As the Scots have voted to remain part of Great Britain, Barry Hunt this month continues his search for the Great British Stones north of the border – in fact, almost as far north as it is possible to go in Britain – to explore the qualities of Caithness stone. It is unequalled as paving but ideal for so much more.

When I decided to start writing this series of articles on the Great British Stones, I had not really considered the possibility of a ‘Yes’ vote in the Scottish referendum and the impact it might have had on the series. Would I have to disregard all 200 or so Scottish building stones used over the past 300 years as not being British?

But then, I thought, before the first of the Acts of Union, those very same stones were not ‘British’. Therefore it was quite simple: we can only consider matters in the context of how they stand today. This series otherwise would have been robbed of the opportunity to discuss some important stones and buildings that help define the cities and landscape of Scotland.

So which stone to discuss first? The Craigleith sandstone that defined Georgian Edinburgh, or possibly the Corncockle and Locharbriggs sandstones? How about the Rubislaw and Kemnay granites used to such great effect in Aberdeen and, in fact, all around the UK? The Ballachulish slate used for so much roofing also deserves a mention.

Today there are fewer than 20 building stone quarries still working in Scotland, although in many of them building stone is only occasionally won. Some produce mainly aggregate; some mainly don’t produce anything.

I have selected Caithness stone because it is dour, tough and unyielding. Yet scratch beneath the surface and there are multiple layers that make the stone interesting to discuss if difficult to classify. Is there an analogy here?

Caithness Stone: Geological History

The Caledonian Orogeny was a major period in Earth’s geological history, when several land masses were brought together to form Laurasia and later the supercontinent of Pangaea.

An ocean between the land masses was lost as they smashed into each other to create both highlands and lowlands. One of these lowland areas is now referred to by geologists as the Orcadian Basin, within which a huge freshwater lake grew that was fed by rivers running down from the surrounding highlands, the waters loaded with sands, silts, muds and clays.

Lake Orcadie was not particularly deep and its chemistry changed with the seasons due to the growth of organisms in the summertime that died later in the year.

It covered an area of around 50,000km2. By comparison, Lake Superior between Canada and the United States covers an area just a little less than 32,000km2. Lake Orcadie covered an area that is now the north-east of Scotland, including the islands of both Orkney and Shetland.

The sediments that built up within the lake became thicker as the basin subsided, resulting in rock formation sometimes well over a kilometre thick. This occurred between about 410 and 370million years ago.

The sediments were found to be quite consistent across a wide area and typically exhibited three different phases of formation:

l Blooms of organisms promoting the growth of carbonate minerals

l Death of organisms resulting in enrichment of the sediments by organic matter

l Laminations of sands, silts, muds or clays.

Closer to the edges of Lake Orcadie the sediments are typically coarser, and some beds in the deeper parts of the lake are rich in fossil fish fragments. Ripple marks from low energy currents and other lacustrine features can also be identified in the rock formed. The result is that the rocks formed are incredibly consistent over a large area with well defined beds and laminations.

So we end up with a rock that has multiple personalities and is hard to define.

The laminations may individually comprise limestone, sandstone, siltstone, mudstone, claystone, marl and other rock types. And these laminations are repeated and vary in size.

Sometimes the stone is mostly sandstone. Other times it is more limestone. But the material that is most commonly used for building purposes is probably best described simply as ‘mudrock’.

Over time and with burial the Lake Orcadie sediments lithified (turned to rock), although this did not include metamorphism, so much of the character of the original sediments has been retained. This means the rock typically splits along bedding and lamination planes.

The silts and clays impart a dark colour with a sheen, mostly provided by flaky minerals such as micas, and the darkness is enhanced by opaque materials derived from the original presence of the organic materials.

Supplies

The rock sequence is found over a considerable area of the north east of Scotland, although it is actually only a fairly small section that is considered to exhibit perfectly planar splitting to allow consistent paving slabs and stone slates to be produced.

The working faces are rarely more than 3m high with minimal overburden, which is a significant factor in determining the final cost of the stone. Another factor is the transportation costs, as the two currently operating Caithness stone quarries are relatively remote. They are in Achscrabster and Spittal.

The Caithness stone is so consistent that the beds exploited within the two quarries barely fluctuate in height across hundreds of metres. The consistency of the deposit is demonstrated by the quarry floors, which are essentially flat, save for the occurrence of jointing structures. This has allowed the quarrymen the luxury of knowing exactly where the best splitting planes of the stone are and the consequence of this is that the beds have acquired individual names (see Fig 1 above).

In some locations the splitting properties of the rock sequences allow slabs to be produced down to 10mm thick, so that a variety of robust, slabbed products can be produced that are more suited to use as roofing.

Some rocks that have been left to weather over many years are sometimes found to delaminate gradually, typically along bands that turn a reddish brown colour that appear to be richer in iron carbonate minerals.

The stone is also found on Orkney, where the local ancient peoples built some of the most astonishing Neolithic sites, the most notable being Skara Brae, which was occupied between 3,200 BC and 2,500 BC.

Here the splitting properties of the stone were used to create walls, shelves, hearths and other items.

The Ring of Brodgar on Orkney is considered to be one of the largest and finest examples of a stone circle and henge in the British Isles. Similarly, the Standing Stones of Stenness are a spectacular use of some very large slabs of Caithness stone. The tallest reaches almost 6m high while being only 250mm thick.

The survival of these and other local structures over such a long timescale has provided considerable testament to the longevity of Caithness stone.

Principal Uses

Caithness stone is best known as a paving material. It is one of the few natural stones that could claim to rival the properties of the various carboniferous sandstones from the Millstone Grit and Coal Measures sequences in Northern England (no-one has disagreed with my assertion that Yorkstone is a figment of our collective imagination – NSS Volume 49 No6). In fact, the Caithness stone can outperform most other paving stones of sedimentary origin.

Its dark grey colour and wide range of natural variation in surface texture make it one of the most aesthetically pleasing paving stones, a reason why it has been used around the world. Notable recent uses include the Cutty Sark project in Greenwich, London, and at the Scottish Parliament Building in Edinburgh, as well as on the Royal Mile.

Unusually, some relatively recently installed slabs in Edinburgh’s East Market Street needed to be lifted in 2012. However, this was not due to some odd failure but because an eagle-eyed geologist had spotted the remains of fossil fish at the surface of some of the slabs. The slabs in this instance had been supplied from Spittal Mains Quarry, which is less than a mile from Achanarras Quarry, an important SSSI that has been preserved due to unique fossil finds. A survey of the Caithness stone paving around Edinburgh resulted in more than 25 slabs being lifted in order to preserve the fossil fish identified. Other towns and cities may yet be searched for important fossils in this new science of ‘rescue palaeontology’.

Locally to the quarries, the stone has been exploited for low level fencing, when it is placed on edge and overlapped with adjacent stones slabs. This is an unusual but obvious regional feature. The same may also be said for its occasional use as roofing slate, the downside being the necessity to build very robust roofs that are able to support the considerable loading requirements.

The Caithness stone would not appear out of place in European alpine environments, where large stone slabs are often employed at low angles for roofing. Waste materials from the paving preparation are used for random rubble walling and thinner random pieces are used for tiling or external veneers.

Caithness stone normally provides excellent service as a cladding material but is rarely used for this purpose. Possibly the fact that the stone is naturally split scares potential users, as there is a traditional belief that slates and other stones that are riven may do exactly the same in service and therefore are too risky to employ.

However, the splitting properties of the stone are so well known that this should not be an issue. Where the stone is used as memorials it performs perfectly well.

The properties of Caithness stone are given in Table 1, which helps to demonstrate why it is a good candidate for cladding in addition to being an excellent paving as well as all round building material. The splitting properties only really limit its use in more traditional statuary and ashlar work.

Slabs of several metres length can be prepared, making it an excellent choice for long run kitchen worktops without joints. Such worktops can command a particularly high premium, although the user must be prepared to accept that the stone will scratch in much the same way as limestone, marble or slate would, and will be susceptible to staining and acidic attack from foodstuffs. But, with the right maintenance, these do not have to be seen as problems.

Concluding Remarks

It is impossible to pin a specific geological name on to Caithness stone because of the changing composition of its various layers, but on average it might be regarded as a carbonate-bearing siltstone.

Its mode of formation should be considered unique for a dimension stone resource, which is why there is no other building stone on the market that comes close to matching it, either in appearance or properties.

Recent events have also demonstrated a geological importance second to none among building stones.

This is a Scottish resident that thrives on splitting but its existence is threatened by globalisation.

Will it now continue to thrive as a great British stone or would it have been better off as a great Scottish stone? I am sure the stone would not care.

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.