Jncc coastal Directories Project Region 11 The Western Approaches

small exposures of possible till have been recorded,
attributed to an Anglian or Wolstonian ice sheet that
occupied the Celtic Sea.
2.1.2  Stratigraphy
Falmouth - Porlock
Thick, alternating sandstone and mudstone sequences of
Middle and Upper Devonian age are seen at the coast
around Falmouth Bay and the south-eastern end of Mount’s
Bay.  They originated in a deep-water basin to the south and
were transported northwards in late Devonian times within
thick thrust sheets (nappes), onto shallow shelf deposits.  At
Falmouth and at Loe Bar in Mount’s Bay they rest upon
younger Upper Devonian mudstones and siltstones, which
are visible along the west side of Carrick Roads and around
Mount’s Bay to just south of Penzance.
The rocks forming the Lizard peninsula are part of a
Devonian ocean plate thrust over contemporaneous
sedimentary rocks during the late Devonian closure of the
so-called Rheic Ocean.  The rocks of Lizard Point itself
originated as mudstones, sandstones and lavas on the deep
basin floor and were then recrystallised under high
Region 11  Chapter 2  Geology and physical environment
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Devonian (Porlock)
Devonian–Carboniferous Transition Group (Braunton)
Devonian
Schist, siltstone, limestone
Sandstone, siltstone, shale, limestone
Sandstone, shale
Slates, grits
Lower Palaeozoic
Lizard
Serpentine, schist
Stratigraphy
Rock types
Igneous intrusive rocks
Granite
Jurassic
Mudstone, limestone
Triassic
Mudstone, sandstone
Permian
Sandstone, breccia
Sandstone, mudstone
Sandstone, mudstone
Upper Carboniferous (Westphalian)
Upper Carboniferous (Namurian)
Lower Carboniferous
Sandstone, mudstone
Map 2.1.1
Onshore coastal geology of the region.  Source: British Geological Survey (1991).

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temperature and pressure to form schists.  The complex of
igneous rocks forming most of the rest of the Lizard
peninsula typifies the rocks forming the earth’s crust
beneath the oceans and is thought to be a fragment of a
Devonian ocean basin floor thrust upwards, together with
basin sedimentary rocks, during the Variscan deformation.
Together they form a complex association of schists,
granulite and serpentinite intruded by gabbro dykes and
granite that is unique in southern Britain.  The serpentinite
is relatively soft, and the shores of the plateau that forms the
peninsula have been eroded into steep cliffs.  
The sweep of Mount’s Bay to the west is largely eroded
into Devonian Mylor Slates, although more resistant
metamorphosed dolerites and granites are seen in places
and often form minor headlands and islands.  
The granites of south-west England were intruded soon
after the Variscan deformation.  They form a minor part of
the coast just north of Porthleven in Mount’s Bay, as well as
the rugged, cliffed coast between Mousehole and Land’s
End.  Some of the granites are traversed by mineralised
veins, and that forming the south side of St. Michael’s
Mount contains greisen (a mineralised rock containing mica
and quartz) veins in which have been found unusual
minerals such as topaz, wolframite and cassiterite.  The
Land’s End peninsula from Mousehole almost to St. Ives is
formed of granite, though metamorphosed slate, commonly
intruded by dykes and mineralised veins, forms a narrow
strip along the coast north of Cape Cornwall.  St. Ives Bay,
the most westerly bay on the northern coast, is incised into
softer Mylor Slates.
The Isles of Scilly are low, flat-topped islands formed of
granite, with the metamorphosed sediment (killas) that
surrounds the granite being exposed only on White Island,
off St. Martin.  Most of the sand on the beaches and between
the islands is derived from erosion of the granite. 
Western Cornwall is formed of a series of plateau
surfaces, at heights of roughly 228 m, 183 m and 131 m
above sea level.  Lower terraces also occur and are
associated with raised beach deposits.  Isolated outliers of
Pliocene deposits (the most extensive are the St. Erth Beds)
are associated with the 131 m level.  The general form of the
area suggests a mature landscape, formed during the
Tertiary, that has suffered some uplift since the Pliocene.
The uplift has led to erosion of the coastal fringes, which
continues today.
The largely cliffed coast northwards to Hartland Point
cuts across the east-west trend of the geological structures to
produce a generally straight coastline, broken only by major
valleys at Padstow and Bude.  In the south around Portreath
the coast is formed of Devonian turbidites (impure
sandstones) with interbedded grey slate.  A small inlier of
granite with many bands of greisen forms Cligga Head, and
from Perran Bay to Newquay the coast is formed of the grey
calcareous slates and thin limestones of the Meadfoot Beds.
A major anticlinal structure exposed at the coast in
Watergate Bay, north of Newquay, has a core of purple and
green slates rich in fish remains.  Devonian slates, with local
intrusions of metamorphosed dolerite, form the coast from
Newquay to Padstow Bay.  At Tintagel the sequence passes
up into folded and faulted Carboniferous sandstones and
shales, which are associated with locally sheared lava and
tuff.  The same strata, but without the lavas, form the coast
northwards to Hartland Point and the southern half of
Bideford Bay.  Cascades of large, recumbent folds occur in
the cliffs at Cambeak, between Boscastle and Widemouth
Bay.  At Instow a bed of fish remains is exposed on the
foreshore.  Devonian rocks, comprising red-brown
sandstones and shales along with grey slates, reappear
north of the sand dunes at Braunton Burrows and form the
spectacular, steeply cliffed coast north to Morte Point.  A
fine exposure of fossiliferous Devonian sandstones and
siltstones is visible at Baggy Point.
Lundy is a steeply cliffed, flat-topped island formed
largely of early Tertiary granite that is locally intruded by
dykes.  Devonian slates form the south-western corner of
the island.  The granite is the most southerly intrusion of the
Tertiary Igneous Province in Britain; the nearest major body
of the same age forms the mountains of Mourne in Northern
Ireland.
The north coast of Devon from Morte Point to Porlock
displays steep cliffs where the high, rounded inland hills
meet the coast.  Narrow linear valleys, some of which are
located along fault lines, reach the coast at Coombe Martin
and Lynmouth, for example.  The cliffs are formed of
Devonian slates and grits with subtle variations in form
related to the structure or chemistry of the rocks. 
Porlock - Kenfig
The form of the coast changes dramatically east of
Minehead, where steep cliffs of Devonian rocks are replaced
by lower cliffs of Triassic and Jurassic sandstones, shales
and mudstones.  The large tidal range in the Bristol Channel
and the rapid erosion of the cliffs has produced a wide
rocky foreshore along parts of this coast.  The axis of a major
east-west syncline passes westwards into the Bristol
Channel from Brent Knoll near Burnham-on-Sea.  Lower
Jurassic rocks (primarily Lias) occupy the core of the
syncline, with Triassic and Carboniferous sediments on the
flanks. 
Triassic rocks forming the southern flank of the syncline
are well exposed at Blue Anchor Bay, and the Rhaetic (part
of the upper Triassic containing an interesting range of
lithologies) is exposed on the foreshore near Watchet.  Local
faulting and folding of these units has produced a foreshore
of great geological complexity.
The inner Bristol Channel, east of a line from Bridgwater
to Cardiff, is flanked largely by wide estuarine flats of
Holocene age underlain by softer Triassic or Jurassic strata.
The Somerset Levels on the south coast and the Wentlooge
Levels on the north coast are the most extensive of these
flats, which are now mostly land-claimed for agricultural
use.  Rising above the levels, and locally forming the coast,
are the steep-sided hills forming Brean Down, Middle Hope
and the area between Portishead and Clevedon.  The islands
of Steep Holm and Flat Holm form similar features.  Most of
these hills or islands occupy anticlinal cores, where
Carboniferous Limestone or Devonian grits, which are more
resistant to erosion than the overlying rocks, reach the
surface.
Along the northern coast of the inner Bristol Channel
there are no bedrock exposures from Sudbrook westwards
to Cardiff except for the small hill at Goldcliff.  However,
bedrock is rarely far below the surface and isolated
exposures are visible in the intertidal zone. 
The coast south and west of the mouth of the River Taff
at Cardiff is formed mainly of steep, low cliffs composed of
the gently dipping, well-bedded Mercia Mudstone Group,
2.1  Coastal geology
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