The Submerged Forests of Cornwall

 These abundant submerged forests, for the most part revealed in the current inter-tidal zone, do not in themselves tell us anything about sea-level positions or crustal stability, but there are very close parallels in South and West Wales and on the Cardigan Bay coast.

Extract from:
French, C. N.,The 'Submerged Forest' palaeosols of Cornwall.
Geoscience in south-west England, 9, 365-369.

The available evidence concerning inter-tidal 'Submerged Forests'
suggest these palaeosols include a variety of habitats such as
woodland, marshland and freshwater/brackish lagoons, and there is no
simple stratigraphy which explains either their formation or the
sequence of events leading to their submergence.
Coastal woodland was clearly an especially important component
of the vegetation at numerous low-lying locations around the coast
and it existed when much of the rest of Cornwall had already been
cleared of forest by Man. For such a woodland to survive suggests that
the area in which it grew was not worthy of clearing - an alder can
with poorly drained marshy soils. There does not appear to be a close
modern analogue of this `Submerged Forest' woodland community in
Cornwall. It seems likely that this woodland developed when the sea
was at a distance, as suggested by the hazel content, and for it to be
maintained over any length of time, the area in which the trees grew,
was not subject to frequent incursions by sea water. However, the
diatom record from Marazion Marsh suggests that prior to final
inundation, the influence of the sea became progressively more
apparent. Furthermore, once submergence had taken place, the
'Submerged Forest' strata remained beneath sea water or marine
sediments, thus ensuring the preservation of the soil and timber
remains.
There is no reason to suppose that the inter-tidal 'Submerged
Forest' beds were all submerged by the same event. It is more likely
that local circumstances controlled the nature and timing of
submergence and the resultant stratigraphic succession. Sediment
barrier structures, which protected the low-lying areas from the sea,
are considered to be especially important in determining the way in
which the rising sea level was able to inundate the coastal lowlands.
Indeed, they may have been instrumental in the formation of the
'Submerged Forest' community itself, by maintaining the poorly
drained conditions which favoured the growth of alder can.

Devensian Isostatic depression in the South-West

 Little Milford, on the shores of Milford Haven.  Sea-level studies in areas such as this show remarkably similar results to studies in Cornwall and on the English Channel coasts, suggesting that the whole of the SW part of the British Isles has a similar eustatic / isostatic history -- and therefore behaves as a block. 


So where does all this stuff about eustasy and isostasy get us?  It's actually quite important.  To summarise.


 This is the overall pattern of eustatic sea-level rise at the end of the Devensian Glaciation, as agreed by most experts from a wide range of disciplines:

Around 14,500  years ago, sea-level stood around -100 m, but a rapid rise of 40 m occurred up to 13,000 BP at a rate of 3.7 m per century.  A second major melting phase at 11,000 BP raised eustatic sea-level to around – 40 m by the beginning of the Holocene (10,000 BP) at a rate of 2.5 m per century, by which time global ice volumes had been reduced by over 50%.  

Holocene sea-level then rose in the Bristol Channel area from -35 m OD at 9,500 BP to 2-5 m OD at 5,000 BP.  The rate of sea incursion gradually declined.  Around 7,000 years BP sea-level was around 8 - 10 m lower than it is today, depending on whose curve you are using (there is generally assumed to be a margin of error of + or - 1m)

After around 6,000 BP the marine incursion into coastal areas of northwest Europe took place more slowly.  In Neolithic times, the sea was at c -6m OD, and in Bronze Age times about -4m OD.  About 2,000 years ago (the time of the Romans in the UK) sea-level was probably about 2m below that of today -- but it may have been higher.  

The configuration of the British coastline around the time of Christ was similar to that of the present day, except that it was more indented due to the drowning of wetlands and estuaries which have subsequently silted up.  As mentioned in earlier posts, the evidence is difficult to interpret in some estuarine and fenland environments because of the effects of storm surges, changes of coastal configuration resulting from the breaching of dune barriers or pebble beaches, and from the compaction of sediments.


In all parts of the UK where isostatic equilibrium has prevailed, or where there has been an overall depression of the crust during the Holocene, it should be possible to "read" this record -- or parts of it -- in the coastal sediments.  In the northern parts of the UK, as indicated in the last post, isostatic effects have caused chaos -- and where the ice load was heaviest isostatic depression was massive, and isostatic recovery has proceeded at a rate that has outstripped the eustatic rise.  But what about the areas south of Cardigan Bay?  This is where, in the models, the hinge line is supposed to have occurred, with virtually no glacio-isostatic depression and therefore no rebound.  It is assumed that this is where the ice margin was located at the time of maximum Devensian ice extent in the domain of the Irish Sea Glacier.  Further to the south, we are into the "forebulge" area, where land is expected to have been HIGHER than it is today around 20,000 years ago and then sinking gradually in compensation for the recovery of the landmass further north.  This is what the maps of current crustal downwarping show (see the last post)..........



This means that the further towards the SW we move, the lower (with respect to a fixed datum) should be the sediments dated to particular phases of the Holocene.  If the Scilly Isles and SW Cornwall really are sinking at a rate of c 2m per millennium, and if the rate of sinking was even faster in the past (in compensation for the faster isostatic recovery further north) then this should show up clearly in coastal sediments.  In other words, there should be pretty large ANOMALIES!


What we find, when we look at the evidence from a vast range of studies from West Wales to the South Wales coast to the Severn Estuary to the Somerset Levels to the south coasts of Cornwall, Devon and Dorset, is that there are virtually no anomalies.  The sea-level curves come out conforming to the normal eustatic curve over and again, with variations of maybe a metre or so on either side of the mean curve -- within the margin of error.  There is an intriguing thought -- namely, that the assumed rate of coastal sinking on the South Coast of around 1 mm per year or 1 m per millennium is more apparent than real, since most of the observations on which this thesis is based have come from areas with compacting sediments!


Two key references:  
Mid- to late-Holocene relative sea-level change in southwest Britain and the influence of sediment compaction, Robin J. Edwards, The Holocene, 16,4 (2006) pp. 575  587
Relative sea-level change and postglacial isostatic adjustment along the coast of south Devon, United Kingdom, Massey, AC et al, J. Quaternary Sci., (2008) (ISSN 0267-8179)

At the moment, the sheer conformity of the observations from across this region suggest to me that this was not a hinge line and forebulge area at all, but an area that acted during and after the Devensian Glaciation as a more or less stable block.  This is what you would expect if Devensian ice really did reach the Scilly Isles, as suggested by Prof James Scourse and others.  The strange tongue of ice that he suggests as flowing down the centre of the Celtic Sea is glaciologically most unlikely, and in the Aberystwyth Devensian modelling work the eastern edge of the Irish Sea Glacier is shown pressing onto the Cornwall and Devon coasts.  If the ice edge had anything like an equilibrium profile, it must have been several hundreds of metres thick in the Bristol Channel -- whether or not it encroached inland.  It all fits..........

Isostatic adjustments in Southern England

We cannot get a reliable answer to the question"Was there ever any glacier ice on Salisbury Plain?" without fully understanding the relative positions of land and sea during past glacial and interglacial episodes.  For the Anglian Glaciation (if that's really the one we are looking at) we have already concluded that the only way to explain the emplacement of giant erratics along the south coasts of Cornwall and Devon -- and further to the east -- is to assume that the isostatic depression of the Channel Coasts must have been considerable at a time when ice floes and icebergs were carrying around piles of glacial debris from the north and west.  That means Southern England was a depressed area rather than a "forebulge" area -- that means ice in the southern counties, whether we like it or not.


What about the evidence from Southern England during the Late Devensian and Holocene (post-glacial) episodes?  What can we glean from the studies of eustasy and isostasy in the literature?  There is a huge literature -- and I think we can say that the eustatic side of the equation is fairly well known.  Previous posts contain the best sea level curves we can find.  I have also added an excellent summary below from Prof Simon Haslett.


The maps above come from the following fascinating presentation:

Locating the Centre of Holocene Glacio-isostatic Uplift in Scotland  -- poster presentation by
C.R.Firth, D.E.Smith and J.Rose
Thje general pattern of isobases ties in, as one would expect, with the pattern of Devensian glaciation and the thickness and weight of ice.  As Prof Haslett says, there has been c 250m of isostatic depression and then recovery beneath the thickest part of the UK ice sheet on the western side of Scotland.  As suggested in the map on the last post, to the south of this area of more or less concentric isobases there is a zone where current uplift is around zero (called the fulcrum or hinge area in somer
literature) and then south of that there is another area (including SW England) where it is assumed that there is current depression of around 1mm per year or 1m per millennium.  In the extreme SW of England it is assumed that the rate of sinking is around 2m per millennium.  There is another major area of sinking or depression in the North Sea, which may be as high as 1.5m per millennium.  This is attributed to recent high sedimentation rates, to the loading of water subsequent to the flooding of the area in post-glacial times, and to the forebulge or balancing effect partly related to the viscosity of the crust.  There may also be a tectonic sinking at play right across the North Sea - Southern England - English Channel area which may owe little or nothing to isostatic factors.

Complicated?  Yes, it is -- and we must remember that many of the maps are based on modelling exercises, backed up by as many coastal observations as possible, for processes that are very slow indeed.

-------------------------------------------
Introduction
The Holocene (or Flandrian in Britain) is the most recent Epoch of the Quaternary Period.
It is an interglacial that has followed the last Pleistocene glaciation, known in Britain as the Devensian Glacial stage.
Upon the melting of the Devensian ice sheets, sea-levels changed through a combination of eustasy and isostasy to achieve their present levels.
Eustasy and isostasy
Changing levels of land and sea reflect the interplay of two major elements:
EUSTASY – global changes in sea-level.
ISOSTASY – localised tectonic activity which results in vertical displacement of the land.
Isostasy refers to the state of balance that exists in the earth’s crust so that depression in one locality will be compensated for by a rise in the crust elsewhere.
The main controlling factor of both eustasy and isostasy is the expansion and contraction of continental ice sheets over successive glacial/interglacial cycles.
Because of this, global sea-level change that results from the repeated extraction of water from the oceans, and its subsequent return on melting, is referred to as glacio-eustasy.
Similarly, crustal deformation caused by loading of glacier ice is termed glacio-isostasy.
Changes in sea-level that take place through the interplay of these factors are known as RELATIVE SEA-LEVEL CHANGES, and are usually local changes in the position of sea-level relative to the land.
In tectonically stable areas, evidence for sea-level change should reflect only the eustatic component and such regions record ABSOLUTE SEA-LEVEL CHANGES.

Devensian glaciation
At the height of the Devensian ice age (the last cold stage) at around 18 ka, enough water had been removed from the oceans by expanding ice sheets to reduce global sea-level by ~130m.
This glacio-eustatic lowering was accompanied by the glacio-isostatic depression of Fennoscandia, northern Britain and Canada through GLACIAL LOADING.
Forebulging occurs where ice loading depresses the crust, which is then compensated beyond the ice perimeter by a bulging of the crust.
Following the melting of the Northern Hemisphere ice sheets, which began around 16 ka, global sea-level rose steadily while melting of the continental ice sheets resulted in rapid glacio-isostatic recovery.
Shorelines that formed around the margins of the melting ice sheets were progressively raised above sea-level as glacio-isostatic rebound outpaced glacio-eustatic sea-level rise.
Detailed analysis of raised shorelines and associated features provides evidence of the extent of glacio-isostatic rebound since deglaciation.
In eastern Scotland, for example, isobase maps (maps showing lines of equal rebound or subsidence) indicate that over 250 m of rebound has occurred since deglaciation, and the amount of rebound further west near the centre of the Devensian ice sheet on Rannock Moor was even greater.

Holocene rising
In all glacially depressed areas, the process of land emergence has continued throughout the Holocene.
In Scotland, glacio-isostatic rebound is still incomplete, and raised shoreline data indicate that in the inner Forth, Clyde and Tay valleys, current rates of rebound range from 1.8 to 2 mm per year.
In southern Britain and the southern North Sea isostatic depression has continued throughout the Holocene, producing submerged forests. Repeated rebound and subsidence results in a see-saw effect around a fulcrum line.
In the North Sea, the Dogger Bank was submerged beneath the rising Holocene sea by 8.7 Ka BP, and the Straits of Dover were breached just before 8 Ka BP. The present configuration of the coastline of southern Britain was more or less established by 7.5 – 7.8 Ka BP.
Holocene sea-level rise in the Bristol Channel area rose from -35 m OD at 9.5 Ka BP to 2-5 m OD at 5 Ka BP at the following rates:
After around 6 Ka BP marine incursion into coastal areas of northwest Europe took place more slowly. The configuration of the British coastline was similar to the present day, except it was more indented due to the drowning of wetlands and estuaries which have subsequently silted up.
Absolute sea-level change
The pattern of absolute sea-level change at the end of the ice age has been difficult to establish, principally because it is difficult to find stable coasts.
However, new approaches using oxygen isotopes from deep-sea cores are beginning to provide an indication of global sea-level trends during the period of ice melting.
The data suggest that at 14.5 Ka BP, sea-levels stood around -100 m, but a rapid rise of 40 m occurred up to 13 Ka BP at a rate of 3.7 m per century.
A second major melting phase at 11 Ka BP raised eustatic sea-level to around – 40 m by the beginning of the Holocene (10 Ka BP) at a rate of 2.5 m per century, by which time global ice volumes had been reduced by over 50%.

Isostatic recovery
During the late glacial period, however, rapid glacio-isostatic recovery in NW Europe outpaced sea-level rise and therefore the shorelines formed during that period now stand well above the present shorelines.
In Scotland, the highest late glacial shorelines, dated at 13 ka BP now stand 50 and 41 mOD on the east and west coasts respectively.
In the early Holocene, however, eustatic rise at rates of 1 cm per year, began to exceed isostatic recovery in many areas. This resulted in a major marine transgression around the coastline of Scotland between 8.5 and 6.5 BP.
After 6 ka BP in Scotland isostatic recovery once again outpaced eustatic rise.

(Prof Simon Haslett, University of Wales, Newport)

Isostasy revisited

Wikipedia map showing gradual ongoing uplift in the north and gradual sinking in the south

This is a revised version of my post of 6th June -- to be read in conjunction with the posts relating to Holocene shorelines etc:

The map above shows that there is still slight uplift going on in the northern parts of the British Isles, but that there is sinking in the south. This sinking might be partly a compensatory isostatic effect, partly tectonic, and partly because of the massive sediment load carried by rivers into the southern North Sea and English Channel. The rate of uplift in the Highlands of Scotland is only about one third of that of the Gulf of Bothnia, since the British-Irish ice mass was that much smaller and since isostatic recovery is now more or less complete.


Another complicating factor is that when a large area like the Bristol Channel or the English Channel becomes submerged as a result of eustatiic rise,  the weight of water that was not there previuously has an isostatic effect -- it causes the crust to sink.



On the measurement front, there are often discrepancies when altitude or sealevel measurements are made in areas like the Severn Esuuary or the Somerset Levels, because sediments are compressed (ie they sink) over time; the degree of sinking or compaction can also be related to water content......

The highest post-glacial or Holocene shorelines in Scotland are nothing like as high as the shorelines I have studied in the Arctic and Antarctic. There are some at c 45m in south-west Scotland and Northern Ireland, although most of those identified have been beneath 30m. In West Wales there is no evidence of post-glacial shorelines above present sea-level; this means that the amount of isostatic recovery following the removal of the ice load has been less than the 120m or so of eustatic sea-level rise.

If we assume that isostatic uplift in South-west Britain is more or less complete, we would expect to find some evidence of prehistoric shoreline occupation BELOW present sea-level. This is just what we do find. During the Palaeolithic sea-level was more than 20m lower than it is today. During the Mesolithic c 7,000 years ago, sea-level was about 10m lower than today, and we find evidence of the Mesolithic sea-level rise in the submerged forests around the coasts of Pembrokeshire. In the Neolithic, sea-level was at c -6m, and in the Bronze Age it was at -4m. Around the time of Christ, it was about 1.5m lower than at present.

This all means that in many locations (for example, on low coastal forelands or in estuarine environments) HWMST would have been located out beyond the position of the present coastline. This is something that does need to be borne in mind by those who argue for the human transport of bluestones by land and sea. To my mind, the degree of difficulty would have been greatly increased -- with extensive boggy and heavily-wooded tracts to be negotiated in places where there is now sea.

Mesolithic footprints

Footptints of a man in bare feet, striding purposefully across an intertidal mudflat in the Severn Estuary around 7,000 years ago

 More and more examples of Mesolithic footprints are now coming to light as research continues around the coasts of the UK.  They are generally preserved on estuarine or riverine muddy sediments which have been later covered by peat.  Peat is easily dated by C14 methods, and so we can say with some certainty that these footprints are probably around 7,000 years old.  Sea level oscillations around this time are difficult to reconstruct,  partly because in the Severn Estuary there is such a vast tidal range and the precise character of the coast is not yet known.

 Acknowledgements to Derek Upton and Severn Estuary Levels Research Committee (SELRC)

Uskmouth

Mesolithic human footprints were first found here on the foreshore by Derek Upton in 1986. This led to a programme of investigation and recording led by Professor Stephen Aldhouse-Green. It was shown that the footprints were stratified in banded sediments below thin peats that are dated to c. 4600 cal BC. The footprints include children as well as adults. They are associated with very extensive footprints of deer and birds. Derek Upton found an antler mattock near the footprints and radiocarbon dating shows it is of similar date to the footprints. The Uskmouth finds may represent a Mesolithic foraging expedition from a site of similar date at Goldcliff where many other Mesolithic human footprints and settlement areas have been found.

The Postglacial sea-level

Found these two excellent illustrations in Bell and Walker, "Late Quaternary Environmental Change."  They are relevant to some our discussions over the past couple of weeks.  Click to enlarge.  Sorry they are a bit fuzzy.

The sea-level rise graphs for the Holocene are all based on research in the areas indicated.  They match very closely the sea-level curves obtained in other stable parts of the world where there is no tectonic or isostatic "interference" -- so they can be taken as pretty reliable.

The map shows Upper Mesolithic sites known in Wales as at 1990 -- in the years that have followed, more mesolithic sites have of course been discovered.  Many of these sites show human settlement at or even below present sea-level.  Some are higher, in river valleys -- but there is of course no reason why Mesolithic travellers should not have also moved into the uplands as well.  They are thought to have been restricted to the coastlands for the most part because they provided a more varied food supply for a hunting, fishing and gathering society, and because movement was generally easier along the coast than it was in the inland jungles.

 The submerged forest is at Borth on Cardigan Bay.  This great expanse of tree stumps, fallen branches and root systems, with peat beds, was a healthy forest, well clear of sea-level c 5,300 years ago, ie about the time when bluestones were starting to be used in monuments (evidence from Boles Barrow) on Salisbury Plain.

More on cobbles and ball bearings

http://www.michaelbradley.info/books/hotair/hotair1.html
DALHOUSIE 1981

Curraghs are fine, but what about the land haulage to get bluestones to and from the water?

This, admittedly, would have been hard work – but much less work if we forget about the engineers’ wooden sledges. Why not lay a narrow "road" or strip of cobblestones? We grease these cobbles with Neolithic domestic animal fat or Neolithic butter whenever we want to move a bluestone. Neolithic rawhide ropes are attached directly around the bluestone and the stone-movers pull along both sides of the narrow cobbled and greasy way. Some stone-movers can push to help guide the stone as necessary. Sturdy wooden levers would have been helpful for this work. Given Welsh topography from the Preseli quarry down to the tidewater at the Bristol Channel, the major problem would have been braking the bluestone, not hauling it, for most of the necessary ten kilometers.  At Stonehenge, the bluestone would have only needed to be hauled two hundred yards.

Using Dalhousie University students in Nova Scotia back in October 1981, we laid a one kilometre strip of cobbles in two days. This was tiring work, but not too brutal because each individual rock was not very heavy. In real megalithic life, this work was actually saving much labour because the cobbled path could be used for many successive bluestones, and over seventy bluestones were eventually moved to Stonehenge from the same quarry in Wales. These cobbles would gradually sink into the ground under pressure and more must be added continually.  But if some care is taken to choose the most rounded  beach rocks available, and if they are liberally greased, then a cobbled strip of virtual megalithic ball bearings is the inevitable result.  We didn't wait for this development.

The very next weekend, a gang of just forty-seven young men and women moved a three and a half ton Nova Scotia "bluestone" (a naturally "squared" boulder of granite) that one kilometre in just five hours and forty-two minutes over the slippery cobbles. The ground was not level, but gently undulated over this distance. It was hard work uphill, and we sometimes had to resort to pushing it just a few inches at a time with levers.  But the "pseudo bluestone" was moved at five times the speed with less than one-fifth the "manpower" (the Dalhousie students were about equally men and women) compared with the British Y2K Millennium Project replication.  And, for that matter and before I forget, greased cobbled roadways were undoubtedly also used when megalithic Britons moved the giant Sarsen blocks from nearby places on the Wiltshire downs to the Stonehenge site.