The Avon Gorge: Thrusting under our noses

This post was originally featured on the University of Bristol Earth Sciences PhD blog "Between a rock and a hard place" http://betweenarock.co.uk/fieldwork/science-snap-7-thrusting-under-our-noses/

As Earth Science researchers, we are extremely fortunate that fieldwork often necessitates trips to exotic and far-flung places. But sometimes we are guilty of ignoring the riches right on our doorstep.

In Bristol, perhaps our greatest geological asset is the Avon Gorge. At the end of the Last Glacial Maximum, torrents of icy meltwater scoured out a 2.5km long gouge through a series of Devonian and Carboniferous limestones and sandstones. The bottom of the 90m deep gorge is now filled with the River Avon and the sheer cliffs of the north side are home to fossil corals, rare plants and challenging climbing routes; they also expose an excellent thrust fault.

This particular example lies at the intersection between Bridge Valley Road and the Portway, just underneath the Clifton Suspension Bridge (see here for map). Compressional forces associated with the formation of the supercontinent Pangea (~290 Ma) caused the the older Clifton Down Limestone to be thrust over the younger Upper Cromhill Sandstone. Friction along the overhanging fault plane deformed the younger sediments, and the resulting instability of the rock face has caused major issues for the adjacent roads.

Thrust fault in the north side of the Avon Gorge where the older grey Clifton Down Limestone (right) has been thrust over the younger red Upper Cromhall Sandstone (left); the intensity and friction of the thrusting is manifest in the deformation of the younger sediments. The fault outcrops at the intersection between Bridge Valley Road the Portway (A4) and is conveniently located adjacent to set of traffic lights and a cycle path – look out for it next time you’re stuck on a red light or peddling past.

Charly Stamper

The Bristol 'tsunami': Flood or fallacy?

This post was originally featured on http://betweenarock.co.uk/

30^th January 1607*.
The day dawns sunny and bright. You are ploughing a field in your smallholding deep in the Somerset Levels. As the sweat drips down your back, you hear a distant rumbling sound but think nothing of it; the wind has been blowing a gale all night. Suddenly, a shout from a neighbour makes you look up in alarm. At the end of the far field you see a great cloud hugging the ground, light dazzling off the whiteness. At first you are confused: is it fog, or smoke from a fire? But then you realise, it's water. Within ten seconds, the tumbling, roaring mass has advanced the length of the paddock. You try to run but it's too late. Knocked off your feet by the force of the wave, your head dips below the surface and you inhale a lungful of salty water...
^{*The exact date depends on whether you have a preference for the Julian or Gregorian calendar}

From eyewitness reports, this is what it felt like to be caught up in the most catastrophic flood ever to hit western Britain. Striking in January 1607*, its effects were felt all over the south-west of England, extending over 570 km of coastline from Barnstaple to south Wales and as far inland as Glastonbury (approximately 22km). Contemporary sources put the death toll at over 2,000, though modern estimates have revised this to 500 - 1000¹. The water flow is said to have been so fast "... that no gray-hounde could have escaped by running before them." But what was the cause?

Contemporary woodcut depicting the scene in Monmouthshire on 30th January 1607.

Prior to a modern-day brush with fame, the Bristol Channel Floods were variously attributed an extreme spring tide (the maximum extent of a tidal range that occurs when the Earth, Moon and Sun are in alignment, roughly every fortnight), a storm surge (high water levels associated with a low pressure weather system) or a combination of both. This type of coastal flooding is relatively common in the UK; a particularly deadly occurrence in 1953 killed 307 people in East Anglia.

The tsunami hypothesis was first proposed in 2002 by two academics (Haslett & Bryant - see references 2,3 and 4), and followed up in a series of subsequent papers by the same authors. Their re-interpretation of the events unintentionally coincided with the devastating Boxing Day tsunami of 2004, and so was perfectly poised to percolate the national consciousness. Numerous media articles publicised the theory, and the floods were featured in two BBC2 TV programmes (Timewatch - "The Killer Wave of 1607" and "Britain's Forgotten Floods").

Flood plaque in Goldcliff parish church, Newport. Reads "1606. On the XX day of January even as it cames to pass it pleased God the flud did flow to the edge of this same bras [brass], and in this parish theare was lost 5000 and od pownds besides xxii [22] people was in this parrish drown.". Photo credit: Robin Drayton.

Of course, publicity is not the mark of whether a theory is right or wrong, but proving this particular watery dispute one way or the other has been hindered by a couple of confounding conundrums: the subjectivity of historical sources and the ambiguous nature of tsunami deposits.

At the turn of the 17^th century, literacy levels in the UK were still relatively low. There were no newspapers (or Twitter!), thus first-hand accounts are mostly limited to privately printed pamphlets which tend to offer contrasting reports. For example, the weather on the day in question is conflictingly described as being "most fayrely and brightly spred", "tempestuously moved by the windes" and in the grip of "a mightie storm". The most supportive evidence for a tsunami comes from "Gods [sic] warning to the people of England" , a publication funded by the Church. Its coverage of the event is predictably zealous, describing the flood as a "universal, punishment by Water."
As geologists, the obvious solution would be to look to the rock record; however, tsunami deposits are notoriously tricky to identify because their physical markers are incredibly hard to distinguish from other sources of coastal flooding. Pro-tsunami authors Haslett & Bryantt cite sand "storm" layers in sediments, erosion of salt marshes, vortex pools, and imbricated boulder dumps as supporting evidence for a 'killer wave'; all features imply rapid deposition from a forceful flow of water. Their proposed mechanism for the tsunami is either a submarine landslide or earthquake in the sea between Ireland and Cornwall.

Prof. Simon Haslett atop imbricated boulders in the Severn Estuary. Photo was taken during filming of the BBC2 programme “The Killer Wave”. Source: http://profsimonhaslett.blogspot.co.uk

Perhaps the most compelling evidence against the tsunami hypothesis is that severe flooding in Norfolk is documented on the same day. Most tsunami models agree that it is geometrically impossible for the effects of a tsunami to wrap around the entire coast of England. It seems like the most plausible cause of the floods is a storm surge imposed on an unusually high spring tide. Indeed, the Severn Estuary has the second highest tidal range in the world. The contemporary reports of windstorms driving up the seas is reminiscent of storm surges in New Orléans during Hurricane Katrina in 2005.

Regardless of the cause, it is important to consider the impact that a repeat of the 1607 floods would have today, in order to mitigate against future disasters. The Severn estuary is home to the (active) Hinkley Point and (closed) Oldbury nuclear power stations, and is the proposed site of the controversial Severn Tidal Barrage. Other notable infrastructure includes two motorway bridges, a working port (Avonmouth) and half a million people living in Bristol alone! One risk assessment puts the cost of such an event at £7 - 13 billion¹.

In the wake of the 2004 Boxing Day tsunami, the UK government recognised they did not have a quantitative assessment of threat to the UK. This was despite another infamous tsunami study⁵ (the results of which are now viewed with scepticism) which predicted that a landslide off La Palma would generate waves "higher than Nelson's column" and smash into the west coast of Britain - mass media loved it. Happily for us, the government reports conclude "tsunami-type events [affecting the UK] are unlikely to exceed those anticipated for major storm surges", and "all major centres of development on coasts and estuaries have defences that have been designed to withstand such surge waves."

Should we have these in Bristol City Centre?

Despite their assurances, a small part of me feels pretty smug about a living and working a good 50 metres above sea level!

Charly Stamper

References
[1] "1607 Bristol Channel Floods: A 400-Year Retrospective" - Online publication by Risk Management Solutions.
[2] Bryant EA & Haslett SK (2007) Catastrophic Wave Erosion, Bristol Channel, United Kingson: Impact of Tsunami? The Journal of Geology: 115, p. 253-269.
[3] Bryant EA & Haslett SK (2002) Was the AD 1607 coastal flooding event in the Severn Estuary and Bristol Channel (UK) due to a tsunami? Archaeology in the Severn Estuary. 13: 163 - 167.
[4] Haslett & Bryant (2004) The AD 1607 coastal flood in the Bristol Channel and Severn Estuary: historical records from Devon and Cornwall (UK). Archaeology in the Severn Estuary. 13: 81 - 89.
[5] Ward, SN & Day, SJ (2001) Cumbre Vieja Volcano; potential collapse and tsunami at La Palma, Canary Islands. Geophys. Res. Lett. 28-17, 3397-3400.

The building stones of Clifton - a walking trail

 Building Stones of Clifton - A Walking Trail

 A thirty-minute ramble through 350 million years of geological time  

The trail includes five stops within Clifton and is approximately 1.5km long (blue trail).
Optional sixth stop is an additional 1 km (pink trail). Begin at Clifton Hill House, Lower Clifton Hill, BS8 1BX.

Bedrock geology

Bedrock geology of Clifton

The oldest rocks beneath Clifton are Devonian Old Red Sandstone, lower Carboniferous limestones and sandstones, and Upper Carboniferous Coal Measures. These are sediments deposited during a long period of fluctuating sea level. In the Permian period, formation of the supercontinent Pangaea caused uplift of existing landmasses which were consequently subject to strong erosional forces. The resulting detritus created the next generation of bedrock, and so the older sediments are unconformably overlain by Triassic conglomerates and sandstones, and Rhaetic limestones.

Site 1 - Clifton Hill House 

Bath Stone (oolitic limestone) - Jurassic
Start the trail at Clifton Hill House at the top of Lower Clifton Hill 

Clifton Hill House - Jurassic oolitic limestone

Built in the 1740s, this former merchant’s mansion is now part of a hall of residence for the University of Bristol. The front of the building is faced with cream-coloured oolitic limestone, a rock not native to Clifton; it was extensively quarried in (and is eponymous to) Bath when it became fashionable in the 18th century. Bath Stone was deposited in a tropical shallow marine environment, similar to that of the Bahamas today. The rock comprises millimetre-sized ‘ooids’, small lithic grains coated in concentric rings of aragonite (preserved as calcite) mud. Other features, such as cross-bedding and calcite veining, are neatly captured in the end stone.

 

Site 2 - Goldney House

Brandon Hill Grit - Upper Carboniferous

Continue Clifton Hill and cross the road at Constitution Hill [150m] 

 

Goldney House coach house - Brandon Hill Grit

Goldney House is also part of a university hall of resi- dence, although the main building is a modern addition to the early 18th century coach house and other outbuildings. The coach house wall is accessible from the pavement and is an irregular patchwork of Brandon Hill Grit, a coarse Upper Carboniferous quartzite sourced from nearby Brandon Hill. The rock was laid down as a deltaic sand coevally to the limestones of the Avon Gorge; coarser horizons in some blocks are evidence for ephemeral stream channels. Its distinctive pink-red colouration is staining from the overlying Triassic sediments. 

Site 3 - Caledonia Place
Pennant Sandstone - Upper Carboniferous
Continue on Lower Clifton Hill as it becomes Regent Street. Walk into Clifton Village and turn left along Royal York Crescent. To the south is Dundry Hill [600m]. Walk all the way along the terrace, turn right at the end into Wellington Terrace, and then second right into Caledonia Place [500m].

Caledonia Place - Pennant Sandstone mounting blocks

Though prevalent as a building stone in the city centre of Bristol, Pennant Sandstone is not as common in Clifton. This grey-coloured sandstone is rich in feldspar and micas, and was deposited in shallow waters in the Coal Measures. The poor cementation between individual grains made the sandstone easy to quarry; however, this is counterbalanced by its relative fragility and vulnerability to weathering. In Caledonia Place it has been employed as mounting blocks (to aid Victorian residents’ ascent into horse-drawn carriages). 

Site 4 - Clifton Suspension Bridge
New Red Sandstone - Triassic
Retrace your steps out of Caledonia Place and continue along Wellington Terrace, then Sion Hill [300m] 
 

New Red Sandstone facings at the Clifton Suspension Bridge

Clifton Suspension Bridge is Bristol’s most iconic land- mark and was designed by Isambard Kingdom Brunel in 1831 (but completed posthumously in 1864) to span the chasm between the Carboniferous limestone cliffs of the Avon Gorge. The base of the gothic towers are attractively faced with New Red Sandstone. Its distinctive red colouration reveals its subaerial formation in the deserts of Pangaea and layering from ancient sand dunes is preserved as cross-bedding.

 

Site 5 - The Observatory

Carboniferous Limestone - Lower Carboniferous

Follow the short footpath up the hill from the Bristol-side toll booth [200m] 

 

The Observatory - Carboniferous Limestone

Originally built as a mill in the late 18th century, Observatory Tower was purchased over fifty years later by a local artist who installed a telescope and camera obscura (to project panoramic exterior views onto a screen). The rounded rubble walls comprise fossiliferous blocks of Carboniferous Limestone hued from the gorge, and provide a reminder of a time when the Avon region was submerged beneath a balmy tropical ocean. Descend to ‘Giant’s Cave’ beneath The Observatory to further explore the strata of the Gorge. 

Site 6 [optional] - The Cumberland Basin

Cornish granite - Lower Permian

For a longer addition to your excursion, retrace your steps towards the Avon Gorge Hotel and take the Zig Zag footpath down to The Portway. Turn left and walk towards Bristol City Centre. Take care when crossing the busy road - it is best to walk over the pedestrian footbridge which begins in Granby Hill [∼1km]. 

 

Cumberland Basin - Bodmin Granite

The Cumberland Basin was excavated in 1809 when the River Avon was diverted to form a floating harbour and granite is used as capping material on the channel walls. Petrolographic analysis has shown it to be Bodmin Granite, part of the Cornubian batholith that is exposed throughout Cornwall and the Channel Island. This igneous rock formed a result of a huge mass of magma intruding into the crust during Variscan orogeny (∼275Ma). Though the surface has weathered to a smooth finish, individual crystals of grey quartz, white plagioclase and pinky- orange orthoclase feldspars, and dark-coloured biotite mica can still be identified. 

Charly Stamper 

References

- Jones D (1992) A History of Clifton. Phillimore, Chichester.
- Mowl T (1991) To build the second city: Arcitects and craftsmen of Georgian Bristol. Redcliffe Press Ltd, UK.
- Savage RJG (1988) Buildling Stones of Clifton. Proceedings of the Bristol Naturalists’ Society, 48: 85-104.

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

The Hot Well, Bristol

The Hot Well spring, Bristol
From rags to riches and back again - 
the story of the Hotwells spa
 

The old Hotwell House on the banks of the Avon. Built in 1696, it held a pump rooms and lodgings for visitors. After the terminal decline of the Hot Wells spa, it was demolished in 1822 and no remnants of its former glories remain, the site being adjacent to The Portway.

INTRODUCTION

The King’s Spring in Bath has been exploited by humans since 836BC, most famously by the Romans who built the first baths, and then later during the spa age of the 18^th century. Less renowned is the eponymous spring of Hotwells in Bristol. At its peak in the Georgian era, the “Hot Well” served a fully functioning pump room and hot baths, and provided the catalyst for much of the development in Clifton. Today, the spring has diminished in flow and is only visible at low tide as a trickle emanating from the banks of the River Avon.

The modern-day resurgence of the Hot Well from the banks of the Avon as visible at low tide. All photo credits: Charly Stamper

HISTORICAL BACKGROUND

The spring was first mentioned in 15^th century historical records, and by the 1630s it was being regularly visited by society. The thermal water emerged on both sides of the Avon, roughly opposite the intersection the The Portway and Bridge Valley Road. It was contemporarily described as being "milky white" and was thought to have restorative properties, particularly for "hot livers, feeble brains and red pimply faces". During the 17^th century the spring was relatively inaccessible, for there was no formal path and a descent from Clifton involved “200 slippery steps”.

As the popularity of spas increased in the Georgian era, so did the number of visitors to Lower Clifton. Initially, development focused on the area adjacent to the natural resurgence, with the building of a spa (old Hotwell House), entertainment complexes (Jacob Wells theatre) and genteel housing (eg. Dowry Square). In the 18^th century the spring’s reputed curing powers extended to venereal disease, tuberculosis and cancer.

Strangers' Burial Ground, Lower Clifton Hill. By the 1750s, the Hot Well acquired a reputation for curing tuberculosis; however, these claims were unfounded and an overflow burial ground was instated to cater for unfortunate commoners who came from outside of the parish.

The main dent in the Hot Well's popularity remained the distance and difficulty of access down the steep sides of the Avon Gorge, and in the mid 1780s Thomas Morgan embarked on an ambitious engineering project to bring the waters to the heart of Clifton. From Sion Row he drilled a shaft some 250ft through the Carboniferous limestone to tap the hot waters, supplying water to a new pump room with hot baths and a reading room, later to become the St Vincent Rocks Hotel (now Avon Gorge Hotel). By 1793, this diversion had become known as the “New Hot Well”.

In the following years, both springs began to cool, almost certainly as result of increased groundwater mixing. This coincided with a nationwide decline in spa popularity in favour of sea bathing, increase in subscription charges and end of the Napoleonic Wars, meaning British people were free to travel abroad. Several revival attempts in the 19^th century failed to capture former glories, and the spring is no longer commercially exploited. 

The Colonnade, Hotwells Road. Originally a shopping arcade, it was built in 1786 as an attempt at reviving the failing fortunes of the Hot Wells spa.

GEOCHEMISTRY AND HYDROGEOLOGY

Modern day studies of the hot springs of the Avon area have tried to shed some light on the source and science behind their existence. In 1993, the yield of the Hot Well was measured at 0.41 x 10^6 litres a day, about a third of the present-day flow recorded at the King’s Spring in Bath and enough to fill an Olympic-sized swimming pool in a week. Compositionally, the two springs are very similar, being rich in calcium and sulphates, though 25% of the Hot Well volume is cold groundwater, reflected in the relatively low average temperature of 24ºC.

The source of the two springs is thought to be rainfall in the Mendip Hills, some 15km to the south-west.  Measured carbon isotopes (∂¹³C) are consistent with storage in Carboniferous Limestone, and hydrogen and oxygen isotopes provide evidence that most of the water is meteoric in origin. The head at this elevated topography is high enough to force the water down beneath the Coal Measures to a depth of around 2.7km in the Bristol-Bath basin and heat the groundwater; silica geothermometers indicate the thermal component of the springs reaches a maximum temperature of 72ºC. 

Cross section showing flow of groundwater through Carboniferous Limestone from source in the Mendip Hills to resurgence in Bath and Hotwells (Andrews et al., 1982).

The overlying stratum of the Coal Measures is a proven aquiclude, so no upward migration can take place. The area north of the Mendips is heavily faulted and folded from both Mesozoic and Tertiary tectonics, so the water migration is unlikely to be direct. Tritium (³H) was produced by thermonuclear weapons testing in the 1950s and is used to identify any modern-day recharge in groundwater. Low tritium levels in the Hot Wells indicate only minor amount of mixing with of ‘recent’ waters, and the majority is likely to be up to 10,000 years old. At the end of its journey, the Hot Well resurges directly from Carboniferous Limestone into the channel of the Avon river.

THE FUTURE

Although the Hot Well spring has had its heyday, the King’s Spring at Bath remains at the heart of the city’s tourist trade. In 2011, two companies (Eden Energy and UK Methane Ltd) were given licenses by Mendip district council to begin a feasibility study for the controversial practice known as “fracking”. Concerns were immediately raised by councilors in Bath and led to a subsequent uproar in the local (and further afield) media. A specially commissioned British Geological Survey report concluded that the risk to the Bath springs was no higher than any other part of the UK, although critics point out that relatively little is still known about the subterranean flow of the groundwater. The energy companies are a long way off obtaining the planning permission needed to begin exploratory drilling, but the authorities would to well to bear in mind the role that human intervention had in the decline of the Hot Well spring.

Charly Stamper

REFERENCES AND FURTHER READING

Andrews JN, Burgess WG, Edmunds WM, Kay RLF & Lee DJ (1982) The thermal springs of Bath. Nature 298: 339-343.

Atkinson TC & Davison RM (2002) Is the water still hot? Sustainability and the thermal springs at Bath, England. Geological Society, London, Special Publications, 193: 15-40.

Clifton and Hotwells Conservation Area Character Appraisal (2010) Bristol City Council http://www.bristol.gov.uk/sites/default/files/assets/documents/clifton-and-hotwells-character-appraisal.pdf

Gallois RW (2007) The formation of the hot springs at Bath Spa, U.K. Geol. Mag. Vol. 144, 741-747

Jones, D (1992) History of Clifton. Phillimore.

Kellaway, GA (1993) The hot springs of Bristol and Bath. Proceedings of the Ussher Society, 8, 83-88.

Mowl, T (1991) To Build the Second City: Architects and Craftsmen of Georgian Bristol. Redcliffe Press Ltd.

Smith NJP & Darling WG (2012) Potential problems within the Bath and North East Somerset Council and surrounding area with respect to hydrocarbon and other exploration and production. British Geological Survey Commissioned Report CR/12/055, 26 pp.

Baryte (BaSO4)

Minerals of the Avon region

Baryte   (Barium Sulphate – BaSO₄)

Colour: White but is often coloured by other minerals to a pink or pale brown

Polymorphism: Forms a series with Selestine

Name: From the Greek for weight due to it’s high specific gravity      

Crystal system: Orthorhombic with up to 70 forms

Specific gravity: 4.5 measured (4.47 calculated)

                           

Hardness: 2.5 – 3.5

Group: Barite Group

Association: Fluorite, calcite, dolomite, rhodochrosite, gypsum,                        sphalerite, galena, stibnite.

Occurrence: A gangue mineral in low-temperature                                         hydrothermal veins; in residual deposits from weathered barite-bearing limestones.

Local location: Stancombe Quarry, Flax Bourton (Working limestone quarry)

Cross section of Baryte vein -Stancombe Quarry

Picture credits Richard Kefford

Larger pictures can be seen here

Paragenesis #1 - Bilbao supergene type

During the Late Triassic, iron rich saline oxidising water leached through the rift basins formed during the early Permian (~290 Ma) to the late Jurassic (~150 Ma).

Paragenesis #2 - Mississippi Valley Type ( MVT )

In the Mendip – Bristol vein field, baryte has been deposited by hydrothermal fluids in tension cracks and fissures in the Carboniferous Limestone. The primary minerals in these fissures are Galena and Sphalerite. Gangue minerals such as Baryte and Calcite occur in banded formations where the veins pinch out. This is known as the Mississippi Valley Type (MVT) and took place during the Middle Jurassic (~170Ma).

Baryte with associated galena and calcite - Stancombe Quarry

Uses:

By far the greatest use of this mineral ~80%, is for the production of drilling mud for use in oil exploration. The main reason for this is that it is very heavy and so helps to prevent blow outs in the drilling stage of the exploitation of an oil reservoir. It is also chemically inert. The specification for drilling mud includes a requirement that the specific gravity should be 4.2 or greater.

Non drilling applications of barytes are comparatively small, although still important because of their higher value. High purity grades of barytes with fine and controlled particles sizes are used as fillers in marine and industrial paints, in brake lining/ friction materials and in plastics. A specialised use of barytes based on its high density and ability to absorb radiation, is as an aggregate in dense concrete for shielding applications in the nuclear industry and hospital radiation departments.

Bladed rosettes of Baryte on associated red ochre

Sample found by Leon Sparrow at Winford

Sources:

Barytes is produced in England and Scotland. In England it is now only produced as a by product of fluorspar mining and processing. In Scotland, barytes is extracted as the sole mineral from the Foss Mine near Aberfeldy. 



Richard Kefford

References:

www.bgs.ac.uk/downloads/start.cfm?id=1349 

http://www.aditnow.co.uk/documents/Bridford-Baryte-Mine/BARYTES-MINE.pdf

Chidlaw, N. (2012) Metamorphism and Mineralisation in the Bristol - Mendip area