32
(Warren 1955). It is suggested that this change in
composition represents the confluence of the early Thames
and the extended Medway (Fig. l).
The lower gravels of eastern Essex do include the Thames
exotic suite as well as Medway stone types, thus
indicating the first appearance of the Thames in the
area. It is known from the work of Wooldridge (1938,1960)
and more recently Gibbard (1977,1979) that the Thames was
diverted by ice during the Anglian (the main glacial per-
iod, when ice reached the London area) from an earlier
course through the Vale of St. Albans and across central
Essex (Green and McGregor, 1978; Rose, Allen and Hey,
1976) into its modern valley. The presence of till
(boulder clay) in the south-draining valleys of the
Ingrebourne (Holmes, 1892) and Roding shows that these
valleys were in existence when the ice reached southern
Essex. In turn, this implies a drainage system extending
at least from the London area to eastern Essex, tributary
to the extended Medway. It is envisaged that it was into
this system that the ice diverted the Thames from its
central Essex course. Certainly a combined Thames and
Medway river seems to have flowed across eastern Essex
in a valley formerly occupied solely by the Medway.
The earliest gravels of this Thames-Medway (Fig. l) are
thought from their altitude and surface gradient to
correlate with the Boyn Hill Terrace of the Lower Thames
(Zeuner 1959; Lake et al 1977), and underlying these
deposits, in a buried channel, is a lower series of
gravels which are thought to represent the Black Park
Terrace (Gibbard 1979) (Fig. 1; 2; 3).
This early Thames-Medway can be traced from Southend,
across the Dengie Peninsula and Mersea Island, to the
Clacton area (Fig. l) where it is represented by low-
lying gravels, including the Clacton Channel gravels, in
which S.H. Warren, twice president of the Essex Field
Club, discovered the type site of the Clactonian
palaeolithic industry (Warren, 1955), these basal gravels
again representing the Black Park Terrace. At this stage,
the earliest evidence for the existence of the River Black-
water has been recognised by tracing gravel heights in