248
THE ESSEX NATURALIST
The island (figure 1) lies in Hamford Water behind Walton-on-
the-Naze and Horsey Island and adjacent to the mainland below
Lower Kirby. The area is one of calm creeks, extensive saltings
and mudflats covered at high water by the smoothly rising tide.
Except for the seawalls, hard substrate areas are almost entirely
absent. The island is joined to the mainland by two causeways
which are exposed at about half ebb tide. One of these, referred
to as "the dam" (figure 1 (a)) is exceptional in that it consists
of a mass of brick rubble, concrete and chalk blocks, held to-
gether by rotting piles and timbers, over and through which the
tidal water moves with sufficient velocity to prevent the sedi-
mentation of mud particles. On the seaward side of the dam,
the level drops steeply through 4 to 5 feet on to the mud of the
creek. Above the dam there is a wide area of gravel in mid-
stream which merges into bordering areas of mud. An iron pipe
of about 1 ft. bore passes through the dam, but drainage is
sufficiently slow to prevent the clearance of water from above the
dam before the flood-tide replenishes the supply. As with the
gravel area above it, the dam merges into mud at its landward
extremities.
Figure 1 (a). Diagram of tho area of the dam. c, positions of the
commensal groups on the piles; d, dam; 1s, lauding stages.