A boggy wetland with scrub and woodland at Ynyshir Nature reserve
Satellite image of a tidal salt-marsh near Ynyshir, Machynlleth -- on the Dyfi estuary
In my assorted ponderings on the relationship between the submerged forest of West Wales and the underlying "blue scrobicularia clay" I've admitted to being puzzled, because on a submerging coast like this it is easy to explain peat beds and "forest layers" being overwhelmed by the sea and then overlain by intertidal or deepwater deposits -- but far more difficult to explain marine silts and clays being overlain by peat beds. That's because there seem to be no clear indicators from the global records of the late-glacial and Holocene marine transgression being interrupted by stillstands or even falls of sea-level.
It gets even more confusing when we look at the sediment sequences exposed around the South Wales coast -- suggesting several phases or cycles with alternating layers of peat and scrobicularia clay.
While admitting that estuarine environments and even open coast environments are very complex, we have to find some mechanism for deepwater or intertidal low-energy environments to be suddenly replaced with environments within which peat and even woodland growth was possible. I did some reading on the Severn Estuary, which has been very well studied over the last 30 years -- while bearing in mind that one vital difference was the exceptionally high tidal range in the estuary of up to 15m.
What seems to emerge is that the sediment sequence (involving mostly intertidal silts and clays and layers of organic materials) changes quite significantly between one site and the next -- and that seems to be mirrored in the studies by Harry Godwin and various others many years ago, centred on the building of the South Wales docks at Barry, Cardiff, Swansea and Port Talbot.
I have been particularly impressed by a paper by JRL Allen and SK Haslett, in which they point out that the precise record for a particular locality will be affected by sea-level change, sedimentation, wave-action, the wandering of estuarine channels and banks, channel meandering, and salt-marsh autocyclicity.
Salt-marsh autocyclicity is explained in the extract below. Essentially, it involves the development of vegetated salt-marshes which tend to "thicken" or grow upwards because the vegetation mat collects sediment at times of high tides and storm surges -- until eventually it becomes almost high and dry, at which point sapping or erosion sets in, with a little sediment cliff developing and eating its way through the salt marsh until a new equilibrium is maintained. This can be repeated over and again -- all other things being equal, which of course they seldom are.........
This resonates with what is observable on salt marshes in West Wales and on the Ceredigion coast, for example at Ynyshir and on the Teifi Marshes near Cardigan. It also resonates with what Harry Godwin and others have observed in association with the submerged forests and the peaty layers that seem to underlie it in some of the deeper borehole records.
Above pics: Teifi Marshes Nature reserve -- saltmarsh and wetland affected by river water and sea water at high tide. Several different environments -- reedbeds, exposed mudbanks, river gravels and sandbanks, which all move laterally and rearrange themselves over time.
Intertidal mud flats (made mostly of silt and clay) in the Carew River, Pembrokeshire. There is a thin veneer of marine algae, but otherwise this mudbank is devoid of vegetation. The surface supports small channels and interconnected pools. This is a low-energy environment with a high tidal range. The river water carries a high sediment load -- it is normally quite cloudy.
Intertidal mud flats (made mostly of silt and clay) in the Carew River, Pembrokeshire. There is a thin veneer of marine algae, but otherwise this mudbank is devoid of vegetation. The surface supports small channels and interconnected pools. This is a low-energy environment with a high tidal range. The river water carries a high sediment load -- it is normally quite cloudy.
So perhaps the sudden shifts (in the sedimentary record) can all be explained because of landscape changes in and around the intertidal zone -- including the existence of pre-existing morainic, scree accumulation or fluvio-glacial topography, development and destruction of sand bars, spits and sand dunes (as proposed for Ynyslas), lateral shifts in river courses and the existence of temporary pools and lakes, changes in sedimentation rates and types, waterlogging and sphagnum growth, and finally salt-marsh autocyclicity.
Maybe the "blue clays" are not as uniform as we are tempted to believe? We already know that some of them appear to be true marine clays containing Scrobicularia shells, and others do not. The sandy silt beneath the peat at Lydstep has even been described as "loess". At Amroth there are quite large stones embedded in the clay, and elsewhere it appears to be stoneless. We will probably only get an answer to the "blue clay"enigma when somebody undertakes a systematic survey, shifting attention from the submerged forest to whatever lies beneath it -- and of course he or she will need a great deal of good luck in the way of gigantic winter storms to expose currently hidden secrets...........
One of Gary's excellent photos from Amroth, showing a stony basal layer with what appear to be till traces, then a roughly bedded set of gravels, and then a sharp break to a thick sticky blue clay. This suggests to me that there might have been an erosional phase between the deposition of the till seen elsewhere at Amroth and the laying down of the clay in a low-energy environment.
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The Severn Estuary and its surrounding wetlands
The Severn Estuary at the head of the Bristol Channel is the largest estuarine system on the British west coast. It lies between the high ground of the Cotswolds-Mendips to the south and southeast and the Monmouthshire hills and Forest of Dean to the northwest. Sediment enters from the seabed to the west and from the substantial catchment of the River Severn and its tributaries. Bordering cliffs and the other rivers that drain into the estuary contribute in a minor way. The estuary is well stirred and highly dynamic, with an extreme tidal range of 14.8m and exposure of prevailing winds. Its waters are notoriously turbid, the amount of suspended mud varying with tidal and weather conditions.
The estuary arose as the periglaciated land area that lay between South Wales and the Cornish Peninsular was transgressed by the rising post-glacial seas (lowstand c.-135m OD). A major consequence of sea-level rise, especially over the last 8,000 years, was that wetlands and their deposits formed close to sea level along the margins of the estuary, creating the extensive Severn Estuary Levels (c.840km2).
The Severn Estuary at the head of the Bristol Channel is the largest estuarine system on the British west coast. It lies between the high ground of the Cotswolds-Mendips to the south and southeast and the Monmouthshire hills and Forest of Dean to the northwest. Sediment enters from the seabed to the west and from the substantial catchment of the River Severn and its tributaries. Bordering cliffs and the other rivers that drain into the estuary contribute in a minor way. The estuary is well stirred and highly dynamic, with an extreme tidal range of 14.8m and exposure of prevailing winds. Its waters are notoriously turbid, the amount of suspended mud varying with tidal and weather conditions.
The estuary arose as the periglaciated land area that lay between South Wales and the Cornish Peninsular was transgressed by the rising post-glacial seas (lowstand c.-135m OD). A major consequence of sea-level rise, especially over the last 8,000 years, was that wetlands and their deposits formed close to sea level along the margins of the estuary, creating the extensive Severn Estuary Levels (c.840km2).
The post-glacial estuarine alluvium preserved beneath the Severn Estuary Levels is dominated by alternating silts, representing intertidal salt marshes and mudflats, and peats, formed in organic marshes in the highest parts of the intertidal zone or above tide levels. Investigations based on exposed sections and borehole records, especially along the Gwent coast, show that the sequence is laterally very variable and that the boundaries between the silt and peat beds can change considerably in age from place to place, depending on such factors as distance from the coast or the main rivers and the thickness of the sediments already present.
The general evidence that relative sea-level has risen in a fluctuating manner since the last glaciation is to a degree blurred by these local variations. The investigations have also demonstrated that the evolving estuarine shoreline was very changeable. From time to time the coast retreated significantly because of erosion, only to advance again when the regime changed. Annual sedimentary banding has proved to be common in the silts, demonstrating that new salt marshes can grow upward very rapidly. Because 'summer' and 'winter' parts of the bands can be distinguished, it is proving possible to show when humans and animals where present on the prehistoric wetlands.
https://archaeologydataservice.ac.uk/library/browse/series.xhtml?recordId=1000381&recordType=Journal
Series: Archaeology in the Severn Estuary
ADS Digital Resource
ADS Collection DOI: https://doi.org/10.5284/1044660
Primary Contact: Severn Estuary Levels Research Committee
ADS Collection DOI: https://doi.org/10.5284/1044660
Primary Contact: Severn Estuary Levels Research Committee
Very informative annual reports (including reprints of relevant articles) from 1990 - 2017.
Goldcliff Mesolithic Archaeology
At Goldcliff, 9km south-east of Newport, Monmouthshire, a promontory projects into the estuary. This marks the site of a former island, the greater part of which has now been eroded away by the sea. Round the edge of this island in the intertidal zone hunter-gatherer Mesolithic sites were excavated in 1992-4 and 2001-3. The sites were occupied at a time of lower sea level and were buried and preserved by silts and peats as sea level rose. At the base of the sequence is a substantial oak forest which existed c. 6000 cal BC. Occupation on the site took place between then and 4700 cal BC. Activity is represented by scatters of flint, chert and tuff artefacts as well as wood, bone and antler tools. Bones of deer, extinct wild cattle, pig, otter and fish (especially eels) represent the animal resources exploited. Animal and plant remains show that activity on these sites took place particularly in the autumn but there is also some activity in spring, at the height of summer and on one site in winter. Stratified within estuarine silts around the edge of the former island are human footprints, many of them children, some as young as 4 or 5. Clearly, children were fully involved in the life of this Mesolithic community. The site has also produced the earliest evidence for human intestinal parasites, showing that the fringes of the settlement area were used for defaecation. The latest Mesolithic occupation is covered by a thick peat and another submerged forest which grew between 4400-4200 cal BC. (Note: this appears to coincide with the main period of submerged forest formation around the West Wales coast, coinciding with the "tipping point" transition from rapid sea level rise (c 1m per century) to a much slower rate of rise of c 8 cm per century.)
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Archaeology in the Severn Estuary 22 (2013), 3-19
SITE FORMATION PROCESSES
IN THE SEVERN ESTUARY LEVELS
by J.R.L. Allen and SK Haslett
Abstract
The Severn Estuary Levels share with conventional dryland archaeological sites such site-formation processes as the anaerobic preservation of organic materials in pits, wells, ponds and moats. Unique to the Levels, however, but to be expected in other tidal wetland areas, are a range of natural physical formation processes, some leading to the concealment and preservation of activity/occupation sites and others to the exposure and destruction of sites, and the modification of the assemblages of cultural debris they contain. These processes are sea-level change, sedimentation, wave-action, the wandering of estuarine channels and banks, channel meandering, and salt-marsh autocyclicity. Human interventions from Roman times onwards also contribute to site formation through the embanking and drainage of salt marshes. These interventions transformed the extensive Severn Estuary Levels from wetlands suitable only for seasonal activities to areas that could be farmed and settled, but at the increasing cost of drainage and sea defence demanded by sea-level rise.
Goldcliff Mesolithic Archaeology
At Goldcliff, 9km south-east of Newport, Monmouthshire, a promontory projects into the estuary. This marks the site of a former island, the greater part of which has now been eroded away by the sea. Round the edge of this island in the intertidal zone hunter-gatherer Mesolithic sites were excavated in 1992-4 and 2001-3. The sites were occupied at a time of lower sea level and were buried and preserved by silts and peats as sea level rose. At the base of the sequence is a substantial oak forest which existed c. 6000 cal BC. Occupation on the site took place between then and 4700 cal BC. Activity is represented by scatters of flint, chert and tuff artefacts as well as wood, bone and antler tools. Bones of deer, extinct wild cattle, pig, otter and fish (especially eels) represent the animal resources exploited. Animal and plant remains show that activity on these sites took place particularly in the autumn but there is also some activity in spring, at the height of summer and on one site in winter. Stratified within estuarine silts around the edge of the former island are human footprints, many of them children, some as young as 4 or 5. Clearly, children were fully involved in the life of this Mesolithic community. The site has also produced the earliest evidence for human intestinal parasites, showing that the fringes of the settlement area were used for defaecation. The latest Mesolithic occupation is covered by a thick peat and another submerged forest which grew between 4400-4200 cal BC. (Note: this appears to coincide with the main period of submerged forest formation around the West Wales coast, coinciding with the "tipping point" transition from rapid sea level rise (c 1m per century) to a much slower rate of rise of c 8 cm per century.)
Archaeology in the Severn Estuary 22 (2013), 3-19
SITE FORMATION PROCESSES
IN THE SEVERN ESTUARY LEVELS
by J.R.L. Allen and SK Haslett
The Severn Estuary Levels share with conventional dryland archaeological sites such site-formation processes as the anaerobic preservation of organic materials in pits, wells, ponds and moats. Unique to the Levels, however, but to be expected in other tidal wetland areas, are a range of natural physical formation processes, some leading to the concealment and preservation of activity/occupation sites and others to the exposure and destruction of sites, and the modification of the assemblages of cultural debris they contain. These processes are sea-level change, sedimentation, wave-action, the wandering of estuarine channels and banks, channel meandering, and salt-marsh autocyclicity. Human interventions from Roman times onwards also contribute to site formation through the embanking and drainage of salt marshes. These interventions transformed the extensive Severn Estuary Levels from wetlands suitable only for seasonal activities to areas that could be farmed and settled, but at the increasing cost of drainage and sea defence demanded by sea-level rise.
EXTRACT
Salt-marsh autocyclicity
Essentially, this intrinsic process relates to the way in which a differential in sedimentation arises on a mudflat being colonized by salt-marsh plants on its higher parts. The halophytes trap and bind tidal silt, so that where they occur the sediment deposition rate is significantly higher than on the adjacent bare mudflat. Consequently, in the course of time, the salt marsh grows up higher than the mudflat and an outward-migrating transition zone of increasingly steep slope develops between the two environments. Eventually, the transition zone becomes sufficiently steep that wave-action begins to cut a cliff in it subject to undercutting and toppling, whereupon a galloping retreat landward occurs, resulting in significant loss of marsh and the formation of an eventually bold cliff. Subsequently, the mudflat to seaward of the marshes grows up again and the cycle is repeated, the marsh cliff stabilizing, on a time-scale of several decades to a few centuries. In a complex system like the Severn Estuary, however, this intrinsic process can be constrained by such external factors as decadal trends in windiness (strength and direction), and modulated by sea-level movements and channel wandering, as discussed above.
Archaeologically, salt-marsh autocyclicity is a two-edged process of site formation. On the one hand, it creates new wetlands for human exploitation while burying through progressive siltation sites initiated during the period of upward and outward marsh growth. On the other hand, during the phase of erosional retreat of the marsh edge, wetland is lost, primary archaeological contexts become exposed to view, and artefacts are released into the intertidal zone, to be dispersed, sorted and modified there.
Essentially, this intrinsic process relates to the way in which a differential in sedimentation arises on a mudflat being colonized by salt-marsh plants on its higher parts. The halophytes trap and bind tidal silt, so that where they occur the sediment deposition rate is significantly higher than on the adjacent bare mudflat. Consequently, in the course of time, the salt marsh grows up higher than the mudflat and an outward-migrating transition zone of increasingly steep slope develops between the two environments. Eventually, the transition zone becomes sufficiently steep that wave-action begins to cut a cliff in it subject to undercutting and toppling, whereupon a galloping retreat landward occurs, resulting in significant loss of marsh and the formation of an eventually bold cliff. Subsequently, the mudflat to seaward of the marshes grows up again and the cycle is repeated, the marsh cliff stabilizing, on a time-scale of several decades to a few centuries. In a complex system like the Severn Estuary, however, this intrinsic process can be constrained by such external factors as decadal trends in windiness (strength and direction), and modulated by sea-level movements and channel wandering, as discussed above.
Archaeologically, salt-marsh autocyclicity is a two-edged process of site formation. On the one hand, it creates new wetlands for human exploitation while burying through progressive siltation sites initiated during the period of upward and outward marsh growth. On the other hand, during the phase of erosional retreat of the marsh edge, wetland is lost, primary archaeological contexts become exposed to view, and artefacts are released into the intertidal zone, to be dispersed, sorted and modified there.
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