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Identification of slates

The Watt Library, the Watt Hall and the Mclean  Museum and Art Gallery, Greenock

The Watt Library, the Watt Hall and the Mclean Museum and Art Gallery is a group of buildings located in the block between Union Street, Kelly Street and Watt Street in the centre of Greenock (Figure 1). The first building to be built was the Watt Library, built in 1832 and extended in 1837. The Watt Hall and the Mclean Museum were built in 1876. The Art Gallery is a modern building, built in 1955.

Figure 1: Aerial view of the buildings. This photo was provided by Collective Architecture.

The Watt library








Figure 2: Watt Library from Union Street.

The Watt Library 

The slates were originally blue-grey in colour which have darkened on the weathered surfaces. The dimensions of the slates are variable which indicates that they are Scottish. The presence of cubic crystals of pyrite and the crinkling surface called crenulation cleavage are characteristic of slate from Ballachulish or one of the Slate Islands. To distinguish between these two sources, it is necessary that XRF trace element analysis be carried out to determine the concentration of barium and other key elements used to distinguish between these sources. Some of the original slates have been replaced with Welsh purple.



Figure 3: The slates on the Watt Library were identified as either Ballachulish or from one of the Slate Islands on the west of Scotland.  Some of the original slates have been replaced with Welsh purple. 

The Library Extension

The slates on the Library extension were not identified.









Figure 4: The north-west extensions to the library.

The Watt Hall and the McLean Museum

Figure 5: Watt Hall and McLean Museum from Kelly Street


The slates on Watt Hall have a purple tint and are all the one size, namely 400x200mm. The uniform dimensions suggests that the slates are not Scottish and the purple colour suggests that they are Welsh from one of the Cambrian group of quarries. There was only limited access to the roofs of the Museum, however those slates that were inspected were similar to those on the Watt Hall. As both buildings were built at the same time, it is not surprising that the slates are from the same source.

Figure 6: The slates on the  the Watt Hall were identified as Welsh, from of the Cambrian group of quarries








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.

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

Welsh slate

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 because of lack of modernisation and shortage of skilled manpower during and after the first World War. Tthe demand for slate in the 1920s was high due to an increase in house building, however  national building firms and  municipal direct labour organisations demanded large volumes of identical slates which few Welsh 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 continued to produce slate from the Blaenau Ffestiniog area for over 180 years.

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.


Penhryn Quarry in the Cambrian slate belt (SH623 650)

Penhryn Quarry in the Cambrian slate belt (SH623 650)

Cambrian deposits were laid down over 500million 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.

Cwt y Bugail Quarry (SH732 456)

Geological Time Scale

The following table shows the geological periods during which the original muds and silt which later became slate were deposited.The approximate ages of these periods are expressed in millions of years before the present day. Similarly the two main orogenies during which the original deposits were metamorphosed to slate are also shown. 

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.


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.

Flagstones are split along bedding planes.

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 25mm 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.


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

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.

Flagstone roof near Housesteads Northumberland

Cornish slate

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.

Delabole Quarry (SX075 840)

Roofing slate is also produced from the Trevillet Quarry Trevillet,Tintagel.   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. The company acquired Longford Quarry in 1984, and the Trevillet Quarry in 1990 However 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.

Cumbrian slate


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 larger scale and more efficient production methods were required. 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 the other 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. Similar to the Welsh industry, production has now recovered and the quarry is producing annually almost 4000 tonnes of Burlington blue slate.

 Geological setting: 

Kirkby-in-Furness Quarry (SD250837)

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 were operated as independent companies in the 19th century before being taken over by larger concerns. Until the 1970s the Lakeland Green Slate Company Ltd operated four quarries and was the largest slate producer in the Lake District until it was taken over by Burlington in 1975.  The Elterwater and Broughton Moor slate quarries, were taken over initially by the Old Delabole Slate Co, and subsequently in 1976  by Burlington. Slate extracted from the Broughton Moor Quarry is not used as a roofing material  but for other architectural uses.

Elterwater Quarry (NY324048)

Geological setting:

Westmorland slate is formed from fine-grained volcanic ash or tuff,  part of the Borrowdale Volcanic Group, This Group is 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 are sufficiently fine-grained to be  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.