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The Group's aim is to identify, survey, protect and promote geological and geomorphological sites in the former County of Avon - the modern unitary authorities of Bath and North East Somerset, Bristol, North Somerset and South Gloucestershire. RIGS are selected for their educational, research, historical and aesthetic value.

Saturday, 5 November 2016

         

South Gloucestershire geology booklet - free download

The story of geology & landscape in South Gloucestershire was published in April 2007 hard copy format as a collaboration between South Gloucestershire Council, the Avon RIGS GroupBRERC and Bristol Museum. Although paper copies are no longer available, this excellent thirteen page booklet is still available as a free-to-download pdf below. 

Front cover

                                                                     
                               Click Booklet to download in PDF format.

       
                                                                   
Fourteen local sites, including Aust Cliff, Huckford Quarry and Wick Golden Valley are featured. Each outcrop is described and interpreted with cross sections, location maps and annotated photos. 

                                                         
 JPEGs of the site interpretation panels in South Gloucestershire can be accessed using 
these links:-



















The archive of the paper copies of OUTCROP can be accessed by clicking Here . This will take you to Issue 15. Later issues can be seen by clicking on the links below.













Tuesday, 24 May 2016

Geology Trail in Badgers Wood Nature Reserve

Backwell Environment Trust

BET was founded in 2004 to protect and preserve the beautiful countryside surrounding the historic village of Backwell, North Somerset. We presently own and manage two nature reserves comprising nearly 22 acres in area. Both reserves contain many rare and endangered species and have a facinating history dating back at least 5,000 years

BET's Nature Reserves

Our two reserves are the 10-acre Jubilee Stone Wood and the 12-acre Badgers Wood. Both reserves are located to the south-east of Backwell, high up on Backwell Hill and offer free access to the whole community. There are sections of level, wheel-chair friendly paths in both reserves leading to magnificent viewpoints taking in the Bristol Channel, Wales and Exmoor on clear days.

OS Explorer Map - Grid Ref (ST681494)



They serve as public amenity woodlands, an educational resource as well as being a superb location for wildlife. Together the two nature reserves cover almost 22 acres in area and are a mixture of broadleaf woodland interspaced with open areas of calcarious grassland. Its many rare species include the Hazel Dormouse, Greater Horseshoe Bat and the Yellow Bird's-nest plant.
The reserves have an amazing archaeological history and contain the ruins of a 14th century rabbit warren and cottage, 17th century lead mines and a (now restored) 19th century limekiln.

Downloadable maps of the trails in the reserves can be seen at:-

http://backwellenvironmenttrust.org/index.php/bet-bulletins/maps

Backwell Hill, where the BET nature reserves are located, is part of an anticlinal structure which forms a dome called Broadfield Down. The two reserves are on the North facing slopes of the Down. The topography of the Down follows the underlying stratigraphy, as can be seen in the face of the disused Coles Quarry from the access path viewpoint.

Photo credit David Moore

View into Coles Quarry and across to Wales from the access path view point.


A geological history of Broadfield Down


The oldest rocks that can be seen exposed in this area are the Upper Old Red Sandstones which were laid down between 380 and 360 million years ago  ( mya ) during the Devonian geological period. ( 420 – 360 mya )
The part of the British Isles that includes Broadfield Down was then drifting slowly North from  20 - 25 degrees South of the equator. It was part of a tectonic plate that included the continents of Baltica and Laurentia. This plate was separated from another plate called Gondwana by the Rheic Ocean. 
Gondwana included the future continents of Africa, South America and the rest of Europe and Asia. Scotland and England had been recently united by a collision between the plates of Laurentia and Avalonia. This collision closed the Iapetus Ocean and caused a wrinkling of the Earth’s crust which resulted in a mountain building process called the Caledonian Orogeny. In Greek mythology Iapetus was the father of Atlas for whom the Atlantic ocean was named, so the Iapetus ocean can be seen as the precursor of the Atlantic.
These massive mountains are called the Caledonides and, as they started being quickly eroded, the material produced was carried South by massive meandering river systems in an arid continental, desert environment. This material was laid down and lithified to form sandstone beds which are known as the Upper Old Red Sandstone. The name comes from the red-brown colour from the oxidised iron, haematite, coating on the quartz grains. Within the Old Red Sandstone are fine grained mica-rich sandstones, shales and mudstones.
These rocks form the cores of the Mendips, Broadfield Down and Failand Ridge. They are known as the Portishead Beds and can be seen exposed along the coast between Clevedon and Portishead. There are also some small exposures in the bed of the track that drops down from Cadbury Camp Lane under the motorway to join Clapton Lane near the Black Horse pub.
The Devonian period came to an end about 360 million years ago ( 360 mya ).  At the start of the following Carboniferous period ( 360 – 300 mya ) all the continents had moved together, adding Laurentia and Gondwana, closing the Rheic Ocean. The British Isles were now moving slowly northwards across the equator into the northern tropics. Shallow seas started to cover the Devonian desert.
Various limestones were deposited in these warm, shallow seas. Some of these limestones are oolitic – consisting of small, < 2mm, egg-shaped particles called ooids. Because they were shallow and warm, there was a high evaporation rate which made the seawater supersaturated.
These were formed in a similar way to rolling snowballs. Calcium carbonate accreted around a particle, of shell or sand,  and the ooids grew in size as they were rolled around the sea floor in the lime saturated water. This process now continues in the Bahamas.
One of these limestones is Clifton Down Limestone which forms the crags alongside which the Geological Trail was built.
Many of these limestones contain fossils, crinoids, corals and brachiopods, and are very common in this area. Fossils can be seen where the host limestones were used for building, such as in the walls along Stockway North and in the parapets of Jackland’s Bridge in Nailsea and in the rocks used for building the drystone wall along the Northern side of Cadbury Camp Lane West, to the West of Cadbury Camp.
Not many of these fossils can be seen on Backwell Hill but an exception is an excellent specimen of Siphondendron Martini ( Lithostrotion ) that was found on a rock pile near the view point at the top of the trail during a geological field trip by the Bristol Naturalists. 


Photo credit David Clegg
Siphonodendron Martinin ( Lithostrotion )

The seas gradually shallowed so the formation of limestones stopped. Rivers brought material to the area and deposited quartzitic sandstones on top of the limestones. These are generally known as Millstone Grit.
The climate had changed from arid to equatorial conditions so the rivers increased in size and flow because of the increase in rainfall. Swamps started to form and the rivers formed deltas on the coasts. Primitive plants such as horse tails  grew in these swamps and as they died, peat started to form. These swamps and deltas were periodically flooded as the sea level rose and fell. The rivers deposited material eroded from the high ground formed by the continuing orogeny which covered the peat with layers of sandstone. These sandstones compressed the peat which was slowly turned to coal. As the rivers meandered and moved away, plants started taking hold again and so there are repeated cycles of  clay, coal and sandstone that can be seen in the Nailsea coalfield. The records of the Watercress Farm borehole, that was drilled in 1903 while looking for a water supply for Tyntesfield demonstrate this. These repeating cycles are called cyclothems. The clay is the relic of the soil that the plants grew in and is invariably found below a coal seam. Coal miners call this ‘seat earth.’ This clay is commonly used to line furnaces as ‘fire clay’ and would probably have been used in the Nailsea glass works’ kilns. It was also used to make bricks in, for example, the brick works that existed adjacent to the shafts of the Dean Lane coal mine in Southville.
The coal seams crop out at the surface on the northern slope of the Nailsea syncline and this is where mining probably started using drift mines to follow the seams. This area was surveyed in 1950 by the Coal Board to assess its viability for surface coal mining. Deep shaft mines followed later, towards the centre of the coalfield.
William Smith – known as the ‘Father of Geology’ - carried out a survey for the mine owners in 1811  to see if the coal output would justify the building of a canal to take the coal to market. In his opinion it was and he sounded very optimistic in his report of 1811 and some of the necessary land was purchased but the canal was never built as the competition increased form the South Wales coalfield.
As the Carboniferous period progressed, the Rheic ocean closed and so there was increasing tectonic pressure from the South West. This is called the Variscan Orogeny. It  resulted in crustal folding in the area and formed the Mendips, Broadfield Down, Failand Ridge and the Nailsea syncline. It continued for some 100 million years into the Permian.
The rivers bringing erosion material from the North reduced so the formation of deltas  slowed and eventually stopped. Rivers then started flowing into the area from the South and East, bringing large quantities of sand that were laid down across an area from Swansea to Oxford. 
This is known as the Pennant sandstone which can be seen in the faces in Conyegar  and Nowhere Quarries. The Pennant is up to 330m thick in the  area and contains thin coal seams. There is also a good exposure of the Pennant at Trooper’s Hill in Bristol.
The direction of the rivers that supplied the material can be inferred from the channels and dune structures seen in the Pennant Quarry faces. This is called cross stratification.
All the land masses then united to form the supercontinent Pangea.
At the end of the Carboniferous period there are no rocks to be seen in the area from the following period, which is the Permian. ( 300 – 250 mya ) This could either be because the conditions were not right for any deposition to take place or they were deposited and have since been eroded away.
The next geological period is the Triassic. ( 250 – 200 mya ) The mountains that were created during the Variscan Orogeny are being rapidly eroded during this time. 
The coarse-grained products of this erosion are known as Dolomitic Conglomerate  which can be seen as a skirt surrounding Broadfield Down. They create an unconformity with the Clifton Down Limestone that can be seen in, for example, the path bed running down from the Jubilee stone to Church Town. It can also be seen in the old quarry face in  the disused Cheston Combe quarry. This unconformity can also be seen in many areas around the area, in the Tyntesfield quarries for example. This conglomerate has been used as a building stone in the area. It is probably best seen in the walls of the Tyntesfield sawmill. A similar rock, from Draycot Quarry on the South slope of the Mendips was famously specified by Brunel for building Temple Meads railway station.
The fine grained deposits are called Mercia Mudstone and can be seen surrounding the area as they merge into the dolomitic conglomerate.
No rocks from the following periods, Jurassic ( 200 – 150 mya ) and Cretaceous ( 150 – 66mya ),  remain in the area so, again, either they were deposited then eroded away or they were not deposited in the first place. 
Recent Quaternary ( 2.6 mya – present ) Drift material covers the low lying areas surrounding Failand Ridge, such as  Nailsea Moor to the South and the Gordano Valley to the North. This is mainly Peat and Alluvium – clay, silt and gravel.
This means that Broadfield Down is an eroded Carboniferous dome structure in a Triassic landscape. It is remarkable how much complex and interesting geology exists in such a small area.

A history of Badgers Wood Geology Trail - from the BET web site

After much discussion by the trustees about the possibility of a high-level cliff path in Badgers Wood, the project was given the go-ahead in the spring of 2013 and was finally completed in October 2014.
In total, the new trail is around 350 metres long and runs along the base of some of the fantastic limestone cliffs to be found in this part of the woodland. The first and last sections have been relatively straightforward, but it was the bit in the middle, close to the cliff faces, that have required the full engineering skills of the trusty BET volunteers. 






We decided to start the trail in the middle, so if we ever felt we couldn’t overcome some of the more challenging technical problems, we could always simply walk away and no one would be the wiser!
The problem was that some sections of the trail had to be constructed on a very rocky, steep slope so the only real solution for us was to attach metal cages to the rock faces, fill them up with many tonnes of rock and then put the trail on top. Although this took a long time to do, I’m very pleased with the finished result as it’s now quite difficult to see the sections of trail that would once have been in thin air just a couple of years ago.  

The Geology Trail branches off the Fern Way close to the start/end of that trail.
One word of warning though.... the trail does have some short, steep sections and you may need a head for heights on the more ‘challenging’ middle section!
The route of the trail is shown on the updated BET Map.
A slideshow of photos taken during the formal opening of the trail can be found here:-


Description of Clifton Down Limestone.

Lithology.
The lithology is dominated by calcite mudstones with a locally abundant but low diversity fossil assemblage. At Burrington Combe the formation is about 170 m thick, and three subdivisions can generally be recognised across the Mendip area. The lowest unit comprises a mixture of calcite mudstones, white oolitic limestones and dark splintery limestones. This interval is relatively expanded in the Cheddar area, where a 38 m thick dark limestone ('Cheddar Limestone Member') is overlain by a 58 m thick white oolitic limestone ('Cheddar Oolite Member'). The middle part of the succession is dominated by fine-grained, grey-black limestone with nodules and bands of chert and abundant remains of the coral Siphonodendron['Lithostrotion'] martini ('Lithostrotion Limestone'). Porcellaneous calcitic mudstones dominate the highest part of the formation, including locally developed algal mudstones and stromatolites, indicating deposition in a very shallow-water, near-shore or lagoonal environment.
Splintery dark grey calcite and dolomite mudstones, pale grey oolitic, dark grey bioclastic and oncolitic limestones and some mudstones. Scattered cherts and silicified fossils in lower half. Sandy limestone at base in Bristol area. Deposited in a barrier/back barrier/shelf lagoon setting.
Age range: Arundian Substage ( CJ ) - Holkerian Substage (CQ ) Early Carboniferous.
Thickness: Up to 266m in the Avon Gorge, thinning Southwards to 150 - 200m in the Mendip Hills.
Parent Unit: Pembroke Limestone Group

References
The BGS Lexicon of Named Rock Units BGS

England and Wales sheet 264, Bristol Geological map BGS

British Regional Geology memoir, Bristol and Gloucester BGS

Backwell Environmental Trust web site BET

http://backwellenvironmenttrust.org/

Grateful thanks to the BET Trustees for their permission to reproduce items here from their web site.








Sunday, 15 May 2016

Saltford RIGS and Geology Trail

Saltford RIGS and Geology Trail : – Exposures Exposing Work Continues in 2016 
Progress Report -- & Advertising of the Exposing Work to Continue to its ‘Cotham Marble Conclusion’, and the further great enjoyable opportunity to be a part of the work: later in the year …. Watch this space! …..As we local keen geo folk : Dig for Marble ! 

About a month back, - earliest Spring (just about!?) -, a very good and keen team spirited group of a few (human! , not JCB) diggers, joined, over a few nourishing and warming flasks of sustenance, and dug happily for several hours, from a nice relaxed 10.30 or 11 am start.

Good progress has now been made, over two work days, to provide a great example exposure to tell the story of Saltford Geology, in a Trail, to be set up by local man, and well known palaeo man, Simon Carpenter, as shown in the photos here. In good relaxed friendly geology keenness fashion all 5 or 6 on the team on the 2nd work day, some weeks back now, near the Bristol to Bath cycle path, gathered and dug. …..Keen to unearth more of the wonders of the special and definitely Regionally Important geology found in Saltford. 

For more detail on the special geology of Saltford there are leads on our Avon RIGS pages & also here linked below, for e.g., you could read up within : 

http://earthwise.bgs.ac.uk/index.php/Geology_of_the_Bath_area:_Introduction 

http://earthwise.bgs.ac.uk/index.php/Geology_of_the_Bath_area:_Jurassic 

https://en.wikipedia.org/wiki/Cotham_Marble 

And page 270 in this reference, on Late Triassic – Early Jurassic strata, for more on the Cotham Marble and other strata of the area: 

https://www.earth.ox.ac.uk/~stephess/Hesselboetal2004.pdf 








Simon (Carpenter) leading the way from the top of the embankment, which lies to the north of the former railway cutting, digging downwards through the strata, exposing a very good section,… towards the Cotham Marble bed….. To be hit very soon now, we hope…next Work Day! 

-Do join us. The more the keen hands the better.

Friday, 19 February 2016

Aust Pylon and Talus Works

With the pylon at Aust coming up close to its 50th anniversary it is time that National Grid and Trant Engineering Limited give the concrete pillars it is stood upon some major renovation. As part of these works, that will continue throughout 2016, the concrete causeway that leads from the old ferry crossing down to the pylon will be in constant use. 

Over many years soil and clay have accumulated at the base of the adjacent cliffs forming a huge bank pushing against the causeway. It was time for the bank to be removed. 

During the first two weeks of February I was on site with a watching brief both for water birds using the foreshore and for fossils as the bank (talus) was removed and laid out across the beach. Two dumper trucks and two diggers were involved in transporting the soil and clay down to the beach. It was laid in two long rows along the beach enabling me to look through it. 

Although there was some rock amongst the talus it was mainly slabs of shelly limestone rock that revealed some lovely bivalve fossils but nothing more. However, time on the beach did give me the chance to find plenty of Rhaetic bonerock – mostly containing the usual mix of tiny teeth, coprolites and fish scales. A few boulders revealed larger bone and shark spines. 

I will be transferring the finds to Bristol Museum & Art Gallery – hand size pieces will be used as handling objects for learning activities. The bank itself has been cut back three metres and to a 45 degree angle. This will help reduce the pressure of future soil and clay building up against the road.  The recent high tides have been washing through the talus and cleaning up any remaining slabs of limestone. 


Ed Drewitt. Thanks also to Joe Keating and Dave Marshall from the School of Earth Sciences, University of Bristol, who covered a day each.



                                                    Talus being cleared at Aust



                                                       Fossil shells at Aust



                                                          Bonerock at Aust



                                                       Shark spine at Aust



                                                Fossilised bone at Aust


Wednesday, 3 February 2016

Student reports new finds of a living fossil


The coelacanth fish, found today in the Indian Ocean, is often called a ‘living fossil’ because its last ancestors existed about 70 million years ago and it has survived into the present – but without leaving any fossil remains younger than that time. Now, some much older coelacanth remains have been uncovered in a fossil deposit near Bristol by a student at the University of Bristol. 



The Jurassic coelacanth Undina, similar to the new finds from near Bristol. Image credit Harry Allard

While working last summer in Bristol’s School of Earth Sciences, Harry Allard, a recent graduate from the University of Exeter, found remains of coelacanth fishes, ranging in size from juveniles to adults, in a section of Late Triassic rocks, dated at about 210 million years old, at Manor Farm, Aust, close to the first Severn crossing.

He discovered the new fossils in a large collection of fish and reptile teeth and bones, representing animals that lived in the shallow seas, and on the neighbouring landmass at that time when Bristol teemed with dinosaurs, and the landscape consisted of numerous tropical islands.
Harry said: “These fossils provide an amazing glimpse of an ecosystem which is so different from the contemporary landscape of south west England. It has been fascinating to look at the changing composition of that long-lost ecosystem.”

The Manor Farm site was created 15 years ago when the second Severn crossing was under construction and contractors excavated there to obtain road-building materials.  After the site was made safe, a section was dug out so geologists, and the public, could visit and learn about the local geology. One of the fossil collectors at the time, the late Mike Curtis of Gloucester, collected batches of sediment, and worked through the material to extract nearly 20,000 teeth and bones.

“Mike Curtis kept such excellent records that Harry was able to separate the collections into findings from five separate bone beds, each perhaps separated by a few hundred thousand years,” said Professor Michael Benton, supervisor of the project.  “This provides unique insight into a turbulent time, when seas flooded across the landscape, submerging much of Europe.  Dry land became shallow seas almost overnight, and the energy of the floods churned up the soil and rock below and deposited bone beds in some places.”

Tracking upwards through the five bone beds, Harry was able to show how the fish faunas changed through time, from being dominated by small sharks at first, and then switching to more thick-scaled bony fishes higher up.

“The coelacanths were smaller than the living coelacanth Latimeria,” said Chris Duffin, a fossil fish expert who was involved in the work, “but these fishes were quite diverse in the Triassic, and only dwindled in importance later.  They are most unusual, having gills and lungs, and moving both by paddling with their gills, and stilt-walking along the seabed as well.

Paper

‘Microvertebrates from the classic Rhaetian bone beds of Manor Farm Quarry, near Aust (Bristol, UK)’ by Harry Allard, Simon Carpenter, Chris Duffin, and Michael Benton in Proceedings of the Geologists’ Association(doi: 10.1016/j.pgeola.2015.09.002)

Sunday, 18 October 2015

Rock clearance day at Salford

Rock clearance day at Saltford

Friday 20 November 

Meet: 10:30am at the clearance site. Finish at 4pm.

Dear volunteers

I am organising a rock clearance day on Friday 20 November to clean up a late Triassic/White Lias site close to the Railway Path at Saltford ST 691668   (see attached map). The site will eventually be part of a new geology trail around the village and I need your help

The rock outcrop occurs along the rim of the cutting and I need a team to help clear ivy and vegetation covering the current rock face as well as removing a substantial talus slope that covers and obscures the lower part of the exposure. It should be possible to complete this work in one day.

If you are interested (volunteer days can be real fun, especially if the weathers nice), please ring or email me to register your interest: 01373 474086 and/or simonccarpenter@gmail.com

You will need to bring a spade, thick work gloves, secateurs and or loppers - some extra tools will be available if you don't have any of your own.  Bring packed lunch or there is the Bird in Hand pub a short distance away.  Bring lots to drink and warm clothing, if its cold. 

There is a regular bus service to Saltford (X39) from Bath and Bristol or the site can be reached by bicycle of walking.  There is some car parking near the River Avon at 'The Shallows' and the rock exposure can be reached from here by a short walk.  There will be signs placed on the railway path adjacent to the rock exposure so that you don't miss it.  

With best wishes Simon Carpenter


Thursday, 15 January 2015

The Geology of Clifton and Durdham Downs

The Geology of Clifton and Durdham Downs        by Andrew Mathieson

Reproduced from Nature in Avon, volume 73 (2013), with the permission of the Bristol Naturalists' Society.

Little has been written about the geology of the Downs, in contrast to the many accounts of that of the Avon Gorge. This is hardly surprising since the Gorge is nationally important for both the exposed rock sequence and the landform itself, but the Downs do have a number of different and special geological features. The two sites are very closely linked since most of the rocks seen in the Gorge also lie under the Downs, but there are several younger rocks on the Downs which are not found in the Gorge, and these add greatly to our understanding of the geological history of the area. 

The Rock Succession
The oldest group of rocks on the Downs is the Carboniferous Limestone, and this underlies most of the area. It is a thick sequence of some 760 m (2,500 ft) of rock, and is mostly made up of different types of limestone, but there are also dolomites, mudstones and sandstones. These all contain the fossil brachiopod shellfish, corals and crinoids, clearly indicating that the rocks formed beneath the sea. The presence of corals and limestone suggests that the water was shallow and warm, and this is supported by studies of palaeomagnetism, which indicate that these rocks formed close to the equator. Radiometric evidence shows that they are between 359 and 343 million years old.

The sequence of Carboniferous Limestone is divided into a series of units, which are mostly named after exposures seen along the Gorge (Kellaway and Welch, 1955). These units were created by the Geological Survey to replace a system of zones based on fossil corals and brachiopds (Vaughan, 1905) which, although revolutionary at the time, had been found to be difficult to use outside the Bristol area. The new system is based on units of types of rock which can be traced across the area from the Mendips to South Gloucestershire. The Gorge is famous as the reference site for both of these methods of dividing the limestone succession (Bradshaw and Frey, 1987; Hawkins, 1987).

The main Geological Survey units present on the Downs are:

  1. Black Rock Limestone is the oldest unit, and it can be traced in a band across the north of the Downs from Sea Walls to Badminton School, and forms the northern edge of the high ground. It takes its name from Black Rock Quarry in the Gorge, where it is well exposed, and is a dark grey, well bedded limestone with many fossil crinoids, corals and brachiopods.
  2. Gully Oolite outcrops across the Downs to the south and parallel to the Black Rock Limestone from near the top of the Gully. It is a light grey coloured oolitic limestone, with few fossils and little evidence of bedding. The rock is best seen in the Gully Quarry in the Gorge. Modern day oolite sediments are found in shallow seas subject to bottom currents.
  3. Clifton Down Mudstone is a relatively softer rock and its course across the Downs runs parallel to the other older rocks, starting from where it was eroded out to form the top of the Gully. The rock is poorly exposed in the Gorge but its base can be seen at the top of the Gully Quarry. The contact with the Gully Oolite beneath is irregular, suggesting that erosion took place at that time and the junction between the units is interpreted as a fossil soil. The presence of mudstones indicates that mud was washed into the sea by rivers from nearby land, but there are also beds of limestone which show that the seawater cleared at times.
  1. Clifton Down Limestone is found both on Durdham and Clifton Downs, since it was displaced by large scale faulting. It is rich in fossils and is sometimes oolitic. There are also some fossil algal structures which suggests an inter-tidal origin for some of the rock.
  2. Hotwells Limestone is again found on both Durdham Down and on the south side of Clifton Down due to the action of the same faulting. It is a well bedded limestone, rich in fossil corals and shellfish, and is best exposed around the foot of the Old Zigzag and the entrance to the former Clifton Rocks Railway. The Upper Cromhall Sandstone, at the top of this unit, is not actually present on the Downs, but is well exposed at the bottom of Bridge Valley Road, and mainly consists of red sandstones and mudstones.

All these rocks in the Gorge and on the Downs were subjected to enormous forces during a major period of earth movements around the end of the Carboniferous period, about 300 million years ago. They were compressed and became part of a massive arched fold (the Westbury-on-Trym Anticline), which extended from Clifton northwards to Kingsweston and Henbury. In addition a number of faults were formed and the largest of these, the Great Fault, can be seen at the bottom of Bridge Valley Road. Here Clifton Down Limestone is pushed up over Cromhall Sandstone with the result that the upper part of the rock sequence in the Gorge (and on the Downs) is repeated to the south. The movement of this major fault is calculated as 335 m (1,100 ft), and it has had the effect of extending the length of the Downs (and the Gorge) by about a third. These earth movements also created sets of joints in the rocks and some of these have since been filled with younger rocks or mineral veins.

The fold must have also included a thick sequence of younger Carboniferous rocks which once covered the Limestone on the Downs. These Coal Measure rocks are found beneath Ashton Vale, where coal seams are found in mudstones and sandstones, and must have been deposited in the equivalent of the tropical rain forest of 315 million years ago. The local Coal Measures are some 600 m (2,000 ft) thick, but much more was probably once present and when they lay on top of the Limestone in the fold, the Downs area must have been on the southern slope of a mountain which could have been over 3,000 m (10,000 ft) high at Westbury on Trym. There followed nearly 100 million years of erosion which almost completely removed the mountain, basically leaving the Downs and Kingsweston Hill as the highest remaining stumps.

Much of the south of the Downs is covered by Dolomitic Conglomerate, a rock composed of pebbles and boulders of limestone in a matrix of sandstone and mudstone. This was created from the eroded remains of the Carboniferous Limestone. The best exposure of this rock is in a cutting on Bridge Valley Road, where it can be seen to lie in horizontal layers. This appears to represent the infill of a fossil valley cut into the relatively softer Cromhall Sandstone. The rock was formed between 250 and 200 million years ago, in the Triassic period of geological time, when the Downs are thought to have been an area of high ground in an arid desert. 

There are some areas of younger Triassic rocks on the Downs. Westbury Beds are found around Clay Pit Road, where they were quarried in the 1780's (Savage, 1999), and also near the centre of Durdham Down. These black coloured mudstones contains fossils which indicate a marine origin, and provide evidence that at least some of the eroded surface of the desert landscape was covered by the sea about 200 million years ago.

The Downs Island

The fossil remains of the “Bristol Dinosaur” Thecodontosaurus antiquus were found in a fissure in a limestone quarry on the edge of the Downs, near the top of Blackboy Hill, in 1834. A quarryman took one or two fragments of fossil bone to the Bristol Institution (which later became Bristol City Museum) where they were examined by the curator and by a number of experts. Men were employed to find more fossils and a large collection was made. This dinosaur was only the fourth named in England, and perhaps the world (Benton, 2012). Sadly some of the fossils were destroyed by bomb damage in the Second World War but 184 specimens are still safely stored in the Geology Department of the City Museum and Art Gallery. It was generally thought that the bones were preserved in Dolomitic Conglomerate which had formed in the fissure, but recent research has found evidence that the fissure fill is equivalent in age to the Westbury Beds. This is based on the presence of some fossil shark teeth and the similarity of many of the fossils to those found in South Gloucestershire where evidence of Westbury BedsS was established. The same research revealed fossil evidence for a number of other dinosaurs and terrestrial reptiles which must have lived alongside Thecodontosaurus on the Downs Island of the time (Foffa, 2014).

Near the Bristol entrance to the Suspension Bridge there is a small deposit of more Triassic, and possibly also Jurassic rock, which appears to have been deposited in a cave or fissure in the Carboniferous Limestone. Tawney (1875, p. 164) described this deposit as: 

a wide fissure (or pocket) in which blocks of Cotham-marble are found imbedded. Lias Septaria too were dug out of it in making the road to the bridge and the excavations for the bridge chains. The bulk of the infilling material seems to be greenish marl, with a little red marl such as occurs in the Rhaetics.

Cotham Marble is found in the Cotham Beds, which are of Triassic age, and younger than the Westbury Beds. The Marble is a well known local limestone formed by fossil algae, suggesting inter-tidal conditions. The Cotham Beds have not been recorded elsewhere on the Downs. The Lias mentioned by Tawney is the lowest division of the local Jurassic rocks. These occurences imply that the site was on the shoreline of an island, sited where the Downs are today, which existed from the time when the Cotham Beds were deposited through to the early Jurassic period. 

Charles Moore (1881) later wrote about the same site:

Close to the toll house, on the Clifton side, there is a deposit of considerable interest, having a face of about forty feet, in which the Rhaetic bone-bed and its asscociated remains are present. It is partly composed of irony and yellow sandy looking marl, with many free crystals of carbonate of lime .. and there are patches of finely lamintated rock, similar to the Rhaetic “White Lias”.... The bone-bed is two inches thick, with teeth of Saurichthys apicalisLophodus minimus, and many fish scales, and the clay on either side contains fish-remains of the same age.

The Bone Bed with its various fossil fish remains is found in the Westbury Beds, which suggests that the coast existed from before the Cotham Beds were formed. 

It is thought that the Island was completely drowned by the sea later in the Jurassic period, since pieces of Lias limestone with fossil Gryphaea and Spiriferina have been found on the Downs (Donovan and Kellaway, 1984, p. 20). These are presumably derived from Jurassic deposits hidden beneath the soil.

Durdham Down Bone Cave
The youngest deposits on the Downs were found in the famous Durdham Down Bone Cave. According to Latimer (1887, pp 265-266):

An interesting geological discovery was made in November 1842, in one of the quarries which then worked in the middle of Durdham Down, the workmen having found an opening into a cavern containing a quantity of the remains of animals for ages extinct in this country. The cavity though narrow, was of some extent, being traceable to a depth of ninety feet. The bones had belonged to about twelve hyenas, a bear, two rhinoceros, several hippopotami, numerous examples of wild bulls, about five deer, and five or six elephants, besides the relics of animals of later date. The bones were nearly all fractured into small pieces, and the proportion of teeth and horns to other parts of the body greatly preponderated. Taking this fact into consideration, together with the marks of gnawing on the bones, and the certainty that the cave could not have accommodated more than a small fraction of the animals represented by the vestiges, scientific observers concluded that the den had been the retreat of hyenas, which had carried to it portions of their prey.

The fauna mainly represents animals which lived in the last warm phase of the Ice Ages (Ipswichian interglacial, between 128,000 and 116,000 years ago). Much of the material was acquired by the Bristol Institution, which later became Bristol Museum, and some was lost in the Blitz of 1940. Despite reports that this entire collection had been lost from the Museum as a result of the event, on the contrary, there is a significant collection present today, including:

Spotted Hyaena Crocuta crocuta (including a specimen on display)
Cave Bear Ursus spelaeus
Brown Bear Ursus arctos
Small-nosed Rhinoceros Rhinoceros leptorhinus
Hippopotamus Hippopotamus amphibius
Cattle’ or Bison  Bos sp or Bison sp
Deer Cervus
Straight-Tusked Elephant  Palaeoloxodon antiquus (including a specimen on display)
Red Fox Vulpes vulpes
Grey Wolf Canis lupus

A model of the cave is also preserved in the Geology Department of the City Museum and Art Gallery.

Natural Landscape
The Downs plateau is a remarkable feature which extends across the area at a hieght of around 100m (330 ft) above sea level, and can be seen to continue across the other side of the Gorge. Most geologists have concluded that it is an ancient surface formed following the very long period of erosion after the late Carboniferous earth movements. They consider that it was planed off at the end of the Triassic and beginning of the Jurassic periods, as the sea advanced across the area. There is certainly evidence for marine sediments of this age on and around the edge of the Downs. The area is then thought to have been covered with a very thick sequence of younger Jurassic and possibly Cretaceous rocks, which would have completely buried the erosion surface. These would have been affected by earth movements in the Tertiary period, with the result that they dip at a low angle towards the south east. Subsequent erosion has removed most of these rocks and exhumed the buried Triassic landscape. 

It is likely that this long period of erosion had some effect on the Downs plateau, and also possible that the postulated presence of an ice sheet during the Ice Ages made further changes. The erosion of the Gully and the New Zigzag valleys must have taken place when the Gorge was created. Given that these are now dry valleys, and that any rainwater that falls on the Downs sinks down through the limestone, it probably required the ground to be frozen to allow water to run across the land surface to erode the features. This could have happened during any of the cold phases of the Ice Ages, but the present shape of the valleys was no doubt completed in the last, the Devensian, between 116,000 and 11,000 years ago. 

There are a number of caves in the area but most are found in the sides of the Gorge. However, the Observatory Hill Cave entrance is on Clifton Down, to the east of Observatory Hill. Its entrance has been blocked up, but it was reported to be 9 m (30 ft) long, 1.5 m (5 ft) high and 3 m (10 ft) wide. There is no known evidence of when it was formed. The Durdham Down Bone Cave was discovered in a quarry but must have once had an entrance on the Downs. There may well be more undiscovered caves and solution cavities on the Downs which were formed as rainwater found its way down through joints and other openings in the limestone. There are several small depressions on the Downs which could be sink holes, but which may turn out to be unrecorded mineral workings. One pit is thought to be a Second World War bomb crater.

It has been suggested that much of the surface of the Downs was formerly covered by limestone boulders and griked bedrock, and that this was mostly removed by lime burners or as ornamental stone for use in rockeries (Kellaway and Welch, 1993, p. 48). Clearly the natural landscape has been considerably altered by quarrying and mining. 

Quarrying




Quarry thought to be to the west of the junction of Ladies Mile and Stoke Road
Watercolour by William Arnee Frank, c.1862 
©Bristol Museums, Galleries and Archives



There have been many quarries which extracted Carboniferous Limestone on the Downs (see the map in Greenacre, The Downs History Trails No. 1). In 1754 it was reported that locals were permitted to take what stone they required from the Downs, and that much was burnt in kilns to make lime for mortar (Savage, 1999). Some was also used as building stone, with the Observatory, for example, almost entirely built of this rock. The only two quarries which have survived are around Observatory Hill. 

In addition to the many small quarries there were four much larger:

  1. Quarry north of Westbury Road and marked by the Seven Sisters pine trees
  2. Quarry to the north west of the junction of Stoke Road and Ladies Mile
  3. Chain Quarry, north of Belgrave Road
  4. Pembroke Road Quarry, north of Clifton Down

Quarry 1 was reported as being nearly 1.2 hectares (3 acres) in area with an average depth of 9 m (30 ft). Quarry 2 is thought to be the site of the Durdham Down Bone Cave, and had an area of nearly 1.6 hectares (4 acres) and also an average depth of 9 m (30 ft). This appears to be the quarry depicted in a watercolour by William Arnee Frank (1862), and, if so, seems to have been much deeper in part. The Clifton and Durdham Downs (Bristol) Act of 1861 established a number of duties, including the closure and infilling of existing quarries. One of the Downs Committee's first actions was to give notice to all quarry users to quit by October 1862. In 1866 the Docks Engineer proposed to the Downs Committee that these now disused quarries should be infilled with the material to be excavated when straightening the course of the River Avon and constructing a new lock at the entrance to Cumberland Basin. This was agreed and the Dock Spoil Tramway was built to carry the excavated material up onto the Downs.

By October 1871 Quarries 1 and 2 were filled and landfill began at the Chain Quarry. The clump of pines known as the Seven Sisters was planted at the site of Quarry 1 in 1872, and the tramway track was removed in 1873, after the completion of the new Cumberland Basin lock. Chain Quarry was finally filled by 1879. In 1890 Pembroke Road Quarry was identified as a landfill site for the material to be excavated during the construction of the Frome Culvert and this was completed in about 1907 (Nichols, 2005). The boundaries of some of these former quarries can be traced due to some settlement of the infill, and no doubt 9 m of river sediment and assorted rocks will have had a significant effect on the type of soil which has developed on these sites.

Minerals and Mining
There is an area of disturbed ground known as the Dumps beween Upper Belgrave Road and Ladies Mile. This is unlike any other feature on the Downs and its origin is unknown. It has been suggested that it was a former lead mining area, but the alignment of the workings are quite different from that of the known lead veins nearby. It has also been suggested that it was a former limestone quarry but it does not have the appearance of any other quarry on the Downs. Other suggestions are that the mineral celestite was worked here, or that the trenches were lead workings which were later enlarged by quarrying to supply limestone (Kellaway and Welch 1993, p. 50).

Several minerals have been found on the Downs. Galena was worked as lead ore, and several veins run north westwards from opposite the Zoo. The remains of some of the “grooves” dug by lead miners as they followed the lines of the mineral veins are still visible.The mineral has been recorded at several other sites on the Downs, including in old workings north and south of the White Tree, in the highly mineralised belt at the northern end of Durdham Down (Kellaway and Welch, 1993). The Romans are commonly thought to have worked lead and there is a reference that the mineral was dug on the Downs in the Anglo-Saxon Charter of 883. The first clearly documented record is for 1611 when the Lord of the Manor of Henbury granted a licence to dig for lead on Durdham Down. In 1712 another lease was granted to mine lead ore as well as iron, manganese and calamine on the Downs (Micklewright and Frost, 1988).

Iron ore was extracted in Clifton, but there is no definite evidence that it was worked on the Downs. In 1872 an iron mine was opened below Royal York Crescent in Clifton and it produced 3,000 tonnes (3,800 tons) of ore in that year. Two beds of hematite, goethite and limonite were worked in red sandstone and shale (Savage, 1999). 

The mineral Calamine (now called Smithsonite) was said to be the “most important mineral in point of frequency and value that the limestone yields. “ (Bright, 1817, p.200). It was a valuable source of zinc which was amalgamated with copper to form brass in local mills. The mineral was found in veins cutting the limestone, together with calcite, barite and galena. Bright records that: “The calamine has hitherto been worked in a very imperfect manner: the vein is broken into, when it meets the surface; a rough windlass is placed over the hole, and a bucket is attached to a few fathoms of rope; two or three men work at the vein as long as the ore is found in abundance, or until the water impedes their progress. The mine is then deserted, but the heaps of rubbish at the mouth of the pit are often so rich in ore that considerable sums are paid for the privilege of washing them.”

Quartz geodes were once extracted from the rocks around Observatory Hill and sold to visitors as “Bristol Diamonds”. Also more prosaically known as Potato Stones, these nodules of quartz are thought to have replaced the mineral anhydrite. The most prized form was a hollow geode with quartz crystals growing in towards the centre. These were very popular with people who came to visit the Hotwells Spa, and could be purchased from shops in the Colonnade. References to Bristol Diamonds go back to 1540 when Camden wrote: in hills about Bristow be found little stones of divers colours counterfeiting precious stones. Probably the largest collection of Bristol Diamonds is in the grotto in the garden of Goldney House in Clifton, which was built in the 1740's. The geodes were found in both the Dolomitic Conglomerate and in iron ore veins cutting through the Carboniferous Limestone (Savage, 1999).

Lead veins found on Clifton Down beneath the Triassic Westbury Beds consist of galena, sphalerite and marcasite, with barite and calcite. Elsewhere the veins in the Carboniferous Limestone show two generations of mineralisation. The first consists of hematite and quartz, followed by galena, barite and calcite, which may be deposited in a central infilling of the vein (Kellaway and Welch, 1993, p. 143). 

Celestite has been recorded found in the local Triassic rocks, but there is no evidence that it was worked on the Downs. However on the other side of the Gorge, in Abbots Leigh, there were extensive workings for the mineral in the late nineteenth and early twentieth centuries. These were considered to be among the richest worked in the Bristol district, with some boulder sized masses weighing up to half a ton (Kellaway and Welch, 1993, p.134).

Conclusion
The 1861 Downs Act certainly preserved a green open space for the citizens of Bristol, but it also unfortunately led to the infilling of most of the quarries which once provided information about the geology of the area. Sadly the Bone Cave is completely buried but at least many of the fossils excavated from the site have survived in the City Museum and Art Gallery. The two remaining quarries on Observatory Hill very clearly display the dip of the rocks and this is much appreciated by the younger generation as a slide. The road cutting leading to the Suspension Bridge exposes a splendid array of fossils and mineral veins, as well as a rock which probably once filled a cave on the coast of the Downs Island of just over 200 million years ago. Hopefully these special places will be conserved for future generations to appreciate.

Andrew Mathieson

References
Benton, M.J., 2012, Proceedings of the Geologists' Association, 123, pp. 766-778
Bradshaw, R. and Frey, A.E., 1987, Proceedings of the Bristol Naturalists' Society, 47, pp. 45-64
Bright, R., 1817, Transactions of the Geological Society, pp. 193-205
Donovan, D.T. and Kellaway, G.A., 1984, Geology of the Bristol District: the Lower Jurassic Rocks, British Geological Survey, 69 pp.
Foffa, D., 2014, Proceedings of the Geologists' Association. (in press)
Greenacre, F.,The Downs History Trails No. 1, Durdham Down
Hawkins, A.B., 1987, Proceedings of the Bristol Naturalists' Society, 47, pp. 65-78
Kellaway, G.A. and Welch F.B.A., 1955, The Bulletin of the Geological Survey of Great Britain, 9, pp. 1-21
Kellaway, G.A. and Welch F.B.A, 1993, The Geology of the Bristol District, British Geological Survey, 200 pp.
Latimer, J., 1887, The Annals of Bristol in the Nineteenth Century, W and F Morgan, Bristol, 552 pp.
Micklewright, S.D. and Frost, L.C., 1988, University of Bristol Avon Gorge Project Report No. 10
Moore, C. 1881,  Quarterly Journal of the Geological Society of London, 37, pp. 67-82
Nichols, G., 2005, To Keep Open and Unenclosed: The Management of Durdham Down Since 1861, Bristol Branch of the Historical Association, Pamphlet No. 116, 40 pp. 
Savage, R.J.G.,1999, Proceedings of the Bristol Naturalists' Society, 59, pp. 65-76
Tawney, E.B., 1875, Proceedings of the Bristol Naturalists' Society, pp. 162-166
Vaughan, A., 1905, Quarterly Journal of the Geological Society of London, 61, pp. 181-307