36
earliest peat might have originated from human activity
in the vicinity. Thus, the radiocarbon dates from these
layers could indicate the date of activity taking place a
little above the contemporary high tide line, or around the
edges of freshwater pools on low lying land not flooded
by the sea.
The variability of the environmental samples taken from
Par Beach suggests that periodic fluctuations between,
terrestrial to maritime habitats occurred throughout
prehistory. These results are suggestive of the formation
of organic deposits behind storm beaches and the
subsequent periodic breaching of such barriers. It is
significant that the earliest deposits from Par Beach,
sample 7661.02 (dating to the Late Mesolithic) and
7761.04 (located stratigraphically below sample 7661.02
and therefore pre Late Mesolithic), show evidence of
marine influence suggesting that when they formed they
were in close proximity to the ancient coastline.
Porth Mellon, St Mary’s
At Porth Mellon, two intertidal deposits were sampled,
dated and analysed (Ratcliffeand Straker 1996, 24-26).
Preservation of palaeo-botanical remains was better at
Porth Mellon than at other intertidal peats on Scilly
allowing detailed analysis (Ratcliffe and Straker 1996,
105-129). The lower peat exposure, located just below
mean low spring tide (MLST), produced a radiocarbon
date of, 3085-2765 cal BC (GU-5394; 4310 ± 60BP)
from its base and a second date of 3015-2709 cal BC
(GU-5393; 4280 ± 50BP) from its upper surface. The
second peat exposure, located higher up the beach than
the first, produced radiocarbon dates of 2470-1981 cal
BC (GU-5392; 3810 ± 80BP) from its base and a date of
2866-2200 cal BC (GU-5396; 3980 ± 100BP) from its
surface. These radiocarbon dates confirm that these
deposits represent two separate phases of peat formation,
both of which began forming around the Late Neolithic,
but that the start date for the lower exposure is at least
500 years earlier than that for the upper. Analysis of
fossil pollen from both deposits demonstrates the
presence of tree and shrub pollen (60% TLPS). In the
lower, slightly earlier peat, birch was dominant with
smaller amounts of hazel, oak, and traces of elm, pine,
alder, ash, holly, lime and willow. Open ground flora was
also abundant within both deposits suggesting the
presence of dry open ground, such as pasture, in the
vicinity. In contrast, plant macrofossils from both
deposits, advocate the presence of standing freshwater or
wet dune slack conditions, suggested by the presence of
aquatics such as stoneworts, waterwort, starwort
pondweed, spearwort, marsh pennywort, sedges, rushes
and gipsywort (Ratcliffe and Straker 1996, 118, table 36).
The presence of disturbed open ground existing nearby
was evidenced from the presence of species such as
chickweed, fat hen, parsley, piert and knotgrass. The best
evidence from the plant remains for marine influence
comes from the upper peat where tasselweed was
identified, at plant characteristic of brackish pools
exposed to salt spray and periodic inundated (Rieley and
Page 1990). Similarly, annual seablite and bittersweet,
were present in both samples, species frequently found
growing above the strand line on the beaches of Scilly
today.
Diatoms were only identified from within the lower
intertidal deposit where they contained a wide range of
well preserved taxa, with high numbers of brackish water
species including Cyclotella meneginiana (Cameron
1994). Similar environmental conditions were suggested
for the formation of the upper peat by the presence of
foraminifera such as Jadammina macrescens a species
typical of lower salt marshconditions (Cameron 1996).
The presence of marine diatoms in the lower peat suggest
that it was subjected to marine influence, at least from
time to time, and the presence of tasselweed and
foraminiferal in the upper peat confirms that it too was
subjected to inundation and salt spray, although the area
may have been protected from direct marine influence by
dunes. The open woodland suggested by the pollen
studies would seem to reflect the nature of drier ground
vegetation behind the brackish and freshwater pools and
marshy conditions that existed within Porth Mellon. It
would seem likely that these deposits were formed in and
around wet dune slacks defined by Rieley and Page
(1990) as ‘the low-lying depressions between dune ridges
where the water table does not fall below 1 metre during
the year, and flooding often occurs in winter and spring.
The peat deposits taken from Porth Mellon, like those
from Crab’s Ledge and Par Beach, suggest that when
they formed they were located in close proximity to the
ancient coastline and not in an open terrestrial
environment as previously postulated (Thomas 1985).
We may now use the data provided by these
environmental studies to construct a model of sea-level
changes on Scilly and assess the implications of such a
model on the character and configuration of the
archipelago during prehistoric.
Producing an independent sea-level curve for
Scilly
Ratcliffe and Straker have produce an altitude curve,
using data derived from the analysis of the intertidal
deposits as outlined above, to correlate the heights and
dates at which these deposits formed (1996, 50, fig. 36).
This curve was produced by plotting radiocarbon
measurements from the deposits against their altitudes in
relation to OD. By drawing a ‘best-fit’ line through, these
points a curve has been produced representing the height
at which deposits formed (Fig. 4.7). It can be seen that
this line (Line A) intersects with cal AD 2000 at
approximately mean high spring tide (MHST) suggesting
the deposits formed at around this altitude. Although a
degree of variation was present within the analysis of the