THE CHANGING COASTLINE OF ESSEX 83
spherical surface (Fig 3). Examples of slumped masses,
each with a characteristic backward tilt at their head and a
protruding toe at their base, can be seen along this part of
the coast during the winter months. One exceptionally large
slump, the effect of which can still be seen at the present
time, took place a quarter of a mile south of Walton Pier
in December, 1897, It measured 70 yards in length and
extended to a depth of 30 yards inland. A similar big slip
took place at Frinton in January, 1901, when hundreds of
tons of clay fell on to the foreshore. (Woodward, 1903.)
Fig. 3. Diagrammatic cliff section showing slumping of the London Clay.
The part played by the sea in this type of coastal retreat
would appear to be limited to removing the debris of the
fall. As the, foot of the slumped mass usually extends some
distance down the beach, it is gradually removed by the sea
at high tide. This inevitably leads to instability, and further
slumping occurs from above until the whole of the loosened
mass is removed. The cycle is then complete and the whole
process can begin again.
Under this form of attack, the coastline from Harwich to
Clacton has retreated a considerable distance during the past
centuries. Evidence for the recession is found in historical
records, whilst the rate of retreat can be estimated by com-
paring the position of the coastline delineated on the large
scale Ordnance Survey Maps of the last century with that
shown on the most recent survey.
In a report to the Harwich Harbour Commissioners made
by Capt. Washington in 1844, he stated that in the area
south of Harwich the encroachment of the sea appears to
have been at the rate of one foot per annum between 1709
and 1756. Between 1756 and 1804 the rate of advance in-