British slate

Slate is derived from mudstone and other fine-grained deposits, which have been metamorphosed by high pressure and temperature associated with shortening of the earth’s crust; the same forces which formed mountains. It is for this reason that slate deposits are generally located in mountainous regions. In Britain slate is located in the mountains of  Wales, Cumbria and the Highlands of Scotland. Slate is also found in Cornwall, although not a mountainous area,  it did experience crustal shortening associated with the Hercynian orogeny. Slates from all of these areas are still in service on British roofs today,  in spite of the fact that  production has been declining since its peak at the end of the 19th century. British roofing slates are still produced in Wales, Cumbria and Cornwall,  although no new Scottish slates have been produced since the 1960s.

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Scottish Slate

Scottish slate is still found on the roofs of traditional buildings  in Scotland over 60 years after the last of the Scottish quarries ceased to operate; a testimony to the quality of the material. Not all Scottish slate is the same, different varieties were produced in different parts of the country. The main types of Scottish slate, still found on roofs today, are (1) Ballachulish  from Argyll, (2) Easdale and the adjacent slate islands also in Argyll,  (3) Highland Boundary from a series of quarries stretching from Arran in the west to Dunkeld in the east and (3) Macduff slate from Aberdeenshire. For more information on Scottish slate see the history  of the Scottish slate industry and  the  geology of Scottish slate

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Welsh slate

Cwt y Bugail Quarry (SH732 456)

Penhryn Quarry in the Cambrian slate belt (SH623 650)

North Wales currently produces 85% of British roofing slates; approximately 35,000 tons per year. However this figure is only a small fraction of production in the 19th century, which reached 450,000 tonnes by the late 1890s, producing slates from over 70 quarries. In the early 20th century, the industry went into decline due to lack of modernisation and shortage of skilled manpower during and after the first World War.  Although the demand for slate in the 1920s was high due to an increase in house building, the demand by national building firms and  municipal direct labour organisations was for large volumes of identical slates which few quarries could meet. These are but some of the reasons for the decline of the Welsh industry which are well documented in several publications including  Alun John Richard’s “ Slate quarrying in Wales” (1995).

In the 1990s McAlpine and Sons Ltd was the most important slate producer in North Wales operating several quarries in the area. In 2007 the present company, Welsh//Slate took over the assets of the company and continues to produce roofing slates from two of its quarries; Penrhyn, Bethesda near Bangor,  and Cwt y Bugail ,  Blaenau Ffestiniog (SH732 456).  An independent company Greaves Welsh Slate Ltd. has produced slate from the Blaenau Ffestiniog area for over 180 years (SH700470).

Geological setting

There are two slate belts in North Wales, the Cambrian and the Ordovician  producing two very different types of slate. The Penrhyn quarry is located in the Cambrian belt while the quarries in the Blaenau Ffestiniog area are located in the Ordovician.

Cambrian deposits were laid down over 500 million years ago superimposed on the Precambrian rocks of North Wales. The oldest deposits were conglomerates which gradually became finer-grained  mudstones and shales.  These fine-grained deposits were metamorphosed into slate during the Caledonian Orogeny during late Silurian Period approximately 400 million years ago.  

Fluctuation in the depth of the basin in which the deposits were laid down, affected the colour and texture of the slate. For example the deep water deposits are finer-grained and have a characteristic red colour due to the presence of the iron ore mineral, haematite. As the basin filled up, the deposits became coarser grained with a higher quartz content.  These slates are sometimes green in colour. At one time there were numerous quarries located in the Cambrian slate belt, producing slates in several colours. Today only the Penhryn Quarry is still in production, producing a  purple blue slate, trading as ‘Heather blue’.  

Quarries in the Blaenau Ffestiniog area are located in the Ordovician slate belt to the SE of the Cambrian.  The original mud and silt were deposited in the Ordovician Period, approximately 450 million years ago, and metamorphosed into slates during the Caledonian Orogeny. They differ from the Cambrian deposits in that the original muds were laid down in a  low-oxygen environment resulting in slates which are dark blue-grey in  colour and containing the iron ore mineral, pyrite. The Cwt y Bugail and Greaves quarries are located in the Ordovician slate belt.

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Geological Time Scale

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Orogeny or mountain building

Orogeny: The earth’s crust is made up of ‘plates’ which are continually in motion relative to each other. As a result some parts of the earth’s crust are under compression while others are under tension. In areas undergoing compression, rocks are deformed by folding and faulting, forming mountains in the process and altering the nature of the constituent rocks by a process known as metamorphism.  Although in many cases the resulting mountains have been eroded away, evidence of their existence remains in the deformation and metamorphism of the rocks exposed at the surface.  Slate is an example of a mudstone which has been metamorphosed due to heat and compressive stress. The conditions required for this to happen can only be found at a depth of  10-15km, hence wherever slate is found close to the surface, it can be inferred that the overlying rock has been worn away.

There were two principal mountain building events in Britain during which slate was formed.  The earlier of these was the Caledonian Orogeny which occurred during the Ordovician, Silurian and Devonian periods, resulting in a mountain belt stretching from Scandinavia in Europe to the Appalachians in North America. In Britain the main activity took place in the Ordovician period resulting in the Highlands of Scotland, and the mountains of North Wales. Most of the slate in Britain was metamorphosed from mudstone during this Orogeny.

The second orogeny affecting Britain was the Variscan or  Hercynian Orogeny which occurred during the Carboniferous and Permian periods. The folding and faulting associated with this orogeny  can be seen in North America and Central Europe. In Britain the main deformation occurred in the south of England and is  associated with the formation of Cornish slate.

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Flagstones are split along bedding planes.

A barn roofed with Caithness flagstone on Hoy, one of the Orkney Islands

Flagstone roof near Housesteads Northumberland

There are also several areas in Britain where the traditional roofing material is locally sourced flagstones; such as  Caithness flagstone in the north of Scotland or Northumberland stone in the north of England.  
Flagstones are sedimentary rocks which are capable of being split along primary bedding planes. They are also referred to as “grey slates” to distinguish them from blue or real slate.  The type of sedimentary rock varies; sandstone, limestone and sandy shales are all used as roofing materials, for example limestone of the Great Oolite in the Cotswolds, and carboniferous sandstone is common in the north of England. Only those in which the original bedding planes are spaced  between 15 and 20mm are suitable  for roofing; closer spaced bedding produces a material which is too friable. Conversely, flagstones produced from seams in which bedding is more widely spaced are too thick and heavy to be widely used as roofing.
Flagstone, being thicker and therefore heavier than real slate, were not normally transported far from  source. As a result, the type of roofing material and hence the vernacular architecture was influenced by the local geology providing variation so important to the built heritage.
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Cornish slate

Delabole Quarry (SX075 840)

Cornish slate has been used as a building material for well over 600 years, and has been quarried continuously since the early 17th century. In general it has been used as paving, walling and decorative purposes, however in the area around Delabole in the north west of Cornwall is it of sufficiently high quality to be used for roofing. In the early 19th century there were many small quarries in the area, five of which joined to form the Old Delabole Slate Company. At a time of a general downturn in slate production the company was liquidated in 1977 and changed ownership several times  The Delabole Slate Company is now  owned by the Hamilton family. The present quarry encompasses the original five quarries and is now 800m x 6000m in area and 140m deep. It is located close to the village of  Delabole (SX075840).

Roofing slate is also produced from the Trevillet Quarry Trevillet,Tintagel  (SX082 882). This quarry is owned by Mill Hill Quarry Ltd.  Tavistock Devon. The company was established in 1959 when the disused Mill Hill quarry (SX452750) was reopened. It acquired Longford quarry in 1984, and the Trevillet quarry in 1990 Only the Trevillet quarry produces roofing slates.    

Geological Setting

 The Delabole and Trevillett quarries are located in the Delabole slate bed formation. The original deposits, which make up the formation, were laid down in the  Upper Devonian over 360 million years ago and  metamorphosed during the Hercynian Orogeny approximately 300 million years ago.  The slate is a blue-grey colour and very durable and is still found locally on  buildings over a hundred years old.

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Cumbrian slate

Kirkby-in-Furness Quarry (SD250837)

Elterwater Quarry (NY324048)


 Slates from Kirkby Moor in the south-west of Cumbria have been used as roofing material for over 400 years. At first production was small-scale carried out by individual tenant farmers.  However by the mid 18th century, the rising population, and rapid growth of cities during the  Industrial Revolution  increased the demand for roofing stone  and hence required larger scale and more efficient production methods. As a result independent slate operators were phased out and replaced by a single authority able to pool resources and improve efficiency. The present company Burlington Slate Co. was  established in 1843 by Lord Cavendish, second Earl of Burlington and later 7th Duke of Devonshire to extract slate from the  Kirkby-in-Furness Quarry  (SD 250837).  The arrival of the railway in Kirkby shortly after the establishment of the company greatly facilitated the distribution. of the slates. Production continued to increase reaching a peak of 15,930 tonnes in 1863.  However, in common with British slate producing areas, annual production declined from just under 10,000 tonnes at the beginning of the 20th century to less than 2000 tonnes in the 1970s. As with the Welsh industry, production has now recovered and the quarry is producing annually almost 4000 tonnes of Burlington blue slate. 

 Geological setting:   Slates are extracted from the Kirkby Moor Flags Formation. This is a thick, sorted homogeneous siltstone of Silurian age, dark blue-grey in colour with occasional beds of finer-grained material cutting across the surface. Slightly calcareous beds are common

Burlington Slate Co. also produces a green slate, trading as Westmorland,  from Elterwater (NY324048) one of several quarries in the Lake District located in the Borrowdale Volcanics.  Many of these quarries operated as independent companies in the 19th century before being taken over by larger concerns.  At one time the Lakeland Green Slate Company Ltd, operating four quarries, was the largest slate producer in the Lake District but was taken over by Burlington in 1975.  The Elterwater slate quarry and Broughton Moor, were bought initially by the Old Delabole Slate Co, and subsequently in 1976 bought by Burlington. However slate extracted from the Broughton Moor quarry is not used as a roofing material  but for other architectural uses.  

Geological setting: Westmorland slate is formed from fine-grained volcanic ash or tuff. This is part of the Borrowdale Volcanic Group, comprised of subaerial lavas, tuffs and agglomerates, which erupted in the Ordovician period between 400 and 450 million years ago. These volcanic rocks form most of the high mountains in the Lake District. Two seams of fine-grained tuffs were worked for roofing material. The lower seam, at the base of the Borrowdale Group near Honister Pass, produced grey-green slabs and the upper seam, to the SE near Ambleside, produces green slabs(Cameron 1996). The material has a high chlorite content which gives them their characteristic green and grey colours. Bedding features such as ripple marks, cross lamination and graded bedding can be seen on the  surface, making them a popular stone both for roofing and other architectural purposes.

Today, the Elterwater quarry, located in the upper seam, is the main quarry producing Westmorland green roofing slates. It employs a workforce of 7 and extracts annually over 400 tonnes of roofing slate and a similar amount for other architectural purposes. All the slate is processed centrally at the Kirkby-in-Furness Quarry.

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Traditional roofing

In a wet climate such as Britain’s, there is an universal requirement to cover buildings  with a suitable waterproof material. In the past the choice of material was depended on local availability. Hence, in the absence of suitable stone, thatch, either of straw or reeds, was commonly used throughout the country, while clay tiles were used  in areas, such as the southeast of England, with  suitable clay deposits. Flagstone was used locally in many parts of Britain, such as Horsham stone in Sussex or Caithness flagstone in the north of Scotland.    Slate, which is generally found in  mountainous parts of Britain,  was used  as a building material in the North Wales,  parts of Scotland and the Lake District.  This use of locally sourced stone led to regional vernacular architecture reflecting the local geology.   

Unlike flagstone which was rarely transported far from its source, the distribution of slate gradually increased in the 19th century, spreading out from its source, often in remote areas, along historic trade routes.  Improved transport systems coinciding with rapid urban growth resulted in slate being transported to all the major cities in Britain and Ireland, eventually becoming the principal roofing material.  The industry expanded rapidly to accommodate this demand, producing over 650,000  tonnes per annum in 1898. In spite of the rationalisation of the industry, and more recently globalisation, it is still possible to recognise the historical trading links. For example, because of the ease of transport across the Irish Sea, Cambrian slate from North Wales was used widely throughout Ireland and is still the preferred slate in that country. Similarly Cumbrian slates were transported northwest along the coast into Ayrshire and are still used extensively in the SW of Scotland.  Scotland too had a significant slate industry, producing from four different areas of which Ballachulish is the best known. 

Production started to decline soon after 1900 and had already dropped to 111,000 tonnes in 1918. The industry partly recovered in the 1920s, to 297,000 tonnes in 1929, but by then manufactured clay tiles had become a major competitor and were taking an increasing proportion of the roofing market. This decline in production continued to the end of the 20th century reaching a low of 25,000 tonnes in 1993 followed by a modest recovery.  Quarries in Wales and England continued to close although production never ceased completely. However, in Scotland the largest quarry at Ballachulish closed in 1955 and the last remaining Scottish quarries closed in the 1960s.



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Mineralogy of slate

The principal minerals present in slate are quartz, white mica and chlorite. However due to the fine-grained nature of the rock, it is not easy to identify any of these with the naked eye.  Other accessory minerals such as pyrite and pyrrhotite (iron sulphides),  may be seen when present in clusters or as individual cubes. However, these minerals usually comprise less than 5% of the total.  Other constituent minerals can only be  detected using more sophisticated methods.

In coarser-grained slates is may be possible to identify the principal minerals as follows:

Quartz. Individual grains of quartz are common in coarse-grained slates and may be seen with a hand lens. They are generally rounded, less than 0.5mm in diameter and have a sugary texture.  There is no alteration in appearance due to weathering.

Chlorite  Chlorite is present in slate with concentrations ranging  from 20 to 50% but is not normally visible even with a hand lens. When present in sufficiently high concentration, it gives the slate a green colour, however this  colour is easily masked by  small amounts of other minerals, such as graphite or haematite.   There is however one important exception to this generalisation;  small specks  of chlorite are visible in Scottish Macduff slate derived from an area closest to a nearby igneous body. These specks are the remains of the mineral biotite, which grew  millions of year ago due to the increased temperature in the surrounding rock at the time of the emplacement of the igneous body.  In most cases, the biotite minerals have subsequently been weathered to chlorite, but the outline of the original biotite mineral remains.  This speckled appearance is one of the characteristics used to identify Macduff  slates.  

 White mica  is a general term covering various minerals from clay to illite to muscovite, the particular type depending on the composition of the rock but more importantly the degree of metamorphism.  With increasing metamorphism easily-weathered clay minerals are gradually replaced by a white mica  with  a composition and structure approaching that of muscovite, which is the least prone to weathering.   Hence the the degree of metamorphism is a useful criterion in assessing the durability of the slate.  

It is not possible to recognise individual mica minerals in  a  slate, but with increasing temperature and pressure, characteristics of  increasing metamorphism, individual mica grains increase in size  giving the rock a  slight sheen as oberved in phyllites such as Ballachulish slate.  As this process continues even further, the rock passes from being a phyllite into a mica schists where mica  grains are easily identified by their flakiness and pale yellow colour.  

Pyrite, pyrrhotite and graphite are all commonly found in slates formed from muds deposited in a stagnant, low-oxygen environment. 

Pyrite      This mineral is found in all shapes and sizes from large metallic crystals with well-defined edges (euhedral) to amorphous powder (anhedral). They are found in clusters or randomly distributed throughout the slate. They may also be associated with a particular bedding layer within the slate.  Metallic euhedral crystals of pyrite are not prone to alteration due to weathering but retain their metallic appearance. In contrast, leaching and brown staining around individual grains is common in amorphous pyrite and pyrrhotite which have been exposed to weathering. In some cases, the whole cluster falls out leaving a hole.

Pyrrhotite  is not normally distinguishable from pyrite by its appearance but can occasionally be identified by its magnetic properties. It is much more prone to alteration from exposure than amorphous pyrite.

 Graphite is present as black greasy powder. Although it is not affected  itself by exposure, it can act as a catalyst accelerating the deterioration of other minerals.

Haematite is not visible to the naked eye. However its presence can be recognised by its purple colour. It is an oxide of iron (Fe2O3) found in slates formed from deposits laid down in oxidising conditions. It is the most durable form of iron and not affected by exposure.

Carbonates: the most common forms of carbonate found in slate are calcite CaCO3.  Its presence can be detected by a drop of acid which makes it fizz. In the presence of pyrite it may react to form gypsum which is detremental to the slate. It is generaly not recommended that slates with carbonate be used in a poluted environment.  On the other hand dolomite CaMgCO3 the carbonate found in Ballachulish and Easdale slates is unaffected by acid and very durable.

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