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Micklegate Bar
Micklegate Bar was historically the main entrance to the city for anyone arriving from the South, and is seen as the most important of York’s four main medieval gateways and the focus for grand events. The name comes from 'Micklelith', meaning great street.
This is an accessible EarthCache that will give you an opportunity to look at the geology here at Micklegate in York.
As this is an EarthCache you need to send me your answers via email or the message centre.
GEOLOGY LESSON:
Micklegate Bar, along with the city walls is built partly of magnesian limestone from near Tadcaster, about 10 miles to the south-west. This is a very variable stone but it often has more dominant shades of light brown and yellow than other, uniformly white/grey limestones. This variable colour is partly because of the metal salts it contains; including magnesium (giving the stone its name), but it is often when it contains iron salts that it has a warmer colour which turns more of a pink when it is scorched by fire, something relevant to the York gatehouses which were particularly damaged by fire in past wars.
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
Magnesian limestone was laid down at the bottom of a shallow, very salty bit of sea where few creatures lived so it has few fossils. It was soaked in chemicals while turning into rock, thus bringing in the magnesium and iron, dissolving shell fossils, and making some parts of the rock vulnerable to acid rain. Blocks made of this vulnerable rock have traditionally weathered quickly, as the stone flakes off or turns to powder when water soaks in rather than just washing over the surface. Carvings and arrow slits on the walls are most likely to get soaked and start weathering in this way. When there are small patches of vulnerable stone in a big block these will become holes when the block suffers from the elements; one way nature produces hollows in the stone of the Walls.
The Romans also used a coarse grained sandstone known as “millstone grit”, seen here at Micklegate Bar. The name "Millstone Grit" came about because certain varieties of Gritstone were commonly used to make millstones. Gritstone is actually a variation of sandstone and is made from sand, grit and rounded pebbles of quartz and some feldspar.
Sandstone is a sedimentary rock made up of tiny grains of mineral, rock, or another organic material. 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.
Sandstone rock is composed mostly of quartz sand, but it can also contain significant amounts of feldspar, and sometimes silt and clay. Sandstone that contains more than 90% quartz is called quartzose sandstone. When the sandstone contains more than 25% feldspar, it is called arkose sandstone and has composition similar to granite. When there is a significant amount of clay or silt, geologists refer to the rock as argillaceous sandstone. The colour of sandstone varies, depending on its composition. Because it is composed of light coloured minerals, sandstone is typically light tan in colour. Other elements, however, create colours in sandstone.
Weathering
The weathering of rocks is quite simply the breaking down of the stone at the Earth's surface, by action of rainwater, extremes of temperature, or biological activity. It doesn't involve the removal of rock material. There are three main types of weathering:
Physical weathering – a physical force breaks down rocks into smaller fragments. Freeze-thaw action is the most well-known type where expanding ice forces cracks to open splitting rocks apart. The growth of salt crystals in cracks in coastal rocks works in a similar way.
Chemical weathering – rocks are affected by acids and corroded by solution weathering; rainfall is weak carbonic acid although the acidity of rainfall can be increased by pollution. Absorbing these substances turns the rainwater into a weak carbonic acid which then corrodes the rocks it comes into contact with.
Biological weathering is the impact of living organisms on the stone. Plants put down roots through joints or cracks in the rock in order to find moisture. As the plant grows, the roots gradually pull the rock apart. Many animals bore into rocks for protection either by scraping away the grains or secreting acid to dissolve the rock. Even the tiniest bacteria, algae and lichens produce chemicals that help break down the rock on which they live, so they can get the nutrients they need.
Questions:
1) Take a look at the colour, texture and general appearance of the main arch, and describe what you see. Based on this, which rock do you think the main arch is made from?
2)The smaller 'double arch' is one part that is built of sandstone, looking at the minerals/grains, do you think that it's argillaceous, arkose or quartzose sandstone?
3) Have a look for signs of weathering; has the stone been affected by physical/chemical/biological weathering or a combination? Please explain your conclusion
Please add a photo of you/your GPS/personal item at the gatehouse though not giving any answers away :)