EVOLUTION OF COASTAL DRAINAGE OF ESSEX. 69
By this time the present-day physiography of South-East
Essex had been etched out to a recognisable degree. The existing
river valleys were present; the broad central Essex valleys had
been developed whilst the coastal slope was in the process of
formation. The reason for the asymmetrical slopes of the valleys
in the south-east of the County can now be stated, for the steep
slopes facing north-west result from the erosion by the ice-sheet
and the uniclinal shifting of the river channels in their migration
to the south-east. Some indication of the cause of the sub-
merged parallel offshore banks can be given. These are, ap-
parently, remains of the south-east divides of the Proto-Medway
and lost Essex rivers.
Fig. 7. Longitudinal Profile of the River Crouch,
showing the "100 foot" and double "50 foot" Terraces.
The Crouch drainage system, whose valleys lie wholly within
the area, was the only system in existence throughout most of the
periods dealt with in the foregoing pages. The profile of the
Crouch3 illustrated by the graph in figure 7 shows the effect of its
vicissitudes during these periods. The remains of the 400 and
the 200 ft. platforms are, of course, missing, but the "knick-
point" at 100 ft. is easily discerned and those representing the
double terrace of Taplow times (about 50 ft.) are apparent. A
further terrace indicates a period of "still-stand" at an altitude
of about 20 ft. O.D.
3 This graph is the result of a survey carried out by the writer in which the true water
level was ascertained by a series of bridge surveys, using the heights of the nearest bench marks.
About eighteen points were selected throughout the fourteen miles, of which seven could be
used merely to check the remaining eleven.