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.