Geoscience Reference
In-Depth Information
as Land's End and westernmost Wales. Since then the sea has advanced to its present
position by flooding this landscape of rivers, valleys and hills. The River Parrett flows
into Bridgwater Bay (Fig. 157) where an array of channels and bars is covered and
modified twice a day by the exceptionally large tides of the Severn Estuary. These tides
have the greatest range of anywhere on the British coast: up to 15 m between low and
high tide. This remarkable range is a result of the unique interaction between the ori-
entation, shape and location of the Bristol Channel, and the patterns of tides that move
around this part of the eastern Atlantic margin.
FIG
158.
Meandering lower reaches of the River Parrett (Fig. 156,
b4
), near Burnham-
on-Sea. (Copyright Dae Sasitorn & Adrian Warren/www.lastrefuge.co.uk)
Away from the channels and bars of the river mouth, the intertidal zone is unusu-
ally extensive, with 2-4 km of land exposed at low tide. Much of the exposed material
is mud, although it tends to be sandy and locally gravel-rich near the high water mark,
where stronger wave action (especially under storm conditions) has sorted and moved
the sediment. Although there is obviously a large amount of sediment movement off-
shore along this coast (as shown by the muddy waters), it is not known whether the
sediment predominantly comes from the land via rivers, or from the floor of the re-
cently flooded Bristol Channel.
Above the high water mark there is commonly a belt of wind-deposited dunes,
up to 1 km wide. The dune belt was built largely under storm conditions after the sea
stabilised at its present position. Low islands of bedrock appear locally from beneath
this spread of young sediment and, as at the mouth of the Parrett, they have sometimes
become incorporated into the coastal bars and spits of the present-day coastline.
