Monthly Archives: March 2011

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.

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 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.


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 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.


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.