Welsh slate

Cwt y Bugail Quarry (SH732 456)

Penhryn Quarry in the Cambrian slate belt (SH623 650)

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 due to lack of modernisation and shortage of skilled manpower during and after the first World War.  Although the demand for slate in the 1920s was high due to an increase in house building, the demand by national building firms and  municipal direct labour organisations was for large volumes of identical slates which few 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 produced slate from the Blaenau Ffestiniog area for over 180 years (SH700470).

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.

Cambrian deposits were laid down over 500 million 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.

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