The Essex Naturalist
17
The Diversion of the River to its Present Lower Thames Course
LOWER ST OSYTH GRAVELS. The diversion of the Thames from its central
Essex course to its present alignment is recorded in the gravels of the St Osyth
area, which are part of the Colchester Formation. These gravels can be broken
down into a lower and an upper unit. The last course of the Thames across
central Essex is represented by the Lower St Osyth Gravels. These Gravels
indicate that the Thames had a sinuous course across Essex (Whiteman and
Rose, 1992) and crossed the coast in the St Osyth area (Bridgland, 1988) (Fig.
2a). They can be identified by their stone counts (Bridgland, 1988) (see below).
UPPER ST OSYTH GRAVELS (UPPER WINTER HILL GRAVELS). These
gravels are associated with the advance of the Anglian ice into the London Basin
and the disruption and diversion of the Thames. The Winter Hill Gravels have a
very gentle gradient compared with the earlier terraces and they grade through to
the Moor Mill lake deposits, which were laid down in a pro-glacial lake at the
western end of the Vale of St Albans (Fig. 2b). The rest of the Vale was blocked
by ice so the Thames was not able to reach East Anglia. However, there is
evidence from the St Osyth area that the lower part of the river, downstream of
the ice blockage, was still operating but depositing a mixture of glacial outwash
and material from the Kentish tributaries, as the Upper St Osyth Gravels.
16-32mm Flint Wealden chert Quart: and Carboniferous
and sandstone quartzite chert chert
Upper St Osyth Gravel 89.8% 2.1% 5.1% 2.1% (ice blockage)
Lower 77.1% 0.5% 18.8% 1.8% (pre-glacial)
With the cutting off of the Thames, the quartz and quartzite decreased
because the connection with the Midlands was temporarily lost, but the supply
of flint and Wealden material increased relatively because the Medway
continued to operate.
There is no direct evidence, but it is thought that the main Thames flowed to
the Moor Mill lake from which the water spilled over into the present valley of
the Thames, presumably into a pre-existing tributary of the Medway and so
created the modern Lower Thames.
The Modern Lower Thames
ORSETT HEATH (BOYN HILL), CORBETS TEY (LYNCH HILL),
MUCKING (TAPLOW) & KEMPTON PARK GRAVELS. These gravels
follow the present line of the Thames through Reading and London to eastern
Essex (Fig. 7). The older gravels show that the river linked with the Medway
and flowed northwards to rejoin its old route near Clacton. The Black Park
Gravels have an unusually steep slope and they sink below the level of the later
gravels by the time the Essex coast is reached. As all the terraces are of the 'cut
and fill' type, the downcutting phase associated with the Gravels created a series