RIVER DEVELOPMENT IN ESSEX.
249
which it can be correlated. On the other hand, 12 miles to the
S.E. and south of the present Thames lies the Wimbledon-
Kingston plateau with a summit elevation of about 180 ft. O.D.,
capped with gravel of the Winter Hill type containing Bunter
quartzites. It has been recognised for many years that this
gravel marks a stage in the evolution of the Thames valley, and
it is now evident that its constitution and elevation links it
with the Winter Hill terrace. Even if we took the Winter Hill
Thames along a more northerly route, we should have to imagine
a stream flowing at about the same level to explain the Wimble-
don gravels. There are no further relics of the terrace in the
London area, but it is clear that the river at this date must
have passed through the relatively narrow gap between Hamp-
stead and the Sydenham Hills, as it does to-day, and thus must
have entered southern Essex.
It is here that the moral of our opening paragraph finds its
application. We find ourselves under the necessity of testing
the conception of the Winter Hill Thames by reference to
Southern Essex. In this connection a most significant feature
is the curious trough of low ground which leaves the Thames
valley near Upminster and runs north-eastwards, past Laindon
and Wickford, to the Blackwater estuary. It was along this
trough that T. V. Holmes carried his "Romford River." As
shown in a previous communication it is of pre-glacial date,
for the glacial deposits descend on to its floor. It appears that
this floor is too low in level to be correlated with the Winter
Hill terrace, even allowing for the seaward drop in the latter ;
but the sheet of gravels at about 150 ft. O.D. south of Warley
Street are at the right elevation and should be examined from
this point of view. The gravels of the Rayleigh Hills (200-250
ft. O.D.) are too high to belong to this stage, and clearly appertain
to an earlier phase, before the trough was cut, in which the
Thames followed an even more northerly route, and received
a Wealden affluent through the Medway Gap.
Our general suggestion is, then, that in Winter Hill times
the Thames followed the line of the present Upminster-Wickford
trough at a level above that of its present floor, and that relics
of the gravels of this stage are possibly to be recognised in the
gravels (coloured as glacial) south of the Warley Hills. The
Q