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Portland stone
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Portland stone is a limestone from the Tithonian stage of the Jurassic period quarried on the Isle of Portland, Dorset. The quarries consist of beds of white-grey limestone separated by chert beds. It has been used extensively as a building stone throughout the British Isles, notably in major public buildings in London such as St Paul's Cathedral and Buckingham Palace.
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Encyclopedia
Portland stone is a limestone from the Tithonian stage of the Jurassic period quarried on the Isle of Portland, Dorset. The quarries consist of beds of white-grey limestone separated by chert beds. It has been used extensively as a building stone throughout the British Isles, notably in major public buildings in London such as St Paul's Cathedral and Buckingham Palace. It is also exported to many countries - Portland stone is used in the United Nations headquarters building in New York City, for example.
The term "Portland Cement" was coined by Joseph Aspdin who produced first in 1824 a hydraulic binder by burning a mixture of limestone and clay resembling the Roman cement and presenting a texture very close to that of the oolitic Portland stone.
Geological Information
Portland stone formed in a marine environment, on the floor of a shallow, warm, sub-tropical sea probably not too far from land (as evidenced by fossilized driftwood, which is not uncommon). As seawater was warmed by the Sun, its capacity to hold dissolved gas was reduced, consequentially, dissolved carbon dioxide (CO2) was released into the atmosphere (as a gas). Calcium and bicarbonate ions within the water were then able to combine, to form calcium carbonate (CaCO3) as a precipitate. The way that in hard water areas, lime scale builds up in a kettle, might be considered to be a loose analogy of the process. Calcium carbonate is the principle constituent of most limestones. Billions of minute crystals of precipitated calcium carbonate (called Calcite) accumulated forming lime mud (called micrite) which covered the sea floor. Small particles of sand or organic detritus, such as shell fragments, formed a nucleus, which became coated with layers of calcite as they were rolled around in the muddy micrite. The calcite gradually accumulated (by accretion) around the fragments of shell in concentric layers, forming small balls (of less than 0.5mm diameter). This process could possibly be likened to the way in which a snowball grows in size; the more it is rolled around in the snow. Over time, countless billions of these balls or, more correctly; "ooids" or "ooliths" became partially cemented together (or lithified) by more calcite, to form the oolitic limestone we now call Portland stone. Fortunately, the degree of cementation in Portland Stone is such that the stone is sufficiently well cemented to allow it to resist weathering but not so well cemented that it can't be readily worked (cut and carved) by masons, this is one of the reasons why Portland stone is so favoured as a monumental and architectural stone. Dr Ian West of the School of Ocean and Earth Sciences at Southampton University completed a detailed geological survey of Withies Croft Quarry before the Portland Beds were quarried by Albion Stone plc.
Quarries & Mines
Jordans is part of the Inmosthay Quarry in the centre of the Island, which also includes Fancy Beach. The quarry has been worked since the late 1800s. Albion Stone leases the southern section from The Crown Estate and purchased the northern part of the site in 2006. The majority of the southern reserves lie under the grounds of the local cricket club. To avoid disturbing the site at surface level, the company has applied and received permission to extract the stone using mining rather than quarrying techniques. The reserves to the north will be quarried using the diamond bladed cutting machines, hydra bags and the wire saws to shape the blocks. This process avoids the use of potentially damaging, dusty and noisy blasting as the primary extraction method, thereby protecting the surrounding environment, which has been designated as a ‘Site of Special Scientific Interest’ (SSSI).
Once quarries have been worked they are then restored. The was formed in 1983. The Trust is dedicated to preserving a knowledge and understanding of stone and the landscape from which it comes.
The quarry at Bowers has been operational since the late 1700’s. It has been leased from The Crown Estate since 1979 and in 2002 it became the site of the first Portland Stone mine. Extraction from this site is now completely underground with the original Bowers Mine in the extreme southern end of the quarry and the High Wall Extraction on the eastern and south east boundaries. High Wall Extraction is a series of small mines that extract otherwise wasted stone that sits between the final faces of the quarry and the actual boundary of the site.
Traditional quarrying methods
Blasting to remove the stone from the Quarry Face:
Small diameter holes (35 mm) were drilled horizontally, under each rock to be removed and were charged with a small quantity of black powder (gunpowder), chosen because of its relative non-shattering properties. When fired the black powder produced a "heave" which dislodged the rock from its natural bed, hopefully undamaged.
Plugs and Feathers to split the stone into blocks:
Stone within the quarry was traditionally cut using plugs and feathers, where a series of short, small diameter (typically 30mm) holes are drilled in a line where a cut is to be made. One plug and two feathers are inserted into each hole. Each plug is hit in turn, with a sledgehammer, until the stone yields to the extreme tensile stresses produced. Most stone is many times weaker in tension than in compression, plugs and feathers utilize this fact. It is also worth noting that stone tends to split much more easily parallel to bedding planes (called graining) than perpendicular to them (called cutting).
New quarrying methods
Saws/Hydro-bags:
In 1999, Italian stone cutting equipment, originally designed for use in Tuscany's marble quarries was imported by Albion Stone and applied to the extraction of Portland stone. This new technology completely eliminates the need for any blasting and has significantly improved the quarries' environmental performance and removed the potential for any possible damage to the stone being quarried through shock. Full account of the local jointing pattern is made when deciding the position and orientation of cuts. Once the quarry faces have been cut, the stone is gently displaced hydraulically. This is done using "hydro-bags", which are thin, flat, steel bags or envelopes that when inflated with water under moderate pressure, are capable of producing the forces necessary to loosen the stone to the point where it can be easily removed using large wheeled loaders.
Splitting stone, using pneumatic drills is arduous work and so wire-saws have been introduced in to the quarries, replacing much of the plug and feather cutting. Additional benefits of wire-sawing are an increase in the quantity of stone produced, squarer blocks and improved quality control, as it is much easier to assess the quality of a block if it has sawn faces.
Mining:
Mining in Portland is done by using a ‘room-and-pillar’ method. The mine will be advanced by extracting the stone using an abrasive tool chain cutter mounted on a machine. Slots are cut into the top, bottom, sides and middle of the stone. A flat steel pillow is inserted into the middle cut and slowly inflated with water. The stones are gently broken off at the back without placing any stress on the resulting blocks. The method of extraction is significantly more expensive than blasting, but results in a higher yield thereby saving valuable reserves for future generations. The environmental benefits are substantial as mining significantly reduces the impact on the local community with reduced noise and dust, not to mention the wildlife that doesn’t get disturbed.
Portland Stone Buildings
Portland's freestone has almost certainly been used as a building material since Roman times. The many well crafted Roman sarcophagi (stone coffins and matching lids, hewn from single large blocks of Portland stone) that have been unearthed locally over the years, testify to the skill of their makers. It is interesting to speculate where the many large blocks of stone needed to make the sarcophagi were obtained and how they were transported. It is possible that stones came from coastal exposures, where they may have been dislodged by the action of the sea, nevertheless the skills necessary to select suitable blocks, retrieve them, shape and hollow them are considerable and hint at an industry of some maturity. Were the sarcophagi "made to order" following someone's death? Considering the amount of work and more critically, length of time involved, this would seem unlikely because of the practical need to carry out a burial promptly after death. Is it possible then, that during Roman times, there was a stone industry on Portland producing "off the shelf" sarcophagi?
The earliest known building to be constructed using Portland stone is Rufus Castle at Church Ope Cove, Portland. The original structure was probably built in around 1080, rebuilt in around 1259 and rebuilt yet again in about 1450 which is the likely date of the walls we see today.
The first known Portland stone quarries were situated on the north eastern coast of the Island, close to Rufus Castle, where huge landslips made the stone more easily accessible and the proximity of the sea, allowed the quarried stone blocks to be moved over relatively large distances by barge.
Portland stone was used to build the Palace of Westminster in 1347, the Tower of London in 1349 and the first stone London Bridge in 1350. Exeter Cathedral and Christchurch Priory, also constructed during the 14th Century are built of Portland stone, its superb characteristics has ensured a popularity amongst masons and architects, that has endured ever since.
Inigo Jones (1573-1652) used Portland stone to build the Banqueting Hall in Whitehall in 1620. Sir Christopher Wren used nearly one million cubic feet to rebuild St. Paul's Cathedral and many other minor churches after the great fire of London in 1666. All of the stone used by Wren was transported by sailing barge from Portland to the centre of London via the Thames. Wren's widespread use of Portland Stone, firmly established it as London's "local stone" and as one of the best loved British building stones.
Other famous London buildings constructed of Portland stone are The British Museum 1753, Somerset House 1792, General Post Office 1829, The Bank of England, Mansion House and the National Gallery.
Following the First World War (1914-1918), Sir Edwin Lutyens (1869-1944), used Portland stone (quarried from the bottom of Wakeham) to construct the Cenotaph in London's Whitehall. Erected in 1920, The Cenotaph commemorates the millions of people killed in this and subsequent conflicts, additionally most of the headstones used to mark the graves of British and Commonwealth war dead, are also of Portland stone.
After the Second World War (1939-1945) the bombed out centers of many towns and cities, such as Plymouth, Bristol, Coventry and London were reconstructed using vast facades of Portland Stone.
Portland Stone has also been used across the world in the UN building in New York, the National Gallery in Dublin and the Casino Kursaal in Belgium.
More recent projects include the new BBC Broadcasting House in London which won the ‘New Build (Modern Non Load Bearing Stone)’ Award in the 2006 Natural Stone Awards.
A major project in 2008 was the Armed Forces Memorial, which is a stunning piece of architecture being formed of a circular structure 43 meters in diameter formed by curved Portland Stone walls. At the 14th Natural Stone Awards ceremony at the Oval Cricket Ground in London the Armed Forces Memorial won the ‘Design & Technology Innovation’ Award. St Paul’s Cathedral South Churchyard Improvements won the ‘Landscaping’ Award. The objective for this work was to provide wheelchair access to the Cathedral, the resulting desire to re-enclose this section of the churchyard presented the opportunity for a more meaningful display of the medieval remains of the building that was there before the Great Fire of 1666.
Memorials
All gravestones for British personnel killed in the First and Second World Wars are made out of Portland stone. Portand stone was used for the £6 million memorial in Staffordshire, England, designed by Liam O'Connor Architects and Planning Consultants, that bears the names of over 16,000 service personnel who died since World War II.
Vertebrate fauna
Ornithischians
| Dinosaurs of the |
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| Taxa | Presence | Description | Images |
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Genus:
- Echinodon sp.
| Geographically located in Wiltshire, England. | |
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Family:
- Indeterminate remains.
| Geographically located in Wiltshire, England. | |
Order:
- Indeterminate remains.
| Geographically located in Wiltshire, England. | |
Suborder:
- Unnamed genus.
| Geographically located in Wiltshire, England. | | |
Saurischians
| Dinosaurs of the |
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| Taxa | Presence | Description | Images |
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Infraorder:
- Indeterminate remains.
- Indeterminate remains.
- Indeterminate remains.
- Indeterminate remains.
| Geographically located in Dorset, England. Geographically located in Oxfordshire, England. Geographically located in Buckinghamshire, England. Geographically located in Wiltshire, England. | "(=Ornithopsis sp.)" "(=Cetiosaurus longus, C. ?longus, C. sp.)" "(=Cetiosaurus longus)" "(=Camarasaurus sp., Diplodocus sp.)" |
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Suborder:
- Indeterminate remains.
| Geographically located in Dorset and Wiltshire, England. | "(=Megalosaurus sp.)" | |
See also
External links
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