10 The Essex Naturalist
Fig. 6. Schematic section through the terraces of the Lower Thames.
(After Bridgland, 1995)
However, a more complex situation is envisaged by Bridgland (Bridgland,
1994; Bridgland and Allen, 1996) (Table 1), with gravel aggradation late in one
cold stage being succeeded by a warm period possibly with interglacial sediments
or organics accumulating on top, to be buried by gravels in the early part of the
next cold stage. Downcutting occurs at a later point within the cold stage,
followed by gravel deposition and so the cycle starts again (Fig. 5). Thus (a) the
gravels in the upper part of one terrace can belong to the same cold stage as the
lower gravels of the next terrace below and (b) the deposits of one terrace can
encompass two cold periods and the intervening warm stage (Fig. 6).
These different views on the process of terrace formation create fundamental
problems in interpreting the terrace sequences.
Stratigraphical procedures
The stratigraphic relationship of the gravels to associated deposits, e.g. interglacial
sediments above, below or within the gravels, particularly those containing floral,
faunal or archeological material, must be established as unambiguously as possible.
Lithostratigraphy
STONE COUNTS. The terrace gravels are sampled extensively and the
lithologies of at least 300 individual clasts per sample are determined by
laboratory examination, using binocular and petrological microscopes as
necessary. It is important that the counts be on the same size fraction to be fully
comparable, e.g. quartz and quartzites have a higher representation in the
smaller size fractions. Because of the long history of work on the Thames, there
has not been a uniformity of approach. While most workers present data for the
16-32 mm size range, others present 8-16 mm, 8-32 mm or 11.2-16 mm.