RIGS of the month [May] - Troopers Hill, Bristol

RIGS of the Month - May
Troopers Hill, Bristol

Fig. 1: Location of Troopers Hill, St George, Bristol. Postcode BS5 8BL.

Click here for directions and public transport options (external website)

SITE SPECIFIC INFORMATION 

Location: BS5 8BL

Accessibility: The entrance at Malvern Road provides the easiest access for those with limited mobility. Some of the slopes on this path may mean that some wheelchair users need assistance. The remainder of the paths on the hill are unsurfaced.

Topography: Hilly, grassed footpaths
Restrictions: Troopers Hill is owned and managed by Bristol City Council, there is free unrestricted access.

This article has been adapted from Andrew's post on the Friends of Troopers Hill website - the original version can be accessed here and includes a detailed stratigraphic log. Eilene Stonebridge also contributed a great deal of material to the site (see section on geology) and wrote an accompanying leaflet which can be downloaded for free - link at the bottom of this post.

Introduction

Troopers Hill’s geology is unusual in Bristol. Much of the city lies on Carboniferous Limestone, but Troopers Hill is dominated by sandstone of the Pennant Measures which are sedimentary rocks formed in tropical swamps some 300 million years ago. In places, the sandstone can be seen on the surface, both as natural outcrops and old quarry faces.

The combination of sandstone and local industry has resulted in acid soils, which are rare in Bristol. This has encouraged a wealth of plants to flourish, that are found nowhere else in the City.

Much of the shape of the hill is the result of quarrying for the sandstone which was used as building stone throughout Bristol. The largest quarry was the gully in the centre of the hill which was worked until the end of the nineteenth centuary. The humps and bumps above Troopers Hill Rd are where the unsuitable stone was tipped.

As well as sandstones the Pennant Measures include mudstones, shales, clay and coal seams. In the past coal has been dug where it outcroped on the hill and both fireclay and coal have been taken from deep mines under the hill.
 

History

Troopers Hill is made up of a thick pile of layers of rock which must have been
well known to the generations of miners and quarrymen who extracted coal, fireclay and sandstone from them for centuries. However, much of that knowledge has been lost and no one now is quite sure what lies under the hill.

There are at least 9 coal seams there and some of these were exposed at the surface. Local geologist Tom Fry recalled one seam 5-6ft. thick revealed on the west side of the hill in 1968, but all the other seams are probably much thinner. He also remembered that a very thin seam was worked by unemployed
men in 1913 on the hill near the bottom of Troopers Hill Road, where sadly a friend of his died after the shaft collapsed. 

The rocks are sandstone, mudstone, coal and clay, and they all dip to the south at between 25 and 45 degrees. They belong to the Upper Carboniferous Coal Measures, and are near the base of the Pennant Sandstone. The rock types, structures and fossils suggest that they were formed when the area was covered by a forested swamp. At this time (about 300 million years ago) the area is thought to have been near the equator with a tropical climate. The rocks were next squeezed by huge forces, which crumpled them into a large arched fold that runs east-west through Kingswood. The Trooper's Hill rocks lie on the south side of this structure. 

Pennant Sandstone

The sandstone at Troopers Hill is called Pennant, since the early geologists took the name traditionally used by miners and quarrymen. The Pennant forms a thick mass of sandstone in the middle of the local Coal Measures around Bristol, as well as in the Forest of Dean and South Wales. The name is thought to have been derived from the Welsh words penn and nant, meaning head of the valley.The rock is well exposed in the Gully Quarry, just south of Trooper's Hill chimney.

The sandstones exposed on the north side of the Gully display cross bedding, indicating that the original sand grains were carried by running water in rivers. 

On the south of the Gully the rocks are coarser grained with coal pebbles and fossil imprints of tree trunks, suggesting that at times the water was flowing fast enough to erode sediment and move large pieces of wood.

 

Fossilised wood in Pennant Sandstone. Photo credit: Steve England

Pennant was quarried at many sites in the local coalfields and provided stone for thousands of buildings, including the chimney at the top of Trooper's Hill. It sometimes splits into thin slabs which were much used for paving, kerbs, steps, gravestones, and, in earlier times, for roofing.

Mudstone
The mudstones were known as duns by the miners. They are less well exposed but can be seen in the paths to the south of Troopers Hill, and they indicate a period when the waters in the swamps must have been still. They had no commercial value. 

Coal 
The miners called the coal seams “veins” and gave them names which were used at each pit in the area where they were found, from Newton St Loe, near Bath, to St Phillips Marsh, in Bristol. Some seams thinned to nothing and others split, which made the naming uncertain in places. Each seam is thought to be the compressed remains of deposits of peat, built up from the remains of the trees of the forest. 

Crewe's Hole Pit was sited at the south end of Troopers Hill, and is marked by the remains of he chimney of the mine engine house. Air Balloon Pit was next to Air Balloon Road, about half a mile north of the Hill. The mining geologist John Anstie was employed to gather information on all the local coal mines for the Royal Commission on Coal. He collected some details of what had been found in these pits, although both were disused when he visited the area in about 1870 and he was concerned that the information might not be completely reliable. He also observed that “the outcrops of the Devil's seam, Buff and Parrot seams, follow parallel lines about 500 yards to the north of those of the Millgrit and Rag seams ... (and at) ..Trooper's Hill .. the shallow works on all of them are clearly traceable.” He noted that the Air Balloon Pit only worked for three or four years as the seams were too variable to be worked economically. 

The engine house in Troopers Hill Rd, c. 1914. Reproduced by kind permission of the Bristol Reference Library.

Tom Fry was brought up near Troopers Hill and knew the area very well. He recalled one seam 5-6ft. thick revealed on the west side of the hill in 1968 before the council used the area as a rubbish tip. Tom recalled that in around 1910, his father used to warn him of the dangers of the hill, telling him that it was all undermined. His father worked in the Fireclay Mine. Tom also recalled the collapse of an old sloping shaft in the floor of the main quarry, the sides of which had been walled with mortared stone, and that he could trace the sites of at least six shallow mines on the Hill.

Fireclay

Fireclay is usually found beneath a coal seam and is basically the clay soil where the trees grew which provided the peat deposits. Fireclay is rich in clay minerals which means that when fired into bricks or tiles, they can withstand high heat. Tom Fry noted that the Troopers Hill fireclay was used to produce high quality terra cotta tiles that were used in many local buildings. 

Three fireclay beds are known to have been worked beneath Trooper's Hill. 

 

Andrew Mathieson

Further reading

Downloadable leaflets on the geology, wildlife and history of Troopers Hill are available at http://www.troopers-hill.org.uk/leaflets/index.htm

December 2016 Update.

Eileen Stonebridge and Andrew Mathieson, of The Avon RIGS Group, have been working for some time with The Friends of Troopers Hill on the interpretation boards in general and the Geological interpretation board in particular. This new board can be seen near the Upper Chimney.

The four boards can be seen on this page of the Troopers Hill web site:-

Troopers Hill Interpretation Boards

References

Anstie, 1873, The Coalfields of Gloucestershire and Somersetshire, London

Bristol Fireclay Company Co Ltd, 1911, Section of the Mines at Troopers Hill

British Geological Survey 6 inch map Sheet ST 67 SW

Buckland and Conybeare, 1824, Transactions of the Geological Society of London (available online through Google Books)

Cornwell, 2003, The Bristol Coalfield, Landmark Publishing

Fry, Accounts of my Earliest Years (up to 1922) www.troopers-hill.org.uk/memories

Kellaway and Welch, 1993, Geology of the Bristol District, HMSO

Prestwich, 1871, Report of the Royal Commission on Coal (in) Gloucestershire and Somersetshire, HMSO (available online through Google Books)

The original walk was funded by our Stepping Forward Sustainability Grant, through the BIG Lottery Community Spaces Programme www.troopers-hill.org.uk/steppingforward

Trendlewood Quarry Nailsea

RIGS of the Month – January 2013

Trendlewood Quarry, Nailsea

Pennant Sandstone quarry face in Nowhere Wood
( 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.

http://www.friendsoftrendlewoodpark.org.uk

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.

http://www.troopers-hill.org.uk/

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

http://en.wikipedia.org/wiki/Pennant_sandstone

http://en.wikipedia.org/wiki/Sandstone

http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=PES

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.

RIGS of the Month [September ] – Redcliffe Caves, Bristol

RIGS of the Month – September 2012.

Recliffe Caves – Bristol

Entrance to caves on Phoenix Wharf.

There are more pictures here.

Location: Phoenix Wharf, near Redcliffe Parade ST 589 723

This area has been a designated RIGS since 1986.

Access: The caves are owned by Bristol Council but access is controlled by the Axbridge Caving Group, who open the caves for tours annually during the Bristol Open Doors Day each September. They can also be approached for Group tours at other times, see here for details:

Risks: Low roofs, rough ground. Hard hats, stout footwear and torches advised. 

Friends Burial Ground showing cliff near hermitage.

The caves are not natural caves but excavated tunnels into the Redcliffe Sandstone. They were originally excavated to provide sand for glass making and have been used over the years for many different storage purposes.

The foundations of what is now the Mercure Hotel can be seen underground, in the caves.

The sandstone underground is massive with little jointing and no cross- bedding can seen although examples can be seen in this formation at the exposure in the Friends Burial Ground adjacent to the hermitage entrance.

Efflorescence from roof surface

There is surface efflorescence in several areas from the roof, which is assumed to be the decalcification referred to in the Bristol memoir.

The headroom varies around a norm of 2m and there is estimated to be some 6m of sandstone between the cave roof and the ground surface above.

The floor is mostly composed of dumped ash and slag – from the old lead works – that have consolidated over time.

Tree roots have penetrated in some areas.

Cross bedding near hermitage

The caves are cut into the Triassic Redcliffe Sandstone Member.

Redcliffe Sandstone Member ( RESA )

Lithological description:

Sandstone, distinctive fine to medium grained, deep red, calcareous and ferruginous. Commonly decalcified at shallow depths below the surface, giving rise to an uncemented sand.

BGS Computer code:

RESA

Definition of Lower Boundary:

Unconformable, at the abrupt base of the red sandstones of the Redcliffe Sandstone Member overlying sandstones and mudstones of Late Carboniferous age. 

Definition of Upper Boundary:

Conformable and gradational, with interdigitation between red sandstone of the Redcliffe Sandstone and reddish-brown mudstone of the Sidmouth Mudstone Formation (undivided).

Thickness:

Up to 65m

Geographical limits:

Crops out in the Bristol area between Bedminster and Winterbourne.

Parent Unit:

Sidmouth Mudstone Formation

Group:

Mercia Mudstone Group

Age:

Triassic

Stratigraphic setting:

Following the Variscan orogeny at the end of the Carboniferous Period, erosion stripped off some of the Coal Measures during the Permian so the RESA was laid down unconformably on the irregular remnant Carboniferous landscape, diachronous with the marginal facies of the Mercia Mudstone Group. RESA was deposited in an elongate depression between Bedminster and Winterbourne and locally exceeds 50m in thickness. The RESA passes laterally into red mudstones and is locally interdigitated with Mercias Mudstone Marginal Facies ( MMMF ). The best exposures of RESA are found in cliffs along the Avon in Redcliffe, the eponymous district of Bristol, and in the New Cut along Coronation Road, Southville.

Applied geology:

RESA was used for glassmaking, the glass being used to make bottles for the thermal water from the five Bristol hot springs which were then largely exported.

The RESA is an aquifer which was an important source of water for Bristol in the past until contamination from local cess pits and burial grounds resulted in several epidemics.

Ground conditions:

In some areas the RESA has become decalcified, losing its cement and becoming friable and difficult to handle when wet. Changes in groundwater regimes can be responsible for this decalcification process, which in turn can cause local subsidence. Weathered RESA , when used for fill material has also been known to provide poor support for buildings.

References:

BGS Lexicon           -           http://www.bgs.ac.uk/lexicon/

BGS map      2004   -           England and Wales sheet 264 – Bristol – Solid and Drift

BGS Memoir 2002   -           Geology of the Bristol District

Memoir 1949 / 2000 -           Geology & scenery of the West of England – AE Turner & N Chidlaw                         

Cave web site         -            http://www.bristoltours.com/Redcliffe.htm

Photo credits          -            Richard Kefford

Richard Kefford

RIGS of the Month [August] - Blaise Castle Estate

RIGS of the Month - August
Blaise Castle Estate

Geological Time Travel in a Land of Giants

A view for all seasons southwest over Henbury gorge from Castle Hill towards Failand Ridge on the horizon. 

Please follow the Geologist's Code.
http://www.brerc.org.uk/rigs_site/geologists_code.htm

SITE SPECIFIC INFORMATION

Location: Henbury, Bristol.

Accessibility: Various access points, see map. From Coombe Dingle car park, on metaled paths through steel gates. Maximum gradient 12%. Some points off main paths are steep and over rough ground. Contact duty ranger on 07795 445999 for car park opening times and gate access for wheelchair users.

Risks: Beware of cliff edges.
Topography: Steep-sided, incised plateau with shallow streams.

All photos from this post can be viewed in a larger format - http://tinyurl.com/blaise-rigs

Blaise Castle Estate is a magnificent Grade II listed parkland.  Jane Austen wrote in Northangar Abbey: “Kingsweston! Aye and Blaize Castle too…The finest place in England – worth going fifty miles at any time to see.”  Here though, Thorpe is somewhat deceitful in his persuasion of the reluctant Catherine to join the excursion from Bath. Even so, Blaise is justly renowned for its picturesque landscape, making it a popular attraction to this day.

18th Century Blaise Castle House and Museum.

Blaise is set mainly onto the Carboniferous Limestone ridge of Kings Weston Hill and Castle Hill, on the western outskirts of Bristol. This dramatic and scenic incised plateau area is a microcosm of the solid geology of the Bristol district. The changing environment and evolution of the landscape can be traced from its sedimentary rock units in the section dating from the late Devonian period, c 360 Ma. Tectonic forces and thrust faults that shaped the region are evident from the folded strata of the Westbury Anticline along with steeply-dipping, overturned and shattered beds in the northern limb of the anticline. Finally, a drainage system etched its channels through the covering sediments and into the Carboniferous rocks to form the spectacular, steep-sided, densely-wooded Henbury gorge: a recent sculpting of the landscape, also seen in similar features across the region, from the nearby Avon Gorge to Burrington Coombe  and Cheddar Gorge in the Mendip Hills.

The Blaise Castle Estate/Kings Weston Ridge RIGS extends from Henbury, ST 562 788 westward along the northern limb of the Westbury Anticline as far as Shirehampton, ST 530 774 and southwest to Coombe Dingle, ST 558 773 including the River Trym valley from Coombe Farm, ST 563 776 to Sea Mills Lane, ST 555 766.  The site is important for education and research into the palaeoenvironment of the region, periglacial landscapes and stream action.  Its rich variety of rock units along with their twisted and contorted structures make for a challenging lithology and stratigraphy.  It’s an area of extraordinary natural beauty, complex geological structure, with some remarkable exposures.

Click to switch map, change view and read bedrock descriptions. 

A walk through geological time starts in Coombe Dingle by the River Trym, see gorge walk, at the base of the succession in the late Devonian, c 360 Ma. From the confluence of the Trym and Hazel Brook the course of Hazel Brook is followed into the shady gorge and a gentle ascent to the top of the early Carboniferous, c 330 Ma and the giant’s footprints on Castle Hill.  From nearby Lover’s Leap, and then from the legendary giant Goram’s Chair opposite, the full panorama of Blaise and the gorge can be admired.

The massive and thick-bedded river cliffs of conglomerate at the Coombe Dingle entrance rest on the Upper Old Red Sandstone of the Portishead Formation.  These 10m high cliffs of Triassic age were formed c 206 to 248 Ma from the debris of shattered and scoured uplands in Carboniferous and Permian times.  Debris collecting in the valleys and wadis below were cemented into the Mercia Mudstone Marginal Facies (dolomitic conglomerate) along shorelines, wedged between and against the limbs of the Westbury Anticline, forming the present day bedrock. 

Outcrops of Triassic dolomitic conglomerate can be seen where the dry stream tributary meets the Trym at Stoke Bishop (above) and also by Hazel Brook near Henbury Church.

The angular unconformity where the Triassic breccia overlies what appear to be vertical beds of Devonian old red sandstone is exposed at ground level to the left of the path c 150m from the Coombe Dingle entrance. A similar unconformity, but with a conglomerate of different origin, is exposed on a far greater scale at Kilkenny Bay, Portishead. At Blaise, the Upper Old Red Sandstone continues along the wooded slopes beyond the Trym floodplain, on the south side of the Trym valley, heading east to Westbury-on-Trym. These rocks were formed by rivers depositing sand and gravels into river channels, c 354 to 364 Ma.

Angular unconformity where the Triassic dolomitic conglomerate overlies vertical beds of Devonian upper old red sandstone.

After a few metres along the path the base of the Avon Group is reached marking a change from fluvial and deltaic to marine environments, c 344 to 354 Ma. Down-slope to the river confluence outlines a small area for the Shirehampton Formation. Surface rock samples show light-grey, medium-grain, crinoidal limestone, possibly formed in a warm shallow sea or lagoons. Across the Hazel Brook weir footbridge and close to the river bank marks the boundary with a thin bed above the Shirehampton Formation. Rock samples show a distinct reddening of finer grained, sparsely fossiliferous limestone and red-brown, siltstone that together may reveal the Bryozoa Bed.

From the main path above the footbridge, an unmade path forks left and leads after c 100m into woodland and to an outcrop of the Maesbury Mudstone Formation, at the top of the Avon Group. 

Maesbury Mudstone Formation beds formed in a warm shallow lime-mud sea with fluctuating levels.

Penny Well has its source c 20m farther along the path, ST 558 778. The spring also marks a termination of a 1 km fault line SW - NE, to where Hazel Brook enters the gorge near Henbury Church. Penny Well feeds into Hazel Brook a few metres from the bridge south of the lily pond. At this point of the succession the base of the Pembroke Limestone Group has been crossed and the main path passes the pond through the Black Rock Limestone Sub-Group, rocks again formed in a warm, shallow, carbonate sea.

A small disused quarry to the right of the path next to the lily pond has been used to extract limestone. North of the pond, by the next bridge, the path meets a shear rock face of Black Rock Limestone beds dipping 86^o NE, climbing away to the east, disappearing into dark woods, upwards to the precipitous Potter’s Point.

As the path winds its way towards the Beech Cathedral the succession passes up through the Black Rock Dolomitic Limestone.  A few metres up the slope, between the beeches, the distinctive white weathered rock of the Gully Oolite Formation crops out. 

The majestic towering columns of the Beech Cathedral is one of the finest sights on the estate.

Coming off the bend, and overlying the Gully Oolite in the succession, the brown mudstones found in the bank prove the presence of the Clifton Down Mudstone Formation.

The appearance along the path of scree deposits marks the boundary with the Clifton Down Limestone Formation and a change in the palaeoenvironment to more open seas. A vertical limestone cliff butts up against the path by the next pond, Tarn Lake (the Giant’s Soap Dish). Above the tree canopy and to the right are the soaring limestone twin towers of Goram’s Chair. On the opposite side of the gorge the limestone cliffs, their beds dipping 30^o E, line up to buttress Castle Hill, screes piled high at their bases.

The uppermost formations of succession are reached at the bridge near Stratford Mill. The paths to Blaise Castle and Rhododendron Walk lead to good exposures of the Oxwich Head Formation (previously the Hotwells Limestone Formation) and the Cromhall Sandstone Formation.

Cromhall Sandstone Formation beds on the path to Blaise Castle.  Limestone, mudstone, siltstone and sandstone may be present suggesting a change in the environment from warm, shallow seas to rivers, marginal marine and estuarine environments, c 327 to 341 Ma. The red colours are caused by oxidation of pyrite. 

A flight of steps leads up to the giant’s footprints, in an outcrop of the Oxwich Head Limestone Formation.  The feature has the appearance of a limestone pavement.  These are usually formed on horizontal beds by slightly acidic conditions dissolving the limestone around cracks, such as bedding planes and joints. They are sometimes associated with glaciation. It’s unusual to find them in southern Britain. 

Limestone pavement on the lower eastern slope of Castle Hill.

Proof of the top of the succession is more reliable in some exciting outcrops along Rhododendron Walk. Rocks more recognisable of the Cromhall Sandstone Formation crop out in places up to the Rustic Lodge.  Oxwich Head Limestone crops out along the path to the gatehouse at Henbury Hill, and before Henbury Lodge the bank leads up to the site of the former Henbury Hill Quarry, reclaimed for the 14^th Green at Henbury Golf Course on Coombe Hill.

Rhododendron Walk makes a dogleg at the Rustic Lodge where rocks of the Oxwich Head Limestone crop out in the bank.  They show recognisable coarse, grey, ooidal and crinoidal limestones that represent high-energy marine deposits expected in open shelf environments.

The path through the rhododendrons runs along the rim of the gorge on Coombe Hill and leads to Goram’s Chair. From one of the cliff towers there is a commanding view across the gorge to Kings Weston Hill, Echo Gate and Castle Hill.  It overlooks the steepest part of the gorge, plunging 60m to the bottom. The Severn Estuary and South Wales can be seen over the col at Echo Gate on a clear day.

View from Goram's Chair northwest across the gorge to Kings Weston Hill, Echo Gate col and Castle Hill to the right.

A local legend says that the giant brothers Goram and Vincent diverted the River Avon, each digging a ravine, Goram at Henbury and Vincent at Clifton. More natural explanations for the origin of the Henbury gorge and the Avon Gorge have been hotly debated from the early 19^th century almost until the present day (Bradshaw 1965).

Hazel Brook makes a curious c 90^o turn south in the Crow Lane Open Space, ST 568 796 and flows in a straight line across the Triassic dolomitic conglomerate plain to where it enters the gorge near to Henbury Church. It follows the straight line of the Henbury Fault along the strike through beds of the Cromhall Sandstone Formation to beyond the bridge at Stratford Mill. From here the brook diverges from the fault line, taking a more westerly course before turning towards the south again below Goram’s Chair, following the gorge, crossing the Henbury Fault near Penny Well.

An alternative course for a river would have been the more direct and easier westerly route across softer Triassic and Jurassic rocks to Hallen. It has been suggested that glacial diversion of the river created a new route that breached the Carboniferous rocks at Henbury, (Hawkins 1977). Evidence in the form of gravels and till found in the Bristol district has been interpreted as glacial deposits (Colborne et al 1973). But there has been no evidence found so far that ice reached as far as the Hallen gap, ST 559 795.

The plateau of Kings Weston Hill and Coombe Hill, c 100m OD, is the result of river or marine erosion, when the steeply-dipping and faulted Carboniferous strata were planed flat. An erosion surface at a similar height in Pembrokeshire is estimated to be Pliocene in age, c 5 – 2 Ma. This has led to the proposal, if the Kings Weston Hill erosion surface is Pliocene, it would have been more extensive, cutting across the hard Carboniferous rocks and adjacent softer Triassic and Jurassic strata that had buried the earlier landscape. During the following 2 million years of the Quaternary, this c 100m hard Carboniferous surface would have been gradually incised by its drainage system, and softer rocks worn down faster.  The Carboniferous rocks being more resistant remained, producing the typical Carboniferous Limestone landscape seen today.

Cheddar Gorge is thought to have been formed in the periglacial, tundra environment of the last glaciation by seasonal meltwater flowing off snowfields and over the surface of the bedrock made impermeable by permafrost.  In the last glacial period during the Devensian stage, c 118,000 to 10,000 years BP, the ice sheet advanced to South Wales. The Bristol district was in the grip of a permafrost and it’s reasonable to suppose that frost action and seasonal melts over frozen ground could have rapidly enlarged the Henbury gorge, in a similar process to Cheddar.

To conclude, it has been possible to map the succession and trace the changing palaeoenvironment from the rocks of Henbury gorge. The precise origin of the gorge remains uncertain. However, the balance of evidence found so far suggests that it's the result  of processes observed in the formation of similar features in the region.

Acknowledgements

Thanks to Eileen Stonebridge and Nick Chidlaw for helping me to identify and understand the processes and possible origins of Henbury gorge.

References

Bradshaw, R. 1965. The Avon Gorge, Proceedings of the Bristol Naturalists' Society, Vol. 31, Part 2, pp 203-220.
Colborne, G.J., Gilbertson, D.D., Hawkins, A.B. 1973. Temporary Drift Exposures on the Failand Ridge, Proceedings of the Bristol Naturalists' Society, Vol. 33, pp 91-97.
Hawkins, A.B. 1971. Some Gorges of the Bristol District, Proceedings of the Bristol Naturalists' Society, Vol. 32, Part 2, pp 167-185.
Hawkins, A.B. 1977. The Quaternary of the North Somerset Area, Geological Excursions in the Bristol District.

Maps

England & Wales, Sheet 264, Bristol, Solid & Drift Geology Map, British Geological Survey, 1:50 000.
Bristol & Bath, Sheet 172, Ordnance Survey, 1:50 000.
Bristol & Bath, Sheet 155, Explorer Map, Ordnance Survey, 1:25 000.


John Byles