Geology Reference
In-Depth Information
Figure 13.15
Factors affecting dune morphology. For explanation, see text.
Source:
Adapted from Pye (1990)
tide levels. As well as major tidal channels,
tidal creeks
flow across tidal flats. These are shallower than tidal chan-
nels and run down to low-tide level. On muddy tidal flats,
tidal creeks often display a dendritic pattern with wind-
ing courses and point bars. On sandy tidal flats, tidal
creeks have ill-defined banks, straight courses, and few
tributaries.
Tidal flats are built up from clay-sized and fine silt-
sized sediments carried to the coast by rivers. On meeting
salt water, particles of clay and silt flocculate (form clot-
like clusters) to become larger aggregates. They then settle
out as mud in quiet coastal waters such as lagoons and
sheltered estuaries. The mud is carried in by the incoming
tide and deposited before the tide reverses. If the mud
continues to build upwards, a part of the tidal flat will
be exposed just above normal high-tide level. This area
is then open to colonization by salt-tolerant plants, and
salt marshes or mangroves may develop.
to form when tidal flats are high enough to permit col-
onization by salt-tolerant terrestrial plants. Depending
on their degree of exposure, salt marshes stretch from
around the mean high-water, neap-tide level to a point
between the mean and extreme high-water, spring-tide
levels. Their seaward edge abuts bare intertidal flats, and
their landward edge sits where salt-tolerant plants fail
to compete with terrestrial plants. Salt marsh sediments
are typically heavy or sandy clay, silty sand, or silty peat.
Many salt marshes contain numerous shallow depres-
sions, or pans, that are devoid of vegetation and fill with
water at high spring tides.
Mangals
'Mangrove'
is a general term for a variety of mainly
tropical and subtropical salt-tolerant trees and shrubs
inhabiting low inter-tidal areas.
Mangals
are communi-
ties of mangroves - shrubs and long-lived trees and with
associated lianas, palms, and ferns - that colonize tidal
flats in the tropics, and occur in river-dominated, tide-
dominated, and wave-dominated coastal environments
Salt marshes
Salt marshes
are widespread in temperate regions, but are
not uncommon in the tropics (Figure 13.19). They start
