RIGS of the Month – January 2013
Pennant Sandstone quarry face in
Nowhere Wood
( Double click on picture for larger view )
( Double click on picture for larger view )
More photos at http://tinyurl.com/b7k53sy Photo credits Richard Kefford
Location: Trendlewood
Park, Nailsea ST 479 702
RIGS citation:
‘Best
surviving exposure of Carboniferous Pennant Sandstone in the Nailsea Coalfield.’
It is designated
as a RIGS because of its aesthetic and education value
Access:
Head
North from Nailsea and Backwell
railway station, passing St
Francis school on the right. Turn right along a public footpath just after the
crescent to enter Tendlewood Park. Follow the path
through the wood then take a path to the right that slopes down on to the
quarry floor. Follow
the path until the quarry faces can be seen ahead and to the right.
Some parking is available in the crescent road.
Risks:
Keep
away from steep rock faces with loose material that can result
in rock falls. Hard hats should be worn when approaching the face.
Topography:
Path
through woodland, unstable rock faces.
General description:
This
is a disused quarry that was used to supply building stone to the local area.
It was in use until 1930.
It is located in Nowhere Wood which is part of Trendlewood Park. This park is owned by North Somerset Council and managed jointly by the owners and a local group of volunteers known as Friends of Trendlewood Park.
Entrance to Trendlewood Community Park
Geological history
The
closure of the Rheic Ocean by the end of the Carboniferous Period (~300 million years ago) caused the Variscan
Orogeny, resulting in folding of the strata in our area when ‘Nailsea’ was
just north of the equator. This produced high mountains which were then quickly
eroded, with the detrital material transported north to be laid down as Pennant
Sandstone in deltaic environments.
Geological chronostratigraphic chart
Geological context
The
quarry is cut into Pennant Sandstone, a lithology that was deposited in a river system with point bar and channel deposits. The cross bedding directions
show that the provenance of the material was from the SSE. The rivers carried
eroded material from high ground that was upthrust during the Variscan Orogeny
that occurred during the late Carboniferous period.
Formation of cross bedding dune structures
The Sandstone is part of the Downend Formation which is up to 660m thick in the Somerset Coalfield. It consists mainly of sandstone with some mudstone. Some coals appear in the lower part; Graces seam in the Nailsea Coalfield for example.
The Pennant deposition took place during the Bolsovian (Westphalian C) time which is 308 – 311 million years ago.
The Downend Formation is part of the Pennant Sandstone Group and is also known as part of the Upper Coal Measures Group. It is exposed as the uppermost strata in the Nailsea syncline. There is also a small exposure at the road cutting at Bucklands batch, which is passed on the road down the hill to the station. This exposure clearly shows the dip of the strata forming the southern leg of the Nailsea syncline.
Other exposures of Pennant Sandstone in the area occur at Conygar Quarry, Clevedon (private land), at a disused quarry, now Cloud Hill Industrial Estate and at Highbury Hill, near the waterfall. Both are near Temple Cloud. There is also a disused quarry where building stone for Bristol was extracted, at Troopers Hill in St George.
Lithological description
Green-grey and blue-grey,
feldspathic, micaceous. Lithic arenites ( “Pennant “Sandstones ) of southerly
provenance, with thin mudstone/siltstone and seatearth interbeds and mainly
thin coals; the lithologies are commonly arranged in fining upwards channel-fill
sequences.
Thickness
c. 275m in the east of the
coalfield [c. SO 25 03 ] to c. 1350m in
the Swansea area [SS 73 94]
330m maximum in the Nailsea area.
Geographical limits
Outcrops widely in the South
Wales Coalfield, from near Llanelly [SN
40 00] in the west to Pontypool [SO 25
03] in the east. It is also present in the Forest of Dean and Bristol
coalfields, and in the subsurface in the Oxfordshire and Berkshire coalfields.
Type area
Formation named after the
predominant “Pennant” sandstone facies of the South Wales coalfield, which provides
a “type area”.
Lithology of Pennant Sandstone
It
is classified as a sandstone or arenite which means that the grains are
0.0625mm – 2mm in size. It
is feldspathic which means it contains clasts ( grains) of feldspar, an aluminosilicate mineral which makes up some 60% of the Earth’s crust. It
is also micaceous, meaning it contains a small proportion of clasts of biotite or
muscovite mica which is a hydrated aluminosilicate mineral.
Quartz makes up the majority of
the grains, which are cemented by silica.
This
composition suggests that Pennant sandstone is composed of the detrital remains
of granitic rocks. Granites are igneous
rocks which form volcanic plutons below the surface which may later be upthrust
or exposed by erosion of their roof. The
grains have polished surfaces which shows that they were transported by water (airborne grains have a ‘frosted’ surfaced). The
quartz grains survived the transport because they are very hard, being composed
of silicon dioxide. The rock itself is fairly soft.
In some areas the grains are covered with different iron oxides which accounts for the different colours seen. Red colouration is from iron whereas purple is indicative of managanese.
Uses of Pennant sandstone
It
has a long history of use as a building stone. As the grains are very hard and resistant to wear it is used as a high skid
resistant road material, especially at bends, traffic lights etc.
Richard Kefford
References
Green, GW. (1992) British
Regional Geology. Bristol and Gloucester region. BGS.
Published by NERC. ISBN 0 11
884482 2
BGS. England and Wales Sheet 264.
Solid and Drift Geology Map. 1:50 000 series.
BGS. Classical areas of British
Geology. Geological sheet ST 47 Solid and Drift.
Clevedon and Portishead. 1:25 000
series.
