8
THE ESSEX NATURALIST
Muddy Sand with Stones and Shells: A very large part of the
middle and lower shore areas, seawards of Bradwell and at Stone
and East Point of Osea Island, are composed of sand and stones
lying over soft mud or alluvial clay. These deposits tend to
become thicker and cleaner towards low-water mark on exposed
places such as Mersea Flat and Colne Point. There also may be
a differentiation between the flat areas of sand and stones and
raised hills of stones and shells. Stones and shell over mud also
occur on oyster layings in creeks, but in these cases there is
generally more surface mud. The infauna and epifauna of these
areas is the most varied and abundant of the Blackwater shores.
The species collected include polychaetes: Harmothoe sp., Nereis
sp., Nephthys sp., Eulalia sp., Glycera sp., Arenicola marina,
Lanice conchilega and Sabella pavonina; crustaceans: Balanus
balanoides, Elminius modestus, Sphaeroma sp., Jaera albifrons,
Gammarus sp., and Carcinus maenas; molluscs: Lepidochitona
cinerea, Gibbula cineraria, Littorina littorea, Littorina saxatilis,
Hydrobia ulvae, Crepidula fornicata, Nucella lapillus, Urosalpinx
cinerea, Acanthodoris pilosa, Nucula nucleus, Mytilus edulis,
Ostrea edulis, Cardium exiguum, Cardium edule, Macoma balthica,
Abra alba, Spisula elliptica and Mya arenaria; echinoderms:
Ophiothrix fragilis and Ophiura sp.
THE SUB-LITTORAL HABITATS
Hard Clay: The clay deposits which occur in the area at ex-
treme low-water mark and below are mostly of London Clay.
These deposits generally have a poor fauna but may contain
Nereis sp., and Nephthys sp., and the burrowing bivalves,
Petricola pholadiformis and Barnea candida. In some places such
as in the Bench Head area, Pholas dactylus also occurs.
Soft Mud: Mild, deposited from suspension, is derived mainly
from the erosion of local shores, but may also be brought in with
easterly winds from the tidal banks of the Thames Estuary. It is
probable that very little, if any, suspended matter brought down
by the River Blackwater can influence areas below Osea Island,
as run-off is relatively small. Soft mud accumulates in creeks and
in hollows on the bottom. Particularly affected are the depres-
sions in the muddy shell bottoms just below low-water mark.
Such a deposition of mud after easterly winds in the winter has
been known to cause severe mortalities of oysters and other
organisms, especially when they have been weakened by low
temperatures. Some areas of mud persist along the south shore of
the Blackwater and to a lesser extent along the north shore. Con-
ditions below the surface of the mud are anaerobic. Buried layers
of dead and blackened shells indicate that some of these grounds
were covered with shells before the deposition of mud.
The fauna is composed mostly of polychaetes, Nereis sp.,
Nephthys sp., Pectinaria koreni, and Clymeme sp., and molluscs,
Abra alba and Macoma balthica, but amphipods, such as Coro-