In some recent posts I have been trying to understand the stratigraphic relationships between the submerged forest which is ubiquitous in most of the embayments around the Welsh coast and the underlying blue clay with Scrobicularia shells in it.
The silty clay is suggestive of deposition on the lower part of an estuarine / beach system, closer to LWM than to HWM. That means quite a sudden transformation from a marine environment to a freshwater environment (allowing peat growth) and to conditions dry enough for a mixed deciduous woodland to thrive. I have suggested that maybe the changing configurations of offshore bars, sand dunes, etc might have been responsible for this apparent sea-level REGRESSION -- just at a time when the eustatic sea-level curve shows that there was a continuous TRANSGRESSION on stable coasts throughout the world, fed by the meltwater released from the melting of the northern hemisphere ice sheets. There is also a suggestion that one can get a temporary or apparent marine regression during an overall transgression because of "salt-marsh autocyclicity", as discussed here:
But over and again I have come back to the significance of the dramatic slowing down of the rate of sea-level rise, from 1m per century to just 8 cms per century, around 7,000 years ago. That's the magic date, give or take a few centuries to account for dating and calibration errors. It's magic because it is a highly significant geomorphological event that must have affected coastal processes worldwide -- but it also seems to mark the beginning of peat and forest growth, as indicated by scores of radiocarbon dates.
Having given this much thought, I'm now convinced that this relatively short-lived 7 ka marine regression on the coasts of Wales can only be adequately explained by reference to residual isostatic rebound following the wastage of Late Devensian glacier ice.
Heyworth doctorate thesis:
Some relevant posts on isostatic rebound:
Relative sea-level changes and crustal movements in Britain and Ireland since the Last Glacial Maximum
by Ian Shennan, Sarah L. Bradley, Robin Edwards
Quaternary Science Reviews 188 (2018) 143-159
Shennan et al 2018 QSR GB&Ireland RSL.pdf
What this map shows is five different RSL regions, all affected by the same eustatic sea-level rise since the end of the last glaciation, but all with different histories of isostatic and tectonic adjustment. The brown dots (sampling areas) mark the area of greatest isostatic uplift associated with the melting of the Celtic Ice Sheet; this was the "core area" and the last to melt. Overall, in this area, RSL has fallen -- see the diagram at top left. The area with the yellow dots was also heavily inundated by ice and shows a similarly complex RSL history, but with isostatic recovery rates more or less in step with the eustatic sea level rise. The area with the green dots experienced some isostatic uplift (the ice was thinner and not so persistent) but an overall rise in RSL until the levelling off which started around 7.000 BP. In the area with blue dots (including West Wales), although there was intense glaciation in places, the ice cover was relatively thin and short-lived, so that the amount of isostatic depression was quite small, leading to a complex interaction between eustatic and isostatic effects up to about 10,000 BP and a relatively straightforward RSL rise since then. The area with the black dots was mostly outside the range of the Devensian glaciation, so although there were some isostatic "forebulge" and other effects, the RSL rise has been relatively straightforward and gradual since 20,000 years ago. This can all be seen in the graphs attached to the map.
Other complicating factors in seeking to define isostatic responses include the "forebulge effect" in which depression of part of the crust may trigger a measurable bulge or crustal uplift some distance away; the variable viscosity of the crust, in which vertical isostatic movements may be associated with lateral transfers of mass (as when you put a weight on a lump of dough and find that it spreads laterally); the isostatic effect of the sea flooding back into the Severn Estuary or Cardigan Bay in the late glacial period, giving rise to some crustal depression; and the crustal depression caused by sedimentation on a large scale in coastal estuaries at the end of the Late Devensian glaciation. Even such things as small water temperature rises can lead to the expansion of the water mass and enhanced depression of the sea bed.
Those who try to model isostatic effects do not have an easy time of it!
N. F. GLASSER, J. R. DAVIES, M. J. HAMBREY, B. J. DAVIES, D. M. GHEORGHIU, J. BALFOUR, R. K. SMEDLEY and G. A. T. DULLER
JOURNAL OF QUATERNARY SCIENCE (2018)
Pembrokeshire must certainly have felt the isostatic depression effects of the Welsh Ice cap, although it was c 40 km away from its southern section. In North Wales there is still a small residual isostatic uplift effect, c 23,000 years after it reaches its LGM.
From Heyworth's thesis (1985):
Leach(1918) describes a forest bed at the south end of the Bay, where trees are rooted in boulder clay. A radiocarbon date of 5960 ± 120 (Q. 530) was obtained herefrom a sample of woody fen peat at -2.0 m OD.