Portland Stone
At the end of the Jurassic Period, around 150 Ma (million years ago), due to continental drift, Britain was located South of its present position. Situated about 35° North of the equator, on a latitude similar to that of modern Florida or Israel, where the climate is much warmer.
Portland stone formed in a marine environment. on the floor of a warm subtropical sea. As seawater was warmed by the Sun, dissolved carbon dioxide CO2 escaped as a gas. Calcium and bicarbonate ions within the water were then able to combine to form calcium carbonate (CaCO3). Calcium carbonate is the principle constituent of all limestones. Billions of minute crystals of CaCO3 (called Calcite) accumulated forming lime mud (called micrite). Small particles of mainly organic detritus, such as shell fragments, became coated with calcite as they rolled around in the micrite. The calcite accumulated around the fragments of shell in concentric layers forming small balls (of less than 1mm diameter). Over time, millions of these balls or ooids became cemented together (or lithified) by more calcite, to form the oolitic limestone we now call Portland stone.
Modern oolitic limestone, similar to Portland stone, is forming today under sub-tropical conditions similar to those mentioned above on the Bahamas Banks.
During the period when the Alpine mountains were thrown up (around 27Ma), tectonic disturbances folded the Weymouth and Portland area into a large dome or anticline, Portland is what remains of it's southern limb, dipping 1.5° to the South East. The Portland beds over Weymouth have long since been denuded and only the older rocks at the centre of the anticline remain under what is now Wyke and Weymouth. There is a narrow exposure of Portland stone to the North of Weymouth; it crops out on the northern limb of the Weymouth anticline at the Ridgeway between Weymouth and Dorchester, dipping steeply North.
New Quarry showing layers of overburden
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 of quite variable thickness ranging from about 1m to 10m or more, across the island. The top 4-5m is usually fairly unconsolidated, comprising thin beds of limestone, clay and marl. The thin limestone or slat beds which occur here contain some very fine sedimentary structures including salt pseudomorphs, ripple marks and desiccation cracks, which all point to a seashore type environment, with high rates of sea water evaporation. Similar conditions exist today in the Middle Eastern sabkhas. The slat also contains many small fossils including gastropods, bivalves, ostracods and the occasional vertebrate bone!
Rock face at Tout showing tree hole (inset)
The lower 5-6m consists of more massive beds of limestone separated by thin beds of fossiliferous soil. It is here that many silicified tree trunks are found, some still in life position. The lignitic soil or "dirt beds" represent ancient forest floors into which the trees once spread their roots. Cap is the name given to the bottom bed of overburden. Cap often contains trace fossils of trees, and the "chaff holes" are what reman of tree branches. The ancient forest in which they once grew must have been flooded quickly. Much of the calcium in the cap was 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 having a period of hundreds of thousands of years. Cap beds are 1.5-2m high and very hard. Cap is of limited use 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 must be loosened by blasting with high explosives before it 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". Roach has been used extensively in the Portland Breakwater and the Cobb at Lyme Regis. When the voids in the stone are filled and the surface polished roach makes a superb decorative stone.
Below the roach is the whitbed (up to 2m). 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. It is capable of being carved with intricate details. It is highly durable and tests upon the stone give a probable weathering rate of 1-2 mm per 100 years, Very large blocks of roach are available up to 4 m³. Fossils occurring here include the large Ammonite Titanites, oysters, echinoids and occasionally, vertebrate bones.
Beneath the roach is the curf (1m). This is really a series of sandy chert beds and shelly limestones. Curf weathers rapidly and is not suitable for use as masonry stone.
At the bottom of the freestone series is the basebed (up to 2m). Basebed is the finest quality Portland stone available. It has a very homogenous texture with a negligible shell content. Basebed can be 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 be found.
Below the freestone is the cherty series (30m), these are thin interspersed limestone and chert beds. Although officially classified as Portland stone, the cherty series is of no use to a quarrier trying to extract dimensional stone, but the beds have been quarried and crushed to produce a low-grade aggregate. There is a superb exposure of the cherty series in Admiralty Quarry.
The cherty series overlie the Portland sand (35m), which is exposed at the base of West Cliff at Blacknor. Under the Portland Sand is Kimmeridge clay which has a total recorded thickness of over 5OOm. Only a small exposure of Kimmeridge clay is evident at the northern and of the island, the majority of it being 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.
Modern quarrying
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. Small diameter holes (35 mm) are drilled horizontally, under each rock to be removed; the holes are usually drilled either in or parallel to a bedding plane. The holes are charged with a small quantity of black powder (gunpowder), chosen because of its non-shattering properties When fired the black powder produces a "heave" which dislodges the rock from its natural bed, undamaged This operation exploits the natural weakness presented by the presence of a bedding plane.
Once removed from the quarry face, large rocks are cut to produce smaller square stones ready for use by masons. Stone within the quarry is cut with plugs and feathers, a series of short, small diameter (typically 3Omm) 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 plugs and feathers, is arduous work, recently wire-saws have been introduced in to the quarries, these have led to an increase in the quantity of stone produced and have assisted quality control. It is much easier to assess the quality of a block after it has been cut with a wire-saw.
Portland stone has probably been used as a building material since at least Roman times. Its superb characteristics as masonry stone has ensured an enduring popularity amongst masons and architects alike.
The earliest known building to be constructed using Portland stone is Rufus Castle at Church Ope, Portland, built around 1080. Indigo Jones used Portland stone to build the Banqueting Hall in Whitehall in 1620. Christopher Wren used nearly one million cubic feet to rebuild St. Paul's Cathedral after the great fire of London in 1666. This established it as one of the best loved British Building stones.
Roach stone from Independent Quarry used at the New Stock Exchange in London, April 2003
Other buildings constructed in Portland stone are The British Museum 1753, Somerset House 1792, General Post Office 1829, The Bank of England, Mansion House and the National Gallery.
After the Second World War the devastated centres of many towns and cities, such as Plymouth, Bristol, Coventry and London were reconstructed using facades of Portland Stone.
During the 1960's and 70's many buildings were 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 resurgence in popularity of more classic building styles, leading to an increased use of traditional 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.
©1997 Mark Godden MIQ
photos by Bob Ford and Paul Crabtree