8
The Essex Naturalist
Geological processes
Former courses of the Lower Thames are marked by terraces (Figs 3,4) which
comprise a sequence of flat surfaces below which there are varied sediments, but
predominantly gravel. They were created by a process of downcutting and then
aggradation of a fill of gravel and other materials, hence they are known as 'cut
and fill' terraces. There has been an evolution of ideas about when and how the
downcutting and filling occurred. In Britain there had been a tendency to
correlate terrace aggradation with interglacial periods, largely because of the
association between the terraces and the finding of temperate Mollusca and
vertebrate bones, flint implements and, in one. case, human remains. Terraces
were thought to be thalassostatic, i.e. that downcutting occurred during cold
periods as a response to lower sea-levels and that aggradation occurred during
warm periods when sea-level rose. A contrary school of thought considered that
the terraces are relatively simple cold stage aggradations separated by warm stage
downcutting. In this case, each terrace can be ascribed to one cold stage, with
occasional warm stage deposits in an associated position. Recent research
suggests a more complex situation.
Fig. 4. Long profiles of the terraces of the Lower Thames. (After Bridgland, 1994)
The field evidence is that the predominant aggradational deposits are gravels
and detailed examination of their distribution and sedimentology shows them to
have been laid down by braided rivers associated primarily with cold, periglacial
climates. During warm periods, the vegetation cover prevents major loss of soil
and other surface materials and reduces the amount of water teaching rivers.
Consequently, rivers tend mainly to transport fine material and to have a
meandering habit, as is the case in the Lower Thames today. In cold climates,
the reduced vegetation cover leaves more of the land surface vulnerable to
erosion and more water reaches the rivers, particularly if there is continuous
winter snow cover leading to several months' precipitation melting over a short
period of time during the spring. Consequently in colder climates, rivers carry