Geology Reference
In-Depth Information
Fig. 8.4
Breached levees as
focus points for small salt
marsh creek systems in
Groves Creek, Georgia, USA
The hydraulic conditions related to the contact
between almost impermeable salt marsh clay in rela-
tively young salt marsh areas and permeable sand
beneath it with a high hydraulic conductivity enable a
phenomenon called piping. Piping consists of small
drainage tunnels formed in the sand beneath the clay.
This type of tunnels or pipes results from an uneven
pressure distribution in the clay-sealed sand after
overmarsh tides. Small fountains of water up to
decimetres high at the upper end of small fi rst-order
creeks have been observed in the Skallingen backbar-
rier salt marsh. As the submerged channels collapse,
they create elongated depressions in the salt marsh
surface (Fig.
8.5
, upper left) that dictate the direction
of the creek's further headward erosion. This kind of
piping in salt marsh environments was fi rst observed
by Kesel and Smith (
1978
) working in the Nigg Bay
salt marsh in Scotland, UK. They discovered a rela-
tion between the disappearance of salt pans and the
formation of such subsurface channels.
Salt pans consist of isolated nonvegetated small
depressions in the marsh surface that need to be drained
before vegetation can invade them. Before this hap-
pens, salt will precipitate on the surface due to evapo-
ration of trapped water after overmarsh tides. The fi rst
to publish on the formation of salt pans was Warming
(
1904
in Danish with a summary in French). He
described the reason for these depressions in salt
marshes as a result of either: (1) an uneven colonizing
of marsh plants resulting in unvegetated spots sur-
rounded by growing vegetated areas, (2) depressions
formed on the exposed coast by beach ridges prior to
the salt marsh formation, (3) uneven coastal erosion
resulting in small bays, later closed by beach ridges
prior to the salt marsh formation, or (4) direct wave
attack on weak spots on the salt marsh surface during
storms. Warming (
1904
) stressed, from observations in
the Wadden Sea, that such depressions will be prone to
collect seaweed and algae which will putrefy and
obstruct the growth of salt marsh plants. He also sug-
gested that the treading of cattle could initiate weak
spots for the waves to excavate. Unfortunately, the
hypothesis of weak spots in interplay with putrefying
organic matter seems to be the only thing remembered
in literature from this paper with many original obser-
vations and interpretations (Redfi eld
1972
; Pethick
1974
; Kesel and Smith
1978
; Boston
1983
) . The paper
was cited by Yapp et al. (
1917
) who stated that Warming
was of the opinion that 'pans may originate in a variety
of ways'. They tested the weak spot hypothesis by
establishing an artifi cial pan as a weak spot, which
they found was not being further excavated. This kind
of test of cause is strongly dependent on the location.
Pethick (
1974
) carried out a statistical analysis of pan
distribution in salt marshes between Scolt Head Island
and Blakeney Point, UK. He used aerial photographs
for the position and levelling for the altitude, and ended
up supporting Warming's weak spot hypothesis. No
doubt salt pans in marsh environments can be formed
in a variety of ways, either (1) by chance because of an
