Granite
Granite is a common type of igneous rock that is granular in texture. Granites can be predominantly white, pink, or grey in colour, depending on their mineralogy. The word "granite" comes from the Latin granum, “ grain”, in reference to the coarse-grained structure of such a rock. Granite has at least 20% quartz and up to 65% feldspar in its makeup, along with mica crystals.
Granite is formed through the cooling and solidification of molten rock called magma or lava. When the magma cools and solidifies, randomly arranged interlocking crystals form. The size of the crystals depends on how quickly the molten magma solidified. The more slowly the magma cools, the bigger the crystals are.
Occasionally, you will see shapes in granite that have a different colour and density than the surrounding rock. These are xenoliths. Xenoliths are formed when the magma rises to the Earth’s surface through deep cracks known as pipes. As the magma rises, it tears off bits of rock from magma pipe in which it is traveling. These bits of rock become trapped in the magma but do not melt into it. Usually they are a different type of rock to that which formed the granite and can have originated deep in the centre of the earth. When the magma cools, the trapped rocks appear as a foreign object in the newly formed granite.
Granite is still quarried in Shap, Cumbria. Shap Granite is Devonian in age (417 to 354 million years old) and is a very distinctive granite. It is used for decorative work throughout the UK, often with a polished finish. Shap Granite is rich in feldspar, mica and quartz with distinctive crystals of pink feldspar which can be up to 5 or 6 cm in length. Two varieties exist; dark Shap which is brownish red with black and grey colouration, and light Shap which is predominantly pink and grey in colour.
Portland Stone
Portland stone is a limestone from the Jurassic period (201.3–145 million years ago). It was formed in a shallow, tropical Jurassic sea towards the end of this geological period. It was quarried on the Isle of Portland, Dorset where the quarries consist of beds of white-grey limestone. There are three distinct beds known as Portland Base Bed, Portland Whitbed and Portland Roach. The Base Bed is relatively free of fossils, the Whitbed contains variable amounts of fossils and the Roach is dominated by shell fossils.
When seawater is warmed by the sun, its capacity to hold dissolved gas is reduced. This means that dissolved carbon dioxide (CO2) is released into the atmosphere as a gas. Then, calcium and bicarbonate ions within the water combine to form calcium carbonate or calcite (CaCO3) which is the main constituent of most limestones. If you live in a hard water area, lime scale build up in your kettle is formed by a similar process.
When Portland Stone was formed, minute crystals of calcite accumulated, forming lime mud or micrite which covered the sea floor. As small particles of sand or shell fragments on the sea bed were rolled around in the muddy micrite, they grew into balls as they became coated with layers of calcite, the way a snowball grows in size as it is rolled around in the snow. Over time, billions of these balls became cemented together to form the limestone we now call Portland Stone. The degree of cementation in Portland Stone is such that the stone is sufficiently well cemented to allow it to resist weathering but not so well cemented that it can't be worked (cut and carved) by masons. This is one of the reasons why Portland Stone is favoured as a monumental and architectural stone.
To claim this Earth Cache, please message me with the answers to the questions below:
1. Name the stone used for the main body of the building and describe its appearance and texture
2. Are there any visible fossils in the rock? Using your knowledge of the building material, which "bed" did it come from?
3. Examine the columns and pilasters along the side of the building and around the porch doorway. Describe their appearance and name the rock they are made from.
4. Look at the columns by the entrance porch. Estimate the size of the largest xenolith and describe its appearance
5. Why do you think these particular building materials were chosen for this building?
6. Optional: Please post a photo of your visit. We like to see visitors to our lovely border city