Sourcing and matching stone by petrography
Ancaster, Ardingly, Barnack, Beer, Kellaways, Ketton, Mansfield White, Monk's Park, Spilsby, Taynton or Totternhoe? Petrographic techniques can help you through the maze.
To find out more about rock description and classification, documents describing the schemes followed by the BGS for each rock type are downloadable free from the BGS website at home.html
A flyer describing current aspects of the uK building stone industry is available at www. mineralsuk. c om/ free_downloads.html
Petrography (the systematic description and classification of rocks by means of microscopic examination of thin sections) is a very powerful tool for identifying, sourcing and matching building stones. However, to extract the most from the technique it should always be used in conjunction with other more traditional approaches. For example, original documents can be a prime source of information regarding stone sources, but the information can often be vague and may be misleading. Petrography can confirm or reject such data.
The location of a building or structure is important, particularly if it is sited near a navigable river or on the coast allowing stones from further afield to be imported. The age of the building will also provide pointers as to whether local or more distant sources were available, particularly following the construction of the canal and railway networks from the late 18th century. In a high-status building the owner or builder may have had the wherewithal to have stone transported over considerable distances, even overland.
Perhaps of greatest importance, however, is to acquire some knowledge of the local geology, of which the petrography of the rocks forms an essential part. Geology can tell us whether the right stone lithologies (limestone, sandstones, granites etc) occur in the area to support a local quarrying industry. All these questions will need to be addressed to determine confidently the source of a building stone. Petrographic analysis must be considered as an integral part of this process.
All rock types, igneous, metamorphic and sedimentary, are described and classified following a series of recognised schemes. Following such standard schemes facilitates the clear communication of information and can prevent misinterpretation of the data presented.
In the uK, sedimentary rocks - sandstones and limestones - are our principal building stones. Sandstones are described and classified on the basis of their mineralogical grain composition. They are composed predominantly of three common grain components which are used in their classification: quartz, feldspar and lithic (or rock fragment) grains. Variations in the proportions of each of these components enable a sandstone to be classified as, for example, quartz-rich, feldspar-rich or lithic-rich, and into a whole spectrum of variations between these end-members.
In determining the source of a sandstone petrographically, however, a classification based on the proportions of the three principal grain types is only a first step. The identification of other minerals (igneous grains, micas, glauconite), their grain shape and cement types all help to determine the source of the stone. The sandstones from different geological units can show very
different grain compositions, shapes and rock textures, thus limiting the search for a potential source to certain stratigraphic intervals.
Carboniferous millstone grit sandstones have high quartz contents (both as grains and natural silica cements), but in contrast pennant sandstones have high lithic grain contents and low silica cement contents. The red and white sandstones of the Permian and Triassic generally have higher feldspar proportions, can have well-rounded eolian (or wind-blown) grains and generally show a more open, porous rock framework (Locharbriggs, Grinshill). Some sandstones from the Jurassic and Lower Cretaceous show very fine grain sizes and high quartz contents (Kellaways and Ardingly Sandstone).
Other minerals identified in thin section, however, can also contribute to the determination of the source of a sandstone. Many of the Lower Cretaceous sandstones of eastern England, for example, contain the green iron silicate mineral glauconite (Spilsby Sandstone, Folkestone Stone, Reigate Stone). Reigate Stone is particularly distinctive as it also contains numerous siliceous spicules derived from the decay of marine sponges.
In contrast the classification of limestones under the microscope is somewhat more complex. Compositionally the grains in a limestone show very little variation in their mineralogical composition. They are all generally forms of calcium carbonate. It is the type and relative proportions of the different grains present that is more important in describing and determining classification. Limestones are broadly divided or classified into ooidal (Ketton, Taynton, Painswick and Portland stones), shelly - or bioclastic - (Ancaster, Barnack, Box Ground, Headington and Caen) and micritic varieties (Blue Lias, Totternhoe chalk) based on the presence or absence of particular grains in their rock framework.
A number schemes for describing and classifying limestones have been developed, some of which also take into account the nature of the intergranular cement present, such as spar cemented (coarsely crystalline), ooidal limestone (Monk's Park and Taynton), or shelly, micritic (finely crystalline) limestone (Beer).
There are also a number of distinctive hybrid stones used for building which are composed of a mixture of carbonate and sand grains (Mansfield White, Kentish Rag, and Reigate stones). These stones do not fit comfortably in either classification scheme.
When a report is commissioned to determine the source of a particular stone, the petrographic parameters outlined above should play a significant part in the determination of its provenance.
Graham Lott is a sedimentary petrographer at the British Geological Survey, responsible for answering enquiries
g the building stone resources of England and Wales.