In his continuing search for the Great British Stones, Barry Hunt this month turns his attention to Cornwall and its granite. In 1858 there were 113 quarries producing the granite. Now, DeLank is the last major quarry still in regular production – and what a fine building stone it offers.
Trinity House, which is in charge of lighthouses in various British territories, this year celebrates the 500th anniversary of being granted a Royal Charter by Henry VIII to manage the safety of shipping and the well being of seafarers.
Today, there are more than 10,000 aids to navigation on and around the British Isles, and of these it is the lighthouses that dwell in the minds and hearts of our island nation.
It is the use of granite that has allowed so many of these structures to remain as historical pointers to our great heritage in engineering as well as the development of the natural stone industry.
The sea is remorseless and unforgiving, but in 1755 it was fire that destroyed the wooden lighthouse at Eddystone, a dangerous rock to the south west of Plymouth.
The task of re-building the lighthouse fell to John Smeaton, who was to cause a true sea-change that captured the imagination of the world.
Smeaton decided to construct a tower made of stone. He based his design on the shape of an English oak tree and employed his newly formulated hydraulic cement (that could set under water). For such a task, he required the toughest local labourers who understood the toughest Cornish stone: Cornish tin miners.
But the lighthouses are a relatively new use of this granite because it could be said that the Cornish granite industry is more than 5,000 years old, as granite ‘moorstones’ were lifted to create many megalithic structures that littered the ancient landscape.
However, a true granite industry was only established following Smeaton’s brilliance in choosing the stone for lighthouses.
The Victorian age ushered in a remarkable era of building as Great Britain’s commercial interests stretched across the globe and safe havens for its many fleets were required. Smeaton’s use of granite had shown that it was the natural choice for harbours, quaysides and bridges, and the South West of England was at the heart of this building.
Competition from abroad, two world wars, the decline of other local mining industries and the advent of concrete were major factors in the decline of the English granite industry during the 20th century.
In 1858 the mineral statistics of the United Kingdom cite 113 granite quarries operating in the South West, but on the eve of the millennium, DeLank quarry on Bodmin Moor was the only major dimensional stone quarry still active on a daily basis in Cornwall.
Geological History of the South West Granites
For the origins of Cornish granite we need to travel back to between 300 and 270million years ago to a great period of mountain building known by geologists as the Variscan Orogeny.
Molten materials invaded the ground at great depth and this eventually led to the creation of a series of massive lumps of rock connected to each other deep below the surface.
The South West of England was once a tropical paradise that allowed the overlying rocks to weather and convert into deep soils. As the climate changed and the earth uplifted, the soils were removed, often to leave large collections of unweathered core stones, or tors, which provide much of the unique character of the south west.
Today, these granite masses are seen in the areas of Bodmin Moor, Carnmenellis, Dartmoor, Exmoor, Hensbarrow and Land’s End on the mainland, and just a bit further west in the form of the Isles of Scilly.
The geological history has been greatly simplified here, but the upshot is that there is a large resource of relatively consistent granite that makes it anything but straightforward to distinguish between the different materials produced by the many quarries.
The granites of the South West are characterized by the presence of large and well developed crystals (or ‘mega-crysts’) of the mineral orthoclase, a type of feldspar, usually seen as long white prisms in a mass of finer grey, white and black crystals.
It is often only the size and distribution of the mega-crysts that can be used to tell the granites from different quarries apart.
There are many local variations for a variety of geological reasons. The granite was also subjected to hydrothermal processes that created kaolin, a mineral important to the china clay industry that is extracted in large volumes from Cornwall.
The DeLank granite has some of the smaller orthoclase mega-crysts, providing it with a more consistent appearance than the rock from some of the Cornish quarries.
Other minerals present in the granite include quartz, plagioclase (another form of feldspar) and biotite mica.
There really is nothing that significantly distinguishes the DeLank stone from the other granite of the region, so it may appear that DeLank has been selected almost by default to be included among my selection of Great British Stone simply because it is the last remaining large dimension stone quarry.
But there is more to it than that. The DeLank quarry has managed to keep going continuously for more than 150 years and arguably produces the most famous of all the granites of the region – although at one time its neighbour of Hantergantick was almost as well known and just as respected.
Properties of DeLank Granite
It is rare that I am swayed by marketing literature, but the description of DeLank granite as being ‘virtually indestructible’ is not that far from the truth.
I have provided in Table 1 some of the typical test results that are required to comply with current legislation, but these are almost superfluous.
The performance of the many structures that have survived hundreds of years in some of the harshest environments is testament enough to the performance of the granite as a building material.
So what is it that makes DeLank granite a material that may be used to great effect for almost any natural stone purpose in any environment? To answer this I have to cover a little more basic geology.
Granite, being an igneous rock (ie formed by the cooling of magma), comprises interlocking crystals without any pores that may be easily exploited by weathering processes.
Quartz is a major component of the rock and is highly resistant to all forms of weathering. It is usually the feldspar minerals that are the weakest link and prone to weathering, although this is typically because they have often been subjected to a degree of geological weathering over a prolonged time period. But DeLank granite has benefited from this slow weathering process because it has removed the more weathered minerals and exposed fresher material.
The Mohs scale of hardness has feldspar at six and quartz at seven, but on an absolute scale there is relatively little difference in hardness between the two mineral forms.
Therefore, when subjected to abrasion, the stone wears reasonably evenly and keeps its form well – something that is excellent for statuary, let alone more standard masonry.
However, this property is also the one weakness of the stone when it is used for flooring, as without a strongly tooled finish it can become slippery over time.
Use of DeLank Granite
Aside from the massive blocks of stone used for lighthouses at Bishop Rock, Beachy Head, Needles and others, and other major maritime structures, one of the most common places to see the DeLank granite is as kerbstones, although it can be difficult to tell it apart from the many other granites used.
However, it may be that there is another way to tell the source of one kerbstone from another from markings frequently carved into the stones. Unfortunately, neither the Geological Society nor the Worshipful Company of Paviors (established 1479) has yet discovered the meaning of the markings, although the Geological Society has established a database that could one day help to answer the mystery.
The Thames bridges of Blackfriars, Putney and Tower all incorporate DeLank granite. Tower Bridge is possibly one of the most recognised structures in the World, although much newer structures in DeLank granite have also received considerable public recognition, including the Diana Princess of Wales Memorial in Hyde Park and the Paternoster Square re-development around St Paul’s Cathedral.
Monumental use of the granite can also be seen in London in the statues of Karl Marx, Winston Churchill and on the base of the Cenotaph, where it marries perfectly with the Portland stone above.
So we have a stone from England that can be used for almost any building purpose and stay the course for many years beyond our own life spans. The density and tightness of the stone means that dirt is unable to penetrate it, so it is easy to maintain.
But if it is so good (and it is) you might ask why do we not use this great resource more and re-open more of the Cornish quarries?
The main reasons are economies of scale and considerable competition from other resources. But there is also the lack of visual variety presented by an essentially grey stone. In some cases this is just what a designer wants. But not always.
This is something that was brought home by the redevelopment of Exhibition Road in London completed in 2012. A million blocks of dark grey and pink igneous rock were shipped all the way from China for the project.
If you want a grey structure, you could, of course, use concrete, although this completely misses the point of using a natural stone such as DeLank, with its natural, lively sparkle from its large crystals and a longevity that will far exceed anything concrete can offer.
Concluding Remarks
It is ironic that John Smeaton’s construction of the third Eddystone Lighthouse that ignited the growth of the granite industry in the South West eventually proved to be the downfall of that industry.
Smeaton’s new cement formula used to such great effect in the lighthouses eventually developed into Portland cement and the concrete that started to replace stone for major civil engineering projects.
DeLank granite was selected by James Douglass for the fourth Eddystone lighthouse that now, just over 130 years later, looks as if it will continue to take the worst the English Channel can throw at it for many more centuries to come. It is likely it will still be standing for the millennium celebrations of Trinity House in another 500 years.
Such longevity of service should not be forgotten in these times of environmental concern, although few seem to have the ability to see beyond the baseline cost and spurious statistics promoted by competing industries.
DeLank granite is a truly great British stone because it is an exceptional granite that has withstood everything that this land has been able to throw at it.
It is the sole survivor of a proud history of stone quarrying in the South West of England and deservedly has been used in some of the most significant and iconic structures in the UK.
We should be bending over backwards to ensure this amazing resource continues to be available for construction – although, thankfully, the spirit of those at DeLank seems to be as tenacious as the stone itself and the stone is currently enjoying a revival under the new owners who took over the operation in 2011.
The author: Barry Hunt
Barry Hunt is a chartered geologist, a surveyor and scientist. He has been awarded the designation of European Geologist and is a Corporate Building, Conservation and Specialist Surveyor. He is also a Member of the Chartered Institute of Building and a Fellow of the Royal Microscopical Society.
Barry has served on a number of professional committees, including the Technical Committee of Stone Federation Great Britain (SFGB), which provides advice on all stone construction issues. He is one of the authors of the SFGB team that has published codes of practice for the installation of stone floors and internal stone finishes.
Barry gained 14 years’ experience working as a consultant for two renowned civil engineering materials consultancies before, in 2001, establishing his own consultancy, IBIS, specialising in the investigation of construction materials.
The specialist knowledge and services provided by Barry have allowed him to be instrumental in the resolution of problems ranging from blast damaged claddings in London’s West End to advice on the quarrying and extraction of stone from abroad for import to the UK. Other areas of experience include the investigation of all types of building finishes, specialist advice on remedial treatments and the preparation of advice for potential and actual use in litigation or arbitration.
Having worked for consultancies that both undertook in-house laboratory investigation to UKAS requirements, Barry is also able to conduct or oversee a wide range of on-site and laboratory techniques and ensure they are carried out to traceable standards.
One speciality in all investigations is Barry’s hands-on approach. Being trained in industrial roped access (abseiling) allows him to get close to the problems with external building envelopes quickly, efficiently and cost-effectively.
Throughout his working life Barry has published findings from the many investigations he has undertaken. He has also authored chapters for two books on building stone and is currently engaged in other book projects in this field. His most important contribution is considered to be the chapter on the repair and maintenance of stone in the landmark Geological Society publication Stone.
Barry is also a regular contributor, covering the full spectrum of natural stone use, for Natural Stone Specialist magazine.