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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.

Thursday, 15 January 2015

The Geology of Clifton and Durham 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

Tuesday, 12 August 2014

Badgers Wood Geological Walk





Part of the Badgers Wood Geological Path          
Photo credit Richard Kefford.
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Monday, 14 October 2013

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



Friday, 27 September 2013

Free public lecture on Stromboli Volcano - 17th October 6pm

Thursday 17 October at 6 pm
Reception Room, Wills Memorial Building, Queen's Road, BS8 1RJ

Free entry, booking not required



ABSTRACT:
Stromboli volcano (Italy) belongs to a class of volcanoes that explode frequently against a background of substantial continuous gas emissions. The explosions are spectacular and the continuous gas emissions have a significant effect on the Earth’s atmosphere. This lecture will consider the processes involved that allow these two modes of behaviour (explosions and gas emissions) to co-exist. In particular, I will show how we can combine results from laboratory flow experiments and computer models with field observations and petrological and textural data from rock samples to advance our understanding of this style of eruptive behaviour.

Wednesday, 10 July 2013

The Bristol 'tsunami': Flood or fallacy?

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

30th 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?

monwoodcut
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").
754586_a96910f6
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 17th 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.


Imbricated boulders
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."

hazards6
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.

Tuesday, 4 June 2013

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.



Thursday, 23 May 2013

Box Rock Circus - official opening




BOX ROCK CIRCUS OFFICIALLY OPENED BY PROFESSOR IAIN STEWART




Photo credits Charles Hiscock
Click on photo to see larger version
On a rather damp Tuesday May 14th 2013 a large number of people from the village of Box, near Corsham, Wiltshire, pupils from the local schools, members of the Bath Geological Society, and many other interested folk gathered at the Selwyn Hall recreation field for the official opening of the Box Rock Circus. The Circus, the brainchild of local geologist and Earth Science Educator Elizabeth Devon, which had been unofficially unveiled on its completion on the 9th August 2012, has since received an interpretation board entitled ‘Box Rock Circus - A magical circle of rocks, fossils and minerals’ and the fossil moulds inserted into a different position, making them more accessible to the smallest child.






            Amongst a colourful array of umbrellas, Elizabeth Devon and the Chairman of the Parish Council welcomed everyone to the event after which Professor Iain Stewart, Professor of Geoscience Communications at the University of Plymouth and well known television presenter officially opened the Box Rock Circus. He enthusiastically praised all those who had the foresight to plan and carry out the project and the referred to the ages and conditions of formation of the rock monoliths. He also suggested that other towns and villages should follow the example of Box. BBC Wiltshire Sound was present to record the event, interviewing the enthusiastic pupils of Box Primary School. Following the opening of the Circus by Professor Stewart, a buffet lunch had been prepared for invited guests in the Box Pavilion.




A full description of the rocks and specimens can be found in the Avon RIGS blog for 2012 - http://avonrigsoutcrop.blogspot.co.uk/2012/08/box-rock-circus.html when the Circus was unveiled following its completion. For more information go the website - www.boxrockcircus.org.uk



Charles Hiscock