EVOLUTION OF COASTAL DRAINAGE OF ESSEX. 39
has, however, been filled and uplifted. The zone of recurring
warping to the north lies within the district under review,
and deserves fuller treatment. The Pre-Pleistocene 200 ft.
platform, discernible throughout the London Basin and passing
beneath the Glacial Clay cover, shows, from the evidence of
well borings, indications of a down-warping north-east of a
line passing north-west to south-east through Braintree. This
line may be regarded as the margin of the East Anglian-North
Sea "sedimentation-subsidence" which is still in the process
of filling up and liable to recurrent warpings. The Braintree
line is thus, in some sense, the "zero-isobase" of the area of
warping dividing the south-western termination of the North
Sea depression from the stable area of south Essex resting on
the palaeozoic basement.5 This difference between south-
western and north-eastern Essex must be borne in mind when
considering the evidence of planation levels. Any peneplanes
found in the north-east of the County must be viewed with
suspicion, and their evidence cannot be utilised for our purpose.
In dealing with the evidence of peneplanes within the stable
area it will be obvious that difficulty arises on investigating
the earlier stages of the coastal drainage of Essex, owing to
the fact that there are few traces of these earlier peneplanes.
The remains of ancient river-gravels are equally rare. It is,
therefore, necessary to reconstruct the courses of the early
Essex rivers from a consideration of other evidence.
The earliest period it is proposed to investigate is the Mid-
Pliocene period consequent upon the land emergence from the
Diestian Sea. After this period there apparently occurred no
further widespread submergence of the land. It would appear
that a reasonably accurate tracing of the lower Proto-Thames
and its Essex tributaries can be formed from a consideration
of the existing watercourses and the sub-surface geology.
Obviously the streams inaugurated upon the land arising from
the Diestian Sea would be directly related to the contours of
the surface of the land. It seems possible to trace in some
existing water-courses many vestiges of Mid-Pliocene streams.
The surface of the original folds by now has been greatly
altered, but it is still possible to arrive at an approximate con-
ception of their shape from a consideration of the uppermost
5 See fig. 4.