Retreat of the Late Pliocene and Lower Pleistocene Crag sea
more fully by Allen (1999) In eastern Essex, a reasonable degree of contiguity allows correlation
with the Ashingdon, Belfairs/Mayland, Canewdon/St Lawrence and Chalkwell/'Caidge Members of
the Lower Thames Formation in the Southend and Dengie areas (Bridgland 1988, 1994, 1995)
(Table 1, Fig. 4).
Dating of the 'Colchester Formation' is rather more secure than for the 'Sudbury Formation'. Although
mapping of the 'Formation'in Suffolk has not been carried out the highest member, the Waldringfield
Member, belongs to Hey's (1980) low level Kesgrave grouping which can be traced to Covehithe.
At Covehithe, the Waldringfield Member overlies the Baventian Westleton Member. At Ardleigh
and Broomfield, in Essex, prolific interglacial material (pollen, Mollusca, Ostracoda, insects, fish
and small invertebrates) has been found within the Ardleigh Gravels belonging to an interglacial, as
yet not defined in Britain (Bridgland 1988, 1994; Gibbard 1996). At Little Oakley, interglacial
deposits yielding molluscs, vertebrates and pollen of Cromerian age are found in a channel cut into
the Ardleigh Gravel (Bridgland 1988, 1994). Thus the Ardleigh Gravel belongs to a cold period
preceding or early in the Cromerian. Nearby, at Wivenhoe, interglacial deposits yielding beetles and
pollen, probably of later Cromerian age, lie within the Wivenhoe Gravels (Bridgland 1988, 1994,
1995). The lower part of the St Osyth Member is known to be immediately pre- or early Anglian and
the upper part Anglian (Bridgland 1988, 1994). Thus deposition of the 'Colchester Formation' is
likely to have been in the later Bccstonian to Anglian. The 'Formation' correlates with the Mundcrslcy
Member of the Wroxham Formation (Rose et al, 2000, 2001).
The history of the later Thames and Medway in Essex and particularly the Ardleigh and subsequent
Members is discussed fully by Bridgland (1988,1994,1995) and is reviewed briefly by Allen (1998).
Tectonics of the Kesgrave Formation
The sequence of ten terraces within the Middle Thames/Kesgrave Formations provides clear evidence
of tectonic activity in the form of uplift during deposition of the Kesgrave Formation. Maddy (1997)
suggests an uplift rate in the order of0.07-0.10m.ka1. Rose et al. (1999) suggest that the hinge line
for this deformation runs roughly north-south then southeast from Cromer to Felixstowe, with a
tectonic tilt to the east. This would explain the migration of the ancestral Thames, though it is
possible that this uniclinal shift was promoted by differential erosion of the relatively softer rocks on
the south-eastern side of the river as it migrated in that direction (Bridgland 1985) or was related to
degradation of the Tertiary scarp (see above).
Summary and outstanding issues
The history of the Crag sea and its retreat is reasonably well documented. The history of the early
Thames is less clear. The Crag sea initially occupied the northern part of the London Basin, with a
southern connection to the Atlantic. Little is known of the Thames at this stage. The local lithologies
of the pebbles in the deposits in the Nettlebed area imply the Thames was a minor river and the
distribution of the coeval Crag suggests that the coast could have been nearby (Fig. 3a). However,
the palynomorphs and stone counts indicate a fluvial influence in the Norwich Crag, although it was
marine, as far east as the present coast in Suffolk. How the fluvial influence was felt so far east is
difficult to understand. The very shallow nature of the sea may have allowed strong floods to push
fluvial sediments far out to sea. Slight drops in sea-level may have allowed rivers to extend temporarily
further east. Both mechanisms may have operated in concert, particularly in the cold periods.
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Essex Naturalist (New Series) 18 (2001)