CITY OF LONDON CEMETERY

TEACHERS' GUIDE TO WEATHERING EXERCISES:

LOCATION 1

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. Its lithology (rock type)
2. Its grain size
3. 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 Stevens Grave (see map).

How long ago was the death of John Stevens? 127 years

Measure the gap between the stone and the lettering. 1.0 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? 0.008mm/year

The calculation you have just carried out is for a gravestone composed of MARBLE. Does the sculpture show well-defined edges? What about the corners of the blocks? What does this show?

The sculpture and block corners are rather rounded because of chemical weathering.

Carved Lettering

Look at the inscriptions on the Holsten Memorial (pink granite) and on the family vault of David Wilkin (marble tablet).

How old is the granite Holsten memorial? 121 years

How old is the Wilkin memorial with its marble tablet? 142 years

Using the basic classification table below, describe the carved lettering on the two gravestones.

Holsten: Level 1 - Lettering is sharp and distinct.

Wilkin: Level 3 to 4 – lettering weathering varies from top to bottom.

What do you think the black marks are on the tablets, and why do they only occur at the top on the side facing Divisional Road ?

The black sooty material is from airborne pollution. This has been washed away by rain in places, but remains where protected by overhanging rock.

Now go to the Peter Allan limestone memorial. How old is it? 57 years

Use the table again to describe the weathering. What kind(s) of weathering have taken place?

Allan: Level 3 to 4 – but it is darker at the top from airborne pollution and lichen growth. Moss has also grown at the bottom, where it is wetter from rainsplash and drawing up of moisture from the soil. Moss growth will give rise to biological weathering.

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.

Have a look at Peter Allan’s Grave, where the fossils obviously protrude.

How old is the gravestone (see earlier)? 57 years

Measure the height of the fossils above the stone. 0.4 mm

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? 0.007 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.

© UCL Earth Sciences 2006.    This page was last updated on 30 January, 2007.   
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