The Essex Naturalist
13
Morphostratigraphy
SURFACES. Formerly the terraces were recognised and mapped primarily on
the basis of the flat surfaces they present. However, recognising terraces by this
procedure can have difficulties, e.g. sometimes portions of terrace are eroded
away or the height difference between two terraces may be slight. Thus on its
own this approach has to be used with caution. By mapping surfaces, four
terraces have been recognised between the Anglian and the Devensian in the
Lower Thames (Fig. 4).
CONTINUITY. The continuity of the various gravel bodies is determined by
having a reasonably close network of quarries and borehole data, so that the
relationship of one data point (quarry or borehole) to the next is as
unambiguous as possible.
ALTITUDES. The heights of the upper and lower surfaces of the gravels are
plotted from closely spaced quarry and borehole evidence. This establishes that
the sand and gravel correlations produce a viable gradient. In the past gradients
of ca. 1m/km were regarded as acceptable, but is now recognised that the early
Thames gradients were probably in the order of 0.5m/km In the past large
"jumps" from one area to another have led to mis-correlations, e.g. Whiteman
(1992) shows that the Westland Green Gravels of Suffolk (Allen, 1984) cannot
correlate with the Westland Green Gravels of the type area on the Essex-
Hertfordshire border (Hey, 1980).
Soil Stratigraphy
PALAEOSOLS are now recognised as having developed on some of the terrace
gravels and their varying properties are helping in environmental interpretation
and relative dating, though there has not been use of this approach in the Lower
Thames.
Biostratigraphy
POLLEN ANALYSIS. Between the Anglian and Devensian cold Stages, only
two types of interglacial pollen assemblage sequences are found. One, the
Hoxnian, represented by sites such as Hoxne, Suffolk, and Marks Tey, Essex, is
characterised by an early influx of Betula, followed by Pinus and then Quercus,
Ulmus, Acer and Tilia. The optimum part of the interglacial had Quercus, Ulmus,
Tilia, Alnus and Corylus. As the climate declined, Carpinus ascended in
importance over Quercus and Ulmus and then Abies came into dominance.
During the Ipswichian, Betula comes in early, but is not followed by Pinus, which
makes a poor showing. During the climatic deterioration, Carpinus assumes a
dominance, but is not accompanied by Abies, Tilia and Picea are not well
represented in Britain though they are common on the Continent in this
interglacial, possibly indicating that sea level rose early in the interglacial
cutting Britain off from the Continent before the species had been able to