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The Essex Naturalist
establish themselves. A similar argument is offered for the lack of human
occupation in Britain at this time (see below). The Hoxnian profile is complete
at Hoxne and several other sites, but the Ipswichian profile is composite, based
on sections from several sites. Although the pollen evidence suggests only two
interglacials between the Anglian and the Devensian, the Oxygen Isotope (OI)
ratios indicate that four occurred and there is growing support for the latter
view. The youngest three OI temperate Stages all appear to be represented by
the one Ipswichian pollen sequence. However, the chance of three warm stages
having similar pollen sequences is regarded as extremely unlikely.
FAUNA. There are significant differences between the faunas of the various
interglacial deposits. Early indications of these variations were given, for
instance, by Snelling (1975), Sutcliffe (1975) and Allen (1977). The Trafalgar
Square deposits are characterised by the presence of Hippopotamus and the
absence of horse and mammoth and of the mollusc Corbicula fluminalis. The last
is considered to be a rapid and prolific coloniser of rivers, so it would not be
overlooked easily. The older interglacials lack Hippopotamus, but do have horse,
mammoth and Corbicula fluminalis. Thus the Trafalgar Square deposits can be
separated out from the other interglacials. It had been thought that C. fluminalis
was rare in the Aveley interglacial deposits and abundant in the Purfleet
deposits, but in fact like was not being compared with like. The Aveley deposits
were generally quiet water, riverside deposits and the Purfleet deposits
represented faster flowing water, a more favourable habitat for Corbicula. Recent
road excavations along the new alignment of the A13 at Aveley have cut
through deposits of both quiet water and faster flow and the latter have yielded
plentiful Corbicula. Research by Schreve (1998) indicates that the older
interglacials can be separated one from another on the basis of faunal changes.
This research may also be able to define climatic oscillations within interglacials
(Schreve, 1997). However, in both cases, the details have not yet been
published.
Several new fauna have been found recently, such as Macaca sylvanus
(macaque monkey) at Purfleet and Felis chaus (misnamed as the jungle cat, in
fact a riverside or marsh dwelling cat, found to-day from Egypt westwards
through southern Asia) at Aveley.
The fauna suggest higher sea-levels during the interglacials, with the hyrobiid
Paladilhia radigueli and the ostracod Cyprideis torosa, both brackish water
indicators at Purfleet and Aveley.
Chronostratigraphy
OXYGEN ISOTOPE RATIOS. Most oxygen has an isotopic value of 16 (16O),
but some has a value of 18 (18O), the ratio between the two being 500:1. During
evaporation of sea-water, the lighter 16O is preferentially taken into the
atmosphere. The rate of evaporation varies according to the temperature, with