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A TOUR OF THE STONE SLATE REGIONS

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

Roofs can be made with any fissile stone but the term stone-slate is only used for those which are sedimentary - mainly limestones, sandstones and shales – or very rarely igneous or higher grade metamorphic rocks. They do not include real slates which are metamorphic. It is important to realise that there is no clear cut division between sandstones and limestones: they grade from the former to the latter through calcareous sandstones and sandy limestones. Also, even though a particular geological formation has a name which indicates it is limestone it can also include sandstones. The Oolite as a formation, for example, contains sandstones whereas oolite as a type of rock is a limestone. Similarly the New Red Sandstone may include the Magnesian Limestone.

The reason for the mixture of rock types within a geological period is that they were laid down over very long time spans during which the conditions of deposition changed radically. In almost all cases the stone-slates were laid down under water. (Exceptions might include sand dunes.) The sandstones were often formed from river beds, on the shores or deltas of lakes or seas or on the sea bed. So their deposition could have taken place in a variety of moving water environments and these influenced factors such as grain size and bedding structures, which in turn affect the structure of the sandstone. Limestones on the other hand are usually formed by chemical processes or deposition of shelly or bony body parts of animals and plants. However, and this is the important point, each of these mechanisms might have occurred and reoccurred repeatedly so that a limestone bed might be buried by beds of sand or gravel. These cycles of deposition lead to sequences of beds, or facies, which can be typical for regions and provide information about the environments in which they were formed.

Stone-slate sources The stone slates are usually named for either the geological formation in which they occur or the place where they are found. The following list includes most of the stone slates of England and Wales and some in Scotland. If you can improve on this list send an e-mail.

STONE SLATES REGIONS, THEIR GEOLOGICAL PERIOD AND FORMATION NAMES

LIMESTONES INCLUDING SANDY LIMESTONES . Dorset Jurassic: Purbeck, Forest Marble South & East Somerset Jurassic: Forest Marble, Lias Sussex Cretaceous: Cyrena Limestone 'The Cotswolds Region' North Wiltshire, East Gloucestershire, North Oxford, South-east Worcestershire Jurassic: Purbeck, Corallian, Forest Marble, Hampen Marly Beds, Trougham & Taynton Stone (Stonesfield Slate), Fullers Earth, Chipping Norton Limestone East Midlands - Northamptonshire, Lincolnshire, Cambridgeshire, Leicestershire Jurassic: Blisworth Limestone, Upper Estuarine Limestone, Chipping Norton Limestone, Lincolnshire Limestone (Collyweston Slate), Northampton Sand, Lias North-east Yorkshire Jurassic: Scarborough Limestone Nottinghamshire to South Shields, East Derbyshire Permian: Magnesian Limestone SANDSTONES INCLUDING CALCAREOUS SANDSTONES . Surrey, Sussex, West Kent Cretaceous: Horsham Stone Gloucestershire, Bristol, South Wales Carboniferous: Upper Westphalian - Pennant Measures Welsh Marches - Gloucestershire, Herefordshire, Worcestershire, Shropshire Devonian / Silurian - Pridoli: Tilestones Old Red Sandstone - Dittonian, Downtonian, Hereford Stone Tiles Ordovician: Hoar Edge Grit, Cheney Longville Flags South Pennines including East Cheshire, North Staffordshire and North Derbyshire Carboniferous: Lower Westphalian - Coal Measures, & Namurian - Millstone Grit Lancashire, Yorkshire, Durham, Northumberland Carboniferous: Lower Westphalian - Coal Measures, & Namurian - Millstone Grit Cumbria Carboniferous: Lower Westphalian - Coal Measures & Namurian - Millstone Grit Permian: New Red Sandstone - Penrith Sandstone  Dumfrieshire  Permian: New Red Sandstone Caithness and Angus Devonian Old Red Sandstone Ross of Mull  Precambrian: Mica-schist IGNEOUS . Shropshire Intrusive: Dolerite: Corndon Hill Slates* . *The attribution of the Corndon Hill stone slate as igneous is now known to be wrong. It is in fact a very fine, laminated siliceous sandstone. More details here.

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