‘Yorkstone’ is probably one of the widest known and used stones, especially for paving. But trying to define Yorkstone turns out to be more difficult than you might imagine, as Barry Hunt discovers as he continues his search for the great British stones.
Charles Caleb Colton, the reputedly eccentric English cleric, writer and collector, had probably had no experience with the natural stone industry when he coined the phrase ‘imitation is the sincerest form of flattery’. For those whose stones are left on the shelf due to another stone with a substituted name being used, imitation seems anything but flattering. And of all the stones available in the UK, possibly the one most imitated has been Yorkstone.
One of the reasons so much stone has been called Yorkstone is that there is no actual definition of Yorkstone. Furthermore, there is no-one to defend the name as no-one has laid claim to it.
We have a conundrum – a seemingly famous British stone that only actually exists in the minds of those who use the term!
This is no illusion, nor a subtext to Rene Descarte’s musings on existence. Yorkstone is firmly embedded in the British psyche as the prime paving stone. It is one of the most recognisable and widely known of all British stones… despite the fact that it does not actually exist.
The plot thickens due to the ambiguities surrounding Yorkstone. There are references to Yorkshire stone, Yorkshire gritstone, York flagstone and other similar variations on this theme.
The origins of these names is not clear but they have been widely used as a name for any buff, grey or blue sandstone from Northern England (not to mention imported stones from much, much further afield) prepared as slabs for external paving.
Yorkstone is clearly a name that should be left to describe stone that actually comes from York (not that there is any these days).
If the reference to ‘York’ is extended to mean Yorkshire, then there are many stones of great variety (and not necessarily sandstone) that could be included, again making it unworkable as a description for a certain kind of paving.
So now it is time either to determine if there is such a thing as Yorkstone and define it or to drop the term completely and start recognising the truly great stones whose names have remained largely shielded from the populace by being lumped together under the generic term of Yorkstone.
So what is Yorkstone?
The present belief is that the first stones classed as Yorkstone were from Elland Edge, a geographical feature above the town of Elland in West Yorkshire. Local records show that Elland Edge was quarried from at least the early 17th century to produce stone for masonry, roofing and paving.
Elland is sandwiched between Halifax and Huddersfield – and Bradford and Leeds are in the way before we even get close to York. There are local structures that prove the stone from Elland Edge was certainly used in medieval times.
If we actually head across to York, we find the land is mostly obscured by recent glacial deposits and the rocks below are found to be pink and red sandstones and mudstones of Triassic age, providing materials that would not be recognised as Yorkstone.
The inference from all this is that ‘York’ in Yorkstone is a contraction of Yorkshire (which is almost expected) and that this term is too generic for use in today’s enlightened times of CE marking and British Standards.
So what is it that everyone has been so readily recognising as York stone?
Quite simply, it is buff stone used for paving. There are also streaks of colour, sometimes with red, pink, blue and grey hues. Sometimes the colours are more dominant than the background buff colour and often there is the presence of silvery flashes, which are crystal flakes of mica (usually a variety called muscovite).
These crystal flakes gave rise to the term ‘flaggy’. Often thin horizons with higher proportions of micas are found to be the natural splitting planes of the stone. Otherwise, many sandstones occur in combination with related stones such as siltstone, mudstone and claystone, which are less competent than the sandstone layers and thus used as planes of extraction.
It is also apparent that material being described as Yorkstone might actually originate from Derbyshire, Northumberland, Lancashire and County Durham, as well as Yorkshire.
Similar rocks are also found in different parts of Wales and Scotland and there are lesser occurrences in several other English counties. What many people are actually recognising in Yorkstone, without realising it, are the persistent features of the underlying geological formations that provide these stones, which are unrelated to our arbitrary land divisions.
We actually find all stones that fit the perceived Yorkstone description are from two very distinct geological sequences – Millstone Grit and Coal Measures. These were both laid down during the Carboniferous geological period.
Now for some brief geological history and what happened between 326million years ago and 304million years ago, during the Namurian and Westphalian stages of the Carboniferous Period.
At that time there was a series of uplands being eroded. Major river systems flowed into what has been termed the Pennine Basin, which covered much of what today is the North East of England.
Here, thick alluvial deposits collected that were subjected to occasional inundation by the sea and which later became marshes.
This is highly simplified, but the upshot was that a huge amount of sand and grit collected in thick but also variable sequences of beds.
Sand, being the weathered product of other existing stones, comprises predominantly those minerals that are more resistant to the weathering processes – notably quartz and, to a lesser degree, feldspar and mica.
The stones formed at this time also included amounts of iron minerals that were able to concentrate and which are responsible for the reddish colour variations we see in the stone.
Because of the large area of original deposition, variations in the parent rocks, local variations in the environment, different geographical features and other factors, there is considerable variety in the stones formed, although often this is not actually readily apparent without a microscope.
The depth of burial and later geological factors has also dictated how the constituent grains are stuck together, particularly the formation of silica that can alter porosity, strength and other factors affecting the stone’s performance as paving or cladding. It is these variations that make every stone unique, despite appearances.
It is clear Yorkstone is not an appropriate descriptor for any stone and should not be used – although I doubt the industry or its customers are likely to drop the term.
There are almost 60 sandstones currently quarried in the UK from the Millstone Grit and Coal Measures sequences. Those actually from Yorkshire or nearby include: Appleton, Bank Top, Bolton Woods, Brackenhill, Branshaw, Cromwell, Crosland Hill, Crossley, Flappit, Gledhills, Greenmoor, Hillhouse Edge, Hunters Hill, Naylor Hill, Ringby, Scoutmoor, Silex, Sunnybank and Woodkirk.
There really is no need to market any of these stones as Yorkstone, although the temptation to do so could remain strong.
Yorkshire Carboniferous sandstone
It is extremely hard not to keep describing all these materials together as Yorkstone because it avoids the mouthful of geological terms and descriptions that otherwise should be applied. But we are dealing with historical subtleties and the pride of Yorkshire, and this is where the great bugbear of imitation really starts to bite.
It seems Yorkshire would have no right to take umbrage against someone describing a material from outside Yorkshire as Yorkstone if it fits the imagined description.
If there is to be no demarcation, then the simplest description we might apply is given in the title of this section – Yorkshire Carboniferous sandstone. This simplified description must encompass gritstone, which essentially is extremely coarse-grained sandstone.
Table 1 provides some typical test results for some of the Yorkshire Carboniferous sandstones listed earlier. Please note that wet and dry strength values have not been differentiated and the figures are for materials tested with the loading direction perpendicular to the natural bedding planes.
The presence of water does not significantly affect the slip resistance test results, so these, too, have not been differentiated.
There are some quite wide ranges in the properties of Yorkshire Carboniferous sandstone (notably the strength) but this must be expected.
What the table is unable to show is that the strength is not necessarily related to either the slip resistance or abrasion resistance.
How a stone maintains its slip resistance is often related to grain shape and interlock, the cementing medium and other subtle factors such as clay particles that weather out rapidly to help keep a surface rough as it abrades at an uneven rate.
There is a point at which sandstone grades into quartzite, when the cement is mostly siliceous, the porosity and water absorption reduce, and density and strength increase, but such apparently higher quality material may not be quite as good for paving.
A good paving stone needs to refresh its surface and not suffer polishing that can reduce slip resistance – and slip resistance is often the most significant feature of paving material. It is its slip resistance that makes the sandstones of the Millstone Grit and Coal Measures – and particularly those from the Yorkshire area – such good paving… along with its ability to withstand frost damage even when saturated, of course.
The imitators of Yorkshire Carboniferous sandstone might promote such qualities as higher strength or better abrasion resistance, but this is missing the point of what makes a good paving material. If a stone is too strong it tends not to refresh its surface so quickly and might eventually be worn to a polish, reducing slip resistance.
Uses of this sandstone
We have naturally focused on paving as the classic use of Yorkshire Carboniferous sandstone, but the stone is also used successfully for many different masonry elements and for roofing.
The natural splitting properties of these stones have been exploited for paving purposes but are a potential downfall of exploiting them for other uses. For ashlar, the different elements of a façade typically require a stone to have a substantial bed height that does not include potential splitting planes. That immediately starts to limit the use of some of the Carboniferous sandstones.
For some long elements that stand vertically (mullions, architraving and columns, for example) the stone can be used face-bedded, although this can leave the stone prone to vertical splitting. For cills, lintles, transoms, strings and other horizontal elements the lamination is a considerable benefit and even if splitting does manage to occur it often does not present a weathering issue.
Some laminated stone does present a potential issue when delicate carved features detach along weaker laminations, resulting in the loss of isolated items such as cornice dentils and parts of more decorative column capitals of the Corinthian and Composite Orders, for example.
There is also a longer term issue as weathering may affect different laminations and there can be highly uneven weathering of faces. That is a consideration that needs to be included in the selection of any laminated stone, although Yorkshire Carboniferous sandstones can be particularly good in this respect, having some large and consistent beds.
In geological sequences where lamination is thinner and grades more rapidly into very fine material, the stone can be more suited to preparation as roofing slates. Do not confuse the term ‘roofing slate’, which is any stone material that can be split and dressed for roofing purposes, with the term ‘slate’ which is used to define metamorphosed mudrocks and similar materials of extremely fine grain size.
The Yorkshire Carboniferous sandstone slates work fine if they are ventilated from below to allow moisture to escape if they become saturated by rain that is then followed by freezing at the outer surface.
Sandstone is typically harder to carve than limestone and is highly abrasive on tools. The Yorkshire Carboniferous sandstones are no exception. The coarser and more angular the grains the more difficult it is to form sharp edges, while detailed work is more likely to suffer granular disintegration over time. So the stone is more suited to brutal architectural details, and while gravestones are easy to form, fine detail, including lettering, is unlikely to last.
Over time, sandstone tends to suffer onion peeling effects from prolonged weathering. The porous structure allows moisture in to a certain depth. It hangs around a bit and then migrates back to the surface to evaporate. While it is hanging around in the stone, the universal solvent that is water dissolves some of it and often the outer surface is gradually depleted of cementing minerals. Now add pollution and all manner of things can start to happen, then add heating and cooling cycles and the outer skin tries to detach itself.
This is a serious problem with many historic structures built using Carboniferous sandstone and detail can be rapidly destroyed by misguided conservation efforts or even basic maintenance such as repointing. The black crust that may commonly develop is often protecting the stone and should not be removed without careful consideration of the consequences.
Although Yorkshire Carboniferous sandstones appear to be the perfect materials for paving, as is the norm with all natural stone, there are a number of things that will try to upset this view that you need to take into account.
The first is that sandstone is porous and will soak up staining media. There is little you can do to combat this. It is often considered better to let all the stone stain over time and develop the weathered patina you see in heavily trafficked areas than to try to clean it. The stone should not be sealed because it makes the behaviour of moisture unpredictable and can considerably reduce slip resistance. Make sure there is positive drainage at the surface if you are not using porous jointing and bedding.
I would contend that sawn or similar flat surfaces should be avoided for paving. Riven or more heavily textured surfaces are safer. The problem here is not the slip resistance of the stone itself, which tests will show to be good. However, in areas of low pedestrian trafficking paving does not get constant abrasion. Dirt remains on the surface and eventually the stone is a wonderful home for algae and other green things that can reduce the slip resistance to next to nothing. Under such conditions a textured surface will provide more grip than a smooth surface.
Patios may need to be jet-washed every six months or so, but not too harshly as the pressure can pluck out the grains of sandstone. Always remove leaf litter as it can promote algal growth in the stone, which can stain it.
Yorkshire Carboniferous sandstones may be wonderful for paving but even they cannot combat all these issues without a bit of regular care.
Concluding Remarks
To paraphrase Descartes, I’ve thought about it and therefore I am… convinced that Yorkstone is a figment of our collective imagination.
This is the prime example of the problems associated with the naming of stone and how it can lead to imitation. We should stop using the term Yorkstone as it serves the industry no good. We should instead promote the many wonderful Carboniferous sandstones that there are – not just from Yorkshire but from around the UK. I urge you not to ask for Yorkstone any more; ask for Carboniferous sandstone with a colour and texture. You’ll be amazed at the variety available.
Thus, in my quest to find distinctive British stones, Yorkstone has failed me. However, Yorkshire Carboniferous sandstones have opened up a whole world of possibilities for inclusion and we might yet be hearing more about Crosland Hill, Scoutmoor, Cromwell, Woodkirk, and many more of these great British stones.