In his continuing search for Great British Stones, Barry Hunt this month turns his attention to Dorset and the peninsular of Purbeck, from which comes limestone laid down in so many layers it is hard to define, but from the polish that some of the beds will take might be said to be the progenitor of the British marble industry.
There are many stones that might claim to be the father or mother of the British stone industry – it is something for archaeologists to argue over. But I am going to stick my neck out and claim that Purbeck stone is the progenitor of the British marble industry, though it took somewhere between one and two thousand years to birth.
The oldest presently known use of Purbeck stone was around 2,800 years ago for a Bronze Age burial cist (or coffin) at Langton Matravers just to the west of Swanage in Dorset.
Around 2,500 years ago the stone was used in the construction of Flowers Barrow, an Iron Age hill fort above Worbarrow Bay, also in Dorset. It is likely that when this hill fort was taken over by the Romans they recognised the qualities of the local Purbeck stone and especially the ability of some beds to take a polish.
The use of Purbeck stone at Camulodunum (Colchester) before AD60 indicates a promptness of exploitation, just a few years after the Romans’ successful invasion of the area that is now Dorset. The dedication slab of the forum in Verulamium (St Albans) from AD79 is another notable example of early Purbeck stone use and its transportation around the early Roman Britain.
The history of quarrying Purbeck appears continuous for 2,000 years, though historical records are sketchy and there are definite periods of fashionable use across Britain in addition to local use.
When the Normans invaded they brought their cathedral building skills and, in time, Purbeck stone was seen as a strong material both for constructing greater structures and for providing a decorative finish. It was one of the strongest materials available for Salisbury Cathedral in the first half of the 13th century, being used for the finish to the four main columns at the nave and transept crossing that eventually supported the main spire, which remains the tallest in Britain.
Approximately 15,000tonnes of Purbeck stone were used, most for flooring – and the floor includes a single slab 3.9m by 1.4m laid as a monument to Robert Wyvil, who was Bishop of Salisbury from 1330 to 1375.
The Ancient Order (or Company) of Purbeck Marblers & Stonecutters is cited as having been granted a Royal Charter by Henry III, who ordered that Purbeck stone be used in the re-building of Westminster Abbey. Work on that started in 1245. Particularly notable is the use of Purbeck for the remarkable inlaid stone decoration known as the Cosmati Pavement.
Originally there were 10 articles governing the Marblers & Stonecutters. In 1698 the Company added a further six, referring to a Royal Charter. No other part of the British stone industry has laid claim to a Royal Charter and, given the dearth of good and commercially viable materials available that genuinely take a polish, similar claims are doubtful in the future.
Therefore the notion that Purbeck stone is the progenitor of the British marble industry is unlikely ever to be challenged.
What is Purbeck Stone?
There is no easy answer to this question other than that it is limestone.
The geological sequence from which Purbeck stone is derived is known as the Purbeck Formation, which spans the end of the Jurassic (Tithonian, 145.0-152.1million years ago) and the beginning of the Cretaceous (Berriassian, 139.8-145.0Ma).
The sequence can reach up to 60m thick in total with more than 200 identifiable beds that are not necessarily consistent in outcrop.
At least 24 different qualities of stone have been named by the quarrymen (see Table 1). Each one represents changing conditions of formation. And this 24 does not include the many other types of stone present (clays, shales and marls) within the sequence.
There are also variations in the bed names between different quarries, and the names rarely agree with the geological divisions that have been assigned, further confusing the situation.
It is generally agreed that the lower part of the Purbeck Formation represents a shallow marine environment, probably lagoonal, that became subjected to drying out, followed by regular inputs of freshwater from rivers.
A large shallow coastal lake may have formed that resulted in changes in the fauna creating distinctive beds. It is only at the top of the sequence that the quality of stone is considered sufficient to take a high polish and thus be termed as marble, in the traditional masonry rather than strictly geological sense. This stone is filled with freshwater snails of the genus Viviparus set within a typically dark calcite matrix.
The base of the sequence is marked by what is known as the Dirt Beds. These contain the silicified remains of tree stumps.
Current Supplies of Purbeck Stone
Historically, Purbeck stone was removed from the surface via open-cast quarrying. As the industry developed, underground workings, known as ‘quarrs’, began to be dug during the 17th century. By the late 19th century there are believed to have been at least 100 quarrs operating. These originally appeared near Swanage and slowly moving west as seams were worked out.
Modern methods of extraction have been able to exploit the deposits more thoroughly, though the supply of stone is now partly governed by the fact that the deposits fall mostly within the UNESCO World Heritage Site of the Jurassic Coast.
The latest edition of the Natural Stone Directory has listings under Purbeck from Blacklands, California, Downs, Keates, Lander’s, Lewis Acton and St Aldheim’s quarries. These are variously owned or run by H F Bonfield & Son, W J Haysom & Son, Keates Quarries, Lander’s Quarries, Lewis Quarries, Lovell Stone Group and Suttle Purbeck Stone.
All the quarries are based in and around Swanage and none of them lists in the Natural Stone Directory the full range of stone types it produces, probably due to the many different beds and their unpredictable availability.
Meaningful test results are difficult to obtain because of the constant changes in the lithologies and thinness of many of the units.
The bulk of the stone is slabbed, due to the typically thin beds and splitting properties that allow it to be variously used for internal flooring tiles, external paving, roofing tiles and cladding.
Thicker beds are used for most masonry ashlar purposes, walling stone and other hard landscaping. Kerbs and steps were a significant traditional use but are now out of general favour due to the current availability of many harder stones that are more easily won. Generally, the stone appears bland unless it is given a polish to release the depth of colour the matrix can exhibit in contrast to the many and varied fossils contained in it. The changes in colour typically provide a natural and interesting variation and many different surface textures can be applied.
The beds of stone are rarely more than 1m thick and, typically, are in the order of only 0.3m thick. There is also the issue of weathering along bedding planes and joints that can reduce the amount of material that can be won. This is why the quarrying of stone can be haphazard and the material is graded as it is quarried rather than there being a predictable reserve of the different types of material present.
The total output for all the quarries varies somewhere between 15,000 and 25,000 tonnes per annum.
Purbeck Stone in Use
There are few reported problems with Purbeck limestone. It has a good level of strength and low porosity. Its longevity in many different uses around the country is a strong testament to its true value, which should be taken into account when carrying out an environmental impact assessment.
It has been noted that grades of marble with more pinkish hues tend to weather more rapidly, which may be due to a higher content of iron minerals resulting in a more open texture with lower strength. This is all relative as the stone offers excellent resistance to both frost and salt effects.
Some have contended that Purbeck Marble does not stand up to internal condensation effects during extended exposure (we are talking centuries). However, it is believed that a problem occurs only when the tooled surface has not been correctly removed prior to final polishing and thus is not a fault of the stone.
The blame has been laid on the bruising that can be imparted when using heavier tools. Deep fracturing loosens the stone matrix and the visual effect is magnified by the dark background colour.
It is also likely that a loss of surface polish results from acids in cleaning materials and human touch, not to mention airborne pollutants that may activate under condensing conditions.
Purbeck stone, like all limestones, has a limit to its hardness and will abrade under foot traffic, but it will resist wear more than most other limestones. The downside to good wear resistance is the greater ability to polish, which can be problematic in external environments. This is why the stone should be textured and more fossiliferous grades should be deployed to provide a variably hard surface that abrades unevenly.
I have recently been involved in an issue where the grade of Purbeck stone supplied for external paving exhibited iron and clay rich bands that resulted in premature fracturing. This was a typical stone quality control problem and an example of why all natural stone producers need to be vigilant.
However, a material like Purbeck stone does not easily fall into the methodologies presently applied to monitor natural stone quality control, and it is very much down to the skill of the quarrier – you should not rely on sporadic test results that may be obtained.
Concluding Remarks
Purbeck stone found its way into almost every ecclesiastical building constructed after the Normans arrived, possibly being more widely used across Britain than any other stone until its Dorset cousin, Portland stone, started to claim prominence in the 17th century. While Purbeck stone is undoubtedly one of the highest quality limestones available in Britain, the difficulties of extraction, lack of substantial bed heights and considerable variability meant it was not destined to be used in large quantities for more general building purposes.
Modern methods have made exploitation easier and the stone is more suited to the current vogue for natural stone floor tiling, while paving and cladding design has developed to allow more widespread use of Purbeck again.
There is renewed interest in the wonders of Purbeck stone but this has to be tempered by the quarrying constraints. Purbeck stone has one of the longest histories of use in Britain and a lack of general recognition should not deny it its rightful place among the greatest of our British stones.