CITY OF LONDON CEMETERY
TEACHERS' GUIDE TO WEATHERING
EXERCISES: LOCATION 2
Gravestone weathering is a big problem, particularly in cities like London.
Define the term “weathering”:
The processes by which rock at or near the earth's surface is disintegrated and decomposed by the action of
atmospheric agents, water, and living things (the breakdown of rock to form sediment)
There are three types of weathering:



Chemical Weathering – e.g. dissolution by acid rain
Physical Weathering – e.g. freeze thaw action
Biological Weathering – e.g. lichen growth
Which type of weathering do you think is dominant in London ? What is the primary cause?
Chemical weathering is dominant in London, caused by acute atmospheric pollution, particularly from vehicle
exhaust fumes, containing sulphur oxides and nitrogen oxides
Name three factors that might affect the weathering of a rock:
1.
2.
3.
Its lithology (rock type)
Its grain size
The environment it is in
We can get an idea of the rate of rock weathering from knowing hold the gravestone is (from
the date on it), and e.g. looking at how well-defined the sculptures are at the present day.
Raised Lead Lettering
Some of the gravestones have inscriptions depicted by lead lettering. When the lettering was
first placed onto the stone, it would have been flush with (at the same level as) the stone
surface. Some newer gravestones still show this. However, can you see how the older
gravestones here have lead lettering that stands away from the gravestones? This is because
the stone has been weathered away, leaving the lettering at the level that the stone would
have been at originally.
Go to the St. Antholin memorial (see map).
What is the oldest possible date of the memorial? 1875
Measure the gap between the stone and the lettering. 1.8 mm
From physics you will have used the simple equation:
Speed = Distance/Time
If you take the gap between the stone and the lettering as Distance and the time period from
the date on the gravestone to now as Time then the Speed should give you the rate of
weathering.
AREA FOR WORKING:
What is the rate of weathering per year? 0.013mm/year
The calculation you have just carried out is for a memorial composed of MARBLE. Look
also at whether the corners of the monument are sharply defined, and how smooth or bumpy
the surface is. This will also give you a clue as to how weathered the stone is. Compare this
gap with any other marble memorials you see, noting the date at the same time.
Carved Lettering
Now go to the St. Mildred Poultry limestone memorial.
How old is it? 133 years
Using the basic classification table below, describe the carved lettering at the top, middle and
bottom “steps” of the gravestone, on the side facing the road.
1. Lettering is sharp and distinct
2. Lettering is slightly rounded but still legible and
clear.
Increase in
weathering
from 1 to 6
3. Lettering is legible but rounded with some edges
clearly removed.
4. Lettering is rounded with all or most original edges
removed. It is still legible, but becoming more
indistinct from the surface of the grave.
5. Lettering is still just about legible, but now almost
indistinguishable from the surface of the gravestone.
6. Lettering has virtually disappeared.
TOP Level 4 to 5 . Lettering very rounded, most can be read with difficulty.
MIDDLE Level 5. Lettering is barely legible. Only a few letters can be seen.
BOTTOM Level 6. Lettering has virtually disappeared.
What type of weathering do you think is taking place? Chemical
Why do you think there is this variation across the gravestone?
The steps closer to the ground are more chemically weathered. This may be because of water being drawn up
into the stone from the soil, and from rain splash from the ground surface.
If we call this face Face No. 1, work your way clockwise around the memorial, looking at
each face in turn, calling them Face No. 2, Face No. 3 and Face No. 4. Can you see any
differences between the weathering on each side?
The vertical faces of the middle and upper “steps” are covered with moss, suggesting more exposure to water,
increased weathering and soil production, possibly from pollutants. Side 3 is water stained, and shows a pale
green lichen coating well. Side 4 has only a little moss. The writing is best preserved on the top and middle
“steps” of face 4.
Can you account for the differences?
Perhaps wind direction may have an effect, as some faces will be more exposed to driving rain than others.
Now choose a granite memorial with carved lettering, such as the pink memorial to St.
James, dated 1874, which is in the shape of a rocky outcrop. Using the same scale of
weathering as you used for the limestone, what value would you give for the granite?
Level 1. Lettering is sharp and distinct.
So, which weathers faster, granite or limestone?
Limestone
Raised Fossils
Fossils in limestone, particularly Portland Limestone, are harder than the surrounding rock,
and therefore weather more slowly. This means that over years of exposure, the fossils end up
protruding from the surface of the gravestones. As with the lead lettering, this allows us to
measure roughly how much of the limestone has weathered away, and how fast it has
weathered.
Now have a look at the memorial to St. Helens , Bishopsgate . Here the fossils obviously
protrude.
How old is the gravestone? 130 years
Using the same method that you did for the raised lead lettering, work out the rate of
weathering of the limestone.
AREA FOR WORKING:
What is the rate of weathering per year? 0.006 mm
Remember, this method will be more inaccurate than the raised lead lettering since lead is
much more resistant to weathering than the lime fossils.
Final thoughts…
If you wanted to ensure your gravestone was still legible in a few hundred years time, what
type of stone would you choose for your headstone? Why?
I would choose granite because it is resistant to weathering, and both carved and metal lettering would last for
a long time in granite.
If you wanted everyone to forget you as soon as possible, what stone would you use then?
What could you do to ensure maximum weathering potential?
I would choose limestone or sandstone, and make sure that it was cut parallel to the bedding planes, to ensure
maximum weathering potential. Carved lettering would probably disappear sooner than lead letters.