"Mark Godden's Little Bit Of Cyberspace Mk.II"

The City of London - A monument to the Portland stone industry and its artisans!

Towards the end of the Jurassic Period, around 140 Ma (million years ago), because of continental drift, the land that makes up much of what is now the south of England, was located far south of its present global position. Situated at about 35^o north of the equator, what is now Portland, was positioned at a palaeolatitude similar to that of modern Florida or Israel, where the "Mediterranean" type climate was likely characterized by warm wet winters and hot dry summers. 

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 all 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.

Portland Stone is one of the youngest Jurassic rocks, laid down just before the end of the period. Modern oolitic limestones, similar to Portland stone, are forming today in warm, lime-rich waters, such as those found in for example, the coastal margin of the Persian Gulf and in the Atlantic, on the Bahamas' Banks.

During the Alpine Orogeny (when the Alpine mountains were pushed up as Africa tectonically "crashed" into Southern Europe), related minor tectonic disturbances effected much of the south of England, This compressed the already folded rocks of South Dorset, resulting in an east-west trending fold structure called the "Weymouth Anticline". Portland, connected to the mainland by chesil beach (forming a tombolo) is what remains of the Weymouth Anticline's southern limb, where bedding dips at about 1.5 ^o  to the south east. The Portland beds which once would have been domed over what is now Weymouth have long since been denuded and only the older middle Jurassic, (generally softer) rocks at the centre of the anticline remain. Were it not for the protective barrier of Chesil Beach, Weymouth and its hinterland would probably have long been eroded away and the whole area would now be under water. There is a narrow, linear exposure of Portland Stone, (running roughly east-west) on the high ridge to the north of Weymouth; it crops out (dipping steeply north) on the northern limb of the Weymouth anticline forming part of the escarpment known locally as the Ridgeway. Portland stone from the Ridgeway tends to be more fractured (compared to that quarried on Portland) because of the more intense folding that occurred here. Despite geological problems, Portland stone was once quarried from the Ridgeway, particularly at Upwey and Portesham.

General view of Bowers Quarry - [click image for a larger annotated version].

Before the quarrying of Portland stone can proceed, the unwanted material, which overlies it, (the overburden) must be removed. Most of the top of Portland is covered with (upper Jurassic) lower Purbeck beds (the Lulworth formation) of quite variable thickness, ranging from about 1m to 10m or more, across the Island. Where the overburden is deepest; the top 4-5m (top rubble) is usually fairly unconsolidated, comprising thin beds of limestone, clay and marl. Stromatolites are common amongst the thin limestone or slatt beds which occur here. These thin limestone beds often contain some very fine sedimentary structures, including salt pseudomorphs, bioturbation, ripple marks, rain prints and desiccation cracks, which all point to a seashore type environment, with high rates of sea water evaporation. Similar hyper-saline conditions exist today in the Middle Eastern sabkhas. The largest of these slatt beds (The thick slatt) is typically 0.5m deep, and is commercially known as "Portland blue". some slabs of thick slatt quarried from the south of the Island have been found to contain dinosaur footprints, these beds also commonly contain some small fossils including gastropods, bivalves' ostracods and rarely, the occasional disarticulated vertebrate bone! The thick slatt's blue colour is a result of staining by naturally occurring iron sulphide which must have been abundant in the depositional environment. A distinctive white or gray margin is often visible at the edges of slabs of quarried thick slatt, this is simply where ground water moving through cracks in the bed, has leached the blue stain from the edges of the rock.

The lower 5-6m of overburden (the bottom rubble) consists of more massive beds of deformed evaporitic limestone (aish tier and soft burr) with thin basal beds of fossiliferous soil. The chalky "aish" was traditionally used by Portland's housewives to whiten hearths and doorsteps. The noun "aish" is thought to be a corruption of ash, a colour term. Some writers have suggested an association with the noun "ashlar" (squared blocks of building stone). Gypsum diapers are sometimes found at these horizons, where buried salts, subject to extreme pressure were forced upwards in a plume disrupting the overlying beds. The "soft burr" found here was traditionally used on Portland to build chimneys and fire places as it has the interesting property of resisting the detrimental effects of heat. It is in the basal dirt beds that many silicified tree trunks are found, some still in life position. These lignitic soil or "dirt beds" represent ancient forest floors into which the trees once spread their roots. Interestingly fossilized charcoal (or fusain) is sometimes found in the dirt beds, indicating that the cupressus-type conifers that once grew here, were affected by occasional forest fires, started by lightning strikes.

Cap is the name given to the bottom bed of overburden, cap often contains trace fossils of trees, and the frequently encountered horizontal "chaff holes" are what remain of now largely decayed tree branches, although careful examination of the holes, occasionally reveals some traces of petrified wood. The ancient forest in which these trees once grew must have been flooded quickly. Much of the calcium carbonate in the cap is probably stromatolitic in origin, produced by the action of blue-green bacteria, living on and around the newly submerged trees. Ostracod fossils are common within the cap. The sequence of beds here tell a story of cyclic flooding and re-emergence of the land, each cycle probably having a period of many hundreds of thousands of years.

Cap beds are typically 1.5-3m high and very hard. The noun "cap" is widely used in mining to describe the rock immediately above a coal or ore seam, on Portland, cap beds are those that immediately overlie the freestone beds.  Cap has very limited potential as a masonry stone, but has been used with great success as reinforcing material on sea-defence projects, it may also be crushed to produce a good quality aggregate for concrete or road building. The lower overburden beds including the cap are usually loosened by blasting with high explosives before they can be removed with mechanical excavators and dump trucks.

At the top of the Portland freestone series is the roach (1m) this is an oolitic limestone full of shell casts such as the Aptyxiella portlandica or "Portland screw" and Laevitrigonia gibbosa or "horse's head". The noun "roach" is possibly derived from the old French "roke" or "roche" in English usage since the middle of the 13th Century, meaning a mass or rock, a cliff or boulder.  Roach has been used extensively in the Portland Breakwater and the Cobb at Lime Regis. When the voids in the stone are filled and the surface polished roach makes a superb decorative stone. Roach is available in very large block sizes.

Below the roach is the whitbed (up to 2m). The noun "whitbed" is almost certainly a corruption of "whitebed". Whitbed is a fine grained oolitic limestone containing a proportion of comminuted shell fragments typically 5 mm across. This is excellent freestone suitable for all external work. Less shelly or "cleaner" whitbed (often occurring at the lower half of the whitbed face) is able to be carved with intricate details. Shelly whitbed while not usually suitable for carving is an excellent for architectural use in ashlar and weathering courses.  Whitbeds in general are highly durable and tests upon the stone give a probable weathering rate of 1-2 mm per 100 years. Very large blocks of whitbed are available up to 8 m³. Fossils occurring here include the large Ammonite Titanites giganteus, oysters, echinoids and occasionally, vertebrate bones.

Beneath the whitbed is the curf (1m). Sometimes called the "little roach", curf is really a series of sandy chert beds and Shelly limestones. Curf from certain areas may weather rapidly (particularly when used externally in exposed locations) and it is not therefore, always suitable for use as masonry stone. The word "curf" or "kerf" is an old verb used in some areas to describe the undercutting of a coal seam it is also a name used in Dorset and Hampshire, to describe or the notch made with an axe or saw when felling a tree. This suggests that in the past, overlying whitbed was removed by making an undercut in the curf.

At the bottom of the freestone series is the basebed (up to 2m). Basebed is often considered the finest quality Portland stone available. Typically basebed has a very homogenous texture with a negligible shell content making it eminently suitable for carving. Basebed can be cut and carved in any direction and as such is a true free stone. It is not quite as durable in exposed locations as whitbed but makes an unbeatable monumental and carving stone used on very many prestigious building projects. Probable rate of weathering is 3-4 mm per 100 years. The average density of all the freestone beds is around 2.4 tonnes/m³. Typically they all consist of >95% CaCO₃ with small proportions of  silica, iron (as Fe₂O₃), magnesium oxide and alumina also present. Identifiable fossils are relatively rare in the basebed, occasional ammonites and the odd piece of driftwood can sometimes be found.

Hard black cherts commonly occur in bands at the junctions between the roach and cap, the whitbed and basebed and also at the base of the basebed (the top of the cherty series). Chert is composed of silicon dioxide, SiO₂ (silica) that is believed to have been biological in origin (biogenic), probably coming from the skeletal remains of diatom like organisms and siliceous sponge needles or spicules. It is believed that chert formed diagenetically, by a complex chemical process that took place in buried voids, after the Portland stone beds were laid down. Cherts were collected (probably from around the coastline) and heavily exploited for tool manufacture by people living in prehistoric times. Interestingly, some archaeologists believe that chert tools originating from Portland were widely traded across the whole of Southern England.

Below the freestone is the cherty series (30m), these are thin interspersed limestone and chert beds. Although geologically classified as part of the Portland stone sequence, the cherty series is of no use to a quarrier trying to extract dimensional stone. The cherty series has however been quarried and crushed to produce a low-grade aggregate. There is a superb exposure of the cherty series (including "ladder cherts") in Admiralty Quarry.

 
The cherty series overlies the Portland sand (35m), which is well exposed at the base of West Cliff at Blacknor. Under the Portland Sand is Kimmeridge clay, which has a total recorded thickness of over 500m. Only a small exposure of Kimmeridge clay is evident at the northern end of the island, The majority of it being at or below sea level.

The quarrying of Portland stone is greatly influenced by the extensive jointing found within the freestone series. Parallel master joints (or gullies) occur at regular intervals of around 25m. Gullies run approximately NNE. The area between the gullies is further divided by two sets of smaller joints running roughly at right angles to one another. "Southers" run roughly North/ South and  "East n' Westers" run WNW. This enables fairly cubic blocks of stone to be extracted with a minimum of cutting by quarrymen.

Plugs & Feathers - Traditionally used to cut a quarry blocks

After the overburden has been stripped and the Portland freestone beds have been exposed, quarrymen can start to establish the local jointing pattern in the area of the quarry, which is to be worked. Traditionally, small diameter holes (35 mm) were drilled horizontally, under each rock to be removed; the holes were usually drilled either in or parallel to a bedding plane. Once drilled, the holes 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. This operation exploited the natural weakness presented by the presence of bedding planes and vertical joints.  

In 1999, Italian stone cutting equipment, originally designed for use in Tuscany's marble quarries was imported by Albion Stone and applied to Portland stone. This new technology is now used to quarry all dimension stone produced by the company, thus completely eliminating the need for any blasting. Full account of the local jointing pattern is made when deciding the position and orientation of cuts. The elimination of all blasting has significantly improved the quarries' environmental performance and removed the potential for any possible damage to the stone being quarried through shock. 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.

Once removed from the quarry face, large rocks are cut to produce smaller square stones ready for use by masons. 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). 

Splitting stone, using pneumatic drills is arduous work and so  wire-saws  have been introduced in to Albion Stones' 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.

Bowers Mine - Portland's first ever underground mine - Opened October 2002

In October 2002 Albion Stone successfully initiated Portland's first ever underground mining operation in Bowers Quarry. This was a precursor to other mining operations, planned for environmentally sensitive areas elsewhere on Portland. It has been possible to translate many the skills learnt using modern stone cutting equipment in open quarries, to an underground mining situation. Mining allows the extraction of stone from beneath sensitive or otherwise inaccessible areas with a minimum of environmental disturbance at the surface, thus helping to ensure a harmonious future for the stone industry on Portland.

The latest "cutting edge" technology being used to extract dimension stone in Bowers Mine.

"Highwall Mining" - A technique now being used to recover otherwise inaccessible stone from terminal quarry faces.

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.

Sir Christopher Wren (1632-1723)- Appropriately carved in Portland stone (Independent basebed)

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.

At the beginning of the Nineteenth Century the output of Portland stone is believed to have typically been 25,000 tons per annum. One estimate suggests that there were 800 men and boys, 180 horses and 50 ships involved in Portland's stone trade at that time.

In the years following the Industrial Revolution, the acid rain, resulting from the heavy burning of coal in cities had the effect of continuously (slightly) dissolving the surface of Portland stone ashlar on buildings. This had the interesting effect of keeping exposed and rain-washed surfaces white as opposed to other (non calcareous) stones which quickly discoloured to black in the smoky atmospheres. This self-cleaning property also helped to enhance the popularity of Portland stone in London.

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.

During the 1960's and 70's there was a change in popular architectural style with many buildings being constructed using synthetic materials such as concrete and glass. The knock-on effect of this caused Portland's quarrying industry to contract to the point where only a handful of men were employed quarrying Portland stone.

Fortunately, recent years have seen a strong resurgence in popularity of more classic building styles, leading to an increased use of traditional, natural building materials which give the buildings constructed with them, a timeless elegance and style which quite simply can not be matched by concrete and glass.

Old and New - London's St Paul's Cathedral viewed from the new Paternoster Square Development, both of Portland Stone!

© 2007 Mark Godden BSc. FIQ.

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