ST PANCRAS GARDENS – ROCK TYPES
St Pancras Gardens is a haven for the geologist with representatives from the
sedimentary, igneous and metamorphic aspects of the rock cycle.
1 LIMESTONE
YOU WILL NEED:
Map of St Pancras Gardens
Two sheets of fossils
Two colouring pencils
Instructions
There are two distinctly different types of limestone within the gardens –
Portland Limestone and Carboniferous Limestone. Each one contains a unique
group of fossils that can be used to identify it. You have been given two sheets
of paper that list some of the fossils found in each.
A number of unshaded gravestones, in the vicinity of St Pancras Old Church,
are also marked on your map. These are made of either one of the two
limestones. Locate these gravestones and try to determine their lithology (rock
type) by finding as many of the listed fossils in each one as you can.
Choose a different colour for each type of limestone and colour in the
unshaded gravestones on your map according to your findings. Don’t forget to
colour in your key too!
While you are doing this:
LOOK at the fossils carefully. Draw what you see and label any features you
can using the sketches provided.
THINK about what you are looking at…
What do these fossilized creatures remind you of? Where do you think they
might have lived? (answer on your notes pages)
What does this tell you about how the limestones were formed?
(answer on your notes pages)
Notes and sketches of Portland Limestone
(use extra paper if necessary)
Notes and sketches of Carboniferous Limestone
(use extra paper if necessary)
2 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 a few examples of marble within St Pancras Gardens, but don’t be
misled by the information plaques on some of the memorials because 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 marble of the Tate Family Grave with the limestone of the Ann
Abbey Memorial (see map).
How are they similar? How are they different?
Colour (try to find relatively clean surfaces)
______________________________________________________________
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Fossil content (don’t try and identify anything, just look at numbers!)
______________________________________________________________
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Surface weathering (which has a rougher, more eroded-looking surface?)
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Texture (are they smooth? rough? crystalline? grainy?)
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3 GRANITE
What type of rock is granite?
(circle correct answer)
SEDIMENTARY
IGNEOUS
METAMORPHIC
There are two distinct types of granite in St Pancras Gardens:
c A pink granite (polished)
d A black and white granite (unpolished)
Mineralogy
The positions of some granite gravestones are marked onto your map. Make
your way to ONE of them (try to avoid all going to the same one!)
You should be able to see three different types of mineral in this rock:
c FELDSPAR = Pink or white crystals with large rectangular outlines, often
crossed by fine parallel cracks known as “cleavage”.
d QUARTZ = Grey or colourless irregularly shaped crystals with a glassy
appearance.
e MICA = Black and/or silver flakes
Black flakes – BIOTITE MICA
Silver flakes – MUSCOVITE MICA (not visible in polished granites)
Quartz is otherwise known as SILICON DIOXIDE.
What is its chemical formula? _______________
Sketches
SKETCH OF THE MINERALS IN GRANITE
Quartz
Feldspar
Biotite mica
Drawing courtesy of Dr Eric Robinson, UCL
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…
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 ___________
Average:
2 ___________
 _____ + _____ + _____ + _____ + _____ 


5
3 ___________
4 ___________
5 ___________
= _____mm
From your average feldspar crystal length, do you think this is a coarse,
medium or fine grained rock? Why?
______________________________________________________________
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Xenoliths
Can you see any patches within the granite that look different from the
surrounding rock? (If you are at the Rhodes Family Grave you may not, so try
one of the other granite stones marked on your map).
What distinguishes them from the rest of the granite?
______________________________________________________________
What shape are they?
______________________________________________________________
(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 became more
rounded as they were eroded by the hot magma (i.e. the edges melted).
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!
4 SANDSTONE
Sandstone is formed by the cementing together of grains of sand. Does this
make it sedimentary, igneous or metamorphic? ________________________
Would you say that this sandstone is coarse grained, medium grained or fine
grained? (use the same criteria that you did for granite)
____________________
Make your way to William Jones’ gravestone (see map).
This gravestone has very well defined bedding planes.
(Bedding = a series of visible layers in a rock that reflect the original surfaces
of deposition)
Has the gravestone been cut parallel or perpendicular to these bedding
planes? ___________________________
Weathering tends to concentrate along the planes of weakness between
sandstone layers (bedding planes). As a result, large flakes of sandstone are
detached from the surface, leaving it very uneven.
Make a sketch of the whole gravestone, marking on the bedding planes and
eroded surfaces:
GRAVESTONE SKETCH
Don’t forget
to add a
scale!
And finally…
Using your map, make your way to the Burdett-Coutts Memorial Sundial. Then
complete the diagram using terms from the list below:
PINK GRANITE
MANSFIELD SANDSTONE
MARBLE
PORTLAND LIMESTONE
GREY GRANITE
CAST IRON
FOSSIL GROUP ONE
Portland Limestone
FOSSIL GROUP TWO
Carboniferous Limestone
KEY
Portland Limestone
Carboniferous Limestone
St Pancras
Hospital
Railway
William Jones’ Grave
(SANDSTONE)
Soane
Mausoleum
(LIMESTONE &
MARBLE)
Flaxman Family Vault
(LIMESTONE)
Jane Grundy
Memorial
(PINK
GRANITE)
Burdett
Coutts
Sundial
Tate Family
Grave
(MARBLE)
St Pancras Hospital
The
Hardy
Tree
Drinking
fountain
Limestone 2
Ann & William
Birch’s Grave
(GREY GRANITE)
St
Limestone
1
Pancras
Ann Abbey
Old
Memorial
Church
(LIMESTONE)
Limestone 3
Limestone 4
Rhodes Family Grave
(PINK & GREY GRANITE)