Slate is a metamorphic mudstone produced when a deposit of fine sediment is heated and compressed due to the movement of tectonic plates, causing the crystals of mica to be compressed and creating new cleavage planes at 90 degrees to the line of compression. These cleavage planes bear no relation to the original bedding planes, which can often still be seen in the material.
Slate has been quarried in areas of Cumbria, with Westmorland being the most famous, in Leicestershire, where Swithland slate is still often seen, and in Devon and Cornwall, where the Delabole slate quarry is one of the largest in England, measuring over 2.5 km in circumference and about 400 metres deep. Historically, each of these areas has had a number of producing quarries, but in England only Westmorland and Delabole are still pro – 116 ducing today.
Almost all English-quarried slates would have been random slates, produced in a range of lengths and widths, and hung using similar techniques to the original stone, slate and wood shingle industries. It is only since the expansion and mechanisation of the Welsh slate supply that tally slating (slates sold by the number of identical sizes – tally) has become common.
Welsh slates are the most common indigenous slate seen across the UK today, but they are only one of a number of varieties that have been used historically, some of which are still in production. Indeed, Welsh slates only became very common following the opening of the railway across North Wales to Holyhead in 1848, which allowed the material to be shipped across the country both rapidly and economically.
It is possible to see the historical development of many areas of the country by assessing the roofscape. The oldest settlements have evidence of the local vernacular roofing materials; with the opening of the canals in the north of England in the eighteenth century, sandstone slates were transported greater distances from their quarries, and they are seen along the routes of the canals; when the railways opened in the mid-nineteenth century, Welsh slates travelled nationwide and are to be seen in many places almost exclusively around railways.