CITY OF LONDON CEMETERY
TEACHERS’ GUIDE TO ROCK TYPE
EXERCISES: LOCATION ONE
The City of London Cemetery is a haven for the geologist, with representatives from the
sedimentary, igneous and metamorphic aspects of the rock cycle.
YOU WILL NEED:
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Map
Magnifying glass
Pencil
Ruler
Instructions
You have four different rock types to study and answer questions about, given in the
following sheets. The rocks are: limestone, marble, granite and sandstone. You will need the
map to help you find the right gravestone, and you will be able to see the rock better if you
use a magnifying glass. It is best to hold the magnifying glass up to your eye, and then move
your head close to the rock, trying not to cast your shadow over it. In order to answer the
questions, you may have to look at all sides of the gravestone. The ruler is needed to make a
scale on your drawings (you may wish to draw an enlarged picture), and also to measure
crystal sizes.
LIMESTONE
1. Look at an example such as the memorial to Peter Allan, shown on your map. It is about 8
metres behind the fourth Chestnut tree on the right hand side of Divisional Road . It has an
ivy leaf pattern on the pale coloured headstone.
Is it made of crystals which lock into each other, or is it made of grains with spaces (pores)
between them? Circle the correct answer below.
Crystals?
Or grains?
Does this mean that the rock is igneous or metamorphic (interlocking crystals), or
sedimentary (made of grains)? Sedimentary
2. Can you see any fossils in the rock? Yes/No
If so, are they whole or broken? Broken
Do you think these fossils are:
a) plants? Yes/No
b) vertebrates (animals with a backbone)? Yes/No
c) invertebrate (e.g. insects, snails) Yes/No
What do these fossilized creatures remind you of? Remember you are looking at them in two
dimensions. Think what they might look like in 3D.
They look like shells that you find on the beach.
Where do you think they might have lived? In the sea.
What does this tell you about where the limestone was formed?
The limestone was formed by deposition (of calcium carbonate) on the sea-floor. The sediments incorporated
sea shells, which were then fossilized.
3. Try to describe the limestone in terms of colour, how big the grains are, and fossil content.
Are the fossils concentrated in any particular area of the stone? Why might this be? Use a
magnifying glass if you have one, and draw the rock in the space below, labelling any fossils
and other features.
Name of rock:
Rock group
(From Q1)
Scale
Description: The rock is cream to greyish in colour. The grains are small. The rock contains lots of broken
fragments of shells, ranging in size from a few mm. to about 5 cm. The shells are concentrated in a layer about
24 cm thick at the top of the gravestone. This reflects the layering, and indicates that there musT have been a
slight change of environment, one being more favourable than the other for life.
Many of the gravestones in the cemetery, just like this one, are made of a limestone which
comes from the Isle of Portland in Dorset , and known as Portland Limestone. This rock was
formed about 150,000,000 years ago.
MARBLE
Marbles are limestones that have been altered by high temperatures and/or pressures. This
process is known as METAMORPHISM, and marble is a METAMORPHIC rock.
There are many examples of marble within the City of London Cemetery. Sometimes the
word marble is used by stonemasons to mean any rock that can take a polish, which is not the
same definition as a geologist would give!
Compare the freshly cleaned marble of the Vigiland memorial with the weathered marble of
the Stevens memorial (see map). Walk around the back of the Vigiland memorial to see the
stone; do not walk on it.
1. Use the table below to fill in your observations on the following properties:
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Colour (try to find relatively clean surfaces)
Fossil content (if any!)
General appearance and feel
(The way rocks weather can be very helpful when trying to identify the rock type).
Property
Vigiland
Stevens
Colour
Whitish with grey patches and
streaks.
Cream when fresh, mostly greyish
and covered with lichen/air
pollutants.
Fossils
None
None
General appearance
and feel
Very smooth surface. (Note:
monument has been cleaned
recently. It was very blackened from
airborne pollution).
Very rough, weathered surface, with
linear ridges and furrows, and
circular bumps. If you stroke the
surface, grains fall off. Note the snail
trails as examples of biological
weathering.
GRANITE
What type of rock is granite? (circle correct answer) SEDIMENTARY IGNEOUS
METAMORPHIC
There are several distinct types of granite in the cemetery. Look at the adjacent memorials to
Dalrymple (a black and white granite) and to Holsten (a pink granite).
Mineralogy
You should be able to see three different types of mineral in granite:
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FELDSPAR = Pink or white crystals with large rectangular outlines, often crossed by
fine parallel cracks known as “cleavage”.
QUARTZ = Grey or colourless irregularly shaped crystals with a glassy appearance.
MICA = Black and/or silver flakes
o Black flakes – BIOTITE MICA
o Silver flakes – MUSCOVITE MICA (not visible in polished granites)
1. Check that you can identify these minerals in the two granite memorials.
Quartz is otherwise known as SILICON DIOXIDE.
2. What is its chemical formula? SiO2
Sketches
Sketch of the minerals in granite
Drawing courtesy of Dr Eric Robinson, UCL
3. Draw an area of granite yourself, identifying and labelling all three minerals and any
features about them that you feel are important. Use the example sketch given to you for help
in identification.
Granite Sketch
Don't forget to add a scale!
Grain size
The granite you are examining is EQUIGRANULAR, i.e. all the crystals are roughly the
same size.
Rocks can have different grain sizes depending on how they formed. There are three main
types, termed simply as:
COARSE GRAINED – crystal diameters >5mm
MEDIUM GRAINED – crystal diameters = 1–5mm
FINE GRAINED – crystal diameters <1mm
Let’s find out the grain size for this granite…
4. Using a ruler, measure the lengths of 5 feldspar crystals (use feldspar crystals because they
have the most regular shape and distinct long axes that can be easily compared):
Length of feldspar crystal:
1. 12mm
2. 7mm
3. 12mm
4. 13mm
Average = 10.4 mm
5. 8mm
From your average feldspar crystal length, do you think this is a coarse, medium or fine
grained rock? Why?
Coarse. Granite is coarse grained, because the melt cools slowly as it intrudes the crust, and the crystals form
over a long time. Where a melt is extruded at the surface, crystals form quickly and are small (e.g. a basalt).
Xenoliths
5. Can you see any patches within the pink granite that look different from the surrounding
rock?
What distinguishes them from the rest of the granite?
They are darker in colour and finer grained.
What shape are they?
They are irregular in shape and rounded at the edges.
(Check that it is not dirt or lichen that you are looking at!)
These are XENOLITHS (Greek word for “strange rock”). Xenoliths are blocks of country
rock (the rock into which the granite would have been intruded when molten) that were
dislodged by the intrusion and “fell” into the hot magma.
Originally the blocks would have been angular in shape but the hot magma would have
caused the edges to melt and became more rounded.
Draw an example of a xenolith, noting its size and its rounded edges. Try to point out how it
is different from the surrounding granite, but don’t worry about its mineral content!
Xenolith Sketch
Don't forget to add a scale!
SANDSTONE
1. Sandstone is formed by the cementing together of grains of sand. Does this make it
sedimentary, igneous or metamorphic?
Sedimentary
Make your way to Henry and Isabella Mead’s memorial (see map).
2. This memorial is made of buff-coloured sandstone, red sandstone and has a marble tablet
for the inscription. Would you say that the sandstones are coarse grained, medium grained or
fine grained? (use the same criteria that you did for granite),
Fine-grained
This memorial has very well defined bedding planes. (Bedding = a series of visible layers in
a rock that reflect the original surfaces of deposition)
3. Look at the different blocks of red sandstone which make up the memorial, and see how
they have been cut so that the bedding planes are sometimes vertical and sometimes
horizontal.
Make a sketch of the whole gravestone, marking on the bedding planes and weathered
surfaces:
Gravestone Sketch
Don't forget to add a scale!
Behind the marble tablet, the bedding planes are vertical. Immediately beneath the buff sandstone, the beds
are vertical, but the rocks below that block show horizontal bedding.
And finally………….
Using your map, make your way along Central Avenue towards the Church. Find the three
memorials listed and for each, label the different materials it is made from, choosing from
following:
GRANITE (PINK)
GRANITE (GREY)
LIMESTONE
SANDSTONE
MARBLE
BRICK
1. The Family Vault of Mr. James Thomas Derby of Ratcliff
2. George James Derby
3. The Private Vault of Joseph Thomas & Jemima Carter of St Peters Road,
Mile End