Retreat of the Late Pliocene and Lower Pleistocene Crag sea
sedimentary sequences described by Bridgland (1994, 1995, 2000) and Bridgland & Allen (1996)
from the 'Colchester Formation' and from the Lower Thames Formation.
Dating the sequence is difficult as there are no associated biogenic deposits and constraining dates
from the underlying material arc not too helpful. At Great Blakenham, the Baylham Common Member
overlies the College Farm Member, so a Baventian or later age is possible (Table 2). At Caistor St
Edmund, dating of the stratigraphy is far from secure but a correlation with the Beestonian is implied
(Postma & Hodgson 1988). As at least six gravel members are involved in the 'Formation', deposition
would have occurred in more than one cold stage. It is likely that early deposition occurred in the
Pre-Pastonian/Baventian. What currently is regarded as the Beestonian may have extended over a
period of 1.2 million years and embraced 20 or more cold stages, so some of the gravel members
could have been deposited in the earlier part of the Beestonian. Although the sediments cannot be
dated per se, they are correlated with the Dobs and How Hill embers of the Wroxham Formation
(Rose et al. 2000, 2001).
'Colchester Formation'
The 'Colchester Formation' can be seen to have a more sinuous course (Fig. 4) and its long profiles
do not clear the Cotswold gaps (Whiteman & Rose 1992; Rose et al. 1999). Thus the catchment
would no longer have extended beyond the Cotswolds. This can be deduced from the stone counts
also. Although broadly similar to the 'Sudbury Formation', the amount of distant exotics (quartz,
quartzite and Carboniferous chert) is lower (Whiteman 1992; Whiteman & Rose 1992) (Table 3).
The implication is that the Thames catchment had contracted and no longer included the area beyond
the Cotswolds. Possibly this can be attributed to glacial interference. Around Bruern on the Cotswold
dipslopc is a poorly sorted stony gravel (diamict), part of the Northern Drift, widely regarded as a
till and therefore indicative of glaciation (e.g. Arkell 1947a, b), though Hey (1986) regarded it as
decalcified solifluction debris. The Bruern diamict is associated with the change from the Sudbury
to the Colchester 'Formations'. The implication is that the glacier could have eroded the softer rocks
beyond the Cotswolds and so effectively beheaded the early Thames. Interpreting the Brucrn diamict
as a till has also been used to explain the continued presence of acid volcanic rocks in the 'Colchester
Formation' despite the loss of a fluvial connection to their provenance in the Welsh borderlands
(Whiteman & Rose 1992). Other evidence that the Thames had lost its headwaters is that the lower
Severn (Mathon River) had become established by this time, cutting across the drainage line of the
Thames (Rose 2000).
The 'Formation' comprises, in Essex, the Waldringfield, Ardleigh, Wivenhoe and St Osyth Members
(Whiteman 1992). The Waldringfield Member can be traced along coastal Suffolk as Hey's (1980)
'Low Level' Kesgrave Gravels. The lower members go offshore, successively southwards, between
Harwich and Clacton. Upstream, the Waldringfield, Ardleigh and Wivenhoe Members arc pinched
out in the Vale of St Albans and the Middle Thames equivalent of all three is the Rassler Member; a
matter which needs further investigation. The Winter Hill and St Osyth Members are correlated not
because they are contiguous but because both show intrusion of Anglian ice. The Lower Winter Hill
Terrace can be traced as a fluvial unit into the Vale of St Albans where it changes upwards to deltaic
deposits, the Upper Winter Hill Member, prograding into the Moor Mill (preglacial lake) Member.
In Essex the Lower St Osyth Gravel has a normal complement of 'Colchester Formation' lithologies,
but higher up the Upper St Osyth Gravel is finer grained and the erratic lithologies show a glacial
input, indicating disruption by ice (Bridgland 1988, 1994, 1995). This sequence of events is reviewed
Essex Naturalist (New Series) 18 (2001)
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