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Monday, 4 February 2013
Earth Heritage magazine
You can download the latest issue for free here.
Thursday, 24 January 2013
Trendlewood Quarry Nailsea
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.
Saturday, 5 January 2013
Free public lecture - 8th January
Mapping of Volcanic Terrains across the Solar System
Dr Ellen Stofan
 |
| Olympus Mons, Mars. Standing at 22km high (three times the height of Earth's own Mt Everest), it is the tallest volcano in the Solar System. Photo credit: European Space Agency |
Venue: @Bristol (Anchor Road, Harbourside, Bristol, BS1 5DB - view map)
Date: 8th January, 18.30 - 19.30
Location: Rosalind Franklin Room, At-Bristol
Admission: Free, lecture suitable over 12s - booking necessary (Book Online
Or phone 0845 4586499 [Monday - Friday, 9am - 5pm excluding Bank Holidays])
Many planets and moons of our solar system show evidence of volcanic eruptions.
The early missions to the Moon, Mars, Venus and Mercury were truly missions of discovery, with great debates in the scientific community on the roles of impacts and volcanic eruptions in shaping their surfaces.
Join Dr. Ellen Stofan to discuss how these alien volcanic features are mapped and interpreted with knowledge and techniques developed from studies of volcanoes on Earth. A perfect way to celebrate BBC Stargazing Live.
This is a free lecture and is supported by the University of Bristol.
Original article: http://www.at-bristol.org.uk/1647.html
Saturday, 22 December 2012
Merry Christmas from Avon RIGS
Season's greetings to all of our readers. Outcrop is having a festive break but will be back in 2013 with some exciting new features.
Merry Christmas from Avon RIGS
 |
| Blaise Castle Estate in snow - from August RIGS of the Month |
Sunday, 4 November 2012
Geo-gardening - Sunday 21st October 2012
John Toller was
joined by a very small group from WEGA to clean up the RIG site at Itchington
which the group had agreed to maintain. The site demonstrates a small but
excellent exposure of the "Bristol Time Gap". This shows the angular unconformity which separates the almost horizontal Triassic Mercian Mudstone
Group, locally Dolomitic Conglomerate, from the older, underlying and tilted
Carboniferous Limestone, which is known locally as Clifton Down Limestone. This was formed approximately 334 to 341 million years ago in the Carboniferous Period.
To admire this site, take the turning from the A38 at the Grovesend road junction near Thornbury signposted to The Slad and Itchington. Keep travelling until you are about to go under the M5 and see the exposure on the right. Close by you can also see a beautifully flat natural bedding plane of limestone forming a bank. The exposures are fenced off and there is always a danger of falling rocks, so keep at a safe distance.
Sandi Shallcross
The Carboniferous
Limestone was deposited in warm shallow seas covering much of the country. This
was followed by the Coal Measures and then thick layers of sandstone lain down
by river deltas until the start of the Permian Period. No Permian rocks are
present here, but during that period the continents were all brought together
to form one huge landmass, called Pangaea.
The Carboniferous rocks here were folded and
compressed to form mountains, then weathered and eroded down to produce more
sediments. The layers overlying the limestone here are products of that
erosion during the Triassic Period, resting on the worn down limestone surface. The environment by then
had changed to mainly desert, resulting in characteristic reddish sands, clays
and debris as seen here, contrasting with the hard grey limestone. The time gap
between the two layers is at least 50 million years, enough time for a whole
mountain range to be created and worn down.
The rock face had
become covered in a fine variety of plants and saplings which were threatening
to completely engulf it. The many ash sapling were especially unwelcome as the
strong roots can quickly penetrate the joints and fissures in the rock to break
down the exposure. After a couple of hours determined clearance the
unconformity was looking very much more obvious, as John is pointing out in the
photo. Next year it will all need doing again or the ash will take hold, so
keep the clippers handy!
To admire this site, take the turning from the A38 at the Grovesend road junction near Thornbury signposted to The Slad and Itchington. Keep travelling until you are about to go under the M5 and see the exposure on the right. Close by you can also see a beautifully flat natural bedding plane of limestone forming a bank. The exposures are fenced off and there is always a danger of falling rocks, so keep at a safe distance.
 |
| Photo credit: Sandi Shallcross |
Sandi Shallcross
Saturday, 20 October 2012
BGS 'GeoBritain' Map
BGS 'GeoBritain' Map
The GeoBritain map acts as an inventory of "fascinating geology providing 'secret' walks through breathtaking scenery, literary inspiration, idyllic holiday destinations and the building materials for historic monuments" that can be found in the British Isles. It also contains details of local geology groups, museums and discovery centres.
Screenshot of the GeoBritain Map
http://www.bgs.ac.uk
Our area is currently looking a little under-represented, so why not put the South West on the map and email some of your suggested additions to the BGS?
Charly Stamper
Sunday, 14 October 2012
Urban geology - Bristol Temple Meads station
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For those who are reluctant to put on their boots or stray too far away from a coffee shop, there are many opportunities to see some geology in the relative comfort of the built environment. Bristol’s main railway station at Temple Meads has plenty to show the urban geologist.
The original Great Western Railway station was designed by Isambard Kingdom Brunel and opened in 1840 as the western terminus of the main line from London, initially consisting of only two platforms. As demand for rail services increased, the original station was quickly outgrown and a major phase of construction between 1870-1898 formed the majority of the current building. Temple Meads then doubled in size during the early 1930s due to the influx of post-WWI holiday traffic.
SITES OF INTEREST
Approach from Temple Gate - the Joint Station
Start off by walking up from Temple Gate. On the left (adjacent to the 8 & 9 bus stop) is the oldest surviving part of the building, the Joint Station of the 1800s. The walling is made of squared, coursed rough Traissic Dolomitic Conglomerate blocks, contrasting with smooth Bath Stone ashlar. The conglomerate consists of rounded pebbles of grey Carboniferous Limestone in a reddish fine matrix that contains the minerals hæmatite and dolomite. It is sourced from quarries at Draycott, near Cheddar, on the other side of the Mendip Hills. Recent repairs were conducted by excavating similar stone from a small old quarry in the Avon Gorge, a good reason in itself for preserving geological sites.
The main façade
Now walk towards the main façade on Temple Gate. The front of the building comprises large blocks of Bath Stone, whereas the side walls are of blue-grey Lower Lias limestone. Both of these lithologies are found locally and the Bath Stone was sourced from the excavation of Box Tunnel; it is likely that the Lias probably came from the cuttings at Saltford and Keynsham.
The Earth beneath your feet...
As you are walking, be sure to look at the flags beneath your feet. Some of the recycled sandstone paving stones on the approaches to the station buildings preserve "fossilised" ripple marks that give clues to the environment when the sand was deposited. They are best seen when the sun is low, or after rain. There is also a great collection of igneous rocks to be seen in the setts in the station approach road. Watch out for taxis!
Inside the station - Plaform 3
You will need a ticket or platform pass for the final stone. Machine-cut slabs of limestone from France were used for paving part of platform 3 in 2000. It is Rocheret Jaune, an Early Cretaceous limestone that comes from Belley, near Lyon. Beautiful sections through fossil shells, especially high-spired gastropods, can be seen in places. The same stone has been used for paving at Paddington Station and at both ends of the Channel Tunnel, as well as in Bristol’s Centre and Millennium Square.
Also in the vicinity
There is much more to see inside and outside the station, including fine surfaces of granite and marble. The station complex really deserves a geological trail of its own; perhaps we should make it a RIGS!
Eilieen Stonebridge (& Charly Stamper)
Geology of Bristol Temple
Meads Station
This post is adapted from an original article written by Eileen Stonebridge in 2003. It appeared in the paper version of Outcrop and you can download the original pdf here.
For those who are reluctant to put on their boots or stray too far away from a coffee shop, there are many opportunities to see some geology in the relative comfort of the built environment. Bristol’s main railway station at Temple Meads has plenty to show the urban geologist.
The original Great Western Railway station was designed by Isambard Kingdom Brunel and opened in 1840 as the western terminus of the main line from London, initially consisting of only two platforms. As demand for rail services increased, the original station was quickly outgrown and a major phase of construction between 1870-1898 formed the majority of the current building. Temple Meads then doubled in size during the early 1930s due to the influx of post-WWI holiday traffic.
 |
| Façade of Bristol Temple Meads Joint Station. Source http://www.networkrail.co.uk/VirtualArchive/bristol-temple-meads |
SITES OF INTEREST
Approach from Temple Gate - the Joint Station
Start off by walking up from Temple Gate. On the left (adjacent to the 8 & 9 bus stop) is the oldest surviving part of the building, the Joint Station of the 1800s. The walling is made of squared, coursed rough Traissic Dolomitic Conglomerate blocks, contrasting with smooth Bath Stone ashlar. The conglomerate consists of rounded pebbles of grey Carboniferous Limestone in a reddish fine matrix that contains the minerals hæmatite and dolomite. It is sourced from quarries at Draycott, near Cheddar, on the other side of the Mendip Hills. Recent repairs were conducted by excavating similar stone from a small old quarry in the Avon Gorge, a good reason in itself for preserving geological sites.
 |
| The Joint Station dates from the 19th century and is made out of coarse-grained Triassic Dolomitic Conglomerate sourced from Draycott, near Cheddar. Photo credit: Eileen Stonebridge |
The main façade
Now walk towards the main façade on Temple Gate. The front of the building comprises large blocks of Bath Stone, whereas the side walls are of blue-grey Lower Lias limestone. Both of these lithologies are found locally and the Bath Stone was sourced from the excavation of Box Tunnel; it is likely that the Lias probably came from the cuttings at Saltford and Keynsham.
 |
| Blue-grey Lower Lias limestone. Photo credit: Eileen Stonebridge |
The Earth beneath your feet...
As you are walking, be sure to look at the flags beneath your feet. Some of the recycled sandstone paving stones on the approaches to the station buildings preserve "fossilised" ripple marks that give clues to the environment when the sand was deposited. They are best seen when the sun is low, or after rain. There is also a great collection of igneous rocks to be seen in the setts in the station approach road. Watch out for taxis!
 |
| Ripple marks in the flags (possibly Pennant Sandstone?) on the station approach pavement. Photo credit: Eileen Stonebridge |
Inside the station - Plaform 3
You will need a ticket or platform pass for the final stone. Machine-cut slabs of limestone from France were used for paving part of platform 3 in 2000. It is Rocheret Jaune, an Early Cretaceous limestone that comes from Belley, near Lyon. Beautiful sections through fossil shells, especially high-spired gastropods, can be seen in places. The same stone has been used for paving at Paddington Station and at both ends of the Channel Tunnel, as well as in Bristol’s Centre and Millennium Square.
 |
| One of the many fossils in the limestone that paves Platform 3. Photo credit: Eileen Stonebridge |
Also in the vicinity
The Jacobean-style Bristol & Exeter House, one-time terminus of the Bristol & Exeter Railway, still stands today and is almost
completely of finely cut Bath Stone ashlar blocks.
There is much more to see inside and outside the station, including fine surfaces of granite and marble. The station complex really deserves a geological trail of its own; perhaps we should make it a RIGS!
Eilieen Stonebridge (& Charly Stamper)
Bristol Temple Meads participates in the Bristol Open Doors Day (usually held in early September) and it is possible to go on tours of the inner workings, including a WWII air raid shelter and extensive tunnel system. For more information, visit the Bristol Open Doors Day website.
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