Andrew A McMillan and Ewan K Hyslop,
British Geological Survey

Scotland is a land of stone. Its diverse landscape owes much to the complicated underlying geological foundation of sedimentary, igneous and metamorphic rocks some of which date back nearly 4000 million years (Table 2). The nature and distribution of the rocks reveal a prehistory which involved the opening and closing of oceans, intermittent volcanic activity, the formation of sedimentary basins, the development of ancient river systems and repeated glaciations. Over the last 5000 years Scotland’s varied rocks have provided a major source of building materials. It is the nation’s geological complexity – the geodiversity – that is responsible for the diversity in its stone-built heritage in terms of both materials and architectural style and which accounts for local distinctiveness. The fundamental geological properties of a particular rock type in an area determine how that material can be shaped and utilised as a building stone.

Structurally Scotland can be divided into three fundamental geological regions separated from each other by major faults. North of the Highland Boundary Fault, extending from Arran to Stonehaven, the Highlands and Islands are dominated by Precambrian (Dalradian, Moine and Lewisian) crystalline metamorphic rocks (Table 2) including metasandstone, quartzite, schist, gneiss, slate and, less commonly, marble. Lewisian gneiss makes up much of the Outer Hebrides and NW Scotland where it is overlain by remnants of hard, bedded, purple Torridonian metasandstone. Generally the crystalline rocks are difficult to work using traditional methods and were used only locally for building purposes. Slate (metamorphosed mudrocks) of the Grampian Highlands (e.g. Ballachulish) were exploited on a major scale for roofing.

Igneous rocks (Table 2) are widespread, with major granite intrusions of Devonian and Carboniferous age present throughout the Grampian Highlands and older Precambrian intrusions in the Northern Highlands. The famous granites of Aberdeenshire were exploited over centuries and exported around the world. Younger Tertiary intrusions underlie western coastal areas and islands, forming large granite and related igneous bodies in the islands of Skye, Mull and Arran.

Devonian sandstones, present in Caithness and Orkney, were deposited in a vast lake, producing both flaggy material and thickly bedded strata which have long been exploited respectively as pavement and building stone. The Mesozoic sandstones which occur in coastal areas on Mull and Arran were also hewn, generally for local use. Sandstones of similar age were worked around the Moray Firth and still supply quality stone (e.g. Clashach).

The fault-bounded lowlands of the Midland Valley of Scotland is underlain by mainly Devonian and Carboniferous sedimentary strata, deposited in former river systems and desert plains. Sandstones were exploited over many centuries to build villages, towns and cities. Many sandstones were relatively easily worked both for pavement and masonry. Small sedimentary basins of Permian red sandstone were exploited in the late 19th century (e.g. at Mauchline, Ayrshire), particularly for the Glasgow market. Devonian and Carboniferous intrusive igneous rocks and volcanic lavas are present in the Lothians and around Glasgow but are difficult to work and were generally used locally for rubble walling, or for roadway construction including setts.

The land of the Borders and Southern Uplands, south of the Southern Upland Fault, is mainly underlain by folded strata of Ordovician and Silurian age. The main rock types are greywacke sandstone (a hard rock made up of mineral and rock fragments, colloquially known as whinstone), siltstone and mudrocks. The greywackes are difficult to work and were generally used only locally for rubble walling or roughly dressed dimension stone. Fissile siltstones provided reasonable stone ‘slates’ for roofing. Stone was also supplied from the inliers of Permian and Triassic red sandstone, in Dumfries and Galloway, and the Devonian and early Carboniferous sandstones of the eastern Borders. The granite masses of Galloway were exploited for building and monumental purposes and harbour construction. Smaller bodies of Devonian and Carboniferous basic intrusive and volcanic rocks are locally present, particularly in the north and east of the Southern Uplands and were mainly used for only local construction.

see map: showing major stone types, illustrating the geological diversity of the country