Retreat of the Late Pliocene and Lower Pleistocene Crag sea
The Lower Thames Formation records the early history of the Medway as it crossed Essex to join
the early Thames in the northeast of the county.
Figure 4. Palaeogeography of the Kesgrave Formation.
(After Whiteman & Rose 1992; Bridgland 1994)
The terms ' Sudbury Formation' and 'Colchester Formation' have not been accepted by the Geological
Society of London, but are retained here informally as they are embedded in the literature.
'Sudbury Formation'
The 'Sudbury Formation' is characterised by quartz and quartzite from the Triassic, Carboniferous
or Devonian rocks of the West Midlands and Welsh borderlands, plus acid igneous rocks from North
Wales (Table 4). Some of the erratics are very large and the surfaces of the sand grains glacially
fractured. Thus there was glacial activity in the catchment. Whether the Thames extended to North
Wales or the Welsh material was introduced into the catchment by glacial activity is open to debate
(Hey 1991). However, the Thames certainly seems to have extended from the Midlands, given the
quantities of material from that provenance, plus the long profile reconstructions suggest that the
Thames headwaters could have passed through gaps in the Cotswolds, though account needs to be
taken that the gap floors may have been higher at that period (Whiteman & Rose 1992; Rose et al.
1999).
The distribution of these gravels shows that the Thames entered the London Basin via the Goring
Gap and flowed high up on the northern limb of the syncline. Progressively the river migrated
Essex Naturalist (New Series) 18 (2001)
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