Salt marshes are widespread in temperate
regions, and are not uncommon in the tropics
(Figure 13.20). They start to form when tidal flats
are high enough to permit colonization by salttolerant
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 depressions,
or pans, that are devoid of vegetation and fill with
water at high spring tides.