Geoscience Reference
In-Depth Information
Muds and sands are widespread just below the surface of the Marshes, and peats
are common a few metres down. It is possible to date these peats using radiocar-
bon methods, and it is also possible to use related fossils to determine whether the
peats formed in wetlands that were freshwater or becoming salty because of flooding
due to sea-level rise. This work has confirmed that all the Marsh deposits have been
formed during the last period of the Flandrian transgression (rise) of the sea. Some
6,000-7,000 years ago, sea level was about 10 m lower than at present and it rose pro-
gressively to reach its present level within the last few hundreds of years. However, it
is also clear that horizontal movement of features such as tidal channels, beach barri-
ers, wetlands and salt marshes can be responsible for local vertical changes, without
requiring any change of overall sea level.
There is no question that the global rise of sea level since the last (Devensian) cold
spell has been the major factor controlling the arrival and deposition of all the sediment
in the Dungeness and the Romney Marshes. However, on a more local scale, it seems
that small-scale variations in the climate have also been important, controlling both the
supply of sediment to the marshes and the power and frequency of storms.
One regional feature that needs to be stressed is the special location and geometry
of Dungeness in relation to the geography of land and sea. The Dungeness promontory
is situated in an exposed position, at the mercy of prevailing Atlantic storms driven
from the southwest up the English Channel, and also from winter storms caused by de-
pressions passing over the northern North Sea. Both of these processes act to transport
sediment into and around the Dungeness promontory. At the same time, the proximity
of the promontory to the coast of France means that winds from the southeast, which
are unusual anyway, do not have sufficient reach to cause erosion that would counter-
act the tendency for Dungeness to grow.
Another striking feature of this Landscape is the
degraded cliff
that marks the
boundary between the areas of bedrock control, to the north and west, and the flat-lying
area of the Marshes covered with surface blanket, to the south and east (Fig. 147). This
cliff feature runs on the landward side of the Royal Military Canal, which was con-
structed for communication and defence purposes under threat of a Napoleonic inva-
sion. As the 'degraded cliff' name implies, this distinctive slope feature is regarded as a
cliff line eroded by the sea before deposition of the young Marsh. Although it is gener-
ally assumed that it formed just before the Marsh sediments themselves, within the last
7,000 years or so, it is also possible that it is much older. For example, in some areas
along the coast of East Anglia, cliff-forming erosion took place during the last (Ipswi-
chian) interglacial, about 125,000 years ago, when sea level was a few metres higher
than it is at present. It is possible that the Romney Marsh cliffs formed by similar pro-
cesses, and that their degradation took place over the 125,000 years that followed.
