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THE ESSEX NATURALIST
have become reduced. Storm surges are well known and the
disastrous flooding of 1949 and 1953 were the result of such
phenomena. When spring tides concur with deep depressional
activity over the North Sea exceptional high tides are produced,
with the resultant breaking of sea defences. These storm surges
are documented as far back as 1552 (Brown, 1927). The alluvial
deposits close to the coastline indicate regions where the sea, at
spring tides and especially during storm surge conditions, was
driven inland to inundate the marshes. Vast drowning of marsh-
land and replenishment of lagoons with saline water is likely to
have occurred with the concurrent distribution of the cockle from
lagoon to lagoon. J. T. Greensmith and E. V. Tucker (personal
communication) have located a bed of C. glaucum shells now
some two miles inland north of Burnham-on-Crouch, Essex. These
shells are regarded as a life assemblage and are found in the bank
of a modern drainage ditch. The creation of sea defences no doubt
effectively reduced the number of, and isolated, separate lagoon
populations of C. glaucum.
The most rational approach to the presence of C. glaucum
round our south-eastern shores would be to consider the sites we
know at present as remnants of a much wider distribution. With
further reclamation and fresh water canalisation, this rare cockle
will undoubtedly become even rarer; but not, I hope, before
marine biologists recognise its specialised existence as one of the
few lamellibranchs to invade and remain in stagnant brackish
water.
References
Brown, E. (1927). An Essex Island Parish. Essex Review, 36: 176.
Eisma, D. (1965). Shell-characteristics of Cardium edule L. as indicators
of salinity. Netherl. J. Sea. Res.: 493-540.
Grieve, H. (1959). The great tide. Essex County Council.
Grossu, A. V. (1961). Variatiile lui Cardium edule L. Marea Neagra
Hidrobiol, 3: 177-91.
Hopner-Petersen, G. (1958). Notes on the growth and biology of the
different Cardium species in Danish brackish water areas. Medd.
Kornm. Pra. Danm. Fisk-og Havunders (N.S.), 11, 22: 1-31.
Mars. P. (1951). Essai d'interpretation des formes generalement groupees
sous le nom de Cardium edule L. Bull. Mus. Nat. Hist. Marseille,
11: 1-31.
Robinson, A. H. W. (1953). The changing coastline of Essex. Essex Nat.,
29: 79-93.
Tebble, N. (1966). British Bivalve Seashells, 97-108, British Museum
(Natural History).