172 THE ESSEX NATURALIST
Adjacent to Creek 7, and near to the end of Creek 21,
there is another area of marshy ground. This extends along
the eastern bank and is similar to the marsh near Creek 3,
except that Scirpus is much more abundant near Creek 7.
This suggests a slightly lower salinity, and this is likely
because more fresh water drains into this area.
As regards Two Tree Island, the drainage channels here
have all been specifically dug for this purpose. Seepage
occurs into the creek following the sea-wall, and the water
here contains 15.7 grams chloride per litre (1.57 per cent).
This high salinity and trampling by cattle again allows for
the colonisation of this creek by halophytes. The dominant
plant in the other ditches is Scirpus maritimus, but one or
two patches of Phragmites communis occur.
COMPARISON OF THE INNED AND NATURAL SALT MARSHES
The effects of salinity and drainage on the natural salt
marsh are constant and distinct, but on the inned marsh the
interaction of these factors with others brings about very
varied conditions. Of the inned marshes, the only area
which can be compared directly with the natural marshes is
the pasture-land. In this area the effect of efficient drainage,
of the removal of salt, and of the stabilisation of the soil due
to the absence of the influence of tides, is clear and has led
to the development of neutral grass-land.
For the rest of the area, it seemed that a consideration
of the main species, taken individually, would give the
clearest comparison with the natural marsh.
Two species, which are found on the natural but not on
the inned marsh, are taken first:—
(i) Halimiorie portulacoides. Since this is found only
along the creek margins and the base of the sea-wall,
the main factor influencing its position is clearly
rapidity of drainage. It could not possibly, therefore,
find a suitable habitat on the inned marshes.
(ii) Spartina Townsendii. This species is also dominant
in areas strongly influenced by tidal movement. It
occurs most extensively in the most exposed parts.