The Sands of Time
Britain's Building Sandstones
Church Horbury, West Yorkshire: Local Carboniferous sandstone
displaying natural cross-bedding to great effect
|Sandstone beds in the UK|
Currently there are around 200 active building sandstone quarries in Britain, more than enough one might think to supply current construction industry needs. However, in the mid 19th century there were more than 850 quarries supplying building sandstone, as well as many more unrecorded small producers meeting local vernacular needs. How has this dramatic decline in the production of building sandstone impacted on our building industry? The most fundamental change has been to restrict the choice of sandstones available, particularly for conservation repair and new build projects.
Sandstone beds (see map, above) occur in all the main geological systems cropping out across Britain (Precambrian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous and Tertiary successions). Although not all beds produce suitable building stone, sandstones from each of these geological systems have been used for building stone to a greater or lesser extent.
In general, most early quarries began as sources of vernacular building stone supplying a few local projects, but by the 19th century a small proportion had grown to become important national sandstone resources, supplying towns and cities far beyond their immediate vicinity. Outside London, in the south of England, the Midlands, Wales and in the north of Britain, our most recognisable historic buildings and structures are commonly constructed using local sandstones, many of which are now no longer quarried.
|Averham Church, Nottinghamshire: The fearsome profile of this massive gargoyle is enhanced by its ‘mask of pollution’|
|Coleorton Church, Leicestershire: Cavernous decay is a common feature of the weakly cemented, fine-grained, white Permo-Triassic sandstones of the English Midlands|
|St Gregory the Great Church, Dawlish: Permian red sandstone from the local Dawlish Sandstone Formation with customised cross-bedding||House, Niton, Isle of Wight: Decoratively coursed and galleted, green-grey, glauconitic sandstone from local Upper Greensand Formation (Cretaceous)|
The past success of the building sandstone industry has in more recent times been tempered by a marked decline in production in some areas. Many important sandstone resources have already been lost. One of the best documented examples is the Craigleith Sandstone, originally quarried and used extensively in the City of Edinburgh in the 18th and 19th centuries. Made inaccessible by redevelopment around the original quarries, the sandstone has not been available in the market place for several decades (Hyslop 2004 – see Recommended Reading).
Elsewhere in the UK, the problem is perhaps less well publicised but equally acute. The lack of active quarries has meant that in sourcing stone for repair and new build projects, architects frequently have to consider using sandstones quite alien to the local area. Stone matching in the heritage field is particularly affected, as it is becoming increasingly a case of using those sandstones that are currently available, rather than selecting a sandstone that best fits the ‘petrological fingerprint’ of the original stone. The risk is that an unsuitable replacement may well weather in a different manner, and in the worst case it may actually be physically incompatible, leading to the deterioration of either the new or the original stone. It is inevitable that sourcing sandstones for sympathetic use in new build projects and for future building conservation will become increasingly difficult should sandstone supplies continue to diminish and become much less diverse.
How, therefore, from a geological point of view, do we persuade the planners and quarry-owners to improve the situation? We clearly need to make sure that mineral planners, producers and users all understand why this diversity is so important. Sandstones do have generic similarities but most show marked differences, from simple colour variations to contrasting mineralogies. We need to explain exactly what a sandstone is, how differences between sandstones arise and the consequences if mismatched sandstones are used in a building. Such differences can subsequently have a significant impact on the rate of decay and long-term durability of a sandstone in the building fabric.
WHAT IS A SANDSTONE?
The term sandstone is used to describe a hard sedimentary rock formed by the weathering, erosion and accumulation of ‘sand-sized’ fragments from any pre-existing rock-type – igneous, metamorphic or sedimentary. Sand accumulations principally form as windblown desert dunes, coastal beaches or the beds of rivers and streams. Geologically, sand includes all particles between 00.063 microns and 2mm in size. Most sand deposits include a range of grain sizes within these limits (deposits which contain a wide range of grain sizes are described as ‘poorly sorted’). Over time, such sand deposits become gradually buried, compacted and cemented to form the sandstone beds quarried today. Geologists refer to this transformation from sand to sandstone as diagenesis, a process which takes place over millions of years.
Understanding the mineral composition of building sandstones is not just of academic interest and should be of value to both the producer and user of the stone. The term sandstone provides no indication of the mineralogical composition of the rock. Most building sandstones comprise a mineralogical mix of grains including quartz, feldspar, mica and diverse rock fragments. They might also include other components in their framework such as fossil fragments, pebbles or clay layers.
The presence of particular minerals in a sandstone can have important implications when considering its end-use as a building stone. Concentrated layers of micaceous clay minerals, for example, give natural fissility to the sandstone, allowing it to be split into flagstones or stone roofing slates. The sand grains are held together by a range of natural mineral cements that may include silica, calcium and magnesian carbonates or iron compounds. The type of cement present can markedly affect not only its visual appearance but its durability and workability. When describing sandstones it is usual to first determine their dominant grain size (coarse-grained, fine-grained, etc) and then to identify any other distinctive features present. These may include colour variations (red, grey, green, purple sandstones, etc), grain composition (feldspathic, pebbly, fossiliferous, etc) or cement composition (calcareous or ferruginous for example).
|Sheriff’s Court buildings, Central Glasgow: Massive stonework using the local Carboniferous ‘blonde’ sandstone||Castle Gates Library, Shrewsbury (Formerly Shrewsbury School, 1630), constructed entirely of white Grinshill Sandstone (Triassic)|
Sandstones dominated by quartz framework grains and cemented by silica, although likely to be hard and very weather resistant, may also prove to be difficult to cut or shape without suitable equipment, which can significantly increase production costs. By contrast, more porous sandstones with a variable mix of grains, together with a less pervasive cement (probably the most typical British building sandstone characteristics), can be worked and dressed by hand and still provide a durable stone cost-effectively.
Sandstone beds may be thick and massive (freestones), show inclined layers or cross-bedding, parallel laminations or may include pebble layers, fossils and mineral concentrations. Such features give many building sandstones a distinctive visual character which in historic structures often helps in the determination of their original quarry source.
POROSITY AND PERMEABILITY
Sandstones commonly show considerable variability in their natural porosity and permeability, thus either restricting or encouraging fluid flow through the rock. Many porous British sandstones are important natural water aquifers, while others form reservoirs holding large reserves of oil and gas. Natural fluid trapped in pores within a sandstone generally reaches a chemical equilibrium with its framework minerals. However, the fluid may also absorb airborne pollutants such as sulphur dioxide. The negative impact of the presence of the airborne pollutants produced by industrial processes during the 19th and 20th centuries in encouraging sandstone decay has been extensively documented. Such acidic or alkaline pore waters can enlarge, damage or block pore systems and cause the disfiguring precipitation of gypsum and other mineral cements on stone surfaces. Mismatched sandstones used as part of a conservation repair can inhibit fluid flow, further enhancing stone decay.
England’s sandstones have been exploited for building stone for hundreds of years and much of its surviving stone heritage is constructed of sandstone. In London, which doggedly remains a limestone-dominated city, other British sandstones, with the exception of Kentish Ragstone and Reigate Stone, were not in general use in the city until the early 19th century, although Yorkshire sandstones were apparently imported for paving as early as the 18th century.
|Southam Church, Warwickshire: Occasionally a building fabric exemplifies what can happen when the local sandstones are no longer available|
|Aislaby Quarry, North Yorkshire: The quarry
remains an important source of Middle Jurassic (Saltwick Formation) sandstone in the North Yorkshire area
In rural counties like Shropshire the need for local building stones was met by quarrying a range of variegated sandstones from the local Cambrian, Silurian or Ordovician successions, none of which ever travelled much further afield. The red-brown and green Devonian sandstones of Herefordshire are ubiquitous in buildings throughout the county and are used for both wall and roofing stones. A comparatively recent revival of the sandstone roofing stone industry has now successfully brought the stone back onto the local market.
Until the 19th century British building sandstone production appears to have remained very localised with each county largely meeting its own needs. Subsequently, however, driven by booming industrial development in the North of England, sandstone production gradually centred on the Carboniferous successions of the Pennines. Today the heartland of Britain’s building sandstone industry is still concentrated on these Carboniferous sandstone resources with numerous active quarries in Derbyshire, Lancashire, Yorkshire, Northumberland and Durham.
In contrast, the Permian and Triassic outcrops that dominate the Midland counties (Worcestershire, Warwickshire, Shropshire, Staffordshire, Nottinghamshire) and the north-west (Cheshire, Lancashire and Cumbria) have provided variegated sandstones for building since Roman times. The red sandstones are displayed in many cathedrals (Worcester, Lichfield, Coventry, Chester, Liverpool) and in countless village churches and houses throughout the area. Equally important but perhaps less evident are the ‘white’ sandstones of this region once extensively quarried in the Forest of Arden, around Warwick, Hollington, Colwich and in the Wirral (Storeton) but currently only available from Grinshill Quarry in Shropshire.
The Jurassic succession in southern England includes many of our principal building limestones, with only a few locally important and distinctive ferruginous sandstone beds such as the Middle Jurassic Northampton Sand ironstones. By contrast, in north east Yorkshire the Middle Jurassic succession is dominated by sandstones which were extensively quarried along its outcrop for building stone in the past, notably around Scalby, Cloughton and in the North York Moors. The Jurassic sandstones of the Whitby area are still actively quarried for local use but in the distant past were occasionally shipped to London. Higher in the sequence, the finegrained sandstones of the Upper Jurassic were also extensively quarried in the past for local use, near Scarborough and in the Howardian Hills, principally for the construction of rural churches of the area. Currently there are no sandstones quarried in these areas.
The Lower Cretaceous successions of southern England include a wide variety of sandstone beds, some of which were once extensively quarried for local use, although they were not generally exported beyond their source areas. These finegrained sandstones are commonly, but not always, characterised by the presence of the green mineral glauconite.
Sandstones are very much in evidence in the vernacular architecture of Sussex, Kent, the Isle of Wight, Wiltshire, Hampshire, Dorset, Bedfordshire and Lincolnshire. However, only a small number of sandstone quarries are still active, including those at Maidstone (Kentish Rag), Chilmark, Tisbury and West Hoathly. The thin Wealden sandstone beds known locally as Horsham Stone are also still actively quarried for roofing stone. In Norfolk the succession is locally characterised by the strongly ferruginous Carstone, which was widely used in many villages and is still quarried at Snettisham.
|Wimborne Minster, Dorset: Coarse-grained, brown, ferruginous sandstone (Heathstone) from the local Tertiary succession|
|English Bridge, Shrewsbury: Discoloration, blistering, salt precipitation and aggressive biological growth, the consequences of poor maintenance of a Grinshill Stone fabric|
|Caerphilly Castle, Caerphilly: Local Carboniferous Pennant Sandstone stonework, with white Jurassic (Sutton Stone) limestone window dressings|
|Crosland Hill Quarry: Using the natural fissility of the sandstone to produce flagstone from the Rough Rock Flags|
Sandstones occur within the Tertiary rocks of southern England and were common sources of vernacular building materials in Dorset, Wiltshire and Hampshire. In the past they have yielded coarse-grained, highly ferruginous sandstones (Heathstones) and the better-known grey Sarsen sandstones for building. The huge upright Sarsen sandstone blocks used in Stonehenge form perhaps one of the most recognisable historic structures in England. However, these siliceous sandstones were also extensively quarried and used locally in the 18th and 19th centuries in farm buildings, houses, churches and field walls around, for example, Marlborough and Avebury.
The building sandstone industry in Scotland has a long and significant history, which more recently has been tempered by a record of decline in the diversity of its sandstone production. The vernacular use of the variegated red-brown sandstones from the Devonian succession in the Orkneys, perhaps best displayed in St Magnus Cathedral (12th century), probably dates back to the Neolithic period. Today, however, despite their widespread use in the past across their outcrop in towns like Dundee, Perth and Inverness, they are no longer quarried to a significant extent. In contrast, however, the Devonian sandstone industry in Caithness is still thriving. Here several quarries actively produce the thinly bedded, dark-grey flagstones that have paved many of Scotland’s cities and towns for centuries and still provide one of its most significant building stone exports.
A rather different story is evident in the Carboniferous sandstone successions that dominate the Midland Valley of Scotland. Currently the production of once ubiquitous Carboniferous ‘blonde’ sandstones has dwindled despite a clear need for adequate supplies of these sandstones for conservation, repair and new build projects in Glasgow and several other large towns in the Midland Valley. Glasgow city centre alone has some of Britain’s finest sandstone buildings. Further east a similar story unfolds, the once famous Craigleith Quarry of Edinburgh is probably the most prominent of its sandstone quarries to have been lost to developments within the city. There is no other British city in which so many of its most notable buildings are constructed of sandstones from a single source. Modern sandstone repair to its historic buildings and new build projects are now heavily dependent on Carboniferous sandstone quarries south of the border.
The extensive red Permo-Triassic sandstone outcrops, most notably in Dumfrieshire and Ayrshire in south west of Scotland, remain one of Scotland’s most important sandstone producing areas. Quarries such as those at Locharbriggs, Spynie and Corncockle have long provided a range of red sandstones for local building and continue to export stone to England and occasionally further afield to the US and even Australia.
Currently with only a few quarries in operation, the modern building sandstone industry in Wales is significantly smaller than in England or Scotland. The quarrying and use of local sandstone for building stone, however, dates back at least to medieval times. The local Cambrian Harlech Grit sandstone was used extensively in Harlech Castle. By contrast, in South Wales such early buildings made extensive use of the local red-brown and grey Devonian sandstones (Tintern Abbey and Raglan Castle) or the ‘blue’ Carboniferous Pennant Sandstone (Caerphilly Castle). While in North Wales the abbeys of Basingwerk and Talacre were constructed using the local yellow-brown Carboniferous Cefn y Fedw sandstones.
By the 19th century, however, the Welsh building stone industry was firmly centred in the rapidly expanding coalfield areas of South and North Wales. In South Wales many small Pennant Sandstone quarries were used almost exclusively for constructing the thousands of terraced houses, shops, clubs and municipal buildings that typify the area. In each valley community extensive quarrying of the local sandstone crags produced the green- or blue-grey sandstones and flagstones that still characterise the architecture of the area. Currently, Pennant Sandstone production is sourced from a few remaining small quarries in the coalfield and nearby in the Forest of Dean. In North Wales a similar demand saw sandstone production focus on the yellow-brown Carboniferous sandstones of the Millstone Grit and the Pennine Coal Measures. Sadly none of these sandstones are currently being worked for building stone in the area and any replacement sandstone is likely to be sourced from quarries in Yorkshire or Lancashire.
In South Wales sandstone present in the Triassic succession was worked for building stone near Bridgend in the Quarella quarries. The fine, grey-green sandstones produced were used extensively for local buildings in the area.
Sandstones occur in the Silurian, Carboniferous and Permo-Triassic successions in Northern Ireland and have been quarried and used locally for building stone at a number of locations. The Silurian sandstones are generally grey, fine- to coarse-grained, poorly sorted and are commonly termed ‘greywacke sandstones’. They have been used in a number of notable historic buildings including Bangor Abbey in County Down. They are principally used as a vernacular rubblestone, but are not currently quarried for building stone. The Carboniferous succession of Northern Ireland includes the variegated, fine- to coarse-grained Ballycastle Sandstone of County Antrim which can range from pink to grey or white in colour. The sandstone was often used for bridge construction, for example in the Glendun Viaduct and the bridge at Ballycastle. The local red sandstone used in Northern Ireland is known as Dundonald Sandstone and was sourced from the Permo-Triassic succession in County Down. In general, however, the lack of indigenous sources suitable for building stone has resulted in extensive use of sandstones imported from mainland Britain and Ireland.
J Curran et al, Stone by Stone: A Guide to Building Stone in the Northern Ireland Environment, Appletree Press, Belfast, 2010
English Heritage, Practical Building Conservation: Stone, Ashgate, Farnham, 2012
JA Howe, The Geology of Building Stones, Edward Arnold, London, 1910
E Hyslop, The Performance of Replacement Sandstone in the New Town of Edinburgh, Technical Conservation, Research and Education Group, Historic Scotland, Edinburgh, 2004
E Hyslop et al, Stone in Scotland, UNESCO Publishing, 2006
GK Lott, ‘The Development of the Victorian Stone Industry’, in England’s Heritage in Stone, English Stone Forum, Folkestone, 2008
GK Lott, ‘The Building Stones of the Edwardian Castles’, in The Impact of the Edwardian Castles in Wales, DM Williams and JR Kenyon (eds), Oxbow Books, Oxford, 2010
GK Lott, Building Stones of the Isle of Wight, Proceedings of the Geological Association, London, 2012 (forthcoming)
English Strategic Stone Study, English Heritage and BGS