Part 3: Deposits of local rivers

D.R. Bridgland

Introduction

It has been shown earlier in this chapter that a large part of Essex was, at different times during the Early and Middle Pleistocene, drained by the lower reaches of the Thames. In the Early Pleistocene the Thames flowed across the northwestern part of the county towards Suffolk, and much of Essex was drained northwards towards it (although not necessarily directly into it), the main north-flowing river being the Medway. By the Middle Pleistocene the south-eastward migration of the Thames, possibly aided by diversion or capture by a Medway tributary (Bridgland, 1988a), had resulted in a north-eastward course for the river across Essex to the Tendring Plateau, where it was joined by the Medway (see Part 1 of this chapter). During the Anglian Stage the Thames was diverted, by way of its present valley through London, into the Medway, thus bringing into being the Thames drainage of south-western and eastern Essex. Subsequent migration caused the river to abandon the north-eastern part of this course, although the northward trend is still represented in the offshore continuation of the Late Pleistocene Thames-Medway valley (D'Olier, 1975; Bridgland and D'Olier, 1989; (Figure 5.5)F).

In the latter part of the Pleistocene, as the Thames moved further towards the south and east, new tributary streams were initiated, draining the areas once occupied by the main river. In particular, the Colne appears to have formed in the old beheaded valley of the Kesgrave (Lower St Osyth Gravel) Thames; this former river is presumably the 'misfit' remnant of the pre-diversion Thames itself (see (Figure 5.5)B). The Chelmer and Blackwater also appear to drain parts of the old Thames valley in central Essex ((Figure 5.4)E), the latter flowing in the opposite direction to the Thames. This part of the old valley was apparently modified by glacial activity between its occupation by the Thames and by the later rivers, since considerable thicknesses of till occur in overdeepened sections of it (Bristow, 1985).

The tributary rivers of the northern and western parts of Essex thus appear to have been initiated immediately following the Anglian glaciation. It is therefore not surprising that they have extensive terrace systems of their own (see Geological Survey Sheet 241, Chelmsford), although these have received comparatively little attention from geologists. A number of important Pleistocene sites have come to light within these terrace systems over the years, but it has not always been easy to distinguish the deposits of these rivers from those of the Thames; there has, for example, been uncertainty about whether the deposits in the Clacton area are the products of the Thames or the local River Colne (see above, Clacton). The richest source of hand-axes in Essex, a gravel at Upper Dovercourt [TM 240 313] (Underwood, 1913; Warren, 1933; Wymer, 1985b), appears to be a Stour terrace deposit banked against the much earlier Oakley Gravel, of (pre-diversion) Thames-Medway origin (Bridgland et al., 1990). Most of the Palaeolithic discoveries in the Chelmsford, Maldon and Colchester areas, carefully catalogued by Wymer (1985b), are probably from the terraces of the Chelmer, Blackwater and Colne. Occasionally collections of mammalian bones have also been made from these deposits (Wymer, 1985b). There are a few sites in areas where the gravels are generally believed to be pre-diversion (Kesgrave Group) Thames deposits, but where important fossil or Palaeolithic discoveries raise doubts about this interpretation. One such is near Thorpe-le-Soken (Daking's Pit — [TM 155 233]), where a rich assemblage of artefacts has been recovered from deposits mapped as part of the Cooks Green Gravel (Warren, 1933; Oakley and Leakey, 1937; Wymer, 1985b). A recent reinvestigation has confirmed the presence of abundant worked flakes, but has also shown the gravel to be perceptibly different to the local Cooks Green Formation. In particular, it contains Rhaxella chert, which is very rare in the Kesgrave Group gravels upstream of the Crag basin (see Part 1 of this chapter), but is present in the Red Crag and in Anglian Stage glacial deposits. This site lies in the valley of the Holland Brook, which suggests that the gravel may be a post-Anglian deposit laid down by that river.

Three GCR sites are included in this part of the chapter, covering very different areas of interest, although all three are associated with the River Blackwater. The first, Maldon Railway Cutting, is an important site for stratigraphical evaluation of the deposits of the Anglian glaciation, since it is the type locality of the controversial Maldon Till. The other two sites are of Late Pleistocene age, dating from the last interglacial/glacial cycle; they therefore represent a part of the Pleistocene for which no Thames deposits are known in Essex (if the various Lower Thames interglacial sites are correctly interpreted as pre-Ipswichian in Chapter 4). The Ipswichian Stage (sensu Trafalgar Square) is represented by hippopotamus-bearing deposits at East Mersea, while a site in a low terrace of the Blackwater at Great Totham has yielded an abundance of palaeontological data that suggests deposition during the Devensian Stage.

References