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

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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