Hugh Prudden
The great unconformity
figured by De la Beche
WELCOME TO SOMERSET
Welcome
to green fields, wild flower meadows, farm cider, Cheddar cheese, picturesque
villages, wild moorland, peat moors, a spectacular
coastline, quiet country lanes……
To
which we can add a wealth of geological features. The gorge and caves at
Cheddar are well-known. Further east
near Frome there are Silurian volcanics,
Carboniferous Limestone outcrops, Variscan thrust
tectonics, Permo-Triassic conglomerates,
sediment-filled fissures, a classic unconformity, Jurassic clays and limestones, Cretaceous Greensand and Chalk topped with
Tertiary remnants including sarsen stones-a veritable
geological park! Elsewhere in Mendip are
reminders of coal and lead mining both in the field and museums. Today the Mendips are a major source of
aggregates.
The
Mesozoic formations curve in an arc through southwest and southeast Somerset
creating vales and escarpments that define the landscape and clearly have
influenced the patterns of soils, land use and settlement as at Porlock. The church building stones mark the outcrops.
Wilder
country can be found in the Quantocks, Brendon Hills
and Exmoor which are underlain by rocks of Devonian age and within which lie
sunken blocks (half-grabens) containing Permo-Triassic sediments.
The
coastline contains exposures of Devonian sediments and tectonics west of Minehead adjoining the classic exposures of Mesozoic
sediments and structural features which extend eastward to the Parrett estuary. The
predominance of wave energy from the west and the large tidal range of the
The
THE
GOOD ROCK GUIDE
Welcome to a quick-reference guide to the best
places of general geological interest in
Please keep to public footpaths. The Ordnance Survey 1:50,000, or better
still, the 1:25,000 maps are most useful for planning visits and locating
sites. A hammer is not required.
The Somerset Studies Library in
Details
about parking, toilets, access and references are provided at the end of each
section. A four-figure grid reference
accompanies each locality. Please telephone for opening times where
necessary. Abbreviations: British
Geological Survey (BGS),
You
are responsible for your safety. The
vertical cliffs between Blue Anchor and Lilstock can
break away at any time (hard hats?). The
shore platforms and beaches are flooded quickly by the incoming tide. Some
mudflats are treacherous. Rocky
foreshores can be excessively slippy. Resist the temptation to climb cliffs. Wear strong footwear. Leave nothing of value
in parked cars. Enjoy
SOMERSET
GEOLOGY GROUP
The
aim of the group is to promote and conserve
Montacute June 2004
1. BERROW ST
Accreting
coastline-Somerset Levels
A
classic lowland coastline of deposition set at right angles to the prevailing Westerlies. The
sandy beach is backed by extensive dunes; post-glacial estuarine clays are
sometimes exposed at low tide below the sands.
Reclaimed siltlands of the Somerset Levels lie
inland. The 13 century church (ST 294
524) is mainly built of Blue Lias mudstones with a
little Carboniferous Limestone and Doulting Stone
dressings. The fine preaching cross is
Ham Hill Stone.
Park at ST 292 542. Crowther 1992, Whittaker and Green 1983. Hour or half-day.
2. BLUE
ANCHOR ST
Triassic and Jurassic
formations-gypsum-tectonics
Proceed eastward along the shore from the eastern
end of the sea wall toward Blue Anchor Point where there is a good display of
folding and fracturing. Red mudstones
and siltstones (‘Keuper Marl’ of the Mercia Mudstone Group) is faulted against grey-green
calcareous mudstones and marls (Blue Anchor Formation). The cliffs further along are famous for the evaporites which were once used in the local alabaster
industry for church monuments. Masses of
pink nodular gypsum parallel to the bedding are thought to represent the
primary anhydrite. Later veins of white
fibrous gypsum, some of which have been deformed by later earth movements, are
seen in tension cracks and along fault planes.
Park at the eastern end of
the sea wall (ST 034 435) and work eastward along the shore. Time your visit for a falling tide. It is possible to travel for one leg of the
distance to or from Watchet on the West Somerset
Railway (Tel. 01643 704996). Edwards, 1999. Half-day/day.
3. BREAN
DOWN ST Weston-Super-Mare
Carboniferous
Limestone-coastal features-Holocene accretion
This
headland, which is in the care of the National Trust, projects into the
Park at the eastern end of Brean Down (ST 296 588); it is unwise to park cars on the
beach as the tide rises quickly owing to the big tidal range! Café and toilets. Crowther, 1992,
Whittaker and Green 1983. Half-or full-day.
4. BRIDGWATER ST
Brick and Tile Museum
The Somerset
Brick and Tile Museum
(Tel. 01278 320200). www.county-museums@somerset.gov.uk
Town Trail
1. The Docks and marina (ST 298376) building stones
include whitish Bath Oolite with cross bedding, red
Otter Sandstone with clasts derived from Devonian
outcrops, granite blocks, and quartzitic sandstone
(Pennant?). There are cobbles of Blue Lias limestone, chert and
sandstone.
2. Blue Lias at Newton Lock
and path with Carboniferous Limestone.
3. Red calcareous Mercia
Mudstone in
4 & 5. Recent silty and
clayey sediments deposited by floods important in the past for brick and tile
making.
6. Carboniferous Limestone used in the flood
protection cages.
7. Note strong tidal flows and consider effects of
0.5m rise in sea level.
8. See Ham Hill Stone opposite the Blake Hall and
blocks of Carboniferous Limestone.in the
9. Blue Lias in the new wall
by the river.
Park in town centre car parks or near docks. Whittaker and Green 1983. Half-day.
5.
BROOMFIELD ST
Morte Slates-building
stones-Wildlife Trust headquarters
Park at
6 CHARTERHOUSE ST
Limestone scenery-mining-soils and vegetation
This
area contains outcrops of both Carboniferous Limestone and Old Red Sandstone
which have influenced the topography, steam pattern, soils and vegetation. There are a number of swallet
holes and stream sinks. The hummocky ‘gruffy ground’ is a reminder of the lead mining dating back
to Roman times. GB Cave and
Park
close to crossroads at ST 505 557. Farrant 1999 highly
recommended. Note his warning on page 2
concerning private land and access to caves. Day/half day.
7. CHEDDAR ST
Carboniferous Limestone-Dolomitic Conglomerate-gorge-caves-tectonics
Cheddar Gorge
The Gorge is
an impressive landscape feature and a popular tourist attraction. The Gorge owes it origin and form to the
steep gradient of the valley, a large catchment area
and the effects of periglacial hillslope
processes acting upon the well-jointed Carboniferous Limestone. There is no
evidence that the Gorge is a collapsed cave. Grey, granular Carboniferous
Limestone (Hotwells Limestone) can be examined at
pavement level below Jacobs Ladder.
Clifton Down Limestone outcrops higher up the
Gorge. Parts of the
Gorge have been altered by quarrying and attempts to stabilise the steep
faces. Cheddar Showcaves
(ST 467 539) are open to the public (tel. 01934 742343) and include a museum
with a collection of Palaeolithic tools.
Panoramic views are to be had from the top of Jacobs Ladder (320 steps). There is a three mile waymarked
trail around the Gorge.
Black Rock Nature Trail
A footpath on the S E side of the gorge leads to the
Somerset Wildlife Trust Reserve and trail. The latter is a quiet oasis at the
head of the Gorge. The old quarry face
400m from the entrance is worth close study.
Note the fissure with solution effects and infilled
with cave earth (a quartz sand). The
Carboniferous Limestone sediments contain ooliths and
fossil debris. Evidence for Variscan tectonics is seen in the sheared rock and cleaved shales. Note the Dolomitic Conglomerate in the bottom of an ancient Triassic
valley just beyond the point where the Trail turns sharply to the right. Nearby outcrops of Carboniferous Limestone
mark the sides of the Triassic valley.
Park in Cheddar; a park and ride operates in
the summer; parking is very limited in the Gorge.
8.
CLOUTSHAM ST
Screes-soils-goyles
A National Trust waymarked nature trail in Exmoor
National Park
Park near
Webber’s Post at SS 993 439.
9 A COLLECTION OF CHURCHES
Building stones-architecture
Medieval churches are a good indicator of
regional variations in the use of building stones besides having much of
architectural interest. We include a selection which illustrate some of the
9a.
9b.
9c.
9d
Norton Fitzwarren Church ST 197 260 North
Curry Sandstone: greenish-grey, calcareous mudstones and siltstones with locally thick
beds of white to pale brown cross-bedded, pebbly sandstones; thought to be channel deposits of
Triassic age.
9e
9f.
See Prudden
2003 for a review of
10.
Jurassic formations-escarpments and dip slopes-
This part of the south Somerset scarplands is a striking example of a double escarpment and
dip slope: Jurassic Junction Bed limestones and Pennard Sands form Corton Ridge
on the west whilst Yeovil Sands and Inferior Oolite limestones underly Corton Beacon to the east.
Proceed up the steep hillside from the church and turn left at the
top. Continue using public footpaths and
the lanes via the Beacon to the
Park
near the church at ST 636 225.
11. COUNTY GATE SS 7948 8 km E of Lynton at the County border with
Hangman Sandstone-screes-tectonics
The NE side of the
Park at
12. CULMSTOCK BEACON AND
Upper
Greensand-Blackdown Plateau-podzol
soil-peat-heathland
The northwest corner of the Blackdown Hills not only provides panoramic views from Culmstock Beacon (ST 110 151) but also much of botanical
and geological interest. There is a
capping of Upper Greensand with bleached, cherty podzolic
soils and heathland which is part of the great Blackdown-East Devon Plateau, an outstanding landform-note
the even skyline. The escarpment has Foxmould Sands (plus badgers) underlying Chert Beds. The
sands rest on the red beds of the Mercia Mudstone
Group and springs are thrown out where the two meet. Waterlogging has
resulted in the formation of peaty deposits and landslips. The nearby
Park
on verge near Crossways Farm at ST 1258 1668. Follow track up past
telecom tower. There is a maze of
footpaths to follow. Avoid at all cost the low boggy patches as they are very
difficult to negotiate. Also remember
that this is a SSSI reserve please. Further
reading: Prudden, 2001.
13. CULVER CLIFF AND MINEHEAD SS 9647 1 km W of Minehead
Hangman Sandstone-tectonics
A somewhat rough but rewarding scramble
over a rocky beach is required to examine the Middle Devonian Hangman Sandstone
at Culver Cliff. The clifs
and fallen blocks show wave-formed ripples on bedding planes and
climbing-ripple cross-lamination interpreted as shallow water deltaic
near-shore sediments. There are tight
and minor folds plus low-angle shears with striated slickensides
and quartz-filled tension gashes beneath.
These give a good idea of the compressive nature of the Variscan Orogeny. Some of the folds were probably formed by
contemporary slumping of the sediments shortly after they were laid down
Combine with a visit to the main shopping
street in Minehead plus the parish church on the hill
above the town centre to study building stones. One can compare the partly
metamorphosed, purple/red Hangman Sandstone with the bright red friable
Triassic sandstones and breccias. There
are also Blue Lias and Ham Hill Stone (from near
Yeovil) to be seen.
Park
by the harbour.
Walk west from the harbour keeping to the foreshore (ST 963 478). A falling tide is important for this
location. Edwards 2000. Half-day.
Morte Slates-Otter Sandstone-Cothelstone Fault
1. This is a good exposure of the Triassic Otter
Sandstone, a thickly-bedded red and cream formation deposited in desert
basins. Note that the beds do not show
any sign that they were affected by the Variscan Orogeny as they followed that event. This sandstone erodes easily where there is
arable and where traffic has worn unmetalled roads. It has been widely used in local buildings,
especially between Bishops Lydeard and Williton.
2 Proceed up the lane and notice
how the character of the cutting changes: brown, silver and grey, slaty,
shiny rocks appear. We have passed from
rock aged some 240 Ma to Devonian Morte Slates aged
some 367 Ma. These older rocks are at a
higher level than the younger Otter Sandstone.
The latter has been let down against the older rocks by the Cothelstone Fault which runs along the southwest side of
the Quantocks. It is not visible in the
cutting as the wrenching has broken-up the rock and therefore the sides have
weathered back. It is a strike-slip
fault with perhaps some 16 km movement in the Variscan
orogeny and some later reactivation.
3 Proceed up the lane and cross to the track
where there are better exposures of the Morte Slates;
note that the cleaved slates have themselves been deformed by earth movements after
the muds were turned into slates possible as a result
of movement on the nearby fault..
4 Return down
the track to the road, turn right and note in the cutting unusual crumpling and
angular folding of the slates in the walls of the cutting. These also may reflect the effects of
sideways motion on the Cothelstone Fault (strike-slip
faulting).
Park at Locality 1 (ST 197 303) Prudden 2001 Ch 19, Edmunds and Williams 1985. Combine with a visit to
15. DUNKERY GATE SS 8940 9 km SW of Minehead
Hangman Sandstone-Mansley Beds-goyles-heathland
This is a good place to explore the south
side of Dunkery Hill.
The high ground is underlain by the quartzitic
Hangman Sandstones (Devonian) that support acidic soils and moorland
vegetation. Dunkery
Gate lies at the head of Mansly Combe
where the finer-grained slates and siltstones of the Mansley
Beds are exposed. A deeply-cut goyle is being actively eroded during storms. Note that
accelerated erosion is continuing upstream of the bridge and parallel to the hedgebank and this suggests that erosion has been
influenced by the presence of an old trackway. There are fine views on a clear day from Dunkery Beacon.
Circular walks can be planned.
Park
at SS 895 406.
The sides of the goyle are very steep so
please take care: Edwards 2000. Hour/half-day.
16.
Carboniferous
Limestone-Quartzitic Sandstone Group-Dolomitic Conglomerate-thrust tectonics-karst
Ebbor
Gorge is a National Nature Reserve with both geological and wild life
importance. Follow the sign-posted trail
from the display boards that leads down into the
Gorge. High up on the right can be seen
highly fractured Carboniferous Limestone which forms the sole of the
Cheddar-Wells thrust and which overlies the younger shales
and sandstones of the Quartzitic Sandstone Group which
crop out on the valley floor. The
footpath continues through exposures of Dolomitic
Conglomerate and Carboniferous Limestone (Clifton Down
Limestone). The latter
shows crinoidal debris, bedding, small-scale tectonic
features and limestone weathering effects. There are also small caves which have yielded
a Late Pleistocene fauna, and rock screes which are
thought to be relicts of a periglacial climate. On a clear day one can see across the
Park
at ST 521 485 for the Gorge. Farrant, 1999, Green
and Welch, 1965. Half-day.
Collection of monoliths from
The European Community of Stone (ECOS) is
an impressive collection of tall monoliths at the Community College close to
the entrance. They were transported by lorry and ship and donated by various
member countries of the EEC funded by numerous commercial organisations. The stones include a basalt from
Park
at the College which is close to the B3090 leading out of Frome to the
northeast (ST 784 4940). Hour.
Junction
Bed-Yeovil Sands-Holocene formations-landforms
Erosion has separated this outlier of
Yeovil Sands from the main escarpment which can be seen some 26 km to the
east. The intervening vale is
Park
in the town.
Avery 1955. Half-day.
19.
GLENTHORNE SSSI
SS 7949 8 km E
of Lynton
Hangman Sandstone-tectonics-goyles
The Geological Conservation Review reads
as follows:
ü ‘This coastal
site has the most accessible and revealing section in the Trentisoe
Formation of the Hangman Sandstone Group.
The Hangman Sandstone represents the Middle Devonian sequence of
Park at
20. GREENALEIGH
SS 9548 2.5 km NW of Minehead
Hangman Sandstone-tectonics-head-arcuate
shingle ridge
Perhaps one of
Park
in Minehead just beyond the harbour. Walk westward from Minehead
Harbour along the road and footpath taking the path on the left which avoids a
rough shoreline trek ascending through the woods at SS 964 477; pass Greenaleigh Farm buildings and make for cove at ST 952 482.
National Trust.
No facilities. Best
at low tide. Edwards 2000. Day/half-day.
21.
Jurassic Ham Hill Stone-Yeovil Sands-tectonics-tufa-gulls-building
stone
This hill-top
Park
near Prince of Wales pub.
Walk north from pub to Monument for rock faces and views. Also follow Lime Kiln Trail to see deep
quarry with gulls and limekiln with adjacent shattered rock. There is a working quarry at the southern end
of the Hill (Tel. 01935 824950). The
Intrusive diorite-Morte Slates-terminal
curvature-building stones-famous gardens
The primary attractions are the formal
Edwardian garden and the recently restored 18th Century landscape garden in the
Combe.
However, there is much of interest for the geologist. The Combe contains the Hestercombe
diorite (the Memoir called it a lamprophyre). There are many small exposures of
cleaved Devonian siltstones and sandstones (Morte
Slates Formation). The overflow car park exposed the contact of the diorite
with the Devonian Morte Slates. An artificial waterfall has been restored
below which are huge quartz boulders collected from
the Devonian formations plus blocks of tufa. Cross-laminated siltstones and sandstones
with ripple marks can be seen in the walls and flagstones of the formal
garden. Thus there are opportunities to
study Devonian sediments, Variscan tectonics and the
mysteries of the igneous intrusion. The Orangery shows Jurassic Ham Hill Stone from near Yeovil at
its best. Other buildings show Doulting Stone and
North Curry Sandstone. The gardens look
out over the Vale of Taunton Deane (Triassic red marls) toward the Blackdown Hills (Upper Greensand).
Parking for cars and coaches, refreshments and toilets at ST 240
288.
Fissures with Triassic and Jurassic infills-tectonics
There are old quarry workings down a track to the right of the pub
(ST 729 452). A large multi-stage
fissure runs the length of an old quarry wall at the eastern end of quarry used
as a car park. Part of the fill has
collapsed enabling Triassic, Rhaetian and Jurassic
sediments plus the mineralisation to be examined at
close quarters. The sediments appear to
have entered the fissures in both a lithified and
partly lithified state. Deformation of the sediments seems to have
been associated with the opening of the fissure and the intrusion of the
sediments. There are footpaths to the
nearby
Park in lay-by to the east of the Bear Inn. Please first contact Mr R