90
THE ESSEX NATURALIST
where the unprotected marsh deposits could persist for a long
time, suffering only a gradual removal by small-sized waves.
Two points with regard to this theory of origin require
further elucidation. The marsh on the inner side of the
ridge has a low elevation at the present time, and but for the
enclosing sea wall, would be periodically inundated. At the
time of formation of the marsh bar, however, it must have
stood at a higher elevation. Its present height is due to a
sinking of the land to half-tide level, consequent upon
artificial draining.
The theory also assumed that the earliest-formed marsh
contained sand and shingle which separated out under
wave attack. The question arises whether the marsh con-
tained these coarser deposits. A mile to the westward,
capping the London Clay, are extensive deposits of Pleisto-
cene Sand and Gravel, and it is not unlikely that some of
this material would find its way into the mud of the flat
accumulating offshore at a time when the shoreline coincided
with the edge of the London Clay upland.
Confirmation of the marsh bar theory of origin can be
found in the present shoreline. At Sales Point, near the site
of the Roman Fort of Othona, there is a ridge of shelly sand
running parallel to the sea wall for one-third of a mile
(Pig. 7). It rises about five feet above the general level of
the saltings and mud flats and has a steep landward slope
(1 in 1) and a more gentle slope seaward (1 in 21/2). The
head of the ridge is bent to the westward and shows at least
one recurved end representing a former position of the limit
of the feature.
The present ridge was initiated following a rise of sea level
when wave action washed out the coarse sand from the
marsh deposits and drove it. on to the remaining marshland
edge. The height of the ridge is due to the vast amount of
shell casts and fragments which has accumulated since the
feature was first formed. Longshore drift may have also
operated since, causing the ridge to extend northwards as
in normal spit development. The slewing round of the head
of the feature under the action of waves approaching from
the north-east is also a recent modification of the initial
form. It will be noted that shoreline processes such as
longshore drifting and upbuilding by waves cause changes