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Sunday 1 March 2020

The submerged forest at Amroth

This shows where the submerged forest and related deposits may currently be seen at Amroth (image from PemBow drone video, with thanks).  Grid ref approx SN16070 06833.

Added 26 Oct 2020: a great pic from Georgina Lewis, taken (I think) just a few days ago.  Very unusual to see exposures at this time of year (late autumn).

Small tree stump and root system partly buried by modern beach sand.

Part of the main submerged forest exposure, with a bounding "trench" filled with water.  The peat is very thin, grading down into blue clay which in places contains sub-angular stones.

Thin peat and part of a tree root system -- in the main exposure.

Close-up of a tangled mass of roots from a tree embedded in the peat.

Thin surface peat layer (c 10 - 15 cm) with bluish clay (partly gleyed) beneath.  There are abundant dark-coloured organic fragments in the clay -- because of inclement conditions I could not ascertain whether these were detrital fragments of remnants of old root systems.

A thin peaty layer grading down into sticky stoneless grey clay.

Here the peaty (dark coloured) layer is c 20 cm thick, and it grades down through a "streaky" band to stoneless grey clay, passing beneath the water level.  There is no sign of an erosional contact, and it appears that depositional conditions have been subject to gradual change.

In a few places sub-angular blocks of local sandstone are contained within the blue clay layer -- and some erratic pebbles are also incorporated in the clay -- but I would not refer to this as a till layer without further evidence.

One of Gary's 2014 photos, showing a thin cap of peaty material passing down to relatively clean grey-blue clay and then to blue-grey clay-rich till full of erratic pebbles and stones -- which 
appears to be in situ.

The enigmatic blue clay layer

Almost all of the old records of the submerged forest refer to an upper peat layer with fallen trunks and branches, and embedded tree stumps, "overlying a stiff blue clay penetrated by roots of trees."   This was recorded by OT Jones at Marros, and at Amroth by Leach and Strahan. There are several references to Mesolithic flint flakes and charcoal BENEATH the peat and ABOVE the grey clay.  This is interesting, and led Leach to assume that the peat layer and the submerged forest were of Neolithic age.  This would make the beds younger than 6,500 BP.  (On the other hand some C14 dating puts some of the peat beds as older than that.....)

Because there seems to be no major erosional break / fossil land surface between the underlying till (and the underlying bedrock slope breccia, in the case of Marros), the grey clay and then the peaty layers, there seems to be a continuum.  The easiest interpretation is that following the LGM and ice wastage, there were exposed till surfaces over thousands of years of permafrost conditions, with ponds and lakes in which sediment gradually accumulated.  A reasonable parallel might be found on the surfaces of the Byers Peninsula and the Fildes Peninsula in the South Shetland Islands today, where there are abundant snow patches, expanses of till, continuous permafrost and virtually no vegetation. Fine-grained sediments would be carried into these standing water areas during the summer months as a result of snow-melt.   Climatic oscillations might account for subtle changes in colour and texture, with "exceptional events" leading to the introduction of stony debris now and then.

A rather hostile tundra environment on Byers Peninsula, Livingston Island, South Shetlands.  Snowmelt feeds a network of small streams that carry fine-grained sediments into a shallow lake (which happens still to be ice-covered in the early summer) at the right edge of the photo.

Some attempts at describing the early post-glacial environment are made in the 2011 volume called "WEST COAST PALAEOLANDSCAPES SURVEY:  Discovering the submerged landscapes of prehistoric Wales," by Simon Fitch and Vince Gaffney.

According to the authors:  "Soil formation and vegetation development lag behind climatic shifts, and vegetation would have been slow to establish. As soil development increased shrubs such as juniper (Juniperus) and willow (Salix) would have been established, closely followed, if the conditions were correct, by small areas of pioneer birch woodland (Betula pubescens). The final upper Palaeolithic stadial period saw a climatic reversal and the area, although not affected by glacial conditions, would have been subject to permafrost. How much this affected the valley (ie the "dry" Bristol Channel) cannot be quantified, but the cold climate would have restricted birch woodland to favourable locations, with open habitat taxa re-establishing elsewhere."

As the climate warmed, around 11,000 years ago, following the Younger Dryas cold snap, tundra vegetation was re-established, and organic materials began to appear more abundantly in the accumulating sediments, until with the arrival of Holocene full interglacial conditions, intermittent and then fully established peat layers were created at the top of the sequence.

Probable climatic oscillations during the period of sediment accumulation at Amroth.  The Irish Sea till was probably emplaced between 24,000 and 20,000 years BP.  The blue clay may have accumulated under permafrost / barren surface conditions between 20,000 and 11,000 years BP.  Organic debris accumulation may then have commenced with coherent peat beds forming after the onset of the Preboreal episode around 10,000 years ago.  With the onset of the Boreal period, around 9,000 years ago, the coastal forest was established which was then overwhelmed bit by bit during the inexorable rise of Holocene sea level.

In an oceanic coastal forested environment with intermittent wetlands, peat development could have occurred throughout the following recognised climatic phases:

Preboreal (10 ka–9 ka BP),
Boreal (9 ka–8 ka BP),
Atlantic (8 ka–5 ka BP),
Subboreal (5 ka–2.5 ka BP) and
Subatlantic (2.5 ka BP–present).

According to Fitch and Gaffney (2011) there was "........ a rapid spread of open, mixed deciduous forest woodland, with hazel (Corylus avellana) and elm (ulmus) establishing an open canopy forest. The canopy closed within 500 years with the establishment of oak (Quercus), Lime (Tilia cordata), alder (Alnus glutinosa) and ash species (Fraxinus excelsior) (Roberts 1998). This strongly seasonal, high productivity woodland would have provided hundreds of edible plant species for human groups - available at a relatively low energy cost. However, the dense closed nature of the woodland would at times have been an obstacle to communities, making hunting and passage difficult. However, the presence of fluvial channels and lakes within the landscape would provide open areas which would have facilitated population movement.......... The regular longitudinal river profiles associated with the Pleistocene- Holocene transition suggest lower energy river systems than the previous period, and these may have been more suitable for travel using the logboats of the period. The rivers still produced high yields of sediment that in-filled deep Pleistocene channels and continued to contribute to the ponding up of lakes within the valley." 

Gradually, as sea level continued to rise, the peat beds and their associated forests were overwhelmed, and the "submerged forest" was covered with storm beach pebbles and beach sand.  The familiar sandy bays of Pembrokeshire were created.  The submerged forest has been seriously neglected in most of the textbooks, but John Evans created a little model of submerged forest development close to an old cliffline -- as at Amroth.  He referred to reed swamp / marshy environments, the existence of migrating sand dunes with "dune slacks" on their landward flanks, and salt marshes developing on the front edge of the advancing (rising) sea along the shoreline.  He assumed that there were "alternating estuarine and peat / forest environments" creating alternating laters of peat and estuarine clays and indeed there is evidence in support of this elsewhere (for example in Swansea Bay) -- but there is no evidence of these alternations here at Amroth, and I am not aware of any local evidence for alternating transgressions and regressions of the rising sea.  Here, at any rate, the sequence is a simple one, with a single transgression punctuated by storm surges.  As at Newgale, we can also assume that during the Holocene marine transgression a storm beach of pebbles and cobbles derived from old glacial deposits marched inexorably across the landscape before settling more or less in its present condition at the shoreline in Amroth Bay.

In summary, these are my suggestions:

Carmarthen Bay and the Bristol Channel in the period after the melting of Late Devensian glacier ice was a vast undulating plain crossed by a number of meandering rivers -- of which the largest was the Severn.  These rivers were fed initially by melting ice and later on by snowmelt in a barren permafrost environment which may have existed for some millennia after 20,000 BP. Not enough is known about the "lost millennia" -- but it is likely that the present intertidal zone, extending southwards from the present-day cliffs, was initially a "coastal wetland" with extensive areas of standing water, lakes and lagoons, moraines and fluvioglacial accumulations and even sand dunes.
There was initially not much vegetation. There must have been extensive destruction on the rock faces of the old cliff line, under the influence of frost processes and gravity.

Rockfall debris mixed with boulders and cobbles derived from old glacial deposits, beneath the cliffline at the western end of Amroth.  Many of the rockfalls probably occurred during a long period of periglacial activity following the melting of the Late Devensian glacier ice.

Later on, tundra vegetation became established, and herds of "Ice Age" animals including woolly mammoth, woolly rhinoceros, red deer, wolf, wild boar, giant ox etc roamed across the landscape. Further substantial river plains ran down from the uplands of South Wales, including the Tywi, Taf and Tawe  to the west of the Glamorgan lowlands.  There are traces of Palaeolithic occupation on Caldey Island and in the caves at Coygan and Paviland.  As the sea-level rose gradually, many of the "Ice Age" animals became extinct as the climate and vegetation changed and the coastline approached its present position.

After 9,000 years BP there must have been an extensive forest,  changing in character as the climate oscillated -- this forest covered both the terrain inland of the present cliff-line and the coastal plain which is now submerged beneath the sea.  There were tree-covered hillocks and ridges between the main drainage routes.  Conditions must have been favourable for the growth of peat within and beyond the tree-covered areas. The fauna changed gradually -- the woolly mammoth and the woolly rhinoceros became extinct, but species like wolf, fox, wild boar, red deer, and giant ox adapted to changing circumstances and survived.  Human hunters now obtained some of their food from the forest, and AL Leach recorded many examples of microliths and other artifacts in the forest indicative of a population of Mesolithic nomads.

A beast of the forest?  In 1913 AL Leach (who was a pretty careful collector and observer) recorded a femur from Elephas primigenius (the woolly mammoth) from an exposure of the submerged forest in Amroth.  This is very strange, since the mammoth was not a forest dweller, but a steppe and tundra dweller, and the mammoth in the British Isles was supposed to have become extinct before 13,000 BP.  The submerged forest, in contrast, is generally dated as younger than 8,000 BP. Could the bone have been recycled from a Palaeolithic context?

Bos primigenius (the aurochs) amost certainly lived in the forested area now represented by the submerged forest.  Quote:  "Throughout much of the last few thousand years European landscapes probably consisted of dense forests, and as such the aurochs were confined to open areas in marshlands along rivers. Comparisons of the ratios of certain mineral isotopes in recovered bones of aurochs from the Mesolithic with domestic cattle has shown they lived in floodplain forests or marshes, areas much wetter than in which modern domesticated cattle live." According to AL Leach, Mr HCR Vickerman found a skull of an aurochs in the peat at Wiseman's Bridge, and presented it to Tenby Museum.

From the recreated eustatic sea-level curves we can assume that the rising Holocene sea-level, associated with the melting of the great ice sheets of the Northern Hemisphere, slowed dramatically around 8,000 years ago when it was located at around -10m, and the rise slowed again around 7,000 years ago when it stood at c -4m.  That means that within the last 7,000 years, with an extreme tidal range of over 8m, forests growing in the current intertidal zone would have been vulnerable to storm surges, tsunamis and extreme weather events.  The shoreline south of the Pembrokeshire coast may have been just a few hundred metres away from its present position............ or maybe several kilometres away.  More research is needed on this.

So bit by bit, as the sea-level rose towards its present position, the forest was overwhelmed, with trees first of all killed by incursions of salt water and then broken and knocked over by storm-force winds and big waves rushing onshore.  In some places (as at Newgale and Abermawr) a migrating storm beach must have moved across the peat beds and the demolished forest, impounding ahead of it a series of lakes and lagoons -- creating conditions for the interbedding of pebble beds and new lagoonal deposits and peat layers.  There must have been great variations both laterally and in stratigraphic terms between one site and another before the sea attained its present level and stabilised at the position of the modern coast.  River channels, areas of sand dunes and even moraines and mounds of fluvioglacial sands and gravels must have existed in the offshore zone, and must have influenced the precise course of events.

Two representations of the bathymetry of the Bristol Channel.  At the onset of the Holocene or post-glacial rise of sea-level, the whole of the area shown on the maps must have been dry land.  Most of the marine area had been re-flooded by the sea by 8,000 yrs BP.  Dense forest growth was probably limited to the area to the landward side of the -10m contour.

This is the first serious study of the submerged forest -- published at about the same time as the Geological Survey Memoirs and the studies around the South Pembrokeshire coast by AL Leach.

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