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: 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)? 2. Can you see any fossils in the rock? Yes/No If so, are they whole or 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. Where do you think they might have lived? What does this tell you about where the limestone was formed?
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: 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: 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 Fossils General appearance and feel
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: 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? 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. 2. 3. 4 Average = ----- ----- ----- mm 5. From your average feldspar crystal length, do you think this is a coarse, medium or fine grained rock? Why? 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? 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 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? 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), 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!
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