Introduction
Fossils are always fun to look at and I'd learned that there was some nice Portland Stone in Manchester and so I decided to pay another visit as I can spend hours perusing rocks and stuff while everyone else is busy shopping 
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I certainly wasn't disappointed - this particular building is crammed with fossilised sea creatures of various types which were alive around 150 million years ago.
You won't need a magnifying glass or anything like that to complete the earthcache but if you have one, or ideally a hand lens or even a magnifying app on your smartphone, you'll be able to see the makeup of the rock and any fossils it includes in much greater detail.
It's important that you get a good look at the whole building frontage if you want to see all the fossils and I recommend starting at the published coordinates and working your way toward the Stage 2 waypoint where you can see around the corner onto the end of the frontage and beyond.
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Based on your study of the building stone and using the information on the cache page please tell me:
- Which particular types of fossilised organism can you find in the walls of this building? Make sure to start at the given coordinates and work your way around to the Stage 2 waypoint where the frontage turns a corner or you might miss some!
- Describe how the individual fossil organisms are grouped / arranged across the building frontage. WHICH fossils do you see WHERE (Consider as you make your way from one end of the building to the other - are the same fossil types present everywhere or do particular types appear in different slabs / locations?)
- Is all of this Portland stone from a single bed or from more than one bed? How do you know?
- Do you think the ooids in this Portland stone are made of aragonite or calcite? How do you arrive at this conclusion?
- 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
Portland stone is a sedimentary rock formed in shallow, tropical seas during the Tithonian stage of the Jurassic period, roughly 150 million years ago, and is quarried on the Isle of Portland, Dorset. The quarries consist of beds of white-grey limestone separated by chert beds. It has been used extensively as a building stone throughout the British Isles and notably in major public buildings in London such as St Paul's Cathedral and Buckingham Palace.
The name Portland Stone refers to a family of closely related limestone variants with differing characteristics rather than a single, uniform rock type. In some types of Portland stone fossilised sea creatures are abundant while in other types fossils are virtually non-existent.
Egg Stone?
Ooids are tiny calcium carbonate sand grains (strictly between 0.25mm and 1mm) which form by accretion i.e. building up in layers. The name ooid is derived from the greek word for egg and it's pretty descriptive in this instance as the grains do look a bit egg shaped - as shown in the image above 
According to scientists, each ooid starts off as a tiny fragment of material which settles in carbonate mud on the seabed, a fragment of shell or a quartz grain for example which acts as a nucleus for the ooid formation. As that nucleus gets rolled in the mud by tidal currents it gets coated by successive fine layers of minerals from the mud (aragonite, calcite or both) - a bit like a snowball getting larger as it picks up more snow - which is what gives ooids their nice rounded shapes.
When individual ooids become cemented together by more sediments from the carbonate mud surrounding them they form a type of rock called an oolite and most oolites are types of limestone - just like the Portland stone which is the subject of this EarthCache.
Portland Freestone is an oolitic limestone with variable shell content. To really appreciate the individual ooids they are best viewed through a hand lens or under some other form of magnfication, although if your eyesight is exceptionally good you might be able to make them out with the naked eye.
The Freestone Member
A freestone is a fine grained stone, typically sandstone or limestone, which can be cut easily in any direction without shattering or splitting, properties which make them particulary suitable for building and for carving.
The Portland Freestone Member forms the upper part of the Portland Limestone Formation and comprises four distinct beds, three of which are extensively used for these purposes:
Roach
The uppermost bed of the Portland Freestone Member is known as Portland Roach. This bed represents a shelly beach dominated by shell fossils. Many of the shells, being formed of aragonite, have been dissolved away by acidic groundwater, leaving behind an empty mould or cast of the shell which was once there.
Whitbed
The fossil content in Portland Whitbed can vary quite considerably, being described as shelly at one end of that spectrum and clean when shells are sparse. Fossil rich varieties tend to be heavy in calcite fossils such as shells of oysters and other bivalves as well as patches of the distinctive reef building alga Solenopora Portlandica.
Basebed
The Basebed is often considered to be the finest quality Portland Stone available.
Typically Basebed has a very clean, homogeneous texture with a negligible shell content making it eminently suitable for carving fine detail in deep relief. It is not quite as durable in exposed locations as Whitbed but makes an unbeatable monumental and carving stone for use on very many prestigious building projects.
Aragonite, calcite or both? (Poly who?)
Polymorph is a word that scientists use to describe distinct substances which have the exact same chemical formula but arranged in a differing atomic and crystalline structure.
The most commonly known examples of polymorphs are diamond - one of the hardest substances known to man and graphite which is very, very soft by comparison. Diamond and graphite are polymorphs of Carbon - two substances with exactly the same chemical formula but with differing atomic and crystalline structures. The individual properties of those two polymorphs - the hardness of diamond and the softness of graphite are entirely down to the way the carbon atoms are arranged in each form.
Calcium carbonate (CaCO3) is one of the most widely existing minerals in nature and has three polymorphs known as vaterite, aragonite and calcite. Vaterite dissolves very easily in water so is useless to sea creatures who need to build skeletons or shells - which is why the skeletons and shells of sea creatures are always formed of either aragonite, calcite, or a combination of both.
Aragonite has a stucture that is more resistant to stress than calcite, explaining why organisms that live in high-enegy environments (e.g. corals) prefer aragonite skeletons over calcite. Calcite, on the other hand, is less prone to dissolution, which is why organisms that live at greater depth (e.g. benthic foraminifera) prefer calcite over aragonite, especially when they live below the Aragonite Compensation Depth (the depth at which the rate at which aragonite dissolves increases drastically).
Bivalves, which rely on strong muscles to pull the two halves of their shell tightly closed, need something strong to anchor those muscles to. For this reason their inner shell may be built from aragonite for strength but some have the added benefit of an outer layer of calcite to protect them from the effects of dissolution.
Fossils
Fossils are any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals, hair and petrified wood.
Fossils are found almost exclusively in sedimentary rocks because the sediments are laid down in such a way that they preserve the living organism's remains, or a cast or mould of its shape during lithification (the process of becoming rock).
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Gastropods - Aptyxiella portlandica - found almost exclusively in the roach and commonly known as portland screws because their spirals make them look a bit like screw threads.
The shells of this particular gastropod were made of aragonite. Aragonite usually reverts back to calcite, the more stable polymorph of calcium carbonate, during the fossilisation process, unless lithification occurs relatively quickly in which case there is insufficient time to allow the conversion to take place and the aragonite remains in the rock.
When the lithified rock was uplifted during a later mountain-building event it was laid open to percolation by acidic groundwater which dissolved the aragonite shells leaving behind moulds and casts of the shells that were once there and these are the fossil forms we see today.
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Cephalopods - Ammonites - just about the easiest fossilised sea creature to recognise thanks to their coiled shells.
Many Ammonite fossils reveal the inner structure of the shell with its multiple chambers separated by walls known as septa with complicated frilled edges which scientists use to identify which particular type of ammonite they are looking at.
The image to the right shows an external view of an ammonite fossil, displaying the numerous ridges on the shell, perpendicular to the shell walls.
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Bivalves - Camptonectes lamellosus - is a variety of pecten. Pecten are scallop shells and characteristically have well developed ‘wings’ along their hinge.
This variety is extremely common among some of the Portland Stone beds and are often well preserved by virtue of their shells, like those of oysters, being built from calcite rather than aragonite.
The small holes in the shell shown to the right have been left by mussels capable of boring through solid rock.
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Bivalves - Laevitrigonia gibbosa - with the exception of one surviving variety that exists in just one place in Australia, Trigonia are now extinct.
Trigoniid bivalves are thick-walled shells which made them very well suited to very high energy environments. The variety most commonly found in the Portland Stone is Laevitrigonia gibbosa which is found in two forms with highly ornamented shells, one of which is ribbed and one of which is decorated with numerous costae, or knobs - if you prefer.
These shells too were built from aragonite which has since been dissolved away by acidic groundwaters, leaving behind external moulds which show the ribs and costae or internal casts which replicate the internal shape of the shell.
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Bivalves - Liostrea expansa - is a thick-shelled oyster of medium size that grew in a somewhat cup-shaped form.
Although unremarkable in appearance, this oyster is quite striking in the Portland Freestone because, like other oysters, it has a shell of bluish calcite which has survived unaltered due to its robust construction of numerous fine calcite layers.
The image to the right shows a cross section through an oyster shell but large collections of shell fragments are quite common in the Whitbed, as shown in the Whitbed example further up this page and in the image at the top of the page.
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Calcareous alga - Solenopora Portlandica - an extinct family of red marine algae important in reef building and numerous in shallow water carbonate environments like the one the Portland Stone was laid down in.
Many forms show millimetre-to-centimetre-sized alternating bands of lighter and darker material which are believed to be seasonal growth bands with lighter Winter layers and darker Summer layers.
Appearance-wise this alga seems to be commonly described as resembling cauliflower and I suppose it does, a little bit, if you took a cauliflower head and cut it straight through the middle vertically and then looked at the cut face - but it's still not a great description. To me it resembles the small-scale ebb and flow of tiny, rounded waves and the patterns they might leave behind, still not a great description but I tried .
Sometimes there are cavities between the layers - as shown in the image to the right. Sometimes there are no cavities. Sometimes the fossilised alga is harder than the surrounding stone and more resistant to weathering and ends up raised above the surface of the softer, more easily weathered stone as it recedes.
If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there 
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