Home > Start > Contents >  A Tour of the Stone Slate Regions > Devonian - Old Red Sandstone
  • Introduction: Stone-slate geology
  • Stratigraphy
  • Rocks and climate
  • Map
  • Cretaceous - Wealden
  • Jurassic
  • Permian and Triassic
  • Carboniferous
  • Devonian - Old Red Sandstone
  • Silurian - Pridoli: The Tilestones
  • Ordovician
  • The Devonian or Old Red Sandstones are found from Cornwall to Shetland and although they were only used for roofing in a few locations some of these were very large industries producing a range of products, especially flagstones for 18th and 19th century towns and cities World-wide. The most important quarries were in Caithness and at Carmyllie in Tayside but they have also been significant sources of stone-slates in the Welsh Marches and South Wales.
    The geology of the Silurian - Devonian boundary is difficult and has taken a long time to unravel. This has lead to a lot of debate about the exact position of the  'boundary' formation, the Downtonian. It was previously placed at the bottom of the Devonian but is now at the top of the Silurian - in the Pridoli, so care is needed when using older texts and maps.
    Devonian geology is differentiated into five main units but one of these is only present in England and one only in England and Wales
      Upper Devonian and Upper Old Red Sandstone  
      Limestone England only
    Devonian Middle Devonian. Middle Old Red Sandstone  
      Lower Devonian England and Wales only
      Lower Old Red Sandstone   
    In Herefordshire and South Wales the sandstones from the St Maughans formation in the Dittonian period of the Lower Old Red Sandstone have been quarried at many sites producing a variety of stone-slates ranging from a grey-green to a distinctive red.  The stone was worked in small surface quarries close to the point of use and roofs are found with single colours or mixtures. These reflect the local geology which varies on a large and small scale so when conserving these roofs it is difficult, and possibly un-necessary, to try to be too precise in defining the colour of stone to be used. There were quarries all over the region. Some of the more well known are around Llanveynoe, Abbey Dore and at Garnons and Dinmore Hills.
    Roofs in the region are always simple, gable to gable, with even valleys being comparatively rare. There are many buildings which need roof renovations and fortunately quarries have recently been established which can supply both colours. However they are only operating on a small scale and it will be important for owners and specifiers not to overwhelm them. It is in situations like this, and there are many examples through the country, where it would be useful if English Heritage, local conservation authorities, church architects, estate owners etc. could develop a plan for roof conservation in the region to ensure supply and demand are in phase.
    Dore Abbey shows the red colour of one of the masonry stones and stone slates of the region.
    Howe recorded stone slates from the Old Red Sandstone marls of Stockholm, Pembrokeshire (where) thin sandy layers are obtained, and formally were made into roof tiles. These were very small and irregular in shape, but nothing could be more lively in effect than their mottled buff, red and grey, with occasional white micaceous sheen. (It is possible that these are from the Downtonian and would now be in the Silurian.)
    The Lower and Middle Old Red Sandstones have both been worked for stone slates in Scotland. The former in Angus, now Tayside, and the latter in Caithness and the Northern Isles.
    East of Arbroath in Tayside (Angus) a large industry developed in the Lower Sandstone. As early as 1678 Robert Edward, Minister of Murroes, wrote that there were many quarries in Angus, producing high quality stone, some of which was shipped to Fife and the Lothians, Holland and North America. (see Mackie).  The industry developed with production increasing substantially in the early 19th century in part due to mechanisation, with the introduction of cutting, planning and edging machines. A machine was even developed to make stone railway sleepers but was abandoned when wooden sleepers became the standard. Problems were encountered with ground water as the quarries deepened and the region became famous for the many windmills used to pump water from the lower levels.
    'The eye is first caught, in approaching them, as we surmount a long flat ridge, which shuts them out from the view of the distant sea, by what seems a line of miniature windmills, the sails flaring with red lead, and revolving at the slightest breeze at more than double the rate of the sails of ordinary mills.' Hugh Miller 1841
    In 1854 a railway was constructed to carry stone from the Carmyllie quarries to Arbroath. By 1859 it was carrying 150 tons a day. Nowadays the quarries are only operated occasionally.
    The quarries produced masonry, including carved and turned products, roofing, flagstones and became famous for the latter shipping large quantities all over the world.
    '.... in various districts of the Seedley Hills there are numerous strata of sandstone flags, which may be raised of various thickness and dimensions. The thinnest of them are from about half an inch, to an inch in thickness. They are formed into plates of about fourteen or sixteen inches broad and eighteen or twenty inches long and are used for roofing houses. These plates are coated with scales of mica, or talc, of a greyish-blue colour, and this occasions the easy separation of the plates from each other; but after long exposure to the air, they assume a grey or brownish colour.'Rev James Headrick 1813
    The size quoted for the roofing slates was typical although they would have been sold as randoms. In 1837 a mix of 15 to 20 inches long by 9 to 12 inches wide cost £4.10s per thousand. Selling random sizes by the thousand must have been the cause of disputes and may explain why Reverend Headrick reported that 'they seldom make a roof that is ..... watertight.' If the roofer received more small sizes than he had reckoned on he would be tempted to reduce the head lap to make them cover the slope with the risk of leaks. Historically the local name for stones slates is sclaits as against sclalyie or skalzie for metamorphics. Stone slates were used throughout the quarrying region but were also shipped to other towns along the Scottish coast.
    The Middle Old Red Sandstone has been used as a local building material in Orkney, Shetland and the north-east of Caithness but it was in the latter that it became a substantial industry. Locally, it was used for many purpose - '... the houses have been built, the roofs slated, the roads paved, the fields fenced and the drains lined so varied are the uses to which the rocks are adapted'.
    In its products and development the Caithness industry mirrors that of Angus although it was later in starting to export - the first shipments were dispatched in about 1824. However, it grew to become very substantial. By 1845, Castlehill Quarry alone, which had opened in 1824, was employing 100 labourers and exporting 3000 to 4000 tons of flagstones. The peak years for the region were 1901/2 when nearly 35,000 tons of all products were manufactured but in the same period employment fell from 500 to 414.
    Two rocks were exploited for roofing: a 'very hard, thin, dark micaceous flag' and a 'pale green, thin flag or "slate" much in demand lately for artistic work on churches and public buildings, and is exported from Thurso to England.' (British Geological Survey Memoir: Caithness. Crompton). The green stone slates were produced at White Moss quarry.
    Eventually the industry declined as metamorphic slates became more readily available and concrete products took the market for flagstones. More recently there has been a revival and stone roofing and all the other products are available again.
    British Geological Survey Memoir: Geology of Caithness C B Crampton and R G Carruthers. 1914 Caithness Flagstones.
    Caithness sandstone being split in the Granit Union quarry at Weydale. Caithness Roofing Products slates