EVOLUTION OF COASTAL DRAINAGE OF ESSEX. 45
Figure 3 purports to be a sketch of the first stage of the
Proto-Thames that need concern us—that of Mid-Pliocene times
consequent upon the land emergence from the Diestian Sea.
For this diagram the available evidence is naturally very scanty,
the surface on which the river flowed (the so-called 400 ft. plat-
form) having been destroyed to a great extent. It has also
been necessary to pursue investigations beyond the area under
immediate review in order to acquire sufficient evidence to
establish these river courses beyond reasonable doubt.
The chief evidence for the river channels shown in the map
has been drawn from a consideration of the sub-surface geology.
Fig. 3. Suggested River Plan of Essex in the Mid-Pliocene :
the "400 Foot Platform" Period.
It will be noticed that the main stream flows consequently with
regard to the buried upper surface of the Chalk. The remaining
evidence lies in the presence of pebble gravel found in localities
in Essex at heights of the order of 350 to 400 ft. above the
ordnance datum.6
In the south-east of the County the Laindon Hills at a height
of 385 ft. bear gravels apparently derived in part from the
Greensand of Kent, a material which is encountered again at
points of a similar height to the west, beyond the Lea. Danbury
Hill (353 ft.) lying to the north of Laindon, bears gravel which
6 South-Eastern Union of Scientific Societies. 1926. Geological section. Wooldridge, S.W.