46
THE ESSEX NATURALIST.
also contains Greensand debris, whilst Warley (360 ft.) has
gravels in which the Greensand chert but no other Greensand
material is found. These indications of a peneplane of the
order of about 400 ft. above O.D. and sloping towards the north
of Essex appear to confirm the suggestions arising from our
consideration of the Chalk, and are supported by the evidence
from localities in Northern Kent. This points to a series of rivers
passing from south of the present Thames to somewhere in mid-
Essex. Evidence to the north of mid-Essex has been destroyed
by the later invasion of the ice-sheet, but it seems a reasonable
suggestion to place the main stream across Central Essex. This
suggestion is made more acceptable by the discovery of the
remnants of the 400 ft. platform to the west of the County
showing a trail of Lower Greensand pebbles converging on this
mid-Essex line. The river probably passed north of Stanmore
along the line of the upper Lea to the Lower Chelmer. It seems
probable, although there is no available evidence, that the
mouth of this river lay off the present coast in the vicinity of
Clacton, and entered an earlier North Sea in the neighbourhood
of the present Blackwater Estuary.
This ancient channel has, of course, influenced the present
landscape to a very small extent, leaving, in the gravels we have
noticed, the only traces of its course that are beyond reasonable
doubt. Its description, however, is a necessary introduction to
the following stage, shown in figure 4.
The map shows the probable Proto-Thames channel at the
close of the Pliocene period. The plan shown is derived from the
evidence of the 200 ft. or Binfield7 platform. Here the line
of the main channel has moved to an oblique course across the
south-east of the County. The cause of this movement is
uncertain. It may have been the result of the first glacial phase;
the southward approach of the ice-front in the north of the
County increasing the volume of the northern tributaries. By
whatever means, the longitudinal stream was forced southwards.
It will be apparent, however, that the main stream shown in the
map is lying above the left arm and foot of the curious "Y"-
shaped depression in the Chalk (fig. 1). Whether this depres-
sion had previously been in existence is unknown. It may be
that it was formed by movements to the north-west, beyond our
7 See Ross, R. B. Unpublished Ph. D. thesis (Univ. of Lond.).