Much of the stone used to build the Cathedral is Portland stone. This limestone, quarried from Portland in Dorset, was formed in a shallow, tropical Jurassic sea towards the end of this geological period (Tithonian faunal stage). This type of stone adorns many of the well-known public buildings of London. The white/grey limestone has been used as a building material in the United Kingdom since Roman times.
There are three main types of rock: sedimentary, igneous and metamorphic.
Formation of sedimentary rocks
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks are deposited. The deposited rocks build up in layers, called sediments. This process is called sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called compaction. The water is squeezed out from between the pieces of rock and crystals of different salts form. The crystals form a sort of glue that sticks or cements the pieces of rock together. This process is called cementation. These processes eventually make a type of rock called sedimentary rock. It may take millions of years for sedimentary rocks to form.
The oldest layers are at the bottom and the youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants trapped in the sediments as the rock was formed.
Formation of igneous rocks
The inside of the Earth is very hot - hot enough to melt rocks. Molten (liquid) rock forms when rocks melt. The molten rock is called magma. When the magma cools and solidifies, a type of rock called igneous rock forms. Igneous rocks contain randomly arranged interlocking crystals. The size of the crystals depends on how quickly the molten magma solidified. The more slowlythe magma cools, the bigger the crystals.
Unlike sedimentary rocks, igneous rocks do not contain any fossils. This is because any fossils in the original rock will have melted when the magma formed.
Formation of metamorphic rocks
Earth movements can cause rocks to be deeply buried or squeezed. As a result, the rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks.
Sometimes, metamorphic rocks are formed when rocks are close to some molten magma, and so get heated up. When a metamorphic rock is formed under pressure, its crystals become arranged in layers. Slate, which is formed from shale, is like this. Slate is useful for making roof tiles because its layers can be split into separate flat sheets. Marble is another example of a metamorphic rock. It is formed from limestone, so has the same chemical composition (they are both calcium carbonate). Metamorphic rocks can be formed from any other type of rock - sedimentary or igneous.
Metamorphic rocks sometimes contain fossils if they were formed from a sedimentary rock, but the fossils are usually squashed out of shape.
TASKS
Your task:
1) Examine the red flagstones at the top of the first flight of steps, describe the fossils you see (colours, shape, total number of fossils, general appearance etc).
2) Do you think that the red polished stones are made from limestone or marble?
3) Explain your answer by using the appearance of the fossils to help you in your judgement.
4) As an optional extra take a picture of yourself and/or your GPSr with St Paul's in the background.