Traditional roofing

A waterproof material to cover buildings is an universal requirement, not least in a wet climate such as Britain’s. In the past the choice of material usually 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 with  suitable clay deposits, such as the southeast of England, . 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.

 
 
 
 

 

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  can be seen with a hand lens  in coarse-grained slates. 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 mineral 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 where originally 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  

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 matamorphism due to increasing temperature and pressure, 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

The mineral pyrite  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  mineral 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

Haematite 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.Haematite is not visible to the naked eye, however its presence can be recognised by its purple colour.

Carbonates:

There are several  common forms of carbonate found in slate, the most common of which is  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 detrimental to the slate. It is generally not recommended that slates with carbonate be used in a polluted environment.  On the other hand dolomite CaMgCO3 the carbonate found in Ballachulish and Easdale slates is unaffected by acid and very durable.

Geology of Scottish slate

 

Slate was produced from four different areas in Scotland: (1) Ballachulish slate from a group of quarries located in Ballachulish near Fort William in Argyll, (2) Easdale slate from a group of islands, including Easdale  from which it gets its name,  south of Oban also in Argyll,  (3)  Highland Boundary slate from a series of quarries just north of the  Highland Boundary line, stretching from Arran in the west to Dunkeld in the east.  These are grouped together by their common geology rather than location and finally (4)  Macduff slate from a range of hills, sometimes referred to as the Slate Hills, just east of Huntly in Aberdeenshire.  The name Macduff refers to the  geological formation from which they are extracted. Although a different type of slate was produced from each of these groups, they are all metamorphic rocks of the Dalradian Supergroup, located between the Highland Boundary Fault and the Great Glen Fault.

The Dalradian Supergroup consists of sediments laid down in the Precambrian Era between 770 and 560 million years ago and metamorphosed during the Caledonian Orogeny over 450 million year ago. Most of Dalradian consist of rocks which have been intensely metamorphosed and hence too course-grained to yield slate. Instead slate is found in areas of low-grade metamorphism,  known as the greenschist facies.

Scottish slate quarries are located withing the low-grade metamorphic zone known as the greenschist facies.

 

 

 

 

 

 

The Dalradian Supergroup is divided into four groups; the Grampian, Appin, Argyll and Southern Highland Groups.  No slate was produced from the Grampian Group. Ballachulish slate is part of the Appin Group and Easdale slate is part of the Argyll Group, while the remaining two types, Highland Boundary and Macduff, are located in the Southern Highland Group.   The characteristics of slate from each group depend on the environment of deposition of the original sediments and on the degree of deformation during the Caledonian Orogeny (Richey & Anderson. 1944, Walsh 2000, 2002).

Easdale slate

Easdale Island can be seen in the background

Ellanbeich quarries. Easdale Island can be seen in the background

The Scottish slate industry probably started on the island of Easdale on the west coast of Scotland. It is not known precisely when quarrying began, but there is a report of a cargo of slates being sent to St Andrews in 1168.  At about the same time, the Norwegians discovered  slate on the Island of Belnahua nearby. Reliable  records began in 1745 when the Earl of Breadalbane established the Marble and Slate Company and opened slate quarries  on Easdale Island. As the demand for slate increased, the company expanded rapidly, establishing several quarries on the  adjacent islands of  Ellanbeich, Luing and Seil. (Ellanbeich is no longer an island but connected to Seil by slate waste.) For the next century Easdale slate dominated the Scottish slate industry until superseded in 19th century by Ballachulish. It survived many disasters; the introduction of a tax in 1799 on slate transported by sea was particularly onerous for quarries on these remote islands.  Natural disasters also threatened the survival of the quarries. In 1879, the night of the Tay Bridge disaster, several of the islands were swept by exceptionally high tides flooding  quarries and houses alike. In this case most of the quarries were pumped out and work continued. However a few years later, in 1881 another storm caused severe damage on Easdale and Ellanabeich, breaching the seawall of one of the Ellanbeich quarries which was never restored.

Production reached a peak at the end of the 19th century, producing 10 million slates annually. However in the 20th century, all of the quarries faced the common problems of the industry, namely competition from imported slate and artificial roofing materials.  Production ceased completely during the First and Second World Wars and in both cases  a few quarries never reopened afterwards. Production finally ended in 1960s when the remaining few quarries closed.

 The geology of the Easdale area is composed of coarse-grained quartzite, superimposed by fine-grained slate, giving way gradually to marble and then phyllite. These are all metamorphosed sediments of the Argyll Group, the second youngest group of the Dalradian Supergroup.  There is an abrupt change from coarse-grained sediments, which have been metamorphosed to quartzite, to fine-grained muds which have been metamorphosed to slate. This rapid change from shallow to deep water sedimentation is probably due to subsidence of the sea floor. As the basin fills up, there is an increasing supply of turbidites, giving way gradually to carbonate deposits which have become limestone.   (Although  metamorphosed carbonate rocks are marble, those in Dalradian Supergroup are usually referred to as limestone.)

Easdale slates  are similar to Ballachulish  in that they contain carbon and iron sulphide minerals. They also have a well-developed crenulation cleavage which is  often used to distinguish between Ballachulish and Easdale slates. However, this identification is not conclusive, as crennulation cleavage is also present in Ballachulish although  generally not as well-developed as in Easdale. Other properties, such as the concentration of barium, need to be taken into account to distinguish between these slates.

Highland Boundary Slate

Aberfoyle Quarry

Southern Highland Group is the youngest member of the Dalradian Supergroup. The rocks of this Group are located  to the north of the Highland Boundary Fault, which extends across Scotland from the Mull of Kintyre in the west to Stonehaven in the east. Associated with this fault zone is a rampart of hills which makes a striking topographical feature marking the bounday betweenthe low rolling countryside of the Midland Valley and the rugged Highlands.  The Highland Boundary Slate quarries of the Group are not from a continuous belt but from different formations within the Group, located at intervals  between Arran in the west and Dunkeld in the east, just to the north of the Highland Boundary Fault.

The original sediments of the Southern Highland Group were deposited by turbidity currents on a subsiding continental shelf, forming major submarine fans of terrigenous sediments. The slate, formed from fine-grained mud, represents the  more distal parts of these fans. Due to the oxidising conditions during deposition of the original mud, Highland Boundary slates do not contain graphite or sulphide minerals. The typical iron ore mineral  present in these slates is haematite and the usual carbonate mineral is calcite. Colour is variable with blue-grey, green and purple often found in the same quarry. Different bands of colour are indicative of primary bedding freatures.  The largest quarries of Southern Highland Group are Aberfoyle, Birnam and  Dunkeld. Other smaller quarries are located in Arran, Bute, Luss, Comrie and Logiealmond some of which no longer appear on the OS maps.  For more on the information on the individual quarries read Scottish Slate Quarries Technical Advice Note 21 published by Historic Scotland in 2000.

Ballachulish Slate

East Laroch, largest of the Ballachulish quarries.

 

Ballachulish slate from the East Laroch and Khartoum quarries is grey-black with a slight sheen. It is coarse-grained, giving the slate a gritty texture. One of the most distinctive characteristics of this type of slate is the strong mineral lineation clearly visible on the surface.  Pyrite grains are common and are usually widely dispersed throughout the slate. The smaller grains are subeuhedral, i.e. they have recognizable but imperfect crystal faces, while the larger grains are anhedral, having irregular faces. In addition there are large clusters of pyrite grains concentrated in quartz veins running through the blocks of slate. The slate is very durable due to the higher than average metamorphic grade and the coarseness of the quartz grains. Pyrite grains when present in an euhedral form, are very resistant to weathering.

 

Not all Ballachulish slate is of the same high quality, in some quarries the pyrite crystals have been altered to a less stable mineral pyrrhotite which is prone to leaching and often fall out leaving a hole.

 

Ballachulish slate containing pyrrhotite are prone to weathering

Macduff slate

Many buildings in the area are still roofed with Macduff slate over a hundred years after production ceased.

Macduff Slate

Slate was extracted from the so called Slate Hills  of Kirkney, Corskie, Foudland, Tillymorgan and others in the NE of Scotland. Of these the most important quarries were on the Hill of Foudland and the name Foudland is sometime used as a generic trem for slates from the area.  Production started in the 1700s and reached a peak of almost 2 million slates in the mid 19th century. Most of the quarries closed during the second half of the century as the development of railways enabled slate from other parts of  Great Britain to be sold competitively in the area.

One of several quarries on the Hill of Kirkney. All of the quarries are very overgrown.

All of the quarries are located within the Macduff Slate Formation.  This Formation outcrops over a large part of the NE of Scotland, from Macduff on the coast, from where it gets its name, to Huntly 50km  to the south.   However slate has only been quarried as a roofing material in a range of hills just south of Huntly.  This is due to the proximity of an igneous intrusion which, due to increased temperatures at the time of emplacement, hardened the surrounding rock. As a result of this hardening, the slate rock forms the high ground, relative to the softer slate to the north.

Macduff slate have a rough gritty texture of a coarse-grained material, rich in quartz. It is possible to see small grains of quartz on the surface. The slates are generally blue-grey in colour often with a purple hue.  Unlike Ballachulish and Easdale slate, there is no pyrite present. Instead, the iron ore mineral  is an oxide, haematite, which gives the slates a purple colour. The most distinctive property of Macduff slate is “spotting”: small dark specks approximately 0.5 mm in size evenly distributed throughout the slate. These dark spots are mainly chlorite with mica intergrowths along the cleavage.

Macduff slate is still found on the roofs of buildings in the area over a hundred years after production had ceased; a testimony to the durability of the material. The Scottish Stone Liaison Group, in an attempt  to find new sources of Scottish slate, selected the Hill of Foudland as one of two locations for the extraction and testing of new slate. In 2003  blocks of rock were extracted  and split into slates from one of the Lower Quarries on the Hill  ( NJ608337), the first new Macduff slate in over a century. In 2005 this exercise was followed by the extraction of two cores, over 40m in length, from the floor of the quarry in order  to assess the resources of slate in the vicinity. (The results of this exercise are recorded in “Macduff Slate; Extraction and testing of slate from the Hill of Foudland, Aberdeenshire.” published by Historic Scotland in 2008.

Scottish Slate Industry

Historic buildings were traditionally roofed with Scottish slate.

A brief history of the Scottish slate industry

Slate quarrying was one of Scotland’s most significant building material industries throughout the 18th, 19th and early 20th centuries. It was concentrated at four geological areas namely: (1) Ballachulish near the Great Glen Fault, (2) Easdale and the surrounding Slate Islands near Oban, (3) a series of quarries just north of the Highland Boundary Fault and (4) the Slate Hills in Aberdeenshire and Banff.

Location of main slate producing areas in Scotland.

In the 18th century Easdale and the adjacent Slate Islands were the centre of the Scottish slate industry, but by the 1860s production of Ballachulish slate had exceeded that of Easdale and it continued to dominate the Scottish industry for the next hundred years. At the time of maximum production, the main Ballachulish quarry at East Laroch was producing 15 million slates per annum (Mineral Statistics 1882-1888), or approximately 18,000 tonnes. The expansion of the railway system in the second half of the 19th century facilitated the transport of cheaper Welsh slates and heralded the decline of the Scottish slate industry as a whole. However, the inroads made by the Welsh industry were not felt immediately and production in Scotland continued to increase, reaching its peak of 45,000 tonnes at the end of the 19th century. Production started to decline soon after 1900 and had already dropped to half its maximum level by 1910. Production ceased completely during World War I due to lack of manpower. The industry partly recovered in the 1920s and 1930s from 3 tonnes per annum in 1920 to 23 tonnes in 1929 (Statistics: Annual Report of the Mines Department, Board of Trade), but by then manufactured clay tiles had become a major competitor taking an increasing proportion of the roofing market. No separate figures for slate production were reported for Scotland between 1945 and 1964 but the last return in the statistical accounts recorded 5 tonnes in 1966. At this time the final quarries closed, although some small-scale production by individual quarrymen continued after that time.

There has been no production of Scottish slate since the 1960s. However in an effort to identify new indigenous sources of slate, the Scottish Stone Liaison Group carried out a programme of tests  from 2002 to 2005 at two locations; Khartoum, one of the Ballachulish group of quarries, and the Hill of Foudland one of the Macduff group in Aberdeenshire in order to assess the feasibility of establishing a new source of Scottish slates. The results of these tests are reported in two research reports Ballachulish Slate. Extraction and testing of slate from Khartoum quarry  and Macduff Slate. Extraction and testing of slate from the Hill of Foudland both published by Historic Scotland in 2008.

Crenulation cleavage

Crenulation cleavage is found in rocks which have undergone multiphase deformation. It is due to folding of the original cleavage during subsequent episodes of deformation. It gives the surface of the slate a crinkled or crenulated effect. It is one of the characteristics commonly used  to identify Scottish Easdale slate.  However it is not conclusive as it is also found in Ballachulish slate,  In most cases polyphase deformation distorts the original cleavage making the rock uneconomic for slate production.