Monthly Archives: February 2011

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.

Cleavage

 Slate belts are usually found in mountainous areas where geological forces have deformed and shortened the earth’s crust.  Rocks respond differently to these compressive forces; some such as quartzite respond by folding while other weaker rocks, such as mudstone, develop a cleavage perpendicular to the direction of maximum stress. Under the microscope, it can be seen that this cleavage consists of  individual grains of quartz, which have been flattened, separated by platey minerals (phyllosilicates) white mica and chlorite.  It is  these zones of platey minerals, called cleavage domains, which enable the rock to be split into slabs suitable as roofing slates.

 

Closely spaced cleavage domians enable this slate to be split less that 4mm thick
Intermediately spaced cleavage domains enable this slate to be split between 7-10mm thick

 

Discontinuous and irregularlly spaced cleavage domains make this slate difficult to split.

The thickness of roofing slates varies from 3mm to over 15mm depending on the spacing of the cleavage domains present.  Shape too affects the ability to cleave the rock; the straighter and more continuous the cleavage domains, the more easily the rock is split Other factors such as the  proportion of rock occupied by the cleavage domains also , affects the ability to cleave into slates.

Quartz-rich rocks shortened by folding, while mudstones nearbye developed a slaty cleavage

Identification of slates of Royal Cottage

Case Study

There is no single procedure used to identify used slates, but rather a combination of methods ranging from local knowledge to more scientific analyses . For a general discussion on the methods used see Identifcation of used slates  The identification of the slates of the Royal Cottage are just one example of the application of some of these methods.

View of the intake of Glasgow’s main water supply

The front of the Royal Cottage

Royal Cottage is situated on the south shore of Loch Katrine at Stronachlachar approximately 20km north-west of Aberfoyle.   Loch Katrine became the primary water source for the city of Glasgow and the surrounding area when in the middle of the19th century an aqueduct was built to transport water to the city. Construction of the aqueduct started under the supervision of James Watt and Thomas Telford and was completed in 1859 (www.incallander.co.uk accessed 08/01/2010). The Royal Cottage was built to accommodate Queen Victoria on the occasion of the inauguration of the aqueduct. However, she never actually stayed in the Cottage as the windows were shattered during the 21 gun salute. 

 In November 2009 the Scottish Stone Liaison Group requested that slates from the roof of the  Cottage be tested in order to establish their provenance.     Using XRD and trace element analyses (as described below), it was found that the slates were from one of Aberfoyle Group of quarries

The slates of the Royal Cottage

Methodology: The slates vary widely in thickness, width and length from which it was inferred that they are Scottish.  However the absence of pyrite crystals or crenulation of the surface excludes Ballachulish and Easdale slates. For these reasons, and substantiated by their grey-green colour, the source of the slates was identified as one of the Highland Boundary Group of quarries.   This Group consists of a series of quarries  located at intervals just north of the Highland Boundary line from Arran to Dunkeld (Scottish Slate Quarries 2000 J A Walsh, Publishers Historic Scotland). However, of these the most likely quarries are Luss and Aberfoyle which are the closest geographically and are known to have produced slates in the 19th  century.  

To distinguish between  slates from the different Highland Boundary quarries,  X-Ray Diffraction  (XRD) scans of the Royal Cottage slates were compared with those in the database. One  of the characteristics of the  XRD scans used in distinguishing between different Highland Boundary slates is the shape of the white mica peak (or peaks) at 9° 2 theta angle.  In some slates there is a double peak at this location due to the presence of the sodium-rich mica, brammulite in addition to the normal potassium-rich mineral     In Bute  slates this secondary peak is well-defined, in Lusss slates less so, while in Aberfoyle slates it appears as a small lip on the usual white mica peak.  It was found that the Royal Cottage samples lacked the double peak of Luss and Bute slates, matching instead the shape of the peak of Aberfoyle slates in the database.  This observation was substantiated by comparing selected  trace element concentrations of the Royal Cottage samples with those in the database.  It was therefore decided that the Royal Cottage slates were from the Aberfoyle group of quarries.