Retreat of the Late Pliocene and Lower Pleistocene Crag sea
The Early Thames of the London Basin
The Crags record the withdrawal of the sea northwards, when western Britain uplifted and/or the
North Sea basin subsided towards the end of Norwich Crag times. As the sea retreated, the Thames
became established.
Nettlebed Gravel
The earliest deposit that can be reasonably ascribed to the Thames is the Nettlebed Gravel (formerly
the 4007122 m Pebble Gravel), found at only three localities (Nettlebed, Lane End and Little Heath)
high on the Chilterns near Henley and High Wycombe. The latter two may also associated with Crag
deposits (Moffat & Catt 1983, 1986) The composition of the gravel is summarised in Table 4.
The nature of the gravels has long been subject to debate, whether they are marine, because of the
high proportion of rounded flint present, or fluvial in origin. The Nettlebed Gravels are now thought
to be fluvial, based on the gradient of the gravel, 0.9m km1 and the indications of a marine origin
(rounded pebbles and scanning electron microscope examination of the sand grain surfaces) to be a
result of the material being derived from nearby Tertiary marine deposits. The presence of quartz
and quartzite and the Carboniferous chert may indicate that the catchment of the river extended to
the Midlands, but the content is very low suggesting that there might not have been a direct connection
and that the river had a relatively local catchment (Fig. 3a). The Greensand chert indicates that
tributaries were coming in from the Weald, to the south of the London Basin. This last point is very
important because it implies that there was no river along the line of the present Thames to interrupt
the northward flow of the Wealden tributaries.
There are interglacial deposits associated with the Nettlebed site. Gibbard (1985) had argued that
these belong to the Cromerian Complex as there are no Tertiary relics, such as Tsuga, present, but
Turner (1983) and Bridgland (1994) point out that Ardleigh and Wivenhoe Members of the 'Colchester
Formation' (sensu Whiteman & Rose 1992) are the equivalent of the Cromerian Complex, hence
the Nettlebed deposits must be much older. However, correlation of the Nettlebed deposits with any
specific member of the Norwich Crag Formation is not yet possible,
Kesgrave Formation
After the Nettlebed Gravel, the sequence is characterised by a significant input of far-travelled
material, particularly quartz, quartzite and Carboniferous chert, forming the M iddle Thames Formation
and its downstream equivalent, the Kesgrave Formation (Table 3). The Formations comprise one of
the largest gravel bodies in the United Kingdom, the older elements, the ' Sudbury Formation' (sensu
Whiteman & Rose 1992) extending from the Goring Gap to Norwich and the Norfolk and Suffolk
coasts, and the younger 'Colchester Formation' (sensu Whiteman and Rose 1992) to the Suffolk and
Essex coasts (Fig. 4). The record is one of southward migration of the river. The Formation was first
recognised as such and described by Rose, Allen & Hey (1976) and Rose & Allen (1977). The
separation into the the 'Sudbury' and 'Colchester Formations' was proposed by Whiteman & Rose
(1992). Over thirty papers have been written on the Middle Thames and Kesgrave Formations, most
have been summarised in reviews by Gibbard (1985), Bridgland (1994), Rose et al. (1999) and
Green & McGregor (1999). Reference should also be made to Lewis and to Gibbard and Preece in
Bowen (1999)
Essex Naturalist (New Series) 18 (2001)
19