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 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.

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.

Urban geology - Bristol Temple Meads station

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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.

RIGS of the Month [April] - Brandon Hill

RIGS of the Month - April
Brandon Hill, Bristol

SITE SPECIFIC INFORMATION 

Location: BS1 5QT (GR = ST 577 728)

Accessibility: Municipal park (open all year round. Small amount of metered parking nearby. Limited access to wheelchair users.

Risks: Minimal
Topography: Hilly, tarmac paths.

Google Earth Map of Brandon Hill. Numbers refer to sites described in the text.

All site photos can be viewed in a larger format

https://picasaweb.google.com/charly.stamper/BrandonHillPhotos

Cabot Tower is a prominent feature in the skyline of central Bristol. It was built in 1897 to commemorate the 400th anniversary of John Cabot’s landing in the new found land (later Newfoundland!) of Canada. Set in the lush surroundings of Brandon Hill park, it also marks the spot of an intriguing geological conundrum.

There are two main lithologies exposed in Brandon Hill: Upper Carboniferous quartzite (the eponymous Brandon Hill Grit); and Triassic Dolomitic Conglomerate.

The quartzite is best viewed in the north-eastern sector of Brandon Hill, close to the entrance at the end of Charlotte Street (see site 1). Within the park there is clear evidence for quarrying, and Brandon Hill Grit was used for many local buildings such as QEH school on Jacobs Wells Road and the Merchant Venturer’s Building at the University of Bristol (perversely, Cabot Tower itself was built using New Red Sandstone from the Midlands).

Site 1: Brandon Hill Grit. The beds are tilted steeply towards the north-east having been deformed at the end of the Carboniferous during the Variscan orogeny, when two continents collided and formed the supercontinent of Pangea.

Brandon Hill Grit is a resistant, coarsely bedded quartzite with a strong siliceous cement indicating that deposition occurred in a shallow deltaic environment. Although similar in appearance to the classic ‘Millstone Grit’ facies, research has shown the Brandon Hill Grit to be diachronous with the local Carboniferous limestone. Intermittent channels in the delta sands are filled with coarser grained material: a good example is visible at the base of the north-west wall of the old bowling green (see site 2).

Site 2: Brandon Hill Grit (channel infill). Coarse-grained sedimentary rock with sub-rounded clasts of up to 10mm in diameter and a matrix comprising 1mm rounded quartz grains.

There is a break of over 50 million years before the next rock unit appears during which the collision of two continents in the Variscan orogeny led to the uplift of existing landmasses. Triassic Dolomitic Conglomerate is exposed on the lower slopes of Brandon Hill near to Jacob’s Wells Road and is basal to the New Red Sandstone. The rock provides evidence that the Carboniferous Brandon Hill Grit was subject to strong erosional forces and that the Triassic conglomerate is the product of rapid deposition of the newly formed upland areas. 

Site 3: Triassic Dolomitic Conglomerate. Coarse-grained sedimentary rock with rounded clasts of up to 20cm in diameter, including large chunks of the Brandon Hill Grit. The unit is extensively iron stained and the haematite cement gives the rocks its distinctive red colour: it has been suggested this is a result of oxidation of pyrites in the underlying Coal Measures.

Although the 1930’s landscaping makes it hard to determine which rocks are in-situ, more tranquil Triassic deposits can be found elsewhere in the park. Sub-horizontally bedded Triassic sandstone are present in amongst the rockery around Cabot Tower. These exposures are authochthonous (in their original place of deposition) though they have been reinforced with concrete between some bedding planes, presumably in an effort to stop the beds crumbling into the path.

Site 4: Triassic New Red Sandstone.  Laid down in a desert environment, its red colour testifies to the rock forming in an aerobic subaerial environment. In contrast to the Brandon Hill Grit at Site 1, it is sub-horizontally bedded.

Further to the north-west at the edge of the pond near to the public toilets, the bedrock is once more exposed as Brandon Hill Grit, which leads to a puzzling conundrum: if Upper Carboniferous quartzite is exposed on the west and east flanks of the hill, but Triassic sandstone is found on the summit, where is the contact between the two different rock units? Logic dictates that the unconformity must be hidden beneath the rocky flowerbeds beneath Cabot Tower, but as yet, the exact location is unknown.

Charly Stamper

Thanks to Andrew Mathieson for sharing his local geological expertise. This post references from an article on Brandon Hill in the paper version of Outcrop (Issue 17) – many thanks to the author, Eileen Stonebridge.

The human history of Brandon Hill park is equally fascinating: it is one of the country’s oldest municipal open spaces and is the site of the only remaining Civil War defences in Bristol. For more details visit the Parks & Gardens UK website (http://tinyurl.com/6uw9yk8).