Objects in flint and chert collected inclued: one hollow scraper, one long flake, ridge-backed and serrated (saw); two shorter flint saws, two conical cores, one core trimmed to yield small flakes, three contiguous flakes, three long cores of cherty flint, two cores of black glossy flint, ten flint pebbles partly chipped into cores, fourteen small blades, twelve large flakes, two calcined flints, some scores of roughly chipped and broken fragments.”

See also Royal Commission Inventory for Pembs 1925.

Leach, 1912:

One locality near Amroth, in Carmarthen Bay, yielded cores and flakes in abundance; the circumstances indicate the existence of a chipping-floor or implement-factory on this part of the submerged land-surface, which now, during spring tides, is covered by not less than 20 ft. of water. In the patch of submerged forest recently exposed at Fresh-water West, in southern Pembrokeshire (see NATURE, March 28, 1912), a few small flint implements were also found.
Ref: Antiquity of Neolithic Man. A. L. LEACH — Nature volume 90, page134 (1912)

This is from the 1925 Royal Commission inventory:

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