Portland stone
The base of the sculpture stands on a plinth of Portland stone. Portland stone is a limestone from the Tithonian stage of the Jurassic period and quarried on the Isle of Portland, Dorset. The quarries consist of beds of white-grey limestone separated by the chert beds. It has been used as a building stone throughout Britiain.
Portland stone is formed in a marine environment, on the floor of a shallow, warm, sub-tropical sea probably near land. When seawater is warmed by the sun, its capacity to hold dissolved gas is then reduced, which consequently dissolves carbon dioxide and is released into the atmosphere as a gas. Calcium and bicarbonate ions within the water are then able to come together and Calcium Carobonate is formed, which is known as a precipitate. Billions of minute crystals of precipitated calcium carbonate known as calcite are then accumulated forming lime mud called micrite which covers the sea floor. Small particles of sand or organic matters such as shell fragments, then form a nucleus, which become coated with layers of calcite. The calcite gradually accumulates around the fragments of shell in concentric layers, forming small balls. Over time, countless billions of these balls, known as ooids or ooliths, became partially cemented together by more calcite, to form the oolitic limestone which we now call Portland Stone. Fortunately, the degree of cementation in Portland Stone is such that the stone is sufficiently well cemented to allow it to resist weathering but can easily be worked by stone masons. This is why Portland Stone is used as bases for many monuments or structures.
Tasks to complete
To log this cache, please complete the tasks below using the information above and your observations at ground zero. Please email your answers to us via our profile page. You can log your find once you have sent the email, there is no need to wait for a reply.
1. Looking at the Portland stone base, describe the general appearance, colour and describe the ooliths within the stone, how big are they? and how do they appear within the stone?
2. Estimate the height and width of the stone base?
3. Portland Stone is a type of limestone formed in which period of history?
4. Why do stonemasons like to use Portland stone?
5. What year was the statue unveiled?
6. As an optional extra, post a picture of yourself or your GPS at the statue.