Jncc coastal Directories Project Region 11 The Western Approaches

This section deals briefly with the geology of the rocks and
sediments at and below the sea bed.  The bulk of the
information is shown on the maps, with some additional
explanation provided by the text. 
2.2.1  Holocene sea-bed sediments
Sea-bed sediments across the region have been deposited in
very different environments and as a result vary
considerably between south Cornwall and the Severn
Estuary.  In the south-west of the region the bulk of the
sediments are sands or thin spreads of gravel, generally less
than one metre thick.  Extensive areas of bedrock are
exposed at sea bed near the coast, particularly off the Lizard,
Land’s End, the Isles of Scilly and Padstow Bay (
Map 2.2.1
).
The coarse nature of the sediments suggests that finer
material has been borne away by wave or tidal action.
An extensive area north-west of Lundy and south of
Dyfed is covered by a major sand wave field.  Individual
sand waves, orientated NNW-SSE, rise to over 20 m above
the level of the surrounding sea bed.  Their asymmetry
indicates that net sediment transport is towards the west,
where finer-grained sediments are found in the Celtic Deep.
The distribution of sediments to the east of Lundy is
complex.  Detailed sediment distributions and bedforms
within the inner Bristol Channel and the Severn Estuary are
related to a series of circulation cells generated by tidal
currents.  Generally the thickness of the sea-bed sediment
decreases eastwards as tidal current speeds increase.  Much
of the inner Bristol Channel has bare rock at the sea bed,
with mobile sediment restricted to isolated linear banks near
the coast.  The location to the north of Nash Sand and the
other sand banks is due to the change in tidal current
velocities as the channel opens out to the west of Nash
Point.  The sand banks are active features, changing form in
response to storm conditions.  
Bridgwater Bay is underlain by an extensive mud sheet
locally over 20 m thick and displaying a sharp seaward
termination; the mud passes landward into a sandy upper
beach.  Similar mud flats are found seaward of the
Wentlooge Levels between Newport and Cardiff, but the
mud here is generally only a few metres thick.  The upper
part of the Severn Estuary is infilled with sand banks
traversed by two major linear channels, the Bristol Deep and
Newport Deep.  
2.2  Offshore geology
23
2.2  Offshore geology
British Geological Survey
S
M
sG
S
sG
M
S
sG
gS
G
G
sG
sG
S
gS
mS
mS
S
S
S
mS
sG
S
m
sG
gS
S
sG
G
gS
S
sG
S
sG
S
mS
mS
mS
sM
M
M
mS
msG
S
S
gS
sG
gS
gS
S
sG
sG
sG
gS
sG
S
S
msG
sG
sG
S
gS
m
sG
S
sG
sG
S
gS
sG
sG
S
gS
sG
gS
msG
Mud
Sandy mud
Sand
Muddy sand
Gravelly sand
Gravel
Muddy sandy gravel
Sandy gravel
Rock outcrop with
patchy sediment cover
Key to symbols
sM
gS
S
M
mS
sG
G
msG
Lundy
Celtic Deep
Nash Sand
Newport Deep
Bristol Deep
Bridgwater Bay
Map 2.2.1  
Sea-bed sediments.  Source: British Geological Survey (1991); sediment classification modified after Folk (1954).

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2.2.2  Pleistocene geology
Determining the extent of the major Pleistocene ice sheets
across the region is problematic.  The last major ice sheet,
which formed during the late Devensian, covered most of
south Wales but reached the coast only at Swansea Bay.
Until offshore investigations commenced some twenty years
ago the southern limit of this icesheet, and thus of
Pleistocene deposits, was drawn from Dyfed across to
southern Ireland.  However, seismic and drilling evidence
indicates that a tongue of the icesheet extended southwards
into the Celtic Sea to reach the Isles of Scilly (Scourse et al.
1990).  This ice sheet, which was probably about 100 m
thick, did not reach the west coast of Devon and Cornwall. 
A Quaternary basin in the Celtic Sea south-west of
Dyfed contains a sequence of tills and sediments over 200 m
thick (
Map 2.2.2
).  Narrow linear deposits of sediment infill
between the basin and the coast are probably related to
erosion at the base of an ice sheet.  Three channel sequences
are recognised within the basin, suggesting that three major
ice sheets traversed the area, the youngest being during the
late Devensian. 
Isolated erratics of lithologies uncommon to the area
suggest that an earlier ice sheet may have reached the coast
of Devon or Cornwall.  One of the best examples is the
‘giant’ boulder of garnetiferous gneiss at Porthleven in
Mount’s Bay, which rests on a rock platform with cemented
beach gravels.  Few definite basal ice-sheet deposits have
been identified along the coast, although outwash gravels,
periglacial deposits and possible interglacial clays have been
found.  The till underlain by beach gravels at Fremington
was probably laid down by an ice sheet, but its age is
uncertain.
Non-glacial Pleistocene deposits in the English Channel
tend to be limited to palaeovalley infills beneath Holocene
sea-bed sediments.  These valleys, now infilled with sand,
clay and gravel, have little topographic expression.  A
deposit of peat recovered from a buried valley in Mount’s
Bay at a depth of 32 m yielded a radiocarbon age of 
12,070 years BP.
2.2.3  Solid (pre-Quaternary) geology
The South-West Peninsula is a basement massif bounded by
Mesozoic sedimentary basins to the south (English Channel
Basin), the west (Celtic Sea Basin) and the north (the Bristol
Channel Basin).  Mesozoic strata approach the shore along
the south coast of Devon and Cornwall (Region 10), but
within this region they are seen onshore only along the coast
of the inner Bristol Channel.
The Mesozoic basins were formed during early Permo-
Triassic times, when thick red marl and sandstone
sequences accumulated in fault-controlled basins.  Later,
Jurassic limestones, shales and clays were deposited more
extensively across the basins and on their flanks.
Region 11  Chapter 2  Geology and physical environment
24
Late Pleistocene to Early Holocene
Middle Pleistocene
Holocene
Predominantly till
Quaternary, undivided, usually >5 m thick
Key to symbols
QMP
QLP
Q
QH
Predominantly mud
Predominantly interbedded mud and sand
Predominantly incision and channel deposits
2
1
6
3
Q
Q
Q
Q
Q
Q
Q
Q
Q
QLP/2
QLP/1
QLP/1
QMP/2
QMP/2
QM
P/
2
QM
P/
3
QMP/3
QLP/6
QLP/6
QLP/6
QLP/6
QH/6
QH/6
Porthleven
Fremington
Map 2.2.2 
Offshore Pleistocene deposits.  Source: British Geological Survey (1994).

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