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the Tollesbury area towards Mersea Island. This trib-
utary provided gravel rich in exotics derived from
earlier gravels which slightly increased the exotic
content of the Thames- Medway gravels in the Mersea
and Clacton areas.
The post-Boyn Hill terraces of the Thames-Medway,
which are associated with another major buried channel
to the west of Rochford, can be traced only as far north
as the Crouch, where the river seems to have turned
eastwards along the line of the present estuary of that
river. The exception is the most recent of the
terraces recognised in the lower Thames valley, the
Lower Floodplain Terrace, which falls to about sea-
level at East Tilbury and is presumably submerged
in the Southend area. It is likely that this is the
terrace recognised from echo-soundings in the Thames
Estuary and off the Essex coast (D'0lier, 1975).
It is worth noting that an important exotic component
of the post-Anglian gravels, Rhaxella Chert, is absent
from the earlier gravels of the area, while being abund-
ant in glacial deposits of the Anglian age, suggesting a
derivation from a northern source during this glacial
period, rather than from Buckinghamshire, as suggested
by earlier authors (Gregory 1922).
It can be seen that the study of the gravels of eastern
Essex has added information about the Pleistocene (Ice
Age) history of the Thames, generally confirming the views
of Wooldridge (1938,1960) and Gibbard (1977,1979) that
the river was diverted by ice from a more northernly
course into the present valley of the lower Thames.
The sequence in eastern Essex shows that this diversion
initially served only to re-route the river between
Slough and Clacton, where it rejoined its original
alignment. The northern part of this course was abandoned
by the river following the formation of the Boyn Hill
Terrace and later courses flowed eastwards along the line
of the Crouch estuary. The modern Thames Estuary is of
comparatively recent origin.
DAVID BRIDGELAND