The Westgate is a medieval gatehouse in Canterbury, Kent. This 60-foot (18 m) high western gate of the city wall is the largest surviving city gate in England. Built of Kentish ragstone around 1379, it is the last survivor of Canterbury's seven medieval gates, still well-preserved and one of the city's most distinctive landmarks. The road still passes between its drum towers. This Grade I listed building houses the West Gate Towers Museum.
The gatehouse is expensively faced in coursed ashlar of Kentish ragstone. It has battered plinths to the drum towers, battlements, machicolations (murder holes!) and eighteen gunloops: a high number for a gateway, and among the earliest gunholes in Britain. The gunloops would have been added by the beginning of the 15th century. It had a drawbridge over the Stour, a portcullis and wooden doors.
Weathering is the process where rock is dissolved, worn away or broken down into smaller and smaller pieces. Once the rock has been weakened and broken up by weathering it is ready for erosion.
Physical weathering is caused by the effects of changing temperature on rocks, causing the rock to break apart. The process Is sometimes assisted by water.
Chemical weathering is caused by rain water reacting with minerals in rocks to form new minerals(clays) and soluble salts. These reactions occur particularly when the water Is slightly acidic.
Erosion happens when rocks and sediments are picked up and moved to another place by ice, water, wind or gravity. In this case it is water. Wave erosion is the wearing away of material from a coastal profile. Waves, generated by storms, wind, or fast moving motorcraft, can cause this erosion, which may include losses of sediment and rocks, or just the temporary redistribution of sediments.
Sedimentary rocks are rocks formed from sediment. They are deposited over time, and often show layers which can be seen in cliffs. Sediments are usually formed from matter which falls to the bottom of oceans and lakes. The matter includes tiny pieces of other rocks, and dead animals, plants and microorganisms. The three most common sedimentary rocks are limestone, sandstone and shale.
Limestone is a sedimentary rock. Historical information as well as buildings still standing point to limestone as a very durable and time-tested building material. It is composed mostly of calcium carbonate(CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the build up of shells, coral, algae and debris. It can also be a chemical sedimentary rock formed by the precipitation (release) of calcium carbonate from lake or ocean water
Sandstone is a sedimentary rock made up of tiny grains of mineral, rock, or another organic material. It also contains a 'glue'-type material that binds the sand grains together and may contain silt or clay particles that fill the gaps between the sand grains. Sandstone also is often mined for use as a construction material or as a raw material used in manufacturing. Inside the Earth, it often serves as an aquifer (underground layer of rock which holds substances) for groundwater or as a reservoir for oil and natural gas.
Shale is a fine-grained sedimentary rock that forms from the silt and mud compacting. This composition places shale in a category of sedimentary rocks known as "mudstones." Shale is different from other mudstones because it is laminated (made of many layers) and fissile (often splits into thin pieces along the laminations).
To complete this EarthCache, you need to answer the following questions:
1) Describe the texture of the rocks on the outside of the arch compared to inside the archway, and why this is.
2) Describe and explain the long term impact that the river is having on the structure of the base of the bridge
3) Using the information in description and your own knowledge of what you can see here, suggest What type of rock Kentish ragstone is and what features tell you this.
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