The most important types of walling stones are described briefly below. Wallers often use loose generic terms to describe the rock found in their area. For example, the term ‘whinstone’ is used in Scotland and elsewhere to describe hard, dark stones which can include greenstone, basalt, chert or quartzose sandstone.
Sandstones
Sandstones consist of grains of sand, mostly quartz, cemented by silica, carbonate of lime or iron oxide. The resulting material may vary considerably in hardness. Colour ranges from white through yellow and brown to red depending on the cementing minerals. The main types of sandstone used in walling are quartzite, grits and flags. Quartzite consists mainly of quartz grains cemented by silica. Grits are hard sandstones containing small pebbles, so that the rock has a very rough surface when broken, which led in the past to their use as millstones. Flags include thin-bedded sandstones which split readily parallel to the bedding.
Sandstones of widely varying ages are found in walls in many parts of the country. Perhaps the oldest is the Precambrian Torridonian Sandstone of Sutherland and Wester Ross, but the earliest widely-used sandstone is the Old Red Sandstone of Devonian age found in many parts of eastern Scotland. The Caithness flag fences use stone of this type. Through large areas of the Pennines and the north-east of England, Millstone Grit and other coarse dark Carboniferous sandstones dominate the walls. Around Carlisle and in Dumfriesshire, the warm New Red Sandstone of Permian age is trimmed into blocks to produce masonry-like dry walls.
Limestones
Limestones are calcareous rocks formed from solidified masses of whole or broken shells, from the remains of coral reefs, from chemical precipitates or evaporates, or from the redeposition of eroded materials from older limey beds. Most limestones are formed almost completely of calcium carbonate, but magnesium limestone contains a high proportion of magnesium carbonate.
All limestones are more or less readily weathered by rain and ground water containing dilute carbonic and sulphuric acids, particularly where the stone has been cracked or has developed bedding due to the pressure of overlying strata. Limestones often intergrade with sandstones and shales so that the division between calcareous sandstone and gritty or sandy limestones, for example, is not always clear.
In Britain, the oldest widespread limestone is Carboniferous limestone, which forms part of the Derbyshire Peak District and the area to both sides of the Craven Fault in the western part of North Yorkshire. This pale grey, lumpy stone is immediately recognisable wherever it occurs, not only in the walls of the north but in the Mendips and the Gower Peninsula as well. North of Wensleydale the Carboniferous limestone becomes increasingly sandy and the walls are more mixed.
Jurassic limestones of different varieties colour the buildings and most of the walls in the Cotswolds. In Oxfordshire, Stonesfield ‘slate’, really a sandy flaggy limestone, provided until recently an important local roofing material. The Jurassic limestone belt runs north-northeast from the Isle of Purbeck to the Cleveland Hills in North Yorkshire, and has been widely used for building. Chalk, a very fine, uniform limestone forms the downs of southern and southeastern England. It is too water soluble for building, but the flints which it contains are used alone, or banded with bricks, in mortared walls.
Shale and slate
Shale belongs to the group of argillaceous or mud rocks, made up of very fine particles of clay which have been dried and hardened by compression. Shale is distinguished from other mud rocks by its well-developed lamination, and it tends to split readily into thin slabs parallel to the original bedding. Most shale is soft and weathers readily.
Slate is mudstone or shale which has been metamorphosed under intense pressure, resulting in the flaky minerals such as mica being shifted so that their flat surfaces lie at right angles to the direction of pressure. The metamorphosed material may retain traces of the original bedding, but it most easily splits along the new cleavage planes.
Most slates or slatey shales are low grade and have little use in building, but have been widely used for field walling in much of central and north Wales, the Skiddaw slate area of the northern Lake District and elsewhere where better walling stone is not available. The common feature of these walls is the fissile nature of the slate or shale, so unless another type of stone is available to form a heavy capping, the walls often tend to flake away layer by layer, or settle sideways if on a slope. High grade slate, such as the waste from slate quarries, can be trimmed into rectangular blocks which are much more resistant to splitting, and can be used to make flag fences for example.
Granite and other igneous rocks
Granite is formed by the slow cooling of acid molten rock deep in the earth’s crust. It is often, but not always, coarse grained and is made up mainly of felspar and quartz with mica and other minerals.
Granite is the toughest and most durable of the building stones. It forms the resistant bulk of many of Britain’s wildest moorlands: the Cairngorms and Rannoch Moor and other parts of Northern Scotland; the Cheviots; parts of the Lake District; and in the south west of England, Dartmoor, Bodmin Moor, and much of Cornwall, including Land’s End.
Detached blocks of granite have been used locally from prehistoric times for tombs and standing stones and, later, for Christian crosses. It was not until the 18th century that granite was widely quarried, so difficult was it to prise from the quarry face. The clearance walls of Zennor, Cornwall, date back to the Iron Age, while the granite walls of Aberdeenshire are of late 18th, 19th and early 20th century date.
The extrusive igneous rocks form a widely varying group, but are little used for walling. An exception are the walls of Borrowdale Volcanics in the Lake District, which are composed of varied Ordovician lavas, ashes and agglomerates, and which can be recognised by their rough surface and warm colours, compared to the dark slate walls elsewhere in the Lakes.
Schists and gneisses
Schists and gneisses are metamorphic rocks which represent an intermediate between sedimentary formations and granite. A distinguishing feature of both schists and gneisses is their foliation, like the tightly packed leaves of leaf-mould. Schists have the foliation closely spaced throughout the body of the rock, so that almost any part of it can be split into flakes. Schists and gneisses are of Precambrian age in Britain, occurring in the Highlands of Scotland, the Outer Hebrides, Anglesey, the Lizard in Cornwall and Start Point in Devon. While some schists produce even rectangular blocks of walling stones, gneisses tend to be rough and irregular. However, even the roughest gneisses are easier to use than granite in dry stone walls.
