EVOLUTION OF COASTAL DRAINAGE OF ESSEX. 47
area; the shift of the main stream thus partaking of the nature
of a uniclinal shift.
Evidence, similar in nature to that brought forward for the
400 ft. surface, demonstrates the former existence of another
platform approaching the 200 ft. level. North of the main
channel, shown in figure 4, abundant evidence has been preserved
beneath the Boulder Clay. This is the gently southward sloping
peneplane formed on the surface of the London Clay. Borings
through the Boulder Clay cover strike the London Clay at
heights varying between 200 and 250 ft. above O.D.8 thus
Fig. 4. Suggested River-system of Essex in the Late Pliocene:
the "200 Foot" or "Binfield" Period.
supplying part of the evidence for the position of the channel
shown on the map.
To the south of the glaciated area a great deal of evidence
of a similar nature has probably been eroded away, but sufficient
material remains to indicate the probable southern flank of the
main stream. The Rayleigh Hills bear gravels of Kentish origin
at a height of 260 ft., whilst further evidence for this 200 ft. plat-
form is forthcoming from the west and south beyond the confines
of the area.
It will be seen that a number of rivers entered the main
stream from the left—the Proto-Chelmer, and three tributaries
8 Essex Naturalist, vol. xxi., Wooldridge, S. W.