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POSTGLACIAL BEDS IN MERSEA, ESSEX.
By W. H. DALTON, F.G.S., F.C.S.
[Read Feb. 29, 1908.]
THE recognition as Postglacial of the gravels and loams of
Clacton and St. Osyth was not extended by the Geological
Survey to those of Walton, eastward, or Mersea, westward. The
Postglacial age of the gravel of Walton Naze has been
practically demonstrated ; that of the underlying loam with
fossil wood, originally regarded as Chillesford Clay from its
overlying the Red Crag, is still only a surmise, though con
sisting best with the evidence we have.
In 1906 a well was sunk through the gravel of East Mersea,
with fairly conclusive results as to the age of the deposit,
hitherto classed as Middle Glacial. A gravel-pit is shown on
the one-inch map of 1838, sheet 48 S.W., a mile east by north of
East Mersea church, by a tiny circlet of converging gravures.
In the floor of this pit was sunk the well referred to, and the base
of the gravel was reached at about 15 feet from the original
surface. Beneath this there should have appeared the London
Clay, as usual throughout the district. In passing near I
noticed some heaps of dark earth here and there in the
surrounding gravelly field, and supposed it to be pond-mud,
distributed thus for its manurial value. Hearing later that
shells had been found in a well there, I revisited the spot,
anticipating something Eocene. I found the material to be a
dark-blue silt, with shells of Cardium edule, Scrobicularia piperata,
and Rissoa. The last species I took to be the familiar R. ulvae
but the Clacton Postglacial Rissoa is R. thermalis, Linn., and the
Mersea specimens call for more precise determination than I can
safely attempt. The silt being found to be salt, the sinking was
at once stopped, and measures taken to retain the fresh water
yielded by the overlying gravel. As the silt is well above the
present tidal range, its salinity is a proof of its slight degree of
permeability, but I think the owner was quite right to sink no
further, as he would soon have reached high-water level, after
which any porous beds found would be charged with at least
brackish water.
The silt much resembles the unweathered condition of the
Clacton Postglacial deposit, as found below the beach ; it is