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they are sensitive to other conditions and have a
predominantly tropical distribution today away from the
turbidity of terrigenous sediment fluxes. Two main forms
exist. Fringing reefs weld themselves to the shore, whilst
barrier reefs parallel the shore beyond an impounded
lagoon. The Great Barrier Reef off Queensland (Australia)
is the modern equivalent of the Palaeozoic Capitan Reef
(Texas/New Mexico) and Wenlock Edge reef along the
Anglo-Welsh border. The latter was formed when 'Britain'
was located at 30
S during the Silurian period 420 Ma ago.
Barrier coasts have developed in mid-Wales, East
Anglia and south-east Yorkshire, with substantial long-
shore drift associated with low tidal range coasts ( Figure
17.18 ). Chesil Beach (Dorset) and Dungeness are formed
in part by the onshore migration of offshore shingle bars.
The southern North Sea and Baltic coasts of mainland
Aber Wnion
0
5
km
Harlech
Rhinog
Mountains
Aber Artro
Barmouth
Aber
Mawddach
Dolgellau
Cardi gan
Bay
COASTAL SEDIMENTS
Exposed shingle
bar
Exhumed forest
(mid-Holocene)
Aber
Dysynni
Estuarine sands
and salt marsh
Tywyn
Figure 17.18
Barrier and estuarine coast,
northern Cardigan Bay, Wales.
Dominant waves from the
south-west help redistribute
terrestrial and offshore late
Quaternary sediments via
longshore drift, diverting or
blocking river mouths (Aber-).
Most sand dune systems are
of medieval age.
Brackish and
freshwater marsh
Sand dunes
Aberdyfi
Low-tide and tidal
channels
Aber Dyfi
Hard coastline -
fossil cliffs, etc.
'ynys' - former
rock island
 
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