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Jurassic Wedding Cake

A cache by Young Thropp Send Message to Owner Message this owner
Hidden : 06/30/2014
1.5 out of 5
1 out of 5

Size: Size: other (other)

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Geocache Description:

One of the country's finest building stones, namely Portland Limestone from the geological Jurassic Period, is the subject of this EarthCache based on Stockport Town Hall.

Please note that this is an Earthcache; a find will only be accepted if the Logging Requirements have been met.

Portland Stone appears on buildings of note throughout the world with Buckingham Palace and St Paul's Cathedral being two high profile buildings in London and Manchester Central Library and Stockport Town Hall being notable examples in the North West

This sedimentary limestone has been quarried for thousands of years from the Isle of Portland in Dorset, for its ease of working and its striking grey/white colour.
From fossil evidence it can be seen that the rock was originally deposited in probably a warm sea water lagoon which was rich in dissolved Calcite, a mineral form of calcium carbonate. The purity of the Portland Stone also suggests that the sea was especially clear water, and the seabed would have been covered by an ever increasing layer of grey/white calcareous mud. In the same way that kettles in hard water areas (high levels of dissolved lime) furr up with limestone deposits, the mud on the seabed in Jurassic times would have built up around small impurities forming small balls of limestone called ooliths. In turn these ooliths would have been cemented together by more calcium carbonate forming solid limestone. A common fossil in the stone is Ostrea (oyster) This Portland Stone proved to be an easy to work and popular building stone. However as with all sedimentary rocks there are variations in quality, surface appearance, resistance to weathering by acid rain, and as a consequence variations in price! Directly above the Portland Stone, geologically, is a workable band of limestone called Portland Roach. This limestone is very resistant to erosion but at some time in the past acidic rainwater or seawater percolated through the rock dissolving the Aragonite/ Nacre (a variety of calcium carbonate known popularly as Mother of Pearl), of fossil shells creating fossil shaped holes in the stone known as casts. This detracts from its visual appeal in high profile buildings, but where the stone has been used for building purposes two of the more obvious fossils can be clearly identified. The two fossils Aptyxiella portlandica a Gastropod (snail) and Trigonia gibbosa a bivalve lamellibranch ((mussel) were named by the quarrymen as Portland Screw and Osses' Ead respectively.

Logging requirements:-
1. Erosion. Look at the ballustrade in Edward Street and explain why there are differences in the surface of the verticals in the ballustrade
2. Fossils. Some of the verticals in the ballustrade clearly display a particular type of marine invertebrate. What do you think it is?

Additional Hints (Decrypt)

Svaq qryrgvba? Cyrnfr ernq yvfgvat

Decryption Key


(letter above equals below, and vice versa)

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