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Wednesday, 1 March 2017

The West Angle Enigma (2): the silt and clay series

 West Angle Bay, near the mouth of Milford Haven.  The main exposure is at the back of the beach, between the car park and the sharp bend in the cliffline near the bottom edge of the photo.  The position of the old brick pit is now very difficult to discern from the air.

The "ridge" of interglacial and glacial sediments exposed in the southern part of West Angle Bay.  The beach is to the left and the brick-pit is to the right.

The sands, silts and clays which overlie the raised beach at West Angle, and which lie beneath the reddish till, are so unusual that they have caused intense (and sometimes acrimonious!) debate among geomorphologists and geologists.  Dixon (1921) referred to them as "loams" but that term is misleading since it is conventionally used for a sandy or silty soil, which implies a period of subaerial evolution beneath a plant cover.  There is certainly some evidence (in the form of a peat bed) of vegetation accumulating and growing -- but it would be a mistake to simply assume that all of the sediments in this sequence were accumulating at the ground surface.  They are very fine-grained, and many researchers have concluded that some of sediment accumulation was beneath water, either in a lacustrine, marshy or estuarine environment. So let's simply refer to this sequence of quite variable sediments as the "silt and clay series".

First, the base of the series.  According to my observations, and those of David Unwin, there are ferruginous cross-bedded sands and gravels above the raised beach.  These are generally less than 1 m thick, but Unwin interpreted them as "dune sands" and I am now inclined to agree with him.  (I originally thought that they might be beach sands laid down on top of the pebble beach during the course of a marine transgression, but I now think that evidence in support of this hypothesis is in short supply.......)  Another interesting fact is that stony grey silts up to 40 cms thick seem to be associated with these sands and with the upper part of the raised beach facies.  This suggests that at the end of the raised beach episode (the Ipswichian interglacial?) sea-level might have dropped slightly, or else there was some other environmental change.  This led to the accumulation of slope deposits or rockfall materials and pre-existing sediments on the valley side, with colluviation extending as much as 30 m from the valley side out onto the valley floor.  Possibly a sand dune system developed on top of the raised beach with a dune slack behind it.  Colluvial materials (incorporating angular rock fragments and also raised beach pebbles and shingle) were then interdigitated with sandy dune materials and with peats, silts and clays in a freshwater environment.

Two examples of flooded dune slacks in England.  In each case, the wetland is trapped behind a sand dune belt at the back of the shoreline.  In West Angle the dry valley has probably existed for hundreds of thousands of years, so without a river to breach the dune belt there are perfect conditions for semi-permanent lakes to exist during interglacial periods, so long as the water table remains high.  There may well have been intermittent tree cover -- this is suggested by the pollen and plant remains in the sediments.
The exposed section at point H, about 25 m away from the rocky valley side at West Angle. Here the top of the raised beach is slightly above the level of the present-day pebble beach.  Above it there is a layer of ferruginous cross-bedded sand, then a thin gravel layer, and then about 30 cms of grey colluvium with angular fragments, and then a peat layer shown with cross-hatching. This has a maximum thickness of c 45 cms.  Above that is the grey silt and clay series, followed by the orange silt and clay series.  At this point the reddish till, sands and gravels are not represented.

Now to the peat and peaty silt layer.  Since this was first discovered it has caused great excitement, since interglacial peat beds in Wales are exceedingly rare.  It has always been assumed that it is linked in some way with the raised beach and with the silt and clay series, which also contains abundant plant remains.

 A more detailed section near cutting H, transposed from a photograph.  The main stratigraphic junctions are shown, together with the peaty silt and peat layer.

 The Silt and Clay Series.  Fragments of well-preserved oakwood, rootlets and leaves are found here and there, and Dixon (1921) referred to "plant fragments and occasional large pieces of wood."   In "Nature" journal in 1968 I reported the conclusions of Dr Ian Simmons and other Durham colleagues regarding samples taken from the sandy and silty layers just above the raised beach.   Ian showed that the pollen and plant remains are fully representative of an interglacial environment.  Quercus (35% of total tree pollen), Alnus (43%) and Corylus (42%) were the dominant constituents, with Pinus only moderately represented (13% of tree pollen).  Non-tree pollen accounted for 85 of the 343 pollen grains counted, with Gramineae, Cyperaceae and Filicales moderately well represented.  Other lesser plant pollen (eg Chenopodiaceae and Armeria) suggesting proximity to littoral conditions.  The vegetation was thus a mixed oak and alder forest with pine and hazel, with salt-tolerant plants close to the water's edge.  Carex, Juncus and other water plants point to an environment that included small open pools with patches of marsh and fen and carr vegetation.    Botanists who have examined my own evidence incline to the view that this was a fresh-water rather then a salt-water (estuarine) environment.

Other researchers tend to agree with this interpretation.  For example,  Bowen (1974) found Alnus tree stumps in a growth position on the surface of the "loam" and dated one of them to >35.5 ka.  This would suggest an Early or Middle Devensian (and possibly an Ipswichian) age for the silt and clay series.   Morey (1997) drew on the research of Stevenson and Moore (1982) and wrote as follows:

"..........Stevenson and Moore (1982) recorded the aquatic taxa, Sparganium, Potamogeton, Typha, Nuphar, Nymphaea and Polygonum amphibium together with Valeriana officinalis, indicating the presence of areas of open, fresh, water bordered by a tall herb- marsh community. Their data also indicate a regional flora with the trees and woodland species characteristic of a temperate (interglacial) environment........... Since estuarine muds deposited in a temperate environment typically contain a substantial fauna with forams, ostracods and small gastropods, as well as larger bivalves etc. the lack of marine fossils makes an estuarine origin for this unit appear unlikely and the earlier interpretations should be questioned. It seems more reasonable to interpret Unit 3 as an alluvial deposit characterised by pools of fresh water. Other taxa present, Hippophae, and Plantago maritima, could be derived from a surrounding dune community.  Stevenson and Moore (1982) also noted a record of Ruppia maritima by Turner, but the presence of a species which is usually associated with brackish pools does not conflict with this interpretation as a marshy valley among coastal dunes."

In 1973 Stevenson and Moore found the following sediment sequence:

Head deposits overlying:

Yellow/orange sandy silt
Grey/yellow sandy clay
Yellow/orange clay with abundant pebbles.
Apparently sharp hiatus
Stiff blue/grey clay with sandy bands at 106 cm. Organic sediments, laterally variable in nature were found within this unit, as was evidence of current bedding in the inorganic sediments.
Grey—brown silty clay. (with included Alnus wood layer). Organic remains become more abundant towards its base; especially obvious are wood fragments
Pebbles in a grey, sandy-clay matrix. Larger pebbles (up to 5 cm diameter) were concentrated in the lower layers.
Very fine white sand, cross-bedded
Very fine yellow sand with an iron-rich layer
Fine yellow and white sands, particles of larger size evident towards the base
Iron concreted, yellow and red sands
Coarse-grained yellow sands with hard bands of ferruginous, cemented material
Pebbly, raised beach deposits

Stevensen and Moore (1982) found four pollen assemblage zones in the silt and clay sequence, all dominated by temperate forest taxa -- so they were convinced that the climate throughout the whole sedimentary episode was temperate rather than glacial or periglacial.  They were inclined to suggest a Hoxnian interglacial age for this episode, but others (such as Campbell and Bowen in 1989) preferred an Ipswichian or Last Interglacial interpretation.

One interesting feature of the work by Stevenson and Moore is the presence of a "hiatus" in the pollen sequence in their Zone WA-2.  They suggest that there was an episode of extensive forest destruction, followed by secondary succession and woodland recovery, but resulting in a changed floristic composition. Such destruction could have resulted from flooding or erosion. The possibility of habitat disturbance as a result of factors associated with local fluviatile conditions is very real in this swamp-carr site. Marine incursion is also possible.

Another suggestion (and it is no more than that) is that towards the top of the sequence there is an increase in Pinus pollen and a decline in some other species -- possibly indicative of a "post-temperate" plant assemblage taking over from a truly interglacial one.  Does this represent the end of the interglacial and the onset of colder -- and eventually periglacial -- conditions?

 Stevenson and Moore pollen diagram for the silt and clay series.  Note that near the junction between the blue clay and the yellow clay there is a sharp decline in oak pollen and a corresponding increase in pinus pollen.  Does this indicate a cooling of temperature at the end of the interglacial episode?

In the absence of accurate dating for the sediments and their plant remains,  all we can say for the moment is that they seem to be linked with the raised beach, that they were probably laid down in a largely freshwater environment of pools and marshy patches, and that the accumulating sediments might have been protected from the open coast by a barrier of sand dunes.  The environment is an unusual one, in that the Angle Valley runs right across the peninsula  as a limestone "dry valley" containing no substantial stream.  This may explain why the circumstances for sediment accumulation here were not at all similar to those which prevailed in other valley mouth situations such as Abermawr, the Nevern Estuary, Newgale, and Goodwick.  But it's reasonable to assume that when the silt and clay series of sediments was accumulating, sea-level was not very much below that of the present day; if it had been, I assume that the water table on the valley floor would have been much lower, and that pools of standing water would not have survived in this vicinity for prolonged periods.  The suggestion of a sudden marine incursion (a tsunami?) high in the sediment sequence also indicates that the sand dune barrier might not have been very strong, and that through most of this chapter in the story, sea-level was at its "interglacial" or high-stand level.

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