Submerged forest exposures at Abereiddi

 

Many thanks to Ruth Crofts for these photos, taken at Abereiddi after one of the winter storms had exposed quite a large expanse of the submerged forest at Abereiddi.

These photos are important bcause they show a stratigraphy of three layers:

3.  Peat bed incorporating tree roots and branches and other detrital debris

2.  Purple clay layer which is clearly "churned"

1.  Buff clay layer which contains many erratic clasts of all shapes and sizes

It is possible that the purple layer is the weathered "cap" of the layer below, but the churning is quite spectacular.  In this situation we cannot be dealing with a violent liquified mud flow -- and the interpretation must be that this is a periglacial feature, created at a time of permafrost.  In the lowest layer I think we are looking at a deposit of the Irish Sea till, similar to that of the Abermawr exposure some miles to the east.  

So my interpretation here would be:  Late Devensian glaciation and deposition of Irish Sea till  >>> late glacial cold climate episode with permafrost and creation of involutions  >>>  Holocene temperate conditions with peat growth and climax woodland  >>> sea level rise and inundation of the woodland and peat bed, probably within the past 5,000 years.

The submerged forest stratigraphy -- multiple sea level oscillations

Relative sea-levels in the Bristol Channel area over the past 12,000 years -- after Bell, Lambeck, 

Shennan and others

This is an interesting thesis concentrating on sites on the Gower and along the Glamorgan coast.  It argues that in the period of Mesolithic and Neolithic settlement there were multiple oscillations of relative sea level -- transgressions and regressions -- leading to coastal changes from saltwater marsh to freshwater lagoons and peat beds with some forest cover.  Although the thesis does not go into mechanisms, this all demonstrates a delicate balance between isostatic recovery rates and the post-glacial (Holocene) eustatic sea level rise. 

This is the context within bwhich we have to try and understand the nature and the sequence of deposits beneath the submerged forest.........

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Changing Tides:
The Archaeological Context of Sea Level Change inPrehistoric South Wales
Thesis submitted for the degree of PhD Cardiff University
Department of Archaeology and Conservation School of History Archaeology and Religion
September 2018

Rhiannon Philp

https://orca.cardiff.ac.uk/id/eprint/118952/14/2019philprphd%20118952%20DPR.pdf

Quote from Introduction:

According to nationally applied models, sea levels rose by around 55m between 10,000 and 6000 years ago (Lambeck 1995; Shennan and Horton 2002). After this period, the models suggest that sea levels stabilised around modern day levels (Bell 2007e,10). Archaeologically this gives the impression that sea level change affected Mesolithic communities more than those in the later prehistoric periods and that coastal ranges were similar to the modern day by the Neolithic period. However, when archaeological evidence is brought into the mix, it is clear that prehistoric experience of sea level change is not so clear cut. Archaeological and palaeoenvironmental evidence from intertidal zones around Britain, including the Severn Estuary (Bell et al. 2000b; Bell 2007b; Bell 2013c), Langstone Harbour (Allen and Gardiner 2000a), Hullbridge (Wilkinson and Murphy 1995) and the Isles of Scilly (Charman et al. 2016c), has shown that despite the apparent reduction in sea level rise, the effects of fluctuating sea levels (both transgressions and regressions) were felt throughout the prehistoric periods, from the Mesolithic through to the Iron Age and beyond within humanly perceivable timeframes (Bell 2000c, 19). This is unlikely to have been in the form of catastrophic events. Rather, despite early Holocene movements appearing significant, archaeological evidence suggests prehistoric coastal populations would have experienced very gradual and fluctuating change with an average of 1cm rise per year (Shennan et al. 2009).

Quote from Conclusions:

The evidence has shown that sea level change affected Gower throughout the Mesolithic and Neolithic periods, but not in a linear fashion. Long-lived inundations did not affect either of the sites during timeframes represented in the studied environmental sequences. However, at Port Eynon in particular, there is evidence for at least five instances of direct marine influence during the late Mesolithic period. This led to the deposition of minerogenic sediments and salt marsh indicators within the pollen record. Importantly these transgressive periods were followed by regressions in sea level, leading to the reinstatement of freshwater environments represented by substantial peat deposits. At Broughton Bay, evidence for contemporary transgressions has not been directly identified in the stratigraphic record, although raised levels of salt marsh indicators in the pollen record towards the base of the organic peat deposits suggests an earlier marine phase.

Gabbro erratic on the Abereiddi shore platform

 

This large erratic made of gabbro (?) has been exposed on the washed surface of the intertidal shore platform,  at the northern end of Abereiddi Bay.  Dimensions -- roughly 1m x 1m x 1m.   Normally it is encased in beach sand, but it is exposed now because the beach has been lowered by a metre or so during recent stormy weather.

New exposures of the Abereiddi submerged forest

 

Following the winter storms there are some new exposures of the submerged forest at Abereiddi -- near the stream at the lower edge of the pubble bank.  Just a few traces of the peat bed, with broken branches exposed at the surface.  Normally these features are buried beneath a metre or more of beach sand.  We can't see any stratigraphic relationships at the moment -- but there is a considerable "boulder bed" which looks as if it lies on top of the peat.  I suspect that there is a stratigraphic inversion there, and that these are "old boulders" that have been incorporated into the storm beach and have migrated with it as it has moved inland over the top of the peat bed during the Holocene.  

Or are the boulders in situ, related to a Devensian till deposit that underlies the peat bed?

More studies required........