Church Monuments Society
The Society is a registered charity. No.279597 Registered Office: The Society of Antiquaries, Burlington House, Piccadilly, London. W1V 0HS Copyright (c) 2016 CMS. All rights reserved.
Stone is the popular and collective term for all the solid constituents of the Earth's crust with the exception of ice. Geologists would use the term rock; however the term stone will generally continue to be used here. Rock is a natural mixture of minerals; for example granites consist of a
mixture of three different minerals. Minerals are naturally occurring part of the
Earth's crust which consist of one single chemical element or (more usually) several
elements combined (compounds); most are a specific crystalline form. Limestones,
for example, are composed primarily of the mineral calcite, a form of calcium carbonate.
A Crystal is a solid with a regular internal structure consisting of strict arrangement
of the smallest particles -
Rocks form in the following ways:
Geological times -
The Jurassic Period mentioned below is from 208 to 146 million years ago. Incidentally we are still living in the Quaternary Period, which began two million years ago.
Below are a series of illustrated articles by geologist Dr Tm Palmer on the various stones. Click on the bookmark below to be taken to the relevant article.
Further articles will appear in due course.
The geological map of England shows, usually marked in dark blue, a thin belt of
stone that runs from Nottingham, northwards past Tadcaster and Ripon, before curling
eastwards and meeting the coast in Teeside. This is the outcrop of the Magnesian
Limestone, a stone that has been of great importance in the architectural and monumental
history of northeast England since Roman times. In the Middle Ages, it provided the
freestone used for the great ecclesiastical buildings of the region, such as the
minsters at York and Beverley as well as numerous lesser buildings, and countless
monuments. For some royal projects it was sent to London (appearing in written records
as ‘Northern Stone’). It is still quarried near Tadcaster to meet restoration demands.
At the southern end of the outcrop it is used in Southwell Minster, and its magnificence
as a building stone there so impressed the commissioners who were recommending the
stone for the new Palace of Westminster in 1839, that it was their first choice.
A terrible mistake, for much Magnesian Limestone decays most horribly in polluted
atmospheres, and within 10 years or so the London sulphur-
Magnesian limestone is strong to pale yellow in colour, some varieties being almost
white. Though readily worked and carved like the more familiar light-
This depositional phase of a stone’s history is followed by further alteration when
the stone becomes buried below the seafloor (often much deeper) and undergoes diagenesis.
The most common diagenetic change is the filling-
One geological scenario that leads to dolomitisation in the sediments below the sea-
A recent well-
Fig. 1. Microscopic views of thin slices of Magnesian Limestone from Nottinghamshire. 1a, general view of fine, sugary dolomite crystals with plenty of intercrystalline space (filled with blue resin in the colour version). The width of view is about 2 mm. 1b, closer view of a specimen with larger dolomite crystals. The original sediment grains of the parent limestone show up as faint outlines within the replacement dolomite crystals; width of view about 0.5 mm.
Many of the medieval monuments in the more southerly counties of England are made
of pale, easily carvable limestone of Jurassic age. The detailed geological character
of these stones may point to both provenance and date of the monument, so an accurate
petrological description is important. The first thing to determine is whether they
are oolitic or non-
The great majority of the limestones that we meet in monuments or buildings started
as shallow seafloor sediments, consisting of clean-
Many Jurassic limestones consist of little other than ooliths, maybe with the odd
fossil or fragment. Others have much more shell debris in them, either scattered
through a mass of ooliths or (more usually) in debris-
Because the smooth spherical character, size-
However, there is a further complication that may confuse neophytes and cause one
oolitic limestone to look rather different from another. This is the disposition
of the natural mineral cement within the stone. Limestones acquire their hardness
by growth, during burial, of calcite crystals in the minute holes (pore space) between
the grains. This material is popularly called ‘spar’. Some oolitic limestones (e.g.
Ketton and Portland) are held together by tiny dabs of spar, principally at the points
where adjacent ooliths touch. This is quite enough to give the stone rigidity, and
a close look will show the ooliths themselves looking like ball-
Each frame (below) is c. 2 cm wide and shows what you would expect to see with a x10 magnifier and a clean surface.
Why, then, are some of the most distinctive non-
A. Portland Stone with grain-
C. Painswick Stone with most ooliths held tightly by the natural cement.
B. Bath Stone with spar-
D. Dundry Stone with no ooliths, only shell fragments.
It was while I was putting out the rubbish that it occurred to me how I might lead
into an explanation of marble as a material. Here in west Wales the plastic wheelie
bin does not yet rule supreme, and we are still allowed to use the traditional dustbin.
The galvanised surface is a thin layer of zinc that has been induced to grow on the
iron vessel by electrolysis. The zinc is in the form of crystals that grew outwards
along the iron surface until they met their advancing neighbours and stopped. The
result, looked at closely, resembles a jigsaw in which the pieces have angular contacts
that abut against each other, rather than interlocking ones. This is the essence
of any mass of material that is made up of intergrown crystals. The ice on a frozen
windscreen is another – but both ice and zinc galvanisation are two-
In marble’s case, the material of which the stone is composed is the mineral calcite,
the most common of the crystalline varieties of calcium carbonate and one of the
2 or 3 most important minerals in the stones that are used by architects and carvers.
The purest white marbles, such as statuary Carrara, are 100% pure calcite. In contrast,
most marbles, including other varieties of Carrara, have other minerals (usually
mixtures of clays or iron-
All marbles started their geological life as limestones, which are also predominantly
composed of calcite, and the formation of marble from limestone principally involved
a textural change. The original limestone sediment was a mix of grains and limey
mud. Shell fragments and ooliths (see the last Newsletter) often featured. Each of
these components was itself a jumble of carbonate crystals, of widely differing shapes,
sizes, and orientations. In addition, there were originally minute spaces between
the constituents. A thought-
In contrast, a sedimentary marble like Purbeck has undergone recrystallisation by
a rather different process. The famous pond-
When a marble is polished, it is the individual calcite crystals that are cut and
smoothed to a high-
Geologists often get rather stuffy about applying the term marble to the polishable limestones, asserting that the word should only be used for the true metamorphic product. I don’t take this view: after all, the term marble has been applied to polishable limestone since well back into the Middle Ages, whereas the subject (and the name) of Geology only started in the 18th century.
Photographs of polished sufaces of Carrara (top) and Purbeck (bottom) marbles seen
under the electron microscope. In the top specimen the width of the view is about
2mm; in the lower one, about 4mm. In both cases the texture is one of closely intergrown
calcite crystals. Purbeck also shows the outlines of the pond-
The great majority of English medieval monuments are made of limestone (either freestone
or polishable marble) or alabaster. However, there is a third type of common sedimentary
rock of which monumentalists should have a working knowledge, and that is sandstone.
Sandstone buildings and monuments predominate in the part of England and Wales that
lies to the north-
Like other sedimentary rocks, sandstones consist of two components: grains that date
from their early geological history as a soft sediment, and a cement that grew between
the grains after they had been deposited and become buried. The grains were originally
transported by water or air, and the sites of deposition were in rivers or deltas,
or on the sea-
Thin section photographs of two sandstones. Above is a sandstone with very angular grains. The spaces between the grains are empty (filled with blue resin to illustrate this feature in the web version of this article). Below the grains are well rounded (originally transported by wind and deposited as a dune in an ancient desert). Their rims have a light covering of haematite (red iron oxide), which gives a pinkish tinge to the stone. Width of view = c.2 mm in both.
By far the most conspicuous variable feature in sandstones, however, is the colour
– white, pale buff, rusty, dark brown, pink, purplish red, dark tan, blue-
It is becoming clear that the freestones that were used in medieval funerary monuments
are largely, and not particularly surprisingly, the same freestones that were used
in the fabrics of contemporaneous buildings. Often, a monument that is caked in
candle grease and dust in a dark recess within a church, will turn out to be made
of the same material that can be seen much more clearly within the structure of the
same building. However, this is not invariably so and traps lie hidden for the monumental
petrographer. I suspect that one stone that was used more commonly in monuments
than is hitherto recognised is Caen Stone from Normandy. It can be obtained in large
blocks and it has a uniform, fine-
Caen stone is a pure, pale buff limestone of the same geological age as Bath stone.
Its texture, however is completely different. Bath stone is made almost entirely
of ooliths, quarter to half a millimetre across, whereas Caen stone contains no
ooliths whatsoever. It is made of a mixture of the minute plates that once comprised
the skeletons of sea-
Caen stone from the 18th C. fabric of Westminster Abbey, seen under the light microscope. The field of view is about 3.5 mm across. The bright grains are skeletal fragments and the dark rounded pellets are invertebrate droppings. The blue areas in between (seen clearly in the web version of this article) are coloured resin that was used to consolidate the sample while it was being cut and mounted on a glass slide. In this sample, the pellets and the skeletal fragments are present in roughly equal quantities, but other varieties of Caen stone are predominately made up of one or the other grain type (see text).
In architectural work, Caen Stone that was imported in the 19th century often shows terrible decay, whereas the medieval variety still survives in many buildings. This has led to a belief that all the Victorian stone was of poor quality, whereas the early material was uniformly good. In fact, a recent study on the Caen stone at Canterbury Cathedral and in other medieval buildings suggests that a mixture of qualities has always been available. The poor quality variety in the older buildings has usually decayed and has been replaced during the course of the building’s history, often with other types of stone, so that the good quality pieces of Caen stone that remain behind are erroneously interpreted as representing the quality of all of the original material. Good quality and poor quality Caen stone looks quite different when viewed microscopically, because they contain different proportions of the 2 different types of grain: skeletal fragments and muddy droppings. The former are rigid and attract a dense calcite cement on them during the stone’s geological history. In samples where they predominate, the stone is hard and weathers very slowly. In contrast, samples in which the droppings predominate are softer and more susceptible to wear and weathering. Externally exposed Caen stone shows this contrast most clearly, but it would probably also be discernable in stone that was used internally, as in monuments. The harder variety tends to be somewhat paler in colour and the skeletal grains tend to stand somewhat proud on a worn surface, often looking (rather confusingly) like protuberant creamy ooliths. The softer variety, in contrast, tends towards a yellower colour and wears back where it has been worn by much touching.
* * *