SURVEY AND COMPARISON 161
parts as a pure dominant. Its power of stabilising the
moving silt and its resistant stems presumably allow for this
extensive colonisation.
THE SEA WALLS
In the area*studied in detail, the sea wall is approximately
seven feet in height and is of clay and not strengthened by
any stonework. The sides of the wall slope fairly steeply,
but the top, which is about 11/2 yards in width, is flat, and a
path runs along the centre of it. Occasionally, the wall is
built up by the addition of a layer of wet clay, which blankets
the upper vegetation for a time but which is gradually
re-colonised.
At the eastern end of the wall, the vegetation is rather
different from that at the western end. From Creek 3 west-
wards, the wall is not much trampled, but towards the
station, where there were house-boats, the wall has suffered
constant interference.
The undisturbed vegetation seen in the western parts of
the sea wall show's some zonation. This is presumably
affected by salinity, because at the base of the wall on the
seaward side the soil shows a chloride content of .014 grams
chloride per gram soil (1.4 per cent), whereas at the top of
the wall no chloride could be detected. In many places
along the base of the wall a narrow line of Suaeda maritima
is found, and Halimione portulacoides also occurs frequently
in a similar position. In this region the roots of plants are
surrounded by sea-water at spring tides. These plants rarely
occur higher than two feet up the side of the wall. From
above this basal region to the top of the sloping side,
Agropyron pungens (Pers.) Roem. & Schult. is generally
dominant. A yard or two up from the base, Beta maritima
(L.) Thell. becomes more abundant and locally replaces
Agropyron, but- is never dominant over large areas. From
about halfway up, to the top of the wall, Atriplex littoralis
L. increases in abundance, and in many places the tall,
woody stems of this plant line the top of the slope.
Atriplex littoralis never, however, covers a large percentage
of the ground.