| Portland Stone Formation | |
|---|---|
| Stratigraphic range:Tithonian | |
 Portland stonequarry on theIsle of Portland | |
| Type | Geological formation |
| Unit of | Portland Group |
| Sub-units | Dorset: Portland Chert Member, Portland Freestone Member Vale of Wardour: Tisbury Member, Wockley Member, Chilmark Member Vale of Pewsey: No formal subdivision |
| Underlies | Lulworth Formation |
| Overlies | Portland Sand Formation |
| Thickness | up to 38 metres (120 ft) in Dorset |
| Lithology | |
| Primary | Limestone |
| Other | Siltstone,Sandstone |
| Location | |
| Region | England |
| Country | United Kingdom |
| Type section | |
| Named for | Isle of Portland |
| Location | Clay Ope, West Weare Cliff |
| Thickness at type section | 25 metres |
Portland stone is alimestonegeological formation (formally named thePortland Stone Formation[1]) dating to theTithonian age of theLate Jurassic that isquarried on theIsle of Portland inDorset, England.[1] The quarries are cut in beds of white-grey limestone separated bychert beds. It has been used extensively as a building stone throughout theBritish Isles, notably in major public buildings in London such asSt Paul's Cathedral andBuckingham Palace. Portland stone is also exported to many countries, being used for example at theUnited Nations headquarters in New York City.
Portland stone formed in a marine environment, on the floor of a shallow, warm, sub-tropical sea probably near land (as evidenced by fossilised driftwood, which is not uncommon). When seawater is warmed by the sun, its capacity to hold dissolved gas is reduced; consequently, dissolvedcarbon dioxide (CO2) is released into the atmosphere as a gas.Calcium and bicarbonate ions within the water are then able to combine, to formcalcium carbonate (CaCO3) as a precipitate. The process of limescale build up in a kettle in hard-water areas is similar. Calcium carbonate is the principal constituent of most limestones. Billions of minute crystals of precipitated calcium carbonate (calledcalcite) accumulated forming lime mud (calledmicrite) 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.[2] Portland stone measures 3.5 on theMohs scale of mineral hardness.[3]
The calcite gradually accumulated (by accretion) around the fragments of shell in concentric layers, forming small balls (of less than 0.5 mm diameter). This process is similar to the way in which a snowball grows in size as it is rolled around in the snow. Over time, countless billions of these balls, known as "ooids" or "ooliths" (from the Greek for "egg-shaped" or "egg-stone"), became partially cemented together (or lithified) by more calcite, to form the oolitic limestone that is called Portland stone. 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 cannot 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. Geoff Townson conducted three years doctoral research on the Portlandian, being the first to describe the patch-reef facies and Dorset-wide sedimentation details. Ian West of the School of Ocean and Earth Sciences atSouthampton University completed a detailed geological survey of Withies Croft Quarry before the Portland Beds were quarried by Albion Stone plc.[2]
Stone has been quarried on Portland since Roman times and was being shipped to London in the 14th century.[4] Extraction as an industry began in the early 17th century, with shipments to London forInigo Jones'Banqueting House.Wren's choice of Portland for the new St Paul's Cathedral was a great boost for the quarries and established Portland as London's choice of building stone. The island was connected by railway to the rest of the country from 1865. Albion Stone PLC has been quarrying and mining Portland stone since 1984. Portland Stone Firms Ltd have been quarrying Portland stone since 1994.
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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 19th century. Albion Stone leases the southern section fromThe 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 thediamond bladed cutting machines, hydro bags andwire saws to shape the blocks. This process avoids the use of dusty and noisy blasting as the primary extraction method, thereby protecting the surrounding environment, which has been designated as aSite of Special Scientific Interest (SSSI). Albion Stone PLC now extract all their stone through mining which dramatically reduces the impact on the environment and local residents. Jordan's Mine is currently the biggest mine on Portland.
Bowers Quarry has been operational since the late 18th century. It has been leased from The Crown Estate since 1979, and in 2002 it became the site of the first Portland stone mine by Albion Stone PLC. 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.
Stonehills Mine is the first completely new mine on Portland (not a mine as an extension from an existing quarry). Albion Stone Plc began the process to open this mine in 2015 and reserves are estimated to last for 50 years (2066).
Independent Quarry and Admiralty Quarry are both owned by The Crown Estate and have been leased since 1982.[5] These quarries have been worked since the mid-19th century with the final dimension stones coming out of Independent in 2006, although some stocks of Portland stone block still remain.
Coombefield Quarry, located near Southwell has been open cast quarried over the last 80 years and is one of three privately owned quarries by Portland Stone Firms Ltd, the largest landholder on the island. The quarry is nearing the end of its life and will be regenerated as a holiday caravan park to boost local tourism on the island.
Perryfield Quarry is found towards the middle of the island and being actively open cast quarried. There are over 20 years of reserves left which is privately owned by Portland Stone Firms. Open cast quarrying provides quicker extraction of raw block dimension stone whilst maintaining its integrity. The majority of buildings in London today use Portland which has been quarried using the same methods over the last 60 years.
Broadcroft Quarry is located on the eastern side of the island and is a part of the open cast quarries used for St Paul's Cathedral. Privately owned by Portland Stone Firms Limited there are over 20 years of reserves left and still being actively quarried.
The coastal strip toward the southern tip of the island has permission to be worked by Portland Stone Firms and will provide in excess of 30 years of reserves. Permission has been granted for this to be open cast quarried.
Once quarries have been worked they are then restored. The Portland Sculpture and Quarry Trust was formed in 1983 and is dedicated to preserving a knowledge and understanding of stone and the landscape from which it comes.[5] The main non-working quarry remainsTout Quarry in which the trust is based, where a workshop is held every year. Tout Quarry has been donated to the Portland Sculpture and Quarry Trust by Portland Stone Firms Limited.
Traditionally small diameter holes (35 mm) were drilled horizontally under each rock and charged with a small quantity ofgunpowder, chosen because of its relative non-shattering properties. When fired the gunpowder produced a "heave" which dislodged the rock from its natural bed, hopefully undamaged. Stone was cut usingplugs and feathers, where a series of short, small diameter (typically 30 mm) holes are drilled in a line where a cut is to be made. One plug and two feathers were inserted into each hole and 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 utilise this fact. Stone tends to split much more easily parallel tobedding planes (called graining) than perpendicular to them (called cutting).
In 1999, Italian stone cutting equipment, originally designed for use inTuscany's marble quarries was imported by Albion Stone and applied to the extraction of Portland stone. This new technology eliminated the need for blasting, 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 wheeledloaders. Splitting stone, usingpneumatic drills is arduous work and so wire-saws have been introduced into the quarries, replacing much of the plug and feather cutting.
Mining in Portland is done by using aroom and pillar method. The mine is 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. This 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 wildlife and the local community with reduced noise and dust.
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Portland'sfreestone has almost certainly been used as a building material sinceRoman times. The many well crafted Romansarcophagi (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.
The earliest known building to be constructed using Portland stone isRufus Castle at Church Ope Cove, Portland. The original structure was probably built around 1080, rebuilt around 1259 and rebuilt yet again around 1450, which is the likely date of the walls seen today. The first known Portland stone quarries were situated on the northeastern coast of the Isle, 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 thePalace of Westminster in 1347, theTower of London in 1349 and the first stoneLondon Bridge in 1350.Exeter Cathedral andChristchurch Priory, also constructed during the 14th Century, are built of Portland stone. Its superb characteristics have ensured a popularity amongst masons and architects that has endured ever since. The East side ofBuckingham Palace, the official London residence ofKing Charles III, including the balcony, was faced with Portland stone, first in 1854 and again in 1913. TheVictoria Memorial (unveiled 1911) is also made of it.
Inigo Jones (1573–1652) used Portland stone to build the Banqueting Hall inWhitehall in 1620. SirChristopher Wren used nearly one million cubic feet to rebuildSt. Paul's Cathedral and many other minor churches after theGreat 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 sea and then up 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 areThe British Museum (1753) with the new WCEC extension inPortland Roach which was short-listed for theStirling Prize in 2017,Somerset House (1792), the General Post Office (1829), theBank of England, theMansion House and theNational Gallery.Tower Bridge is partly clad in Portland stone (along withCornish granite). Portland stone was used in 1923 to build the supporting pillar of theGrace Gates atLord's Cricket Ground.
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Recently, Portland stone has been used in the building ofChelsea Barracks, Wilkins Terrace at UCLUniversity College London, St James's MarketHaymarket, London andGreen Park tube station.
Portland stone is prevalent inManchester despite the historical preferred use of hard-wearing materials, such asBurmantofts andsandstone to resist the harsh industrial environment.[6] Portland stone was mostly used in Manchester during the 1930s. Manchester buildings with Portland stone exterior include100 King Street (1935),Arkwright House (1937),St. James Buildings (1912),Manchester Central Library (1934),Kendal Milne (1939) andSunlight House (1932).
Two of Liverpool'sThree Graces, theCunard Building and thePort of Liverpool Building, are clad in Portland stone which surrounds theirreinforced concrete frames.[7]
TheNottingham Council House, completed in 1929, is also built with Portland stone, as are the public buildings in Cardiff's civic centre. ArchitectCharles Holden significantly used the stone in his major commissions of the 1920s and 1930s, includingSenate House and55 Broadway, the home of theLondon Underground. The 1929 steel framed building (one of the first erected in the United Kingdom) of theLondon School of Hygiene & Tropical Medicine, based in Keppel Street, has a Portland stone façade to complete its stripped Classical design.[8] After the Second World War (1939–1945), the bombed out centres of many English towns and cities, such asPlymouth,Bristol,Coventry and London were reconstructed using vast facades of Portland stone.
Many of the buildings surrounding the flagshipParkinson Building at theUniversity of Leeds are clad in Portland stone, including theMichael Sadler Building, the Chemistry and Engineering buildings and the newLaidlaw Library.
Oxford typically uses oolitic limestone in its buildings, and theAshmolean Museum has been refurbished using a large amount of Portland stone.
Portland stone has also been used across the world. Examples include the UN building in New York, the Casino Kursaal in Belgium and theAuckland War Memorial Museum.[9] Most of the important civic and administrative buildings which survive from 18th and 19th centuryDublin, Ireland, known then as "the second city of the Empire", are of Portland stone, includingCity Hall (1779), theHouses of Parliament (1767),the Custom House (1791), theNational Gallery (1864) and theGeneral Post Office (1818). More recent projects include theBBCBroadcasting House in London, which won the "New Build (Modern Non-Load-bearing Stone) Award" in the 2006 Natural Stone Awards.
Portland stone has been designated by theInternational Union of Geological Sciences as aGlobal Heritage Stone Resource.[10]
Following the First World War, SirEdwin Lutyens used Portland stone (quarried from the bottom of Wakeham) to construct theCenotaph in London's Whitehall. Erected in 1920, The Cenotaph commemorates the millions of people killed in this and subsequent conflicts.TheRAF Bomber Command Memorial in London'sGreen Park commemorates the 55,573 crew members of the RAF's Bomber Command who were killed between 1939 and 1945.
Thegravestones forBritish personnel killed in the First and Second World Wars were made out of Portland stone.[11] TheCommonwealth War Graves Commission use Portland Limestone supplier Albion Stone's Portland Basebed.
Portlandstone was used for theArmed Forces Memorial inStaffordshire, England. Designed byLiam O'Connor Architects and Planning Consultants, it was completed in 2007 at a cost of over £6 million and bears the names of over 16,000 service personnel of the British Armed Forces killed since the Second World War.[12]
Portland stone is recognised to be of a high quality, but is somewhat expensive. In the case of the refurbishment of theBritish Museum's central court, the planned Portland stone was controversially substituted withAnstrude Roche Claire stone from France, another similar (but cheaper) oolitic limestone.[13]
The term, "Portland cement", was coined byJoseph Aspdin who in 1824 patented a hydraulicbinder created by burning a mixture of limestone andclay, resembling the previously existingRoman cement and presenting a material which he hoped looked like the famous Portland building stone.[14]
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