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The Geology of Leeds Central Library...

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Hidden : 03/06/2019
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Geocache Description:

The Geology of Leeds Central Library.

This building is a magnificent place, and one which is best taken time to explore. It has many a wonder. Since it was built, it has played a role in the history of the city of Leeds. This EarthCache, will take you to some of the wonders of the library and the stone that it is built from. 

Please be aware that this earthcache is only accessible during the opening times of the Leeds Central Library. Leeds Central Library Calverley Street, Leeds LS1 3AA. 

Opening times :

Mon – Wed 09:00 - 19:00.

Thu 09:00 - 18:00.

Fri 09:00 - 17:00.

Sat 10:00 - 17:00.

Sun 11:00 - 15:00.

The Entrance Steps

This is the original entrance to the building, and is the start of your journey. The steps are made of Shap Granite, and a part of a way up you will have once encountered a portcullis, which was winded into place by a mechanism at the end of the day by the porters from the library, that is now long gone. So lets look at the Shap Granite.

Granite is a common type of igneous rock. Igneous rock is one of the three main rock types , the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava, which then forms crystals which make up the rock. Granite is formed of crystals, of which some can be described as phenocrysts. Igneous rock forms by crystallisation either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks. Intrusive rock forms within the crust from the crystallization of magma. As the magma pushes up from deep, it eventually slowly cools into a solid, and forms rock. An intrusive rock is any form of igneous rock that forms within the crust of the planet. Large areas of magma that solidify underground before they reach the surface are known as plutons .Granitic rocks mainly consist of feldspar, quartz, mica and amphibole minerals, which form an interlocking matrix of feldspar and quartz with scattered darker biotite mica and amphibole (often hornblende) peppering the lighter colour minerals.

A phenocryst is a crystal in igneous rock, that is larger than the surrounding crystals of the rock that it forms part of. Though shap granite is known for its large pink orthoclase feldspar, which form Megacrysts. These are crystals that are larger than the surrounding crystals, indeed it could be said that a megacryst is a phenocryst.


1. Please look at the steps, what percentage of the steps are formed from the pink megacrysts?

2. Please look at their shape, is there a particular shape that is more prominant?


As you get to the top of the steps, look up you can see the date stones from when the building was started and completed. Now look straight ahead, at the ornatley carved features infront on you. What you are looking at is Alabaster.

What is Alabaster?

Alabaster is a fine grained rock, that is formed of gypsum and other minerals, it is soft enough to carve, and can be transformed into very ornate features. Gypsum is a sedimentary rock, that is formed of sulphate of calcium. Its is usually a clear translucent white colour, but can be coloured and veined by the prescence of  iron oxide, which produces a brown colour. 


1. Please tell me, what colour the alabaster is here, what has caused that colour?

2. Looking at the carved frame, please measure the width of the thickest vein, how wide is it?


We are now infront of the lending library, which used to be where the people of Leeds came to pay their taxes, but no fear, no money to be paid here. First lets look at the granite pillar at the bottom of the stairway. You will notice some carving on it, this relates to when the police were also based in the building. There are two types of granite here, one which is from Peterhead (the pillar) in the North East of Scotland, and the other  (the top of the plinth) from Rubislaw in Aberdeen, the granite city. So what has caused the different colours?

Granite comes in a range of colours. Pink or white feldspar is known as as orthoclase, whilst grey is known as plagioclase. The pink colour, comes from K-Spar, which is also known as potassium feldspar. 


1. Please tell me which parts are orthoclase, and which parts are plagioclase?

2. What is inscribed on the pillar?


This stone is Caen stone, which comes from Normandy in France, it is what is known as Oolithic limestone. Oolitic limestone is a carbonate rock made up mostly of ooliths (or ooids) which are sand-sized carbonate particles that have concentric rings of Calcium Carbonate. These rings are formed around grains of sand or shell fragments that were rolled around on the shallow sea floor, gathering layer after layer of limestone.This limestone formed in a shallow sea. The rock has an even structure rather like cod roe and it can therefore be cut or sculpted in any direction. This feature, coupled with hardness, colour and durability, gives the limestone its quality as a building stone. Its colour can be white, grey or yellow, with a fine grained texture.


1. At one side of the doorway are two carved birds, this block of stone has been cleaned. Please tell me what colour the Caen stone is?

2. Why was it used here?

If you look up, you will see a craving showing people paying their taxes, and on one is the Owl of Leeds, and on the other the sheep, representing wool. 


This is an amazing place, one which you can only wonder about, the tiles and the ceiling are worth just sitting and looking at. We are here to look at the granite pillars, and to have a look for xenoliths and autoliths.


Xenoliths are odd shaped, which are a different colour than the bulk of the granite rock. Xenolith is greek for a foreign rock, bascially it is a piece of rock trapped in magma. As explained above, granite is an igneous rock, this is a type of rock formed through the cooling and solidification of magma. A xenolith is a rock embedded in magma while the magma was cooling. As the molten material rises, it tears off bits and pieces of the magma pipe, or roof / wall of a pluton in which it was travelling in which it is traveling. These bits and pieces, trapped in the magma but not melting into it, become xenoliths. Crystals that are torn from the sides of magma pipes are called xenocrysts. As the magma flows out on to the surface of the Earth, or remains as a body within the crust, it is cooled, and various types of igneous rocks could be formed. Xenoliths usually have a different colour and density when compared to the surrounding rock, they can be small or large.


An included rock of similar type is called an autolith or a cognate inclusion, this was formed from a rock similar to the igneous rock, and can often be seen as a shape similar to the surrounding granite, but one which stands out from the rest of the crystal structure.


1.Please find an xenolith on one of the of the pillars, and describe its colour, shape, length and width.

2,On one of the pillars is a large autolith, please tell me how wide it is, what shape it is, and what colour it is.

This being an EarthCache, in order to log it, I ask that you answer the above tasks. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile.


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