The Search for Stone
Identifying, sourcing and matching Britain's building stones
Graham Lott
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| St Radegund Church, Maplebeck, Nottinghamshire built of Triassic Skerry Sandstone (below right) |
Britain's long and complex geological history has produced a very diverse range of rock types, many of which have been quarried for building purposes over the centuries. Some of the country's best known building stone quarries have operated continuously since earliest times. Many more, however, have long since closed and their stones are no longer readily available. Original quarry sources may often be difficult to locate as agricultural development and the expansion of cities, towns and villages have gradually changed the land surface.
Sourcing suitable stone for the conservation and repair of historic churches and other buildings can be difficult and, like so many aspects of conservation, is increasingly becoming the preserve of professional experts. Nevertheless, it is important that everyone concerned with the upkeep and maintenance of our buildings or monuments should themselves be aware, if only at a very basic level, of the procedures that the experts are likely to follow. While some areas of this work are best left to the professionals, others require closer study and with a little detective work can be an informative pastime.
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One of the most important sources of information in this field is The British
Geological Survey (BGS) as it maintains a comprehensive archive of the geology
of Britain's building stones. Following its formation in 1839, one of the
first commissions of the BGS was to locate a suitable building stone for
the construction of the new Palace of Westminster buildings in 1839. Since
then the BGS has documented over 1,000 different building and decorative
stones which have been quarried and used in Britain and has undertaken the
collection of representative samples from working quarries. The organisation
now maintains the country's most comprehensive database of geological maps,
samples and archives. This dataset is used in a wide variety of applied
geological research, including the sourcing of building stone.
Where a building or monument is concerned, the criteria for sourcing new
stone may be considered under the following headings: stone identification,
sourcing and matching.
IDENTIFYING THE STONE
All natural stones are classified into one of three groups of rock types
- igneous, sedimentary or metamorphic. Igneous
rocks
are hard and crystalline and make up the primeval material of the earth.
They are formed directly by the cooling of hot molten magma. The best known
igneous rock used for building or monumental purposes is probably granite.
Granites are coarse grained rocks in which large crystals of glassy quartz,
opaque white or pink feldspars and dark brown or black ferromagnesian minerals
(mica, hornblende etc) can easily be identified with the naked eye. The
coarse nature of the crystals tell us that the rock formed from a magma
that cooled very slowly deep in the earth's crust allowing individual crystals
to grow to a large size. Some well known granites include Cumbrian Shap
with its large pink feldspars and Cornish De Lank which has large white
feldspar crystals. Many other types of coarse grained igneous rock occur
(such as the granodiorites of Mountsorrel in Leicestershire), but few are
used commonly for building purposes and most need to be identified by experts.
If molten magma manages to rise through the crust and emerge at the earth's
surface, most familiarly in the form of a steep sided volcanic cone or as
a fluid lava flow, then it will cool very quickly to form an igneous, volcanic
rock. Such rapid cooling prevents large crystals developing and the resulting
rocks are fine grained with individual minerals difficult to distinguish
except by hand lens or microscope. There are examples of such fine grained
volcanic rocks in the UK, such as the black basalts of Antrim, Northern
Ireland, but in general few have been used for building in this country.
Intermediate in origin and crystal size to these two extremes are magmas
injected as molten sheet-like bodies into the earth's crust and cooled just
beneath the earth's surface. They are termed sills or dykes. The best example
is probably the Whin Sill in Northumberland, formed of a very hard, dark
greenish grey dolerite that was used extensively in parts of Hadrian's Wall.
Although these rocks occur widely they rarely provide more than a local
building stone. However, because of their hardness, they are commonly quarried
for aggregate.
The Church of St John the Baptist, Muston, Leicestershire, which is built of two contrasting local limestones: ironstone (orange-brown) of the Lower Jurassic Marlstone rock formation and Middle Jurassic, Lincolnshire Limestone (cream coloured) |
The sedimentary rocks include our most common building stones - the
sandstones and limestones. Sandstones are formed by the weathering and erosion
of pre-existing rocks. They consist of small fragments or grains held together
by natural cements such as calcium carbonate (calcite) or silica. Most sandstones
consist of grains of quartz, feldspar and small rock fragments. Their high
quartz content makes them hard, durable building stones. By measuring the
average size of the grains, sandstones can be divided into fine, medium
or coarse grained types. Some sandstones can be identified by the presence
of distinctive mineral components, at least at a regional level. Two examples
of this are the sandstones from the Lower Cretaceous rocks of south east
England which commonly contain the green mineral glauconite, and the very
fine grained Triassic sandstones from the East Midlands locally known as
'skerry', which are commonly cemented by dolomite.
A wide variety of limestones are used as building stones. They are principally
composed of calcium carbonate (calcite) and are relatively soft in comparison
to most sandstones. Their softer nature allows them to be easily carved
and they are our best freestones. Most limestones were formed by the accumulation
of the broken shells of marine organisms on the seabed in tropical or sub-tropical
environments. Good examples include the Clipsham and Ancaster stones. In
coarse grained limestones, fossil shell fragments are easy to see with the
naked eye. The white chalk of southern and eastern England is a good example
of a fine grained limestone. It is composed entirely of the calcite skeletons
of microscopic organisms known as coccoliths. A distinctive group of limestones
which form perhaps our best known building stones are the oolitic limestones
from the Bath and Portland quarries. They were formed at the seabed as concentrations
of millimetre-sized spherical ooliths. Their distinctive spherical shape
and concentrically laminated structure developed when microscopic algae
precipitated successive coatings of calcium carbonate around a tiny nucleus,
which may have been a shell fragment or quartz grain. In some limestones
the fossils they contain may be distinctive enough for an expert to be able
to determine which quarries they may have come from.
Also included in this group of rocks are the magnesium-rich limestones or
dolomites. Dolomites are formed by the chemical alteration of an original
calcium-rich limestone. This alteration process may completely destroy the
original shelly limestone fabric to produce a crystalline rock, or it may
preserve it. Well known dolomitic building stones include the Mansfield,
Cadeby and Tadcaster limestones.
An important feature of sedimentary rocks is their porosity. Pores are natural
holes in the rock which allow fluids like rainwater to enter and leave the
fabric. Some free fluid flow through a rock is necessary to maintain the
rock's durability, and it is not always advisable to block such flow by
using incorrect mortar mixes or by injecting unsuitable synthetic fluids.
Very high porosities, however, may allow excessive volumes of corrosive
fluids such as acid rainwater to enter and cause severe damage to the rock.
Thin section rock analysis can identify where such problems are likely to
occur. Most durable sedimentary building stones commonly have moderate porosity.
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Detail of the fossiliferous ironstone from the Lower Jurassic Marlstone |
| Micrographs of two different Middle Jurassic Lincolnshire limestones: the famous Barnack (above) and Clipsham (below) stones. | |
The third group of stones are the metamorphic rocks formed by the
alteration of other rock types by heat and pressure. They show some of the
characteristics of both igneous and sedimentary rocks, often having a hard,
crystalline structure but retaining traces of original sedimentary bedding
surfaces. Metamorphic rocks are not widely used as building stone but they
are the main source of roofing slates in the UK. Such slates are still quarried
from the older metamorphic rocks of North Wales, Cornwall and Cumbria. Important
slate industries in Leicestershire (Swithland) and throughout Scotland are
long since closed or very much reduced in scale.
Slates are formed by the recrystallization of fine grained sedimentary or
igneous rocks under extremes of temperature and pressure. Under such conditions,
which develop over many millions of years, new minerals (most notably micas)
grow and the characteristic slaty cleavage is formed. It is the alignment
of these new minerals that enables the slates to be easily split into thin
sheets. An important feature of metamorphic slates is their lack of porosity
which makes them impervious to fluid flow. So-called stone-slates are not
metamorphic slates but thinly bedded sedimentary limestones and sandstones
which naturally split into thin layers or slabs.
Included in the metamorphic rocks are the true marbles. Geologists only
apply this term to limestones that have been altered by metamorphism. Metamorphosed
limestones show very different textures from the original stone but mineralogically
they are still principally composed of calcium or magnesium carbonate. One
thing you can be sure of is that if you have identified a metamorphic marble
in a building or monument, it is almost certainly an import from overseas.
The only marbles that have been produced in the past in Britain are those
from the islands of Iona, Tiree and Skye in Scotland and Glen Tilt, Tayside;
the quarries at Ledmore near Ullapool, producing metamorphosed Durness limestone,
now provide Britain's only indigenous marble.
In the building stone trade the term marble is commonly used to describe
any sedimentary limestone which is hard enough to polish. Close examination
of the polished surface of sedimentary 'marbles' such as Purbeck, Bethersden
or Frosterley will usually reveal the unaltered fossils that make up much
of the rock.
SOURCING THE STONE
Having made a preliminary identification of the building stone, the next
step is to find the original quarry source for the stone. The first place
to look for information on the provenance of a building stone is in original
documents or archives related to the building or monument. If, as is often
the case, this becomes a fruitless search then a little more detective work
is required.
The first step is to establish the age of the structure in which the stone
occurs. Stone buildings constructed from medieval times or earlier (this
includes a large proportion of our churches) to the late 18th century used
local stone sources where possible, because of the difficulties and costs
of transportation. Obvious exceptions are high status buildings where expense
was no object, and buildings in coastal situations or close to navigable
river systems where water transport was available. Particularly attractive
stones for decorative and monumental work, such as the black marble from
Tournai (Belgium) have been imported from medieval times. For most early
buildings, however, local stone sources should be investigated first. The
geological maps and archives held by the BGS identify the different types
of stone that outcrop across the country, and are an invaluable resource
for research into potential stone sources in any given area.
From the latter part of the 18th century to the early part of the 19th century,
the construction of the canal network provided the first national transportation
system for moving bulky goods such as stone across the country. Existing
stone quarries soon took advantage of the canals and many new quarries quickly
opened up. The canals were, however, soon eclipsed by the development of
the railways. Transportation costs rapidly plummeted as the rail network
extended nationally and by c1850 stone could be supplied to almost any part
of the country at a reasonable cost. Welsh slate, for example, rapidly appeared
on roofs throughout the country, displacing many equally durable but more
expensive locally quarried varieties.
The 19th century was a boom time for the British stone industry, with thousands
of new quarries opening across the country to supply the ever-growing urban
centres, factories and houses. 19th century buildings are therefore more
likely to contain stone from outside the local area, often supplied by the
much larger scale quarrying operations that developed in Derbyshire, Yorkshire
or Lancashire.
Builders and architects currently use stone from even further afield. Recent
buildings in the London area have used Indian marbles and Australian sandstones.
Today you can even view and order your stone via the internet from anywhere
in the world and, in some cases, have it on site, prepackaged in polythene,
within days.
MATCHING THE STONE
Finding the original stone source solves only part of the problem. In many
cases it soon becomes clear that the original stone is no longer quarried.
Few of the stone sources that supplied Britain's early ecclesiastical buildings,
for example, have survived as producing quarries into the 21st century -
some only operated long enough to complete the church and a few local buildings.
Fortunately, however, if the stone has been correctly identified and its
source properly located then other options can be considered. Re-opening
the original quarry may be a possibility, providing the local planners can
be convinced. The economics of re-opening even a small quarry on a temporary
basis can be somewhat daunting, so it may be useful to assess whether there
is a wider market for the stone which would make better economic sense.
In the East Midlands area, for example, one local stone which has been widely
used for church buildings and village housing is the red-brown and orange-brown
sandy ironstone, known to geologists as the Lower Jurassic, Marlstone Formation.
This stone was widely quarried for building purposes from medieval times
but no quarries now exist in the area. Existing buildings are commonly in
a sad state of repair simply for lack of readily available matching stone.
BGS maps show that there are still sources of the stone available in the
area, and a concerted effort by interested parties is probably needed to
develop a new quarry for conservation purposes. Traditional building stone
quarries can be small concerns and if properly planned and managed can be
worked using low-tech methods so as not to damage the stone or the environment.
In many instances it may not be necessary to go as far as re-opening an
original quarry to supply stone. In Britain at present there are more than
350 building stone quarries still in operation, quarrying a wide variety
of good quality, durable stones. Some quarries, such as those for the Bath
and Ancaster limestones, have a history stretching back to Roman times.
Careful matching of an original stone in terms of its geology, texture,
mineralogy (by thin section analysis) and colour with one or other of these
stones should provide a suitable matching replacement in many cases.
The BGS maps and archives are a primary source of geological information
where some of these questions can be readily answered. However, there is
no real substitute for experience, and looking at stone in buildings of
varying ages and in as many different locations as possible is still the
best way to become an expert in the subject.
