156 THE ESSEX NATURALIST
It is clear, from the Ordnance Survey Map, that the area
is separated from the main channel of the Thames by
Canvey Island. This island measures five miles by two
miles, and is very flat. Attempts to enclose the land were
made as early as the Roman occupation, but no efficient
sea-wall had been built because flooding of the island
occurred at every spring tide. In 1623, however, new walls
were built by Dutch workers under an agreement with Sir
Henry Appleton, a local landowner. This protection of the
island proved adequate, and has been maintained ever since.
The salt marsh of the mainland is separated from the
main channel of Hadleigh Ray by Two Tree Island. This
measures 11/2 by 11/3 miles, and was partially inned at the same
time as Canvey. Enclosure of Two Tree Island and of the
mainland marshes was, however, completed in 1653.
Heavy clay underlies the whole area, and the mud of the
salt marshes is made up of fine silt and is extremely slimy.
THE NATURAL SALT MARSHES
Natural salt marsh occurs in the area to seaward of the
mainland sea-wall and on the south side of Two Tree Island.
There is little marsh on the north side of this island, and
this is presumably because of the curvature of Creek 141.
Two Tree Island lies on the concave part of this curve, so
that no silt is being deposited here.
The best-developed salt marsh is that lying adjacent to
the mainland. This area is protected from the strongest
currents of the river and from the wash of shipping, by the
presence of Two Tree and Canvey islands.
Fig. 3.
Creek 14, which separates this marsh from Two Tree, is
approximately '25 yards wide and 10 feet deep at its deepest,
1 The position of all sites mentioned will be seen from Fig. 1.