292 THE ESSEX NATURALIST
stream bed opens out and the stream is fordable. The main
stream flows round the south side of this island but a small
tributary stream is present on the north side, with its source at
the western end of the island.
DISTANCE IN YARDS
Figure la. The profile of the transect across area B. The stations
marked on the profile are those given in Tables II and III.
Figure lb. The profile of the transect across area G.
The form of the stream as described above, is, in part, re-
sponsible for the nature of the substratum of the creek at low
tide. Although it may be described generally as estuarine mud,
the substratum varies greatly in detail, as will be shown later.
Away from the main channel, towards High Water Mark, the
muds are very soft and sticky. Nearer the main channel, and at
intervals along its whole length, one finds well-defined areas of
relatively clean sand which are quite firm to walk on. These
'sand banks' occur usually where there is a small meander in the
stream. Just after the turn of the tide the incoming currents are
swift (3 ft./sec.) in the incised channel, too swift to allow for the
deposition of the silts but sufficiently slow to allow for the deposi-
tion of the sand. As the tide rises the main stream overflows
its incised channel, the current is checked and slackens, thus
allowing for the deposition of the finer material. This is well