Monthly Archives: November 2011

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.

Flagstones

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