Introduction
Blackburn Cathedral is a Grade II* listed stone building and its site has been home to a church for over a thousand years. The stone used in its construction though is much, much older. Look at the stone floor inside the cathedral and you're looking at limestone which was laid down on the bed of a shallow sea some time between 300 and 360 million years ago and is filled with the fossils of sea creatures which where alive at that time.
This EarthCache is placed with the kind permission and full support of cathedral staff. The cathedral is open to the general public between 9.00am to 5.00pm Monday to Friday and 9.00am to 3.30pm on Saturday. Please be respectful of the primary use of the building and of any services which might be taking place.
Most services between Monday and Saturday inclusive are held in a separate area which means that, provided any noise is kept to a respectful minimum, the general public are still free to explore the fossil floors in the nave and transepts (click here for a floorplan of the building).
Visiting on a Sunday would probably require careful planning as it is the busiest day for the cathedral in terms of the number of services held. Click here to visit the full timetable on the cathedral website.
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Based on your on-site study of the floor of the building and using the information on the cache page please tell me:
- Which three fossilised sea creatures listed on the cache page are most easily found in the limestone of the cathedral floor and of those three, which is most abundant?
- Are there any crinoid fossils composed of a) Articulated ossicles b) Disarticulated ossicles or c) A mixture of both articulated and disarticulated ossicles?
- Are all the limestone slabs the same colour or are there various colours and shades?
- Do you think the ingredients in the limestone slabs vary from slab to slab or are they all the same?
- A very small number of slabs include distinct patches of a brighter colour as a result of the ingredients including a particular mineral that isn't in the other slabs - what is this colour? Extra points if you can name the mineral in these coloured patches
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- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
The Carboniferous is a geologic period that spans 60 million years from the end of the Devonian Period 358.9 million years ago (Mya), to the beginning of the Permian Period, 298.9 Mya.
During the late Carboniferous the north of England, as we know it now, was actually positioned just south of the equator and was covered by a warm, shallow sea - a perfect environment for the sea creatures which we see today as fossils in the limestone floor.
Limestone
Limestone is a sedimentary rock, most of which originally formed by the accumulation of sediments on the sea floor (although some formed in fresh water). These sediments, which were afterwards turned into limestone rock, are composed of over 50 percent carbonate minerals.
Lots of sea creatures build shells or skeletons of calcium carbonate (chemical formula CaCO3) which is made from ingredients they extract from the sea water they live in. These sorts of sea creatures usually live in warm, shallow seas where the water is rich in those ingredients and the creatures can easily extract them. When those creatures die their shells and skeletons settle on the sea bed. Some of this carbonate material turns into a calcium rich mud but some of the fragments remain whole. Over millions of years this mixture of calcium rich mud and solid fragments gets deeper and deeper and eventually the lower layers get squeezed under the weight of the higher layers and turn into solid limestone rock in a process called lithification.
Pure calcium carbonate is bright white in colour and quite soft. You may have come across calcium carbonate like this before - it's called chalk 
. Most limestone isn't pure calcium carbonate though - it has other stuff mixed in with it like sand and silt which results in limestone of different colours - off whites, beiges, greys, blues and some is almost black. Most limestone is much harder than chalk which is good because it wouldn't make such great flooring material otherwise .
What is a fossil?
Fossils are the preserved remains of plants or animals. For such remains to be considered fossils, scientists have decided they have to be over 10,000 years old. The remains of sea creatures you can see in the limestone of the cathedral floor are millions of years old so they definitely qualify as fossils .
There are two main types of fossils, body fossils and trace fossils. Body fossils are the preserved remains of a plant or animal’s body. Trace fossils are the remains of the activity of an animal, such as preserved trackways, footprints, fossilised egg shells, and nests.
The fossils you need to look at to complete this EarthCache are all body fossils - but there are probably trace fossils in the limestone too, if you know what to look for.
Time to look at some of the types of fossilised sea creature commonly found in limestone. AT LEAST THREE of these fossil types are present in the limestone of the cathedral floor and you'll need to find an example of each of the three most obvious fossil types.
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Coral - Corals still inhabit our oceans today and at first glance, you may think that coral reefs are made up of rocks, but they are actually made up of tiny living organisms called polyps.
Some coral polyps build hard carbonate skeletons and also reproduce by making lots of copies of themselves, so that what starts off as a single coral polyp gradually grows into a whole community, with each polyp adding to the growing mass of carbonate material.
If you see a coral fossil in carboniferous limestone you're seeing the hard carbonate material left behind by the many polyps which built it millions of years ago.
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Gastropod - from the Greek gastér meaning stomach and podòs meaning foot - so you could call them stomachfoots
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Gastropods belong to the same family as snails and there are thousands of species still alive today. Over millions of years gastropods have evolved to be able to live in all sorts of environments. There are species which live in sea water, others which live in fresh water and still others which live entirely on land. You may have snails in your garden. Those found as fossils in limestone though will almost certainly have lived in the sea .
Sea snails, at least adult ones, usually have coiled or spiral shaped shells - which makes their fossils very easy to identify.
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Oyster - is the common name for a number of different families of salt-water bivalve molluscs.
Bivalves have two shells or valves, a left valve covering the left hand side of the body and a right valve covering the right hand side of the body. The valves are joined together by a ligament which allows the valves to open up, so the creature can feed for example, or close tightly together to protect the soft body from predators or to retain moisture when out of the water, at low tide for example. This ligament though decays when the creature dies and so the two valves are rarely fossilised together.
If you've ever seen a cockle or a mussel at the seaside or at the local fishmonger's stall, you've seen a bivalve.
Bivalves have inhabited the Earth for over 500 million years. They first appeared in the Mid Cambrian, about 300 million years before the dinosaurs.
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Brachiopod - a modern Latin word derived originally from the Greek brakhion meaning arm and podòs meaning foot. So if we think of gastropods as stomachfoots we might think of brachiopods as armfoots .
Brachiopods get their name because they have two muscular arms which they use to collect food.
Brachiopods have two valves or shells, just like bivalves but where bivalves have a left valve and a right valve which tend to be a mirror image of each other, brachiopods have an upper valve (which covers the upper side of the body) and a lower valve (which covers the lower side of the body) - which are not symmetrical.
In brachiopods one shell is larger than the other which has led to them being commonly known as lamp shells because they resemble early Roman oil lamps.
Brachiopods have existed for around 550 million years and at one time were one of the most abundant forms of life in the sea. The fossil record includes around 12,000 individual species but most of them became extinct a long time ago.
It isn't too difficult to identify brachiopod fossils in cut stone but they can be different shapes depending on the direction of the cut made when the rock was quarried - which is why there are two images to the right showing the most common shapes so you'll know what you're looking for
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Crinoid - also known as sea lilies because some species look more than a little like the flowers of the same name (as shown in the image to the right of a rare fossilised crinoid in which the soft parts were preserved as well as the hard parts). The ancient Greeks must have thought so anyway because crinoid is from the Greek words krinon and eidos which together translate roughly as lily form or lily shaped if you prefer.
Crinoids are very much animals though and definitely not plants. Though crinoids appeared in the Ordovician (488 mya), they survived the Permian mass extinction and diversified into hundreds of species which survive, today - one of which is shown in the image at the top of this page .
Crinoids can very basically be described as upside-down starfish with a stem. The stem of a crinoid extends down from what would be the top of a starfish, leaving the mouth of the organism opening skyward, with the feathery arms splayed out to catch any passing food particles. At the bottom of a crinoid stem there's a part called a holdfast which helps to anchor the crinoid to the sea bed.
Crinoid Stem Anatomy
The soft parts of ancient crinoids were very rarely fossilised and all that tends to remain - as with all the other fossilised sea creatures we've looked at so far - is the hard carbonate parts, so most crinoid fossils are just parts of the stem.
The stem is made from disc-shaped pieces of carbonate material called ossicles which are stacked upon each other and are hollow in the middle. In the living crinoid the ossicles are held together in a long column by ligaments and skin and the column of ossicles is described as articulated (see A in the above diagram).
After death the ligaments and skin decay and the ossicles start to break apart from each other (see B and C in the above diagram) - becoming disarticulated. If the ossicles are quickly covered up by mud or sand they stand a better change of staying together in a column like they were in life and fossilised together as the mud becomes rock. If ocean currents get to the ossicles before they are covered up by sand or mud though the ossicles tend to get separated from one another and scattered around. This is one of the reasons why crinoid fossils can appear as individual ossicles or as tube-like columns of ossicles, short ones and sometimes longer ones.
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Typical crinoid fossil appearance - so the only parts of crinoids that are fossilised most of the time are the hard parts - the ossicles of carbonate material which, in life, are stacked one on top of the other in the long stem. Because they are little tube-like structures with a hole through the middle, some people think that they look like polo mints - the ossicle in the top-left corner of the image on the right is a good example of this.
If you're lucky enough to see a cylindrical stack of articulated ossicles that haven't been cut through, the narrow grooves around the cylinder make the stack look a bit like a bolt or a screw - which is why people sometimes call these fossils screws
More often than not though, when the limestone is cut into slabs the saw blade cuts through the length of any articulated stacks of ossicles, revealing the inside parts and you get what looks a bit like a zip with two rows of jagged teeth pointing in towards each other - like the example toward the right-hand side of the image to the right.
Sometimes you'll see crinoid fossils which have more of a pointed oval shape. This happens when the rock is cut into slabs and the saw blade cuts through the fossil on an oblique angle.
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