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Sunday, 26 February 2017

The West Angle enigma (1): the stratigraphy



Three photos of the drift cliff / main exposure at the back of West Angle beach.  As we can see, the vegetation cover has varied over the decades -- and every now and then it is "freshened up" by extreme winter storms.  If you are a geologist or a geomorphologist, what you see is partly a matter of luck.......


One of the most difficult sites in the British Quaternary is at West Angle, near the mouth of Milford Haven.  It is considered by all workers (and there are many who have published) to span a very large part of the Late Quaternary.  Some, including DQ Bowen, think that there are signs of TWO glacial episodes represented here, with glacial and periglacial deposits separated by a raised beach of probable Ipswichian (last interglacial) age.  In the early 1900's there was a clay-pit here, worked for clay which was used in an adjacent brick-making kiln, but following the closure of that operation the clay-pit has been abandoned and is overgrown, and the adjacent sedimentary exposures at the back of the beach have become degraded and very messy.  There are many slumps, and many places where human interference has destroyed the natural stratigraphy.  It's difficult to see what on earth the section looks like, let alone to decide what it all means.....

The earlier workers, including Leach and Dixon (1921) had the advantage of being able to examine the stratigraphy in the clay-pit.  They recorded the sequence as follows:

8.  Angular stony loam - head?  3 ft
7.  Sand with abundant rounded bedrock flakes (?!!) plus erratics (including one from N Pembs) -- 4 ft
6.  Well bedded gravel and sand -- 2 ft
5.  Buff laminated loam including erratics -- 5 ft
4.  Dark blue loamy clay with wood and plant fragments -- 5 ft
3.  Irregular loam, sand and fine gravel  - 3 ft
2.  Grey clay or loam -- 1.5 ft
1.  Clean buff sand -- 1 ft

This sequence has been mis-reported by other researchers to suggest that till was found in the pit beneath raised beach materials.  However, the Geological Survey surveyors were very experienced, and  they did not refer to the gravel and shingle (6) as a raised beach deposit; and neither did they refer to the grey clay and loam (2) as a glacial deposit.

Back in the 1960's and 1970's David Unwin, David Bowen and myself, all working independently with other fieldworkers, measured and described the site, and undertook excavations, but we all seem to have seen slightly different things!



When I organized excavations here in 1965, we managed to dig a couple of metres below present beach level, exposed the raised beach, and found just "basal sand" beneath it.  The sand acts as an aquifer, and is disturbed by flowing water -- so our trenches filled with water so quickly that we could not discern any structures.

David Unwin also described the raised beach, and showed its relationship with other deposits as follows:

David Unwin's sketch of the main exposure at West Angle (north part).  RB = raised beach.

David Unwin's section (south part)

David shows nothing beneath the raised beach deposit, and he shows a complex unconformity on one side of which is a reddish till (?) on which he undertook various fabric analyses.  I have not seen those results, and I don't think they were ever published.  There is one big mistake on the southern part of his sketch section, and that is his portrayal of all the beds as dipping southwards.  In fact the beds are virtually horizontal, and some of them have a very slight dip northwards, from the valley side out into the valley floor.  This is an important point for the interpretation of the sequence.  

My own detailed section for the site is very long and thin, and is difficult to reproduce  --  but here it is:


Click to enlarge.  The locations of our six exploratory trenches are shown, and it is possible to match at least some of the deposits in the sequence with the Unwin diagrams.  The sands, silts and clays exposed in the southern part of the section (and including organic-rich layers) are highly enigmatic -- are they estuarine, lacustrine, colluvial or fluvial?  

The sharp and unconformable contact between these fine sediments and the more varied reddish deposits with clasts occurs close to point E on my diagram; I studied this closely, and here is my record of what the junction looks like:

Unconformable contact between non-glacial and glacial (?) deposits close to point E in the cliff exposure.  The metre scale on the right shows surveyed altitude above OD.  The displacement of the sedimentary layers is about 1m on slip face (2) and about 30 cms on slip face (3) -- suggestive of fracturing and slippage under permafrost conditions.

 My own observed stratigraphic sequence is this:

10.  Made ground and soil
9.    Dark red stratified horizon
8.    Dark red diamicton (non-stratified)
7.    Orange silt and clay series
6.    Grey silt and clay series
5.    Peat and peaty silt
4.    Stony grey silts -- up to 1.5 m thick
3.    Ferruginous bedded sands and gravels -- up to 1.5 m thick
2.    Rounded pebbles / beach shingle in a sandy and gravelly matrix -- up to 1.8 m thick
1.    Sand layer -- more than 1 m thick

It's difficult to tie in this sequence to that of Dixon (1921) -- although Bowen (1977, Table 3) tried to do just that.  

David Bowen has published extensively on the Pleistocene correlations of South and West Wales, and has referred in a number of publications to the fact that like Dixon he has seen 1.5 m of till at the base of the succession.  He says (1977) that "Dixon's till has been re-exposed and a succession directly comparable to his re-established" -- but no adequate supporting evidence has ever been published, and the bulk of Bowen's mentions of a West Angle till simply consist of references to his own past assertions.   As for the sequence described by myself and David Unwin, a promised "radical revision" written by Ribbon and Bowen has never been published......... so we do not know whether an Irish Sea till has been securely tied in through stratigraphy to the base of the sediment sequence.  I suspect that Bowen's observed till is the same as that which I observed adjacent to cutting B.  I am quite certain that the till which I observed is considerably YOUNGER than the raised beach.

I now think that the band of orange clay shown by Dave Unwin and myself is not a primary sedimentary feature but a weathering or secondary feature. I have learned from looking at scores of other sites in West Wales that horizons such as this exist beneath glacial deposits -- not because these horizons have been exposed to the atmosphere, but because this is where there has been water concentration, leaching and also precipitation of chemicals once held in solution. A virtually identical orange-coloured layer exists beneath the Irish Sea till at Abermawr.  But clearly there has been a "cut" here, with pre-existing sediments eroded away and replaced with later sediments that are themselves slumped or faulted. Are we to interpret these as glacial deposits incorporating overridden sediments and far-travelled erratics laid down in a highly fluid ice-marginal environment? I suggest just that.

The reddish till is seen in the cliff face for about 15m north from point E, unstratified towards its base and stratified above. In a QRA field visit in 1969, some experienced Quaternary experts came to the view that because of the relative lack of large clasts in this material, and the frequency of weathered fragments, it must be a re-deposited ancient till, more properly defined as a slope deposit. This view was repeated by DQ Bowen in a number of papers. However, during one later visit to the site, I was lucky enough to discover, close to cutting B (in the gully down to the beach) an excellent exposure of blue clay till with thrust structures and inclusions of rotted bedrock debris. Adjacent to this was another exposure in which the till had a reddish colour with many ORS fragments included, with clearly striated clasts and with lignite and shell fragments too. This was a classic exposure of Irish Sea till, confirming that the ice of the Devensian Irish Sea Glacier did indeed affect this area.

Deposits from this glacial episode are also seen in the northern coves at West Angle -- including a wedge of in situ reddish till in cove number 3 which is packed with fresh striated cobbles, many derived from the ORS.  The till contains flow structures and fluvioglacial lenses, suggesting sedimentation in an ice-marginal environment.

At this stage we should mention two other publications:  one a chapter by Campbell and Bowen in the GCR volume called "Quaternary of Wales" in 1989, and the other  a short paper by CR Morey published in 1997:
Morey, C.R. 1997. A re-examination of the valley-fill deposits at West Angle Bay, Pembrokeshire. Proceedings of the Ussher Society, 9, 164-167.

Campbell and Bowen do not publish any sections or other diagrams of the stratigraphy, and Morey's illustration of the sequence is so generalised and inaccurate as to be effectively useless.  Morey does not accept that there is a genuine raised beach deposit near the base of the succession at West Angle, and nor does he accept that his "reddish brown" layer 4(i) is a till layer, in spite of recording within it clasts of ORS marls and sandstones, dolerites and rhyolites.  He prefers to follow Bowen (1974) in interpreting this as a periglacial head or slope deposit accumulated under permafrost conditions.  He does not explain where all this erratic material might have come from.  Nor does he seem to have examined the fresh till that is exposed in the northern coves of West Angle Bay; such an examination might well have convinced him that Late Devensian ice did indeed reach this locality and must have affected the sedimentas at the head of the main bay.  Campbell and Bowen accept the presence of the raised beach, and its dating as Ipswichian, but are ambivalent on the matter of the "reddish till".  They do however accept the contention by Bowen that there is indeed an exposure of Irish Sea till BENEATH the raised beach.  No evidence is cited in support of this thesis  -- so the jury is still out on the matter.

In 2000 Kenneth Rijsdijk argued that the whole of the sediment sequence at West Angle is of Devensian age, with different layers representing  differing environmental and climatic episodes. 

So we have -- by less than common consent -- a raised beach that appears to be of Ipswichian or Last Interglacial age, and above it a reddish diamicton (which I consider to be till) that must be related to the Devensian Irish Sea till which is found in many locations around the northern and western coasts of Pembrokeshire.  In between those two recognisable deposits we have a suite of clays, silts, sands and gravels with an incorporated layer of peat and peaty silt.  These deposits occupy an equivalent stratigraphic position to the "lower head" or stratified slope deposits assumed elsewhere to be of Lower and Middle Devensian age.  So what were the climatic and environmental conditions which led to the accumulation of these deposits at West Angle?

There are two big questions:

1.  Are there really two tills of different ages at West Angle?
2.  What is the nature of the layers of sands, silts, and peaty loams between the raised beach cobbles and the reddish till?

To be continued.......

----------------------------------

Some key references:

BOWEN, D. Q. (1970). South-east and central South Wales. In: The Glaciations of Wales (Ed. by C. A. Lewis). Longmans, London.

BOWEN, D. Q. (1980). The Pleistocene scenario of Palaeolithic Wales. In: Culture and Environment in Palaeolithic Wales (Ed. by J. A. Taylor), pp. 1-14. British Archaeological Report, no. 76.

Dixon, E.E.L.  (1921)

JOHN, B. S. (1968). Age of raised beach deposits of South-western Britain. Nature, 218, 665-667. 
MITCHELL, G . F . (1960). The Pleistocene history of the Irish Sea. Advancement of Science, 68, 313-325.

MOREY, C.R. (1997(. A re-examination of the valley-fill deposits at West Angle Bay, Pembrokeshire. Proceedings of the Ussher Society, 9, 164-167.

PHILLIPS, L . (1974). Vegetational history of the Ipswichian/Eemian Interglacial in Britain and Continental Europe. New Phytologist, 73, 589-604.

RAWITSCHER, (1945). The hazel period in the post-glacial development of forests. Nature, 156, 302-303.

Stevensen, A.C. and Moore, P.D. (1982). Pollen Analysis of an Interglacial Deposit at West Angle, Dyfed, Wales. The New Phytologist. Vol. 90, No. 2, pp. 327-337.

TURNER, C. (1970). The Middle Pleistocene deposits at Marks Tey, Essex. Philosophical Transactions of the Royal Society of London, B, 257, 373-440.

TURNER, C. & WEST R. G. (1968). The subdivision and zonation of Interglacial periods. Eiszeitalter und Gegenwart, 19, 93-101.

WEST, R . G . (1980). The Pre-glacial Pleistocene of the Norfolk and Suffolk Coasts. Cambridge University Press, Cambridge.

2 comments:

Dave Maynard said...

Have you been here recently? Is there much erosion since the earlier work?

Dave

BRIAN JOHN said...

Hi Dave -- haven't been there for quite a while. I am planning to pop over one day to take a look..... the reason for my refreshed interest in this site is the possibility (no more than that) of two tills here, separated by an interglacial raised beach.....